14 results on '"Fred F. Pollitz"'
Search Results
2. 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
3. Coseismic compression/dilatation and viscoelastic uplift/subsidence following the 2012 Indian Ocean earthquakes quantified from satellite gravity observations
- Author
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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
4. GPS measurements across the Northern Caribbean Plate Boundary Zone: Impact of postseismic relaxation following historic earthquakes
- Author
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Fred F. Pollitz and Timothy H. Dixon
- Subjects
business.industry ,Crust ,Mantle (geology) ,Plate tectonics ,Geophysics ,Caribbean region ,Large earthquakes ,Relaxation effect ,Global Positioning System ,General Earth and Planetary Sciences ,business ,Geology ,Seismology ,West indies - Abstract
GPS measurements in the northern Caribbean suggest that the rate of Caribbean plate motion relative to North America is about 10 mm/yr faster than predicted by global plate motion model NUVEL-1A. Several of the key sites used in the GPS study are located in the Dominican Republic, near the rupture zones of large earthquakes in 1946 and in the previous two centuries. Postseismic relaxation of the crust and upper mantle is a possible explanation for the plate velocity discrepancy. We explore a range of fault mechanisms and crustal and mantle theology to place an upper bound on postseismic relaxation effects. The upper bound velocity contribution in the southern Dominican Republic is 5-6 mm/yr, and the most plausible contribution is 1-2 mm/yr, suggesting that postseismic effects cannot account for the discrepan- cy. This implies that the NUVEL-1A model underesti- mates the rate of motion of the Caribbean plate.
- Published
- 1998
5. Fossil strain from the 1811-1812 New Madrid Earthquakes
- Author
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Paul A. Rydelek and Fred F. Pollitz
- Subjects
High rate ,Geophysics ,Shear (geology) ,Deformation (mechanics) ,Intraplate earthquake ,General Earth and Planetary Sciences ,Fault model ,Present day ,Strain rate ,Viscoelasticity ,Seismology ,Geology - Abstract
Observations of postseismic deformation generally suggest that the effects of postseismic viscoelastic relaxation may persist for many decades after an earthquake. In particular, strain effects from the three great earthquakes that occurred in the New Madrid seismic zone in 1811–1812 may influence present day measurements of ground deformation in this active seismic region of the central United States. Forward calculations using an earth rheology that may be appropriate for a continental intraplate region and a simple fault model for the 1811–1812 sequence suggest that postseismic relaxation may be an important factor in driving the unexpectedly high rate of shear deformation recently observed in the southern New Madrid Seismic Zone.
- Published
- 1994
6. Correction to 'Coseismic slip distribution of the February 27, 2010 Mw 8.8 Maule, Chile earthquake'
- Author
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Jean-Claude Ruegg, Jaime Campos, James Foster, S. Cimbaro, Christophe Vigny, Sergio Barrientos, Michael Bevis, Roland Bürgmann, B. A. Brooks, Fred F. Pollitz, H. Parra, M. Blanco, Robert Smalley, Juan Carlos Báez Soto, Anne Socquet, and X. Tong
- Subjects
business.industry ,Inversion (geology) ,Geodesy ,Geophysics ,Distribution (mathematics) ,Epicenter ,Interferometric synthetic aperture radar ,Global Positioning System ,General Earth and Planetary Sciences ,Seismic moment ,business ,Joint (geology) ,Aftershock ,Seismology ,Geology - Abstract
[1] In the paper “Coseismic slip distribution of the February 27, 2010 Mw 8.8 Maule, Chile earthquake” by Fred F. Pollitz et al. (Geophysical Research Letters, 38, L09309, doi:10.1029/2011GL047065, 2011) the captions for Figure 1–3 are ordered incorrectly. The correct captions for Figures 1–3 appear here. [2] Figure 1. Rupture areas associated with historic earthquakes along the Andean megathrust and aftershocks of the February 27, 2010 event. Historical epicenters are provided by Centro Regional de Sismologia para America del Sur and aftershock locations are provided by the National Earthquake Information Center. White lines indicate the surface projection of the fault plane used to model the 2010 event. Nazca–South America relative plate motion vector is from Ruegg et al. [2009]. [3] Figure 2. Observed coseismic GPS offsets (black vectors) with 95% uncertainties compared with model horizontal offsets using the coseismic slip model obtained by the joint InSAR/GPS inversion, which is contoured in gray (values in meters). White lines indicate the surface projection of the fault plane. [4] Figure 3. Coseismic slip models derived from (a) InSAR data only, (b) GPS data only, and (c) combined GPS and InSAR data, with seismic moment indicated for each model. Contour interval is 3 m. Star symbol indicates the Global CMT epicenter.
- Published
- 2011
7. Coseismic slip distribution of the February 27, 2010 Mw 8.8 Maule, Chile earthquake
- Author
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Jean-Claude Ruegg, Fred F. Pollitz, H. Parra, B. A. Brooks, M. Blanco, Sergio Barrientos, X. Tong, Jaime Campos, James Foster, Christophe Vigny, S. Cimbaro, Juan Carlos Báez Soto, Robert Smalley, Michael Bevis, Anne Socquet, and Roland Bürgmann
- Subjects
Geophysics ,Epicenter ,Interferometric synthetic aperture radar ,General Earth and Planetary Sciences ,Geodetic datum ,Seismic moment ,Submarine pipeline ,Moment magnitude scale ,Slip (materials science) ,Geodesy ,Spherical Earth ,Seismology ,Geology - Abstract
[1] Static offsets produced by the February 27, 2010 Mw = 8.8 Maule, Chile earthquake as measured by GPS and InSAR constrain coseismic slip along a section of the Andean megathrust of dimensions 650 km (in length) × 180 km (in width). GPS data have been collected from both campaign and continuous sites sampling both the near-field and far field. ALOS/PALSAR data from several ascending and descending tracks constrain the near-field crustal deformation. Inversions of the geodetic data for distributed slip on the megathrust reveal a pronounced slip maximum of order 15 m at ∼15–25 km depth on the megathrust offshore Lloca, indicating that seismic slip was greatest north of the epicenter of the bilaterally propagating rupture. A secondary slip maximum appears at depth ∼25 km on the megathrust just west of Concepcion. Coseismic slip is negligible below 35 km depth. Estimates of the seismic moment based on different datasets and modeling approaches vary from 1.8 to 2.6 × 1022 N m. Our study is the first to model the static displacement field using a layered spherical Earth model, allowing us to incorporate both near-field and far-field static displacements in a consistent manner. The obtained seismic moment of 1.97 × 1022 N m, corresponding to a moment magnitude of 8.8, is similar to that obtained by previous seismic and geodetic inversions.
- Published
- 2011
8. Geodetic slip model of the 2011 M9.0 Tohoku earthquake
- Author
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Roland Bürgmann, Fred F. Pollitz, and Paramesh Banerjee
- Subjects
Earth structure ,Geodetic datum ,Slip (materials science) ,engineering.material ,Geodesy ,Geophysics ,Interplate earthquake ,Trench ,Displacement field ,engineering ,General Earth and Planetary Sciences ,Submarine pipeline ,Far East ,Seismology ,Geology - Abstract
[1] The three-dimensional crustal displacement field as sampled by GPS is used to determine the coseismic slip of the 2011 M9.0 Tohoku Earthquake. We employ a spherically layered Earth structure and use a combination of onland GPS, out to ∼4000 km from the rupture, and offshore GPS, which samples the high-slip region on the interplate boundary along the Japan trench. Inversion of the displacement field for dip slip, assