129 results on '"Fred F. Pollitz"'
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2. Kinematic Slip Model of the 2021 M 6.0 Antelope Valley, California, Earthquake
- Author
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Fred F. Pollitz, Chuck W. Wicks, and William C. Hammond
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Geology ,QE1-996.5 - Abstract
We present a kinematic slip model of the 8 July 2021 Antelope Valley earthquake from a finite-source inversion based on regional seismic waveforms and static offsets from Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR). Seismic waveforms are employed at 6 s dominant period out to 100 km from the epicenter, and the combined GPS and InSAR datasets cover the near field and far field out to ∼100 km and constrain the overall rupture size. The aftershock pattern defines a nearly north-striking, 50° east-dipping fault plane. We find a unilateral rupture along this fault plane propagating southward and updip with predominantly normal slip up to ∼1.5 m. The estimated seismic moment of 8.47×10^17 N·m is equivalent to Mw 5.92. A finite-source inversion that retains seismic waveforms and GPS static offsets but omits InSAR range changes yields a seismic moment of 1.08×10^18 N·m (Mw 5.99). Despite vigorous aftershock activity between 10 km and Earth’s surface, coseismic slip is concentrated in the depth interval 7–10 km.
- Published
- 2022
- Full Text
- View/download PDF
3. Preface to the Focus Section on Deformation Models for the U.S. National Seismic Hazard Model
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Fred F. Pollitz, Alexandra E. Hatem, and Kaj M. Johnson
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Geophysics - Published
- 2022
4. Western U.S. Deformation Models for the 2023 Update to the U.S. National Seismic Hazard Model
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Fred F. Pollitz, Eileen L. Evans, Edward H. Field, Alexandra E. Hatem, Elizabeth H. Hearn, Kaj Johnson, Jessica R. Murray, Peter M. Powers, Zheng-Kang Shen, Crystal Wespestad, and Yuehua Zeng
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Geophysics - Abstract
This report describes geodetic and geologic information used to constrain deformation models of the 2023 update to the National Seismic Hazard Model (NSHM), a set of deformation models to interpret these data, and their implications for earthquake rates in the western United States. Recent updates provide a much larger data set of Global Positioning System crustal velocities than used in the 2014 NSHM, as well as hundreds of new faults considered as active sources for the 2023 NSHM. These data are interpreted by four geodetic models of deformation that estimate fault slip rates and their uncertainties together with off-fault moment release rates. Key innovations in the 2023 NSHM relative to past practice include (1) the addition of two new (in addition to two existing) deformation models, (2) the revision and expansion of the geologic slip rate database, (3) accounting for fault creep through development of a creep-rate model that is employed by the four deformation models, and (4) accounting for time-dependent earthquake-cycle effects through development of viscoelastic models of the earthquake cycle along the San Andreas fault and the Cascadia subduction zone. The effort includes development of a geologic deformation model that complements the four geodetic models. The current deformation models provide a new assessment of outstanding discrepancies between geologic and geodetic slip rates, at the same time highlighting the need for both geologic and geodetic slip rates to robustly inform the earthquake rate model.
- Published
- 2022
5. Viscoelastic Fault-Based Model of Crustal Deformation for the 2023 Update to the U.S. National Seismic Hazard Model
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Fred F. Pollitz
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Geophysics - Abstract
The 2023 update to the National Seismic Hazard (NSHM) model is informed by several deformation models that furnish geodetically estimated fault slip rates. Here I describe a fault-based model that permits estimation of long-term slip rates on discrete faults and the distribution of off-fault moment release. It is based on quantification of the earthquake cycle on a viscoelastic model of the seismogenic upper crust and ductile lower crust and mantle. I apply it to a large dataset of horizontal and vertical Global Positioning System (GPS) interseismic velocities in the western United States, resulting in long-term slip rates on more than 1000 active faults defined for the NSHM. A reasonable fit to the GPS dataset is achieved with a set of slip rates designed to lie strictly within a priori geologic slip rate bounds. Time-dependent effects implemented via a “ghost transient” have a profound effect on slip rate estimation and tend to raise calculated slip rates along the northern and southern San Andreas fault by up to several mm/yr.
- Published
- 2022
6. Postseismic Relaxation Following the 2019 Ridgecrest, California, Earthquake Sequence
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Fred F. Pollitz, Charles W. Wicks, Jerry L. Svarc, Eleyne Phillips, Benjamin A. Brooks, Mark H. Murray, and Ryan C. Turner
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Geophysics ,Geochemistry and Petrology - Abstract
The 2019 Ridgecrest, California, earthquake sequence involved predominantly right-lateral strike slip on a northwest–southeast-trending subvertical fault in the 6 July M 7.1 mainshock, preceded by left-lateral strike slip on a northeast–southwest-trending subvertical fault in the 4 July M 6.4 foreshock. To characterize the postseismic deformation, we assemble displacements measured by Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar. The geodetic measurements illuminate vigorous postseismic deformation for at least 21 months following the earthquake sequence. The postseismic transient deformation is particularly well constrained from survey-mode GPS (sGPS) in the epicentral region carried out during the weeks after the mainshock. We interpret these observations with mechanical models including afterslip and viscoelastic relaxation of the lower crust and mantle asthenosphere. During the first 21 months, up to several centimeters of horizontal motions are measured at continuous GPS and sGPS sites, with amplitude that diminishes slowly with distance from the mainshock rupture, suggestive of deeper afterslip or viscoelastic relaxation. We find that although afterslip involving right-lateral strike slip along the mainshock fault traces and their deeper extensions reach a few decimeters, most postseismic deformation is attributable to viscoelastic relaxation of the lower crust and mantle. Within the Basin and Range crust and mantle, we infer a transient lower crust viscosity several times that of the mantle asthenosphere. The transient mantle asthenosphere viscosity is ∼1.3×1017 Pa s, and the adjacent Central Valley transient mantle asthenosphere viscosity is ∼7×1017 Pa s, about five times higher and consistent with an asymmetry in postseismic horizontal motions across the mainshock surface rupture.
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- 2021
7. Seismic and Geodetic Analysis of Rupture Characteristics of the 2020 Mw 6.5 Monte Cristo Range, Nevada, Earthquake
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Fred F. Pollitz, Thorne Lay, Chengli Liu, Xiong Xiong, and Jiao Xu
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Geophysics ,010504 meteorology & atmospheric sciences ,Geochemistry and Petrology ,Range (biology) ,Geodetic datum ,010502 geochemistry & geophysics ,01 natural sciences ,Geology ,Seismology ,0105 earth and related environmental sciences - Abstract
The largest earthquake since 1954 to strike the state of Nevada, United States, ruptured on 15 May 2020 along the Monte Cristo range of west-central Nevada. The Mw 6.5 event involved predominantly left-lateral strike-slip faulting with minor normal components on three aligned east–west-trending faults that vary in strike by 23°. The kinematic rupture process is determined by joint inversion of Global Navigation Satellite Systems displacements, Interferometric Synthetic Aperture Radar (InSAR) data, regional strong motions, and teleseismic P and SH waves, with the three-fault geometry being constrained by InSAR surface deformation observations, surface ruptures, and relocated aftershock distributions. The average rupture velocity is 1.5 km/s, with a peak slip of ∼1.6 m and a ∼20 s rupture duration. The seismic moment is 6.9×1018 N·m. Complex surface deformation is observed near the fault junction, with a deep near-vertical fault and a southeast-dipping fault at shallow depth on the western segment, along which normal-faulting aftershocks are observed. There is a shallow slip deficit in the Nevada ruptures, probably due to the immature fault system. The causative faults had not been previously identified and are located near the transition from the Walker Lane belt to the Basin and Range province. The east–west geometry of the system is consistent with the eastward extension of the Mina Deflection of the Walker Lane north of the White Mountains.
- Published
- 2021
8. Coseismic Fault Slip and Afterslip Associated with the 18 March 2020 M 5.7 Magna, Utah, Earthquake
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Fred F. Pollitz, Jerry L. Svarc, and Charles W. Wicks
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Geophysics ,010504 meteorology & atmospheric sciences ,Fault slip ,010502 geochemistry & geophysics ,01 natural sciences ,Geology ,Seismology ,0105 earth and related environmental sciences - Abstract
The 2020 Magna, Utah, earthquake produced observable crustal deformation over an ∼100 km2 area around the southeast margin of Great Salt Lake, but it did not produce any surface rupture. To obtain a detailed picture of the fault slip, we combine strong-motion seismic waveforms with Global Positioning System static offsets and Interferometric Synthetic Aperture Radar observations to obtain kinematic and static slip models of the event. We sample the regional seismic wavefield with three-component records from 68 stations of the University of Utah Seismograph Stations network. We find that coseismic slip and afterslip, with predominantly normal slip, distributed on a shallowly west-dipping plane, possibly augmented by afterslip on a steeply northeast-dipping plane, best fits the joint dataset. The west-dipping plane locates near previously inferred sources of interseismic creep at depth. Hence, the earthquake may have occurred on the down-dip extension of the Wasatch fault and activated further slip (afterslip) at shallow depth east of the hypocenter. This inferred afterslip may have driven the vigorous aftershock activity that was concentrated east of the hypocenter.
- Published
- 2021
9. Rupture Process of the M 6.5 Stanley, Idaho, Earthquake Inferred from Seismic Waveform and Geodetic Data
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Charles W. Wicks, Fred F. Pollitz, and William C. Hammond
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Geophysics ,010504 meteorology & atmospheric sciences ,Process (computing) ,Waveform ,Geodetic datum ,010502 geochemistry & geophysics ,01 natural sciences ,Geology ,Seismology ,0105 earth and related environmental sciences - Abstract
The 2020 M 6.5 Stanley, Idaho, earthquake produced rupture in the north of the active Sawtooth fault in the northern basin and range at depth, without any observable surface rupture. Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data yield several millimeters of static offsets out to ∼100 km from the rupture and up to ∼0.1 m of near-field crustal deformation. We combine the GPS and InSAR data with long-period regional seismic waveforms to derive models of kinematic slip and afterslip. We find that the coseismic rupture is complex, likely involving up to 2 m combined left-lateral strike slip and normal slip on a previously unidentified ∼south-southeast-striking fault. This slip is predominantly left-lateral strike slip, different from the dominant east-northeast–west-northwest normal faulting of the region. At least one ∼northeast-trending fault, likely associated with the Trans-Challis fault system, is inferred to have accommodated a few decimeters of right-lateral afterslip, consistent with vigorous aftershock activity at depth along northeast-trending lineations.
- Published
- 2020
10. Kinematics of Fault Slip Associated with the 4–6 July 2019 Ridgecrest, California, Earthquake Sequence
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Jessica R. Murray, Benjamin A. Brooks, Fred F. Pollitz, Kenneth W. Hudnut, Evelyn Roeloffs, Sarah E. Minson, David Mencin, Katherine J. Kendrick, Katherine M. Scharer, Charles W. Wicks, Jerry L. Svarc, and J. M. Nevitt
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Geophysics ,010504 meteorology & atmospheric sciences ,Geochemistry and Petrology ,Kinematics ,Fault slip ,010502 geochemistry & geophysics ,01 natural sciences ,Geology ,Seismology ,0105 earth and related environmental sciences ,Sequence (medicine) - Abstract
The 2019 Ridgecrest, California, earthquake sequence produced observable crustal deformation over much of central and southern California, as well as surface rupture over several tens of kilometers. To obtain a detailed picture of the fault slip involved in the 4 July M 6.4 foreshock and 6 July M 7.1 mainshock, we combine strong-motion seismic waveforms with crustal deformation observations to obtain kinematic and static slip models of both events. We sample the regional seismic wavefield for both the foreshock and mainshock with three-component records from 31 stations of the California Integrated Seismic Network. The deformation observations include Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), and borehole strainmeter recordings of the dynamic strain field. These data collectively constrain the kinematic coseismic slip distributions of the events, with measurements variously observing coseismic slip from one event (e.g., seismic waveforms, kinematic solutions from continuous GPS, and strainmeter time series) or coseismic slip from both events combined (InSAR). We find that the foreshock ruptured two separate faults, one with left-lateral strike slip on a northeast–southwest-trending fault and the other with right-lateral strike slip on an orthogonal fault, with unilateral rupture propagation along both. The mainshock ruptured a series of northwest–southeast-trending faults with right-lateral strike slip concentrated in the uppermost 6 km with exceptionally low-rupture velocity averaging 1.0–1.5 km/s. A possible explanation for the low-rupture velocity is that the mainshock rupture expended relatively high energy, generating secondary fractures in off-fault deformation, which is consistent with field and seismic evidence of plastic deformation on small fault strands adjacent to the main rupture trace.
- Published
- 2020
11. Rapid Geodetic Observations of Spatiotemporally Varying Postseismic Deformation Following the Ridgecrest Earthquake Sequence: The U.S. Geological Survey Response
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Kang Wang, Janis L. Hernandez, Brian Olson, Sarah E. Minson, Jessica R. Murray, Evelyn Roeloffs, T. L. Ericksen, Ryan Turner, Benjamin A. Brooks, Roland Bürgmann, Kenneth W. Hudnut, Jerry L. Svarc, Fred F. Pollitz, J. M. Nevitt, M. H. Murray, and Eleyne L. Phillips
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Sequence (geology) ,Geophysics ,010504 meteorology & atmospheric sciences ,Geological survey ,Geodetic datum ,Deformation (meteorology) ,010502 geochemistry & geophysics ,Geodesy ,01 natural sciences ,Geology ,0105 earth and related environmental sciences - Abstract
The U.S. Geological Survey’s geodetic response to the 4–5 July 2019 (Pacific time) Ridgecrest earthquake sequence comprised primarily the installation and/or reoccupation of Global Navigation Satellite System (GNSS) monumentation. Our response focused primarily on the United States’ Navy’s China Lake Naval Air Weapons Station base (NAWSCL). This focus was because much of the surface rupture occurred on the NAWSCL and because of NAWSCL access restrictions only permitting Federal and State of California personnel. In total, we measured or are still measuring at 24 sites, 14 of which were on the NAWSCL and, as of this writing, operational. The majority of sites were set up as continuous stations logging at either 1 sample per second or 1 sample per 15 s. Two stations were recording a 200 m cross-rupture aperture starting ∼10 hr after the M 6.4 event, and they recorded the coseismic displacements of the M 7.1. Approximately, 1 hr after the M 7.1 event, two new stations were recording a ∼200 m cross-rupture aperture of the surface rupture. In the days following, we established the rest of the stations ranging to a distance of ∼15 km from the M 7.1 principal rupture trace. The lack of differential displacement across the M 6.4 rupture during the M 7.1 event suggests that it did not reactivate the M 6.4 plane. The lack of differential cross-fault displacement for both events suggests that rapid shallow afterslip did not occur at those two locations. The postseismic time series from these stations shows centimeters of horizontal displacement over periods of a few months. They record a mixture of fault-parallel and fault-normal displacements that, in conjunction with analysis of more spatially complete Interferometric Synthetic Aperture Radar displacement fields, suggest that both poroelastic and afterslip phenomena occur along the M 6.4 and 7.1 rupture planes. Using preliminary data from these and other regional stations, we also explore the Ridgecrest sequence’s effect on regional GNSS time series and the differentiation of long-term postseismic motions and secular deformation rates. We find that redefining a common-mode noise filter using different GNSS stations that are assumed to be unaffected by the earthquakes results in small but systematic differences in the regional velocity field estimate.
- Published
- 2020
12. Coseismic and post-seismic gravity disturbance induced by seismic sources using a 2.5-D spectral element method
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Fred F. Pollitz
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Gravity (chemistry) ,Geophysics ,Disturbance (geology) ,010504 meteorology & atmospheric sciences ,Geochemistry and Petrology ,Spectral element method ,010502 geochemistry & geophysics ,01 natural sciences ,Seismic cycle ,Geology ,Seismology ,0105 earth and related environmental sciences - Abstract
SUMMARYI present a prescription for computing free-air coseismic and post-seismic gravity changes induced by seismic sources in a viscoelastic earth model. I assume a spherical earth geometry and a 2.5-D calculation, that is, 3-D motions that satisfy the equations of quasi-static equilibrium on a 2-D viscoelastic structure. The prescription permits application to regional gravity computations where a 2-D structure adequately represents the structural heterogeneity. I use a hybrid approach where deformation is computed on a discretized domain and the resulting density perturbations are expanded with spherical harmonics to produce the free-air gravity field. Starting with a solution to the equations of quasi-static displacements in the Laplace transform domain for a given dislocation source, I solve Poisson’s equation using Lagrangian interpolation on spectral element nodes to compute the required deformation quantities that contribute to free-air gravity. A numerical inverse Laplace transform then yields time domain results. This methodology is tested with analytic solutions on a spherically stratified viscoelastic structure, then applied to evaluate the effect of a descending slab of relatively high viscosity on post-seismic gravity in a megathrust faulting setting.
- Published
- 2020
13. Coseismic Slip and Early Afterslip of the M6.0 24 August 2014 South Napa, California, Earthquake
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Jerry L. Svarc, Fred F. Pollitz, Jessica R. Murray, Benjamin A. Brooks, Sarah E. Minson, and Charles W. Wicks
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NAPA ,Geophysics ,Space and Planetary Science ,Geochemistry and Petrology ,Earth and Planetary Sciences (miscellaneous) ,Coseismic slip ,Seismology ,Geology - Published
- 2019
14. Exploring GPS Observations of Postseismic Deformation Following the 2012 M W 7.8 Haida Gwaii and 2013 M W 7.5 Craig, Alaska Earthquakes: Implications for Viscoelastic Earth Structure
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Han Yue, Katherine A. Guns, Fred F. Pollitz, and Thorne Lay
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business.industry ,Earth structure ,Deformation (meteorology) ,engineering.material ,Geodesy ,Viscoelasticity ,Geophysics ,Space and Planetary Science ,Geochemistry and Petrology ,Earth and Planetary Sciences (miscellaneous) ,Global Positioning System ,engineering ,business ,Geology - Published
- 2021
15. Induced Seismicity Reduces Seismic Hazard?
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Fred F. Pollitz and Andrew J. Barbour
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Wastewater disposal ,Geophysics ,Seismic hazard ,Hydraulic fracturing ,General Earth and Planetary Sciences ,Induced seismicity ,Geology ,Seismology - Published
- 2019
16. Fault Slip Associated with the 2 September 2017 M 5.3 Sulphur Peak, Idaho, Earthquake and Aftershock Sequence
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William L. Yeck, James Evans, Charles W. Wicks, and Fred F. Pollitz
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Geophysics ,Geochemistry and Petrology ,Fault slip ,Aftershock ,Geology ,Seismology ,Sequence (medicine) - Published
- 2019
17. Sea Level Rise in the Samoan Islands Escalated by Viscoelastic Relaxation After the 2009 Samoa‐Tonga Earthquake
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Shin-Chan Han, Richard D. Ray, Jeanne Sauber, and Fred F. Pollitz
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geography ,Gravity (chemistry) ,geography.geographical_feature_category ,010504 meteorology & atmospheric sciences ,Subsidence ,010502 geochemistry & geophysics ,Quarter (United States coin) ,01 natural sciences ,language.human_language ,Geophysics ,Sea level rise ,13. Climate action ,Space and Planetary Science ,Geochemistry and Petrology ,Asthenosphere ,Archipelago ,Earth and Planetary Sciences (miscellaneous) ,language ,Samoan ,Tide gauge ,Geology ,Seismology ,0105 earth and related environmental sciences - Abstract
The Samoan islands are an archipelago hosting a quarter million people mostly residing inthree major islands, Savai'i and Upolu (Samoa), and Tutuila (American Samoa). The islands haveexperienced sea level rise by 2-3 mm/year during the last half century. The rate, however, has dramaticallyincreased following the Mw 8.1 SamoaTonga earthquake doublet (megathrust + normal faulting) inSeptember 2009. Since the earthquake, we found largescale gravity increase (0.5 Gal/year) around theislands and ongoing subsidence (8-16 mm/year) of the islands from our analysis of Gravity Recovery AndClimate Experiment gravity and GPS displacement data. The postseismic horizontal displacement is faster inSamoa, while the postseismic subsidence rate is considerably larger in American Samoa. The analysis oflocal tide gauge records and satellite altimeter data also identified that the relative sea level rise becomesfaster by 7-9 mm/year in American Samoa than Samoa. A simple viscoelastic model with a Maxwellviscosity of 2–3×10(exp 18) Pa s for the asthenosphere explained postseismic deformation at nearby GPS sites aswell as Gravity Recovery And Climate Experiment gravity change. It is found that the constructiveinterference of viscoelastic relaxation from both megathrust and normal faulting has intensified thepostseismic subsidence at American Samoa, causing ~5 times faster sea level rise than the global average.Our model indicates that this trend is likely to continue for decades and result in sea level rise of 30-40 cm,which is independent of and in addition to anticipated climaterelated sea level rise. It will worsen coastalflooding on the islands leading to regular nuisance flooding.
- Published
- 2019
18. Surface Imaging Functions for Elastic Reverse Time Migration
- Author
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Fred F. Pollitz
- Subjects
Surface (mathematics) ,Geophysics ,Space and Planetary Science ,Geochemistry and Petrology ,Earth and Planetary Sciences (miscellaneous) ,Seismic migration ,Mechanics ,Geology - Published
- 2019
19. Shallow microearthquakes near Chongqing, China triggered by the Rayleigh waves of the 2015 M7.8 Gorkha, Nepal earthquake
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Jing Wu, Lu Li, Baoshan Wang, Qiang Li, Libo Han, Chris Johnson, Fred F. Pollitz, Zhigang Peng, and Hongmei Wei
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geography ,geography.geographical_feature_category ,010504 meteorology & atmospheric sciences ,Event (relativity) ,Magnitude (mathematics) ,Induced seismicity ,010502 geochemistry & geophysics ,Waveform matching ,01 natural sciences ,symbols.namesake ,Geophysics ,Volcano ,Space and Planetary Science ,Geochemistry and Petrology ,Rate change ,Earth and Planetary Sciences (miscellaneous) ,symbols ,Rayleigh wave ,Geothermal gradient ,Geology ,Seismology ,0105 earth and related environmental sciences - Abstract
We present a case of remotely triggered seismicity in Southwest China by the 2015/04/25 M7.8 Gorkha, Nepal earthquake. A local magnitude M L 3.8 event occurred near the Qijiang district south of Chongqing city approximately 12 min after the Gorkha mainshock. Within 30 km of this M L 3.8 event there are 62 earthquakes since 2009 and only 7 M L > 3 events, which corresponds to a likelihood of 0.3% for a M L > 3 on any given day by a random chance. This observation motivates us to investigate the relationship between the M L 3.8 event and the Gorkha mainshock. The M L 3.8 event was listed in the China Earthquake National Center (CENC) catalog and occurred at shallow depth (∼3 km). By examining high-frequency waveforms, we identify a smaller local event ( ∼ M L 2.5 ) ∼ 15 s before the M L 3.8 event. Both events occurred during the first two cycles of the Rayleigh waves from the Gorkha mainshock. We perform seismic event detection based on envelope function and waveform matching by using the two events as templates. Both analyses found a statistically significant rate change during the mainshock, suggesting that they were indeed dynamically triggered by the Rayleigh waves. Both events occurred during the peak normal and dilatational stress changes (∼10–30 kPa), consistent with observations of dynamic triggering in other geothermal/volcanic regions. Although other recent events (i.e., the 2011 M9.1 Tohoku-Oki earthquake) produced similar peak ground velocities, the 2015 Gorkha mainshock was the only event that produced clear dynamic triggering in this region. The triggering site is close to hydraulic fracturing wells that began production in 2013–2014. Hence we suspect that fluid injections may increase the region's susceptibility to remote dynamic triggering.
- Published
- 2017
20. Connecting crustal seismicity and earthquake-driven stress evolution in Southern California
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Camilla Cattania and Fred F. Pollitz
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Quake (natural phenomenon) ,010504 meteorology & atmospheric sciences ,Crust ,Geophysics ,Induced seismicity ,010502 geochemistry & geophysics ,01 natural sciences ,Mantle (geology) ,Viscoelasticity ,Transient stress ,Space and Planetary Science ,Geochemistry and Petrology ,Earth and Planetary Sciences (miscellaneous) ,Common spatial pattern ,Stress evolution ,Geology ,Seismology ,0105 earth and related environmental sciences - Abstract
Tectonic stress in the crust evolves during a seismic cycle, with slow stress accumulation over interseismic periods, episodic stress steps at the time of earthquakes, and transient stress readjustment during a postseismic period that may last months to years. Static stress transfer to surrounding faults has been well documented to alter regional seismicity rates over both short and long time scales. While static stress transfer is instantaneous and long-lived, postseismic stress transfer driven by viscoelastic relaxation of the ductile lower crust and mantle leads to additional, slowly-varying stress perturbations. Both processes may be tested by comparing a decades-long record of regional seismicity to predicted time-dependent seismicity rates based on a stress evolution model that includes viscoelastic stress transfer. Here we explore crustal stress evolution arising from the seismic cycle in Southern California from 1981 to 2014 using five M≥6.5 source quakes: the M7.3 1992 Landers, M6.5 1992 Big Bear, M6.7 1994 Big Bear, M7.1 1999 Hector Mine, and M7.2 2010 El Mayor-Cucapah earthquakes. We relate the stress readjustment in the surrounding crust generated by each quake to regional seismicity using rate and state friction theory. Using a log-likelihood approach, we quantify the potential to trigger seismicity of both static and viscoelastic stress transfer, finding that both processes have systematically shaped the spatial pattern of Southern California seismicity since 1992.
- Published
- 2017
21. Geodetic Slip Model of the 3 September 2016Mw 5.8 Pawnee, Oklahoma, Earthquake: Evidence for Fault‐Zone Collapse
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Fred F. Pollitz, Martin Schoenball, Mark Murray, Charles W. Wicks, and William L. Ellsworth
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geography ,geography.geographical_feature_category ,010504 meteorology & atmospheric sciences ,Hypocenter ,business.industry ,Geodetic datum ,Slip (materials science) ,Fault (geology) ,010502 geochemistry & geophysics ,Geodesy ,Strike-slip tectonics ,01 natural sciences ,language.human_language ,Geophysics ,Pawnee ,Interferometric synthetic aperture radar ,Global Positioning System ,language ,business ,Seismology ,Geology ,0105 earth and related environmental sciences - Abstract
The 3 September 2016 M w 5.8 Pawnee earthquake in northern Oklahoma is the largest earthquake ever recorded in Oklahoma. The coseismic deformation was measured with both Interferometric Synthetic Aperture Radar and Global Positioning System (GPS), with measureable signals of order 1 cm and 1 mm, respectively. We derive a coseismic slip model from Sentinel‐1A and Radarsat 2 interferograms and GPS static offsets, dominated by distributed left‐lateral strike slip on a primary west‐northwest–east‐southeast‐trending subvertical plane, whereas strike slip is concentrated near the hypocenter (5.6 km depth), with maximum slip of ∼1 m located slightly east and down‐dip of the hypocenter. Based on systematic misfits of observed interferogram line‐of‐sight (LoS) displacements, with LoS based on shear‐dislocation models, a few decimeters of fault‐zone collapse are inferred in the hypocentral region where coseismic slip was the largest. This may represent the postseismic migration of large volumes of fluid away from the high‐slip areas, made possible by the creation of a temporary high‐permeability damage zone around the fault.
- Published
- 2017
22. A Note on Adding Viscoelasticity to Earthquake Simulators
- Author
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Fred F. Pollitz
- Subjects
State variable ,010504 meteorology & atmospheric sciences ,Deformation (mechanics) ,business.industry ,Structural engineering ,Induced seismicity ,010502 geochemistry & geophysics ,Fault (power engineering) ,01 natural sciences ,Viscoelasticity ,Physics::Geophysics ,Stress (mechanics) ,Geophysics ,Earthquake simulation ,Geochemistry and Petrology ,Precomputation ,business ,Geology ,0105 earth and related environmental sciences - Abstract
Here, I describe how time‐dependent quasi‐static stress transfer can be implemented in an earthquake simulator code that is used to generate long synthetic seismicity catalogs. Most existing seismicity simulators use precomputed static stress interaction coefficients to rapidly implement static stress transfer in fault networks with typically tens of thousands of fault patches. The extension to quasi‐static deformation, which accounts for viscoelasticity of Earth’s ductile lower crust and mantle, involves the precomputation of additional interaction coefficients that represent time‐dependent stress transfer among the model fault patches, combined with defining and evolving additional state variables that track this stress transfer. The new approach is illustrated with application to a California‐wide synthetic fault network.
- Published
- 2016
23. Persistent slip rate discrepancies in the eastern California (USA) shear zone
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Wayne Thatcher, E. L. Evans, Jessica R. Murray, and Fred F. Pollitz
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010504 meteorology & atmospheric sciences ,Geology ,Slip (materials science) ,Active fault ,010502 geochemistry & geophysics ,Geodesy ,01 natural sciences ,Block model ,Shear zone ,Fault slip ,Seismology ,0105 earth and related environmental sciences ,Slip rate - Abstract
Understanding fault slip rates in the eastern California shear zone (ECSZ) using GPS geodesy is complicated by potentially overlapping strain signals due to many sub-parallel strike-slip faults and by inconsistencies with geologic slip rates. The role of fault system geometry in describing ECSZ deformation may be investigated with total variation regularization, which algorithmically determines a best-fitting geometry from an initial model with numerous faults, constrained by a western United States GPS velocity field. The initial dense model (1) enables construction of the first geodetically constrained block model to include all ECSZ faults with geologic slip rates, allowing direct geologic-geodetic slip rate comparisons, and (2) permits fault system geometries with many active faults that are analogous to distributed interseismic deformation. Beginning with 58 ECSZ blocks, a model containing 10 ECSZ blocks is most consistent with geologic slip rates, reproducing five of 11 within their reported uncertainties. The model fits GPS observations with a mean residual velocity of 1.5 mm/yr. Persistent geologic-geodetic slip rate discrepancies occur on the Calico and Garlock faults, on which we estimate slip rates of 7.6 mm/yr and
- Published
- 2016
24. Lithospheric rheology constrained from twenty-five years of postseismic deformation following the 1989 M 6.9 Loma Prieta earthquake
- Author
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Fred F. Pollitz, Roland Bürgmann, and Mong-Han Huang
- Subjects
010504 meteorology & atmospheric sciences ,Deformation (mechanics) ,Crust ,010502 geochemistry & geophysics ,01 natural sciences ,Geophysics ,Rheology ,Space and Planetary Science ,Geochemistry and Petrology ,Lithosphere ,Interferometric synthetic aperture radar ,Earth and Planetary Sciences (miscellaneous) ,Groundwater-related subsidence ,Aseismic slip ,Shear zone ,Seismology ,Geology ,0105 earth and related environmental sciences - Abstract
The October 17, 1989 M w 6.9 Loma Prieta earthquake provides the first opportunity of probing the crustal and upper mantle rheology in the San Francisco Bay Area since the 1906 M w 7.9 San Francisco earthquake. Here we use geodetic observations including GPS and InSAR to characterize the Loma Prieta earthquake postseismic displacements from 1989 to 2013. Pre-earthquake deformation rates are constrained by nearly 20 yr of USGS trilateration measurements and removed from the postseismic measurements prior to the analysis. We observe GPS horizontal displacements at mean rates of 1–4 mm/yr toward Loma Prieta Mountain until 2000, and ∼2 mm/yr surface subsidence of the northern Santa Cruz Mountains between 1992 and 2002 shown by InSAR, which is not associated with the seasonal and longer-term hydrological deformation in the adjoining Santa Clara Valley. Previous work indicates afterslip dominated in the early (1989–1994) postseismic period, so we focus on modeling the postseismic viscoelastic relaxation constrained by the geodetic observations after 1994. The best fitting model shows an elastic 19-km-thick upper crust above an 11-km-thick viscoelastic lower crust with viscosity of ∼ 6 × 10 18 Pa s , underlain by a viscous upper mantle with viscosity between 3 × 10 18 and 2 × 10 19 Pa s . The millimeter-scale postseismic deformation does not resolve the viscosity in the different layers very well, and the lower-crustal relaxation may be localized in a narrow shear zone. However, the inferred lithospheric rheology is consistent with previous estimates based on post-1906 San Francisco earthquake measurements along the San Andreas fault system. The viscoelastic relaxation may also contribute to the enduring increase of aseismic slip and repeating earthquake activity on the San Andreas fault near San Juan Bautista, which continued for at least a decade after the Loma Prieta event.
- Published
- 2016
25. Seismic velocity structure of the crust and shallow mantle of the Central and Eastern United States by seismic surface wave imaging
- Author
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Walter D. Mooney and Fred F. Pollitz
- Subjects
USArray ,geography ,Earthscope ,geography.geographical_feature_category ,Rift ,010504 meteorology & atmospheric sciences ,010502 geochemistry & geophysics ,01 natural sciences ,Craton ,Geophysics ,Seismic tomography ,Core–mantle boundary ,Beijing Anomaly ,General Earth and Planetary Sciences ,Low-velocity zone ,Geology ,Seismology ,0105 earth and related environmental sciences - Abstract
Seismic surface waves from the Transportable Array of EarthScope's USArray are used to estimate phase velocity structure of 18 to 125 s Rayleigh waves, then inverted to obtain three-dimensional crust and upper mantle structure of the Central and Eastern United States (CEUS) down to ∼200 km. The obtained lithosphere structure confirms previously imaged CEUS features, e.g., the low seismic-velocity signature of the Cambrian Reelfoot Rift and the very low velocity at >150 km depth below an Eocene volcanic center in northwestern Virginia. New features include high-velocity mantle stretching from the Archean Superior Craton well into the Proterozoic terranes and deep low-velocity zones in central Texas (associated with the late Cretaceous Travis and Uvalde volcanic fields) and beneath the South Georgia Rift (which contains Jurassic basalts). Hot spot tracks may be associated with several imaged low-velocity zones, particularly those close to the former rifted Laurentia margin.
- Published
- 2016
26. Interpretation ofSWaves Generated by Near-Surface Chemical Explosions at SAFOD
- Author
-
Justin L. Rubinstein, Fred F. Pollitz, and William L. Ellsworth
- Subjects
Geophysics ,Amplitude ,Synthetic seismogram ,Geochemistry and Petrology ,Observatory ,Attenuation ,Geophone ,Near and far field ,Classification of discontinuities ,San Andreas Fault Observatory at Depth ,Geology ,Seismology - Abstract
A series of near-surface chemical explosions conducted at the San Andreas Fault Observatory at Depth (SAFOD) were recorded by high-frequency downhole receiver arrays in separate experiments in November 2003 and May 2005. The 2003 experiment involved ∼100 kg shots detonated along a 46-km-long line (Hole–Ryberg line) centered on SAFOD and recorded by 32 three-component geophones in the pilot hole between 0.8 and 2.0 km depth. The 2005 experiment involved ∼36 kg shots detonated at Parkfield Area Seismic Observatory (PASO) stations (at ∼1–8 km offset) recorded by 80 three-component geophones in the main hole between the surface and 2.4 km depth. These data sample the downgoing seismic wavefield and constrain the shallow velocity and attenuation structure, as well as the first-order characteristics of the source. Using forward modeling on a velocity structure designed for the near field, both observed P - and S -wave energy for the PASO shots are identified with the travel times expected for direct and/or reflected phases. Larger-offset recordings from shots along the Hole–Ryberg line reveal substantial SV and SH energy, especially southwest of SAFOD from the source as indicated by P -to- S amplitude ratios. The generated SV energy is interpreted to arise chiefly from P -to- S conversions at subhorizontal discontinuities. This provides a simple mechanism for often-observed low P -to- S amplitude ratios from nuclear explosions in the far field, as originating from strong near-field wave conversions. Online Material: Figures of observed and synthetic seismogram record sections, snapshots of synthetic wavefields, and models of crustal structure.
- Published
- 2015
27. Coseismic compression/dilatation and viscoelastic uplift/subsidence following the 2012 Indian Ocean earthquakes quantified from satellite gravity observations
- Author
-
Jeanne Sauber, Fred F. Pollitz, and Shin-Chan Han
- Subjects
Indian ocean ,Geophysics ,Oceanic crust ,General Earth and Planetary Sciences ,Moment tensor ,Spatial localization ,Geodesy ,Vertical motion ,Viscoelasticity ,Mantle (geology) ,Seismology ,Geology - Abstract
The 2012 Indian Ocean earthquake sequence (Mw 8.6, 8.2) is a rare example of great strike-slip earthquakes in an intraoceanic setting. With over a decade of Gravity Recovery and Climate Experiment (GRACE) data, we were able to measure and model the unanticipated large coseismic and postseismic gravity changes of these events. Using the approach of normal mode decomposition and spatial localization, we computed the gravity changes corresponding to five moment tensor components. Our analysis revealed that the gravity changes are produced predominantly by coseismic compression and dilatation within the oceanic crust and upper mantle and by postseismic vertical motion. Our results suggest that the postseismic positive gravity and the postseismic uplift measured with GPS within the coseismic compressional quadrant are best fit by ongoing uplift associated with viscoelastic mantle relaxation. Our study demonstrates that the GRACE data are suitable for analyzing strike-slip earthquakes as small as Mw 8.2 with the noise characteristics of this region.
- Published
- 2015
28. Postearthquake relaxation evidence for laterally variable viscoelastic structure and water content in the Southern California mantle
- Author
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Fred F. Pollitz
- Subjects
geography ,geography.geographical_feature_category ,Crust ,Strain rate ,Mantle (geology) ,Igneous rock ,Geophysics ,Volcano ,Space and Planetary Science ,Geochemistry and Petrology ,Lithosphere ,Asthenosphere ,Earth and Planetary Sciences (miscellaneous) ,Water content ,Geology ,Seismology - Abstract
I reexamine the lower crust and mantle relaxation following two large events in the Mojave Desert: the 1992 M7.3 Landers and 1999 M7.1 Hector Mine, California, earthquakes. Time series from continuous GPS sites out to 300 km from the ruptures are used to constrain models of postseismic relaxation. Crustal motions in the Mojave Desert region are elevated above background for several years following each event. To account for broadscale relaxation of the lower crust and mantle, the Burgers body model is employed, involving transient and steady state viscosities. Joint afterslip/postseismic relaxation modeling of the GPS time series up to one decade following the Hector Mine earthquake reveals a significant rheological contrast between a northwest trending “southwest domain” (that envelopes the San Andreas fault system and western Mojave Desert) and an adjacent “northeast domain” (that envelopes the Landers and Hector Mine rupture areas in the central Mojave Desert). The steady state viscosity of the northeast domain mantle asthenosphere is inferred to be ∼4 times greater than that of the southwest domain. This pattern is counter to that expected for regional heat flow, which is higher in the northeast domain, but it is explicable by means of a nonlinear rheology that includes dependence on both strain rate and water concentration. I infer that the southwest domain mantle has a relatively low steady state viscosity because of its high strain rate and water content. The relatively low mantle water content of the northeast domain is interpreted to result from the continual extraction of water through igneous and volcanic activity over the past ∼20 Myr.
- Published
- 2015
29. The Mw 6.0 24 August 2014 South Napa Earthquake
- Author
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Thomas M. Brocher, Fred F. Pollitz, Katherine M. Scharer, A. L. Llenos, John Boatwright, Benjamin A. Brooks, Jessica R. Murray, Kenneth W. Hudnut, Daniel J. Ponti, Douglas S. Dreger, Annemarie S. Baltay, Brad T. Aagaard, David R. Shelly, William D. Barnhart, David P. Schwartz, Keith L. Knudsen, Jeanne L. Hardebeck, Timothy E. Dawson, Victoria E. Langenheim, David J. Wald, Richard M. Allen, James J. Lienkaemper, and James Luke Blair
- Subjects
NAPA ,Tectonics ,Geophysics ,Sinistral and dextral ,Mercalli intensity scale ,Echelon formation ,Slip (materials science) ,Quaternary ,Geology ,Aftershock ,Seismology - Abstract
The M w 6.0 South Napa earthquake, which occurred at 10:20 UTC 24 August 2014 was the largest earthquake to strike the greater San Francisco Bay area since the M w 6.9 1989 Loma Prieta earthquake. The rupture from this right‐lateral earthquake propagated mostly unilaterally to the north and up‐dip, directing the strongest shaking toward the city of Napa, where peak ground accelerations (PGAs) between 45% g and 61% g were recorded and modified Mercalli intensities (MMIs) of VII–VIII were reported. Tectonic surface rupture with dextral slip of up to 46 cm was observed on a 12.5 km long segment, some of which was along a previously mapped strand of the West Napa fault system, although the rupture extended to the north of the mapped Quaternary strand. Modeling of seismic and geodetic data suggests an average coseismic slip of 50 cm, with a maximum slip of about 1 m at depths of 10–11 km. We observed up to 35 cm of afterslip along the surface trace in the week following the mainshock, primarily along the southern half of the surface rupture that experienced relatively little coseismic offset. Relocation of the sparse aftershock sequence suggests en echelon southwest‐ and northeast‐dipping fault planes, reflective of the complex fault geometry in this region. The Napa basin and historic and late Holocene alluvial flood deposits in downtown Napa amplified the ground motions there. Few ground failures were mapped, reflecting the dry season (as well as a persistent drought that had lowered the groundwater table) and the short duration of strong shaking in the epicentral area. The South Napa fault rupture lies within an 80 km wide set of major north‐northwest‐trending faults of the San Andreas fault system, forming the boundary between the Pacific and North American tectonic …
- Published
- 2015
30. Rare dynamic triggering of remoteM ≥ 5.5 earthquakes from global catalog analysis
- Author
-
Roland Bürgmann, Fred F. Pollitz, and Chris Johnson
- Subjects
Indian ocean ,Geophysics ,Transient stress ,Shock (fluid dynamics) ,Space and Planetary Science ,Geochemistry and Petrology ,Earth and Planetary Sciences (miscellaneous) ,Global change ,Induced seismicity ,Geology ,Seismology - Abstract
Probing the effects of a transient stress on the timing of an earthquake occurrence is necessary for understanding the remote interaction of large-magnitude events. Global catalog data containing 35 years of M ≥ 5.5 earthquakes allow us to explore for periods of enhanced or suppressed seismic activity. We consider 113 M ≥ 7.5 main shocks between 1977 and 2012 and focus on seismic activity on time scales from seconds to days following these main shocks. We search for evidence of dynamic triggering of large-magnitude events similar to the previously observed global increase during the first few days following the 2012 M8.6 Indian Ocean main shock. We restrict the analysis to regions of elevated strain during the passage of surface waves. Using a threshold of 0.1 microstrain (~3 kPa) and a temporal window of ±1 year, we stack daily seismicity rate curves using the exclusion-zone declustered M ≥ 5.5 catalog events in order to resolve deviations from the background rate. Our results do not indicate a significant change in activity for at least 10 days when considering the collective set of 113 main shocks and subsets at M8.0 and M8.5 thresholds. The results also do not indicate immediate triggering of M ≥ 5.5 events. We do find two instances of increased seismicity in the elevated strain region within 10 days. These increases are subsequent to two main shocks, the 1977 M8.3 and 2012 M8.6, both located in the Indian Ocean. We conclude that a global change in M ≥ 5.5 earthquake rates following a transient stress from distant earthquakes is a rare occurrence.
- Published
- 2015
31. Seismic versus aseismic slip: Probing mechanical properties of the northeast Japan subduction zone
- Author
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Naoki Uchida, Fred F. Pollitz, Roland Bürgmann, Toru Matsuzawa, Manoochehr Shirzaei, and Yan Hu
- Subjects
Subduction ,Moment magnitude scale ,Aseismic creep ,Geophysics ,Slip (materials science) ,Seismic wave ,Space and Planetary Science ,Geochemistry and Petrology ,Interplate earthquake ,Earth and Planetary Sciences (miscellaneous) ,Episodic tremor and slip ,Aftershock ,Seismology ,Geology - Abstract
Fault slip may involve slow aseismic creep and fast seismic rupture, radiating seismic waves manifested as earthquakes. These two complementary behaviors accommodate the long-term plate convergence of major subduction zones and are attributed to fault frictional properties. It is conventionally assumed that zones capable of seismic rupture on the subduction megathrust are confined to between about 10 to 50 km depth; however, the actual spatiotemporal distribution of fault mechanical parameters remains elusive for most subduction zones. The 2011 Tohoku Mw 9.0 earthquake ruptured with > 50 m slip up to the trench, thus challenging this conventional assumption, and provides a unique opportunity to probe the mechanical properties of the Japan subduction zone. Drawing on the inferred distribution of coseismic and postseismic slip, it has recently been suggested that portions of the megathrust are capable of switching between seismic and aseismic behavior. Kinematic models of the coseismic rupture and 15-month postseismic afterslip of this event suggest that the coseismic rupture triggered widespread frictional afterslip with equivalent moment magnitude of 8.17–8.53, in addition to viscoelastic relaxation in the underlying mantle. The identified linear relation between modeled afterslip, slip inferred from repeating earthquakes on the plate interface, and the cumulative number of aftershocks within 15 km distance of the subduction thrust suggests that most aftershocks are a direct result of afterslip. We constrain heterogeneous rate-state friction parameters of the subduction thrust from the computed coseismic stress changes and afterslip response. Our results indicate a variable pattern along dip and strike, characterizing areas down-dip and south of the main rupture zone as having velocity-strengthening properties. In agreement with seismic tomographic models of plate boundary elastic properties and geologic evidence for previous M > 8.5 megathrust earthquakes on this section of the plate boundary, we suggest that the obtained pattern of the frictional properties is characteristic of subducted material and thus persistent in time and space.
- Published
- 2014
32. LITHOSPHERE AND SHALLOW ASTHENOSPHERE RHEOLOGY FROM OBSERVATIONS OF POST-EARTHQUAKE RELAXATION
- Author
-
Fred F. Pollitz
- Subjects
geography ,Rift ,geography.geographical_feature_category ,010504 meteorology & atmospheric sciences ,Physics and Astronomy (miscellaneous) ,Subduction ,Astronomy and Astrophysics ,Crust ,Fault (geology) ,010502 geochemistry & geophysics ,01 natural sciences ,Mantle (geology) ,Tectonics ,Plate tectonics ,Geophysics ,Space and Planetary Science ,Lithosphere ,Asthenosphere ,Geology ,Seismology ,0105 earth and related environmental sciences - Abstract
In tectonically active regions, post-earthquake motions are generally shaped by a combination of continued fault slippage (afterslip) on a timescale of days to months and viscoelastic relaxation of the lower crust and upper mantle on a timescale of days to years. Transient crustal motions have been observed following numerous magnitude >~7 earthquakes in various tectonic settings: continental rift zones (Basin and Range), continental plate boundary zones (San Andreas fault corridor; Alaska; Turkey), subduction zones (Japan, Chile, Sumatra), ongoing continental collision zones (Arabia; Tibet), and mid-ocean rifting zones (Iceland). When afterslip can be discriminated from viscoelastic relaxation and when temporal coverage of the postseismic measurements is broad (i.e., geodetic surveys of at least several years duration are available), a wide spectrum of relaxation timescales are usually identified. Current temporal resolution and modeling approaches (e.g., Burgers body analog) allow identification of transient (Kelvin) and steady-state (Maxwell) viscosities that are operable in the short-term and long-term, respectively. I compile results from 40 studies of post-earthquake motions, augmented by ten studies of contemporary surface loading or unloading, that illuminate current estimates of transient and steady-state viscosity of the lower crust and/or uppermost mantle. Lower crust viscosity estimates range from ~1018 to 1021 Pa s, with most estimates near the upper end except in areas of overthickened crust. Mantle lithosphere and asthenosphere viscosity estimates are particularly abundant and yield a picture of transient viscosity ranging from ~1016 to 1019 Pa s and steady-state viscosity ranging from ~1018 to 1021 Pa s. To first order, both transient and steady-state viscosities are well correlated with regional heat flow, and steady-state viscosities are comparable with temperature and strain-rate dependent rock viscosities from laboratory-based flow laws.
- Published
- 2017
33. Connecting crustal seismicity and earthquake-driven stress evolution in Southern California
- Author
-
Fred F. Pollitz and Camilla Cattania
- Published
- 2017
34. Seismic imaging east of the Rocky Mountains with USArray
- Author
-
Fred F. Pollitz, Robert W. Porritt, and Richard M. Allen
- Subjects
USArray ,Subduction ,Geophysical imaging ,Archean ,Crust ,Oceanic plateau ,Geophysics ,Space and Planetary Science ,Geochemistry and Petrology ,Lithosphere ,Earth and Planetary Sciences (miscellaneous) ,Farallon Plate ,Seismology ,Geology - Abstract
Article history: Accepted 23 October 2013 Available online xxxx Editor: P. Shearer USArray has facilitated significant advancement in tomographic models and methodologies. While there persists a fundamental tradeoff between horizontal and vertical resolution due to the components of the wave train analyzed, advances in joint inversions are continuing to refine the tomographic images generated with USArray. The DNA13 model incorporates teleseismic P observations, independent SH and SV observations, and surface-wave phase velocities from both teleseismic earthquakes and ambient noise to constrain the relative wave-speed from the crust down into the lower mantle. We address the validity of our models through forward prediction of observables and compare the predictive power of the DNA13 models to other models. In the shallow portion of DNA13, we image the Archean age Wyoming Province, which exhibits evidence of ocean closure at its northern and southern ends. The Llano Province in Central Texas is of Grenville age and still contains lithospheric evidence of subduction as the province accreted to North America. Comparison of these two provinces highlights the role of fossil slabs as part of the cratonic architecture. Analysis of the deep portion of the models highlights variations within the Farallon plate, including two distinct high wave-speed anomalies in the eastern U.S. and a shallow feature in the center of these two anomalies. We propose this is evidence of an oceanic plateau, which provides the necessary positive buoyancy to promote flat-slab subduction of the Farallon plate. Published by Elsevier B.V.
- Published
- 2014
35. Seismic structure of the Central US crust and shallow upper mantle: Uniqueness of the Reelfoot Rift
- Author
-
Fred F. Pollitz and Walter D. Mooney
- Subjects
Rift ,Earthscope ,Trough (geology) ,Crust ,Mantle (geology) ,Geophysics ,Space and Planetary Science ,Geochemistry and Petrology ,Earth and Planetary Sciences (miscellaneous) ,Intraplate earthquake ,Rift zone ,Aulacogen ,Geology ,Seismology - Abstract
Using seismic surface waves recorded with Earthscope's Transportable Array, we apply surface wave imaging to determine 3D seismic velocity in the crust and uppermost mantle. Our images span several Proterozoic and early Cambrian rift zones (Mid-Continent Rift, Rough Creek Graben—Rome trough, Birmingham trough, Southern Oklahoma Aulacogen, and Reelfoot Rift). While ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences, the Reelfoot Rift is unique in its association with low mantle seismic velocity. Its mantle low-velocity zone (LVZ) is exceptionally pronounced and extends down to at least 200 km depth. This LVZ is of variable width, being relatively narrow ( ∼ 50 km wide) within the northern Reelfoot Rift, which hosts the New Madrid Seismic Zone (NMSZ). We hypothesize that this mantle volume is weaker than its surroundings and that the Reelfoot Rift consequently has relatively low elastic plate thickness, which would tend to concentrate tectonic stress within this zone. No other intraplate ancient rift zone is known to be associated with such a deep mantle low-velocity anomaly, which suggests that the NMSZ is more susceptible to external stress perturbations than other ancient rift zones.
- Published
- 2014
36. Post-earthquake relaxation using a spectral element method: 2.5-D case
- Author
-
Fred F. Pollitz
- Subjects
Geophysics ,Condensed matter physics ,Transient deformation ,Geochemistry and Petrology ,Spectral element method ,Relaxation (physics) ,Geology - Published
- 2014
37. The Profound Reach of the 11 April 2012 M 8.6 Indian Ocean Earthquake: Short-Term Global Triggering Followed by a Longer-Term Global Shadow
- Author
-
Fred F. Pollitz, Ross S. Stein, Roland Bürgmann, and Volkan Sevilgen
- Subjects
Global system ,Indian ocean ,Tectonics ,Geophysics ,Geochemistry and Petrology ,Quiescent period ,Induced seismicity ,Seismology ,Geology ,Aftershock ,Stress level ,Term (time) - Abstract
The 11 April 2012 M 8.6 Indian Ocean earthquake was an unusually large intraoceanic strike‐slip event. For several days, the global M ≥4.5 and M ≥6.5 seismicity rate at remote distances (i.e., thousands of kilometers from the mainshock) was elevated. The strike‐slip mainshock appears through its Love waves to have triggered a global burst of strike‐slip aftershocks over several days. But the M ≥6.5 rate subsequently dropped to zero for the succeeding 95 days, although the M ≤6.0 global rate was close to background during this period. Such an extended period without an M ≥6.5 event has happened rarely over the past century, and never after a large mainshock. Quiescent periods following previous large ( M ≥8) mainshocks over the past century are either much shorter or begin so long after a given mainshock that no physical interpretation is warranted. The 2012 mainshock is unique in terms of both the short‐lived global increase and subsequent long quiescent period. We believe that the two components are linked and interpret this pattern as the product of dynamic stressing of a global system of faults. Transient dynamic stresses can encourage short‐term triggering, but, paradoxically, it can also inhibit rupture temporarily until background tectonic loading restores the system to its premainshock stress levels.
- Published
- 2014
38. Annual modulation of non-volcanic tremor in northern Cascadia
- Author
-
Roland Bürgmann, Aaron G. Wech, Honn Kao, and Fred F. Pollitz
- Subjects
geography ,geography.geographical_feature_category ,Subduction ,Crust ,Mantle (geology) ,Geophysics ,Volcano ,Space and Planetary Science ,Geochemistry and Petrology ,Transition zone ,Earth and Planetary Sciences (miscellaneous) ,Shear stress ,Episodic tremor and slip ,Precipitation ,Seismology ,Geology - Abstract
[1] Two catalogs of episodic tremor events in northern Cascadia, one from 2006 to 2012 and the other from 1997 to 2011, reveal two systematic patterns of tremor occurrence in southern Vancouver Island: (1) most individual events tend to occur in the third quarter of the year; (2) the number of events in prolonged episodes (i.e., episodic tremor and slip events), which generally propagate to Vancouver Island from elsewhere along the Cascadia subduction zone, is inversely correlated with the amount of precipitation that occurred in the preceding 2 months. We rationalize these patterns as the product of hydrologic loading of the crust of southern Vancouver Island and the surrounding continental region, superimposed with annual variations from oceanic tidal loading. Loading of the Vancouver Island crust in the winter (when the land surface receives ample precipitation) and unloading in the summer tends to inhibit and enhance downdip shear stress, respectively. Quantitatively, for an annually variable surface load, the predicted stress perturbation depends on mantle viscoelastic rheology. A mechanical model of downdip shear stress on the transition zone beneath Vancouver Island—driven predominantly by the annual hydrologic cycle—is consistent with the 1997–2012 tremor observations, with peak-to-peak downdip shear stress of about 0.4 kPa. This seasonal dependence of tremor occurrence appears to be restricted to southern Vancouver Island because of its unique situation as an elongated narrow-width land mass surrounded by ocean, which permits seasonal perturbations in shear stress at depth.
- Published
- 2013
39. How do 'ghost transients' from past earthquakes affect GPS slip rate estimates on southern California faults?
- Author
-
Wayne Thatcher, Fred F. Pollitz, Celia Tiemi Onishi, and E. H. Hearn
- Subjects
Seismic gap ,business.industry ,Geodetic datum ,Perturbation (astronomy) ,Slip (materials science) ,Elastic-rebound theory ,Geodesy ,Physics::Geophysics ,Geophysics ,Seismic hazard ,Geochemistry and Petrology ,Asthenosphere ,Global Positioning System ,business ,Geology ,Seismology - Abstract
[1] In this study, we investigate the extent to which viscoelastic velocity perturbations (or “ghost transients”) from individual fault segments can affect elastic block model-based inferences of fault slip rates from GPS velocity fields. We focus on the southern California GPS velocity field, exploring the effects of known, large earthquakes for two end-member rheological structures. Our approach is to compute, at each GPS site, the velocity perturbation relative to a cycle average for earthquake cycles on particular fault segments. We then correct the SCEC CMM4.0 velocity field for this perturbation and invert the corrected field for fault slip rates. We find that if asthenosphere viscosities are low (3 × 1018 Pa s), the current GPS velocity field is significantly perturbed by viscoelastic earthquake cycle effects associated with the San Andreas Fault segment that last ruptured in 1857 (Mw = 7.9). Correcting the GPS velocity field for this perturbation (or “ghost transient”) adds about 5 mm/a to the SAF slip rate along the Mojave and San Bernardino segments. The GPS velocity perturbations due to large earthquakes on the Garlock Fault (most recently, events in the early 1600s) and the White Wolf Fault (most recently, the Mw = 7.3 1952 Kern County earthquake) are smaller and do not influence block-model inverted fault slip rates. This suggests that either the large discrepancy between geodetic and geologic slip rates for the Garlock Fault is not due to a ghost transient or that un-modeled transients from recent Mojave earthquakes may influence the GPS velocity field.
- Published
- 2013
40. ViscoSim Earthquake Simulator
- Author
-
Fred F. Pollitz
- Subjects
Stress (mechanics) ,Geophysics ,Earthquake simulation ,Coulomb ,Shear stress ,Earthquake rupture ,Geometry ,Slip (materials science) ,Induced seismicity ,Viscoelasticity ,Seismology ,Geology - Abstract
Synthetic seismicity simulations have been explored by the Southern California Earthquake Center (SCEC) Earthquake Simulators Group in order to guide long‐term forecasting efforts related to the Unified California Earthquake Rupture Forecast (Tullis et al. , 2012a). In this study I describe the viscoelastic earthquake simulator (ViscoSim) of Pollitz, 2009. Recapitulating to a large extent material previously presented by Pollitz (2009, 2011) I describe its implementation of synthetic ruptures and how it differs from other simulators being used by the group. ### Generation of Synthetic Slip Events Model faults are assumed to reside in an elastic crust that overlies a viscoelastic crust and mantle with a Maxwell viscoelastic rheology. The evolving physical variable is the Coulomb failure function, which is a linear combination of the shear stress and normal stress resolved upon an a priori slip direction along the considered fault surface (Simpson and Reasenberg, 1994). Following Ben‐Zion et al. (2003), a static stress threshold σ s determines the initiation of a slip event. During an event, stress on an initially failing patch drops by an amount Δ σ (a stress‐reduction parameter assigned to a given patch) to the arrest stress level σ a (more precisely, the slip is that which would reduce the stress to σ a in the absence of slip on other patches). After a set of slip events so determined, stress is transferred from slipped patches to neighboring patches, which may fail first at the static stress threshold σ s and subsequently (during the same event) at the dynamic stress threshold σ d . It is useful to define a dynamic overshoot coefficient D =Δ σ /( σ s − σ d ), where Δ σ = σ s − σ a (i.e., the reduction in stress on one initially failing patch). The parameter D characterizes how far the stress in each subevent …
- Published
- 2012
41. A Comparison among Observations and Earthquake Simulator Results for the allcal2 California Fault Model
- Author
-
M. K. Sachs, E. M. Heien, Fred F. Pollitz, M. Burak Yikilmaz, Terry E. Tullis, Donald L. Turcotte, Steven N. Ward, Louise H. Kellogg, John B. Rundle, Keith Richards-Dinger, Edward H. Field, Michael Barall, and James H. Dieterich
- Subjects
Engineering ,Geophysics ,Earthquake simulation ,business.industry ,Earthquake hazard ,Probability distribution ,Covariance ,Fault model ,business ,Scaling ,Seismology ,Simulation - Abstract
Online Material: Supplemental figures of space‐time and frequency‐magnitude relations, scaling plots, mean and covariance plots of interevent times, probability distribution functions of recurrence intervals, and earthquake density plots. In order to understand earthquake hazards we would ideally have a statistical description of earthquakes for tens of thousands of years. Unfortunately the ∼100‐year instrumental, several 100‐year historical, and few 1000‐year paleoseismological records are woefully inadequate to provide a statistically significant record. Physics‐based earthquake simulators can generate arbitrarily long histories of earthquakes; thus they can provide a statistically meaningful history of simulated earthquakes. The question is, how realistic are these simulated histories? This purpose of this paper is to begin to answer that question. We compare the results between different simulators and with information that is known from the limited instrumental, historic, and paleoseismological data. As expected, the results from all the simulators show that the observational record is too short to properly represent the system behavior; therefore, although tests of the simulators against the limited observations are necessary, they are not a sufficient test of the simulators’ realism. The simulators appear to pass this necessary test. In addition, the physics‐based simulators show similar behavior even though there are large differences in the methodology. This suggests that they represent realistic behavior. Different assumptions concerning the constitutive properties of the faults do result in enhanced capabilities of some simulators. However, it appears that the similar behavior of the different simulators may result from the fault‐system geometry, slip rates, and assumed strength drops, along with the shared physics of stress transfer. This paper describes the results of running four earthquake simulators that are described elsewhere in …
- Published
- 2012
42. Postseismic gravity change after the 2006–2007 great earthquake doublet and constraints on the asthenosphere structure in the central Kuril Islands
- Author
-
Fred F. Pollitz, Jeanne Sauber, and Shin-Chan Han
- Subjects
Gravity (chemistry) ,010504 meteorology & atmospheric sciences ,Subduction ,010502 geochemistry & geophysics ,Geodesy ,01 natural sciences ,Seafloor spreading ,Viscoelasticity ,Article ,Viscosity ,Geophysics ,Asthenosphere ,General Earth and Planetary Sciences ,Relaxation (physics) ,Spatial extent ,Seismology ,Geology ,0105 earth and related environmental sciences - Abstract
Large earthquakes often trigger viscoelastic adjustment for years to decades depending on the rheological properties and the nature and spatial extent of coseismic stress. The 2006 Mw8.3 thrust and 2007 Mw8.1 normal fault earthquakes of the central Kuril Islands resulted in significant postseismic gravity change in GRACE but without a discernible coseismic gravity change. The gravity increase of approximately 4 micro-Gal, observed consistently from various GRACE solutions around the epicentral area during 2007-2015, is interpreted as resulting from gradual seafloor uplift by (is) approximately 6 cm produced by postseismic relaxation. The GRACE data are best fit with a model of 25-35 km for the elastic thickness and approximately 10(exp 18) Pa s for the Maxwell viscosity of the asthenosphere. The large measurable postseismic gravity change (greater than coseismic change) emphasizes the importance of viscoelastic relaxation in understanding tectonic deformation and fault-locking scenarios in the Kuril subduction zone.
- Published
- 2016
43. The earthquake-source inversion Validation (SIV) project
- Author
-
Wenyuan Fan, Haruko Sekiguchi, Luca Passone, Jagdish Chandra Vyas, Cedric Twardzik, Martin Käser, K. K. S. Thingbaijam, Mathieu Causse, Fred F. Pollitz, Martin Galis, Gaetano Festa, Jean-Paul Ampuero, Martin van Driel, Walter Imperatori, Danijel Schorlemmer, Dmytro Malytskyy, Olaf Zielke, Surendra Nadh Somala, Seok Goo Song, Kimiyuki Asano, Yuji Yagi, Morgan T. Page, Ryo Okuwaki, František Gallovič, Hoby N. T. Razafindrakoto, Rongjiang Wang, P. Martin Mai, Susana Custódio, Mai, P. Martin, Schorlemmer, Danijel, Page, Morgan, Ampuero, Jean Paul, Asano, Kimiyuki, Causse, Mathieu, Custodio, Susana, Fan, Wenyuan, Festa, Gaetano, Galis, Martin, Gallovic, Frantisek, Imperatori, Walter, Käser, Martin, Malytskyy, Dmytro, Okuwaki, Ryo, Pollitz, Fred, Passone, Luca, Razafindrakoto, Hoby N. T., Sekiguchi, Haruko, Song, Seok Goo, Somala, Surendra N., Thingbaijam, Kiran K. S., Twardzik, Cedric, Van Driel, Martin, Vyas, Jagdish C., Wang, Rongjiang, Yagi, Yuji, Zielke, Olaf, GeoForschungsZentrum - Helmholtz-Zentrum Potsdam (GFZ), Institut des Sciences de la Terre (ISTerre), Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux (IFSTTAR)-Institut national des sciences de l'Univers (INSU - CNRS)-Institut de recherche pour le développement [IRD] : UR219-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes [2016-2019] (UGA [2016-2019]), Département Géotechnique, Environnement, Risques naturels et Sciences de la terre (IFSTTAR/GERS), Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux (IFSTTAR)-Université de Lyon-PRES Université Nantes Angers Le Mans (UNAM)-PRES Université Paris-Est-PRES Université de Grenoble, Institut de Physique du Globe de Paris (IPGP), Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-IPG PARIS-Université Paris Diderot - Paris 7 (UPD7)-Université de La Réunion (UR)-Centre National de la Recherche Scientifique (CNRS), Faculty of Mathematics, Physics and Informatics [Bratislava] (FMPH/UNIBA), Comenius University in Bratislava, Department für Geo-und Umweltwissenschaften [München], Ludwig-Maximilians-Universität München (LMU), US Geological Survey [Menlo Park], United States Geological Survey [Reston] (USGS), Disaster Prevention Research Institute (DPRI), Kyoto University [Kyoto], and German Research Centre for Geosciences - Helmholtz-Centre Potsdam (GFZ)
- Subjects
Engineering ,010504 meteorology & atmospheric sciences ,Inversion methods ,[SDU.STU]Sciences of the Universe [physics]/Earth Sciences ,Imaging problem ,010502 geochemistry & geophysics ,Source inversion ,computer.software_genre ,01 natural sciences ,Slip-rate function ,Soil structure interaction ,ComputingMilieux_MISCELLANEOUS ,0105 earth and related environmental sciences ,Collaborative software ,Sourcereceiver geometrie ,business.industry ,Inversion (meteorology) ,Multiple source ,Quantitative waveform comparisons. tables of scalar source parameters and dissimilarity value ,Geophysics ,Input and inverted rupture model ,Figures of velocity-density structure ,Data mining ,Artificial intelligence ,business ,computer ,Strengths and weaknesses - Abstract
Finite-fault earthquake source inversions infer the (time-dependent) displacement on the rupture surface from geophysical data. The resulting earthquake source models document the complexity of the rupture process. However, multiple source models for the same earthquake, obtained by different research teams, often exhibit remarkable dissimilarities. To address the uncertainties in earthquake-source inversion methods and to understand strengths and weaknesses of the various approaches used, the Source Inversion Validation (SIV) project conducts a set of forward-modeling exercises and inversion benchmarks. In this article, we describe the SIV strategy, the initial benchmarks, and current SIV results. Furthermore, we apply statistical tools for quantitative waveform comparison and for investigating source-model (dis)similarities that enable us to rank the solutions, and to identify particularly promising source inversion approaches. All SIV exercises (with related data and descriptions) and statistical comparison tools are available via an online collaboration platform, and we encourage source modelers to use the SIV benchmarks for developing and testing new methods. We envision that the SIV efforts will lead to new developments for tackling the earthquake-source imaging problem.
- Published
- 2016
44. The 11 April 2012 east Indian Ocean earthquake triggered large aftershocks worldwide
- Author
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Fred F. Pollitz, Ross S. Stein, Roland Bürgmann, and Volkan Sevilgen
- Subjects
Seismic gap ,Remotely triggered earthquakes ,Tectonics ,Multidisciplinary ,Subduction ,Seismic moment ,Earthquake swarm ,Far East ,Seismology ,Geology ,Aftershock - Abstract
Although strong remote aftershocks are exceedingly rare, their rate increased fivefold during the six days following the 2012 east Indian Ocean earthquake, perhaps as a result of the strike-slip nature of the 2012 event or a build up of close-to-failure nucleation sites.
- Published
- 2012
45. Source Characterization of Near-Surface Chemical Explosions at SAFOD
- Author
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William L. Ellsworth, Fred F. Pollitz, and Justin L. Rubinstein
- Subjects
Moment (mathematics) ,Geophysics ,Source characterization ,Explosive material ,Geochemistry and Petrology ,Attenuation ,Implosion ,Ranging ,San Andreas Fault Observatory at Depth ,Geology ,Seismology ,Seismic wave - Abstract
A series of near‐surface chemical explosions conducted at the San Andreas Fault Observatory at Depth (SAFOD) main hole were recorded by high‐frequency downhole receiver arrays in April 2005. These seismic recordings at depths ranging from the surface to 2.3 km constrain the shallow velocity and attenuation structure as well as the first‐order characteristics of the source. Forward modeling of the explosions indicates that a source consisting of combined explosion, delayed implosion, and second‐order moment‐tensor components (corresponding to a distribution of vertical shear dislocations in the rock directly above the explosion) is sufficient to characterize the generated seismic wave fields to first order. Grid searches over source parameters controlling the nonexplosive components allow for the quantification of distributed vertical shear above the source and the estimation of the moment and time delay of the implosive component relative to the explosion. An estimated implosive to explosive moment ratio of 0.34 to 0.43 indicates a net static moment and positive macroscopic volume change.
- Published
- 2012
46. High-frequency Born synthetic seismograms based on coupled normal modes
- Author
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Fred F. Pollitz
- Subjects
Physics ,Wave propagation ,Mathematical analysis ,Geophysics ,Integral equation ,Physics::Geophysics ,symbols.namesake ,Fourier transform ,Geochemistry and Petrology ,Normal mode ,Frequency domain ,symbols ,Time domain ,Born approximation ,Seismogram - Abstract
SUMMARY High-frequency and full waveform synthetic seismograms on a 3-D laterally heterogeneous earth model are simulated using the theory of coupled normal modes. The set of coupled integral equations that describe the 3-D response are simplified into a set of uncoupled integral equations by using the Born approximation to calculate scattered wavefields and the pure-path approximation to modulate the phase of incident and scattered wavefields. This depends upon a decomposition of the aspherical structure into smooth and rough components. The uncoupled integral equations are discretized and solved in the frequency domain, and time domain results are obtained by inverse Fourier transform. Examples show the utility of the normal mode approach to synthesize the seismic wavefields resulting from interaction with a combination of rough and smooth structural heterogeneities. This approach is applied to an ∼4 Hz shallow crustal wave propagation around the site of the San Andreas Fault Observatory at Depth (SAFOD).
- Published
- 2011
47. Lower crustal relaxation beneath the Tibetan Plateau and Qaidam Basin following the 2001 Kokoxili earthquake
- Author
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Fred F. Pollitz, Roland Bürgmann, and Isabelle Ryder
- Subjects
Shear modulus ,Geophysics ,Satellite geodesy ,Geochemistry and Petrology ,Interferometric synthetic aperture radar ,Stress relaxation ,Crust ,Slip (materials science) ,Shear zone ,Geology ,Viscoelasticity ,Seismology - Abstract
SUMMARY In 2001 November a magnitude 7.8 earthquake ruptured a 400 km long portion of the Kunlun fault, northeastern Tibet. In this study, we analyse over five years of post-seismic geodetic data and interpret the observed surface deformation in terms of stress relaxation in the thick Tibetan lower crust. We model GPS time-series (first year) and InSAR line of sight measurements (years two to five) and infer that the most likely mechanism of post-seismic stress relaxation is time-dependent distributed creep of viscoelastic material in the lower crust. Since a single relaxation time is not sufficient to model the observed deformation, viscous flow is modelled by a lower crustal Burgers rheology, which has two material relaxation times. The optimum model has a transient viscosity 9 × 1017 Pa s, steady-state viscosity 1 × 1019 Pa s and a ratio of long term to Maxwell shear modulus of 2:3. This model gives a good fit to GPS stations south of the Kunlun Fault, while displacements at stations north of the fault are over-predicted. We attribute this asymmetry in the GPS residual to lateral heterogeneity in rheological structure across the southern margin of the Qaidam Basin, with thinner crust/higher viscosities beneath the basin than beneath the Tibetan Plateau. Deep afterslip localized in a shear zone beneath the fault rupture gives a reasonable match to the observed InSAR data, but the slip model does not fit the earlier GPS data well. We conclude that while some localized afterslip likely occurred during the early post-seismic phase, the bulk of the observed deformation signal is due to viscous flow in the lower crust. To investigate regional variability in rheological structure, we also analyse post-seismic displacements following the 1997 Manyi earthquake that occurred 250 km west of the Kokoxili rupture. We find that viscoelastic properties are the same as for the Kokoxili area except for the transient viscosity, which is 5 × 1017 Pa s. The viscosities estimated for the Manyi and Kokoxili areas are consistent with constraints obtained from other earthquakes in the northwest and south central parts of the Tibetan Plateau.
- Published
- 2011
48. Epistemic Uncertainty in California-Wide Synthetic Seismicity Simulations
- Author
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Fred F. Pollitz
- Subjects
Geophysics ,Geochemistry and Petrology ,Cascade ,Conditional probability ,Crust ,Slip (materials science) ,Uncertainty quantification ,Induced seismicity ,Geology ,Seismology ,Viscoelasticity ,Mantle (geology) - Abstract
The generation of seismicity catalogs on synthetic fault networks holds the promise of providing key inputs into probabilistic seismic-hazard analysis, for example, the coefficient of variation, mean recurrence time as a function of magnitude, the probability of fault-to-fault ruptures, and conditional probabilities for foreshock–mainshock triggering. I employ a seismicity simulator that includes the following ingredients: static stress transfer, viscoelastic relaxation of the lower crust and mantle, and vertical stratification of elastic and viscoelastic material properties. A cascade mechanism combined with a simple Coulomb failure criterion is used to determine the initiation, propagation, and termination of synthetic ruptures. It is employed on a 3D fault network provided by Steve Ward (unpublished data, 2009) for the Southern California Earthquake Center (SCEC) Earthquake Simulators Group. This all-California fault network, initially consisting of 8000 patches, each of ∼12 square kilometers in size, has been rediscretized into patches, each of ∼1 square kilometer in size, in order to simulate the evolution of California seismicity and crustal stress at magnitude M ∼5–8. Resulting synthetic seismicity catalogs spanning 30,000 yr and about one-half million events are evaluated with magnitude-frequency and magnitude-area statistics. For a priori choices of fault-slip rates and mean stress drops, I explore the sensitivity of various constructs on input parameters, particularly mantle viscosity. Slip maps obtained for the southern San Andreas fault show that the ability of segment boundaries to inhibit slip across the boundaries (e.g., to prevent multisegment ruptures) is systematically affected by mantle viscosity.
- Published
- 2011
49. Lithosphere-asthenosphere interaction beneath the western United States from the joint inversion of body-wave traveltimes and surface-wave phase velocities
- Author
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Fred F. Pollitz, Richard M. Allen, Mathias Obrebski, and Shu-Huei Hung
- Subjects
USArray ,Geophysics ,Geochemistry and Petrology ,Lithosphere ,Seismic tomography ,Asthenosphere ,Hotspot (geology) ,North American Plate ,Crust ,Mantle (geology) ,Seismology ,Geology - Abstract
SUMMARY The relation between the complex geological history of the western margin of the North American plate and the processes in the mantle is still not fully documented and understood. Several pre-USArray local seismic studies showed how the characteristics of key geological features such as the Colorado Plateau and the Yellowstone Snake River Plains are linked to their deep mantle structure. Recent body-wave models based on the deployment of the high density, large aperture USArray have provided far more details on the mantle structure while surface-wave tomography (ballistic waves and noise correlations) informs us on the shallow structure. Here we combine constraints from these two data sets to image and study the link between the geology of the western United States, the shallow structure of the Earth and the convective processes in mantle. Our multiphase DNA10-S model provides new constraints on the extent of the Archean lithosphere imaged as a large, deeply rooted fast body that encompasses the stable Great Plains and a large portion of the Northern and Central Rocky Mountains. Widespread slow anomalies are found in the lower crust and upper mantle, suggesting that low-density rocks isostatically sustain part of the high topography of the western United States. The Yellowstone anomaly is imaged as a large slow body rising from the lower mantle, intruding the overlying lithosphere and controlling locally the seismicity and the topography. The large E–W extent of the USArray used in this study allows imaging the ‘slab graveyard’, a sequence of Farallon fragments aligned with the currently subducting Juan de Fuca Slab, north of the Mendocino Triple Junction. The lithospheric root of the Colorado Plateau has apparently been weakened and partly removed through dripping. The distribution of the slower regions around the Colorado Plateau and other rigid blocks follows closely the trend of Cenozoic volcanic fields and ancient lithospheric sutures, suggesting that the later exert a control on the locus of magmato-tectonic activity today. The DNA velocity models are available for download and slicing at http://dna.berkeley.edu.
- Published
- 2011
50. Rayleigh-wave phase-velocity maps and three-dimensional shear velocity structure of the western US from local non-plane surface wave tomography
- Author
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J. Arthur Snoke and Fred F. Pollitz
- Subjects
Earthscope ,Subduction ,Mantle wedge ,Geophysics ,Mantle (geology) ,Physics::Geophysics ,Geochemistry and Petrology ,Lithosphere ,Seismic tomography ,Shear velocity ,Phase velocity ,Geology ,Seismology - Abstract
SUMMARY We utilize two-and-three-quarter years of vertical-component recordings made by the Transportable Array (TA) component of Earthscope to constrain three-dimensional (3-D) seismic shear wave velocity structure in the upper 200 km of the western United States. Single-taper spectral estimation is used to compile measurements of complex spectral amplitudes from 44 317 seismograms generated by 123 teleseismic events. In the first step employed to determine the Rayleigh-wave phase-velocity structure, we implement a new tomographic method, which is simpler and more robust than scattering-based methods (e.g. multi-plane surface wave tomography). The TA is effectively implemented as a large number of local arrays by defining a horizontal Gaussian smoothing distance that weights observations near a given target point. The complex spectral-amplitude measurements are interpreted with the spherical Helmholtz equation using local observations about a succession of target points, resulting in Rayleigh-wave phase-velocity maps at periods over the range of 18–125 s. The derived maps depend on the form of local fits to the Helmholtz equation, which generally involve the non-plane-wave solutions of Friederich et al. In a second step, the phase-velocity maps are used to derive 3-D shear velocity structure. The 3-D velocity images confirm details witnessed in prior body-wave and surface-wave studies and reveal new structures, including a deep (>100 km deep) high-velocity lineament, of width ∼200 km, stretching from the southern Great Valley to northern Utah that may be a relic of plate subduction or, alternatively, either a remnant of the Mojave Precambrian Province or a mantle downwelling. Mantle seismic velocity is highly correlated with heat flow, Holocene volcanism, elastic plate thickness and seismicity. This suggests that shallow mantle structure provides the heat source for associated magmatism, as well as thinning of the thermal lithosphere, leading to relatively high stress concentration. Our images also confirm the presence of high-velocity mantle at ≳100 km depth beneath areas of suspected mantle delamination (southern Sierra Nevada; Grande Ronde uplift), low velocity mantle underlying active rift zones, and high velocity mantle associated with the subducting Juan de Fuca plate. Structure established during the Proterozoic appears to exert a lasting influence on subsequent volcanism and tectonism up to the Present.
- Published
- 2010
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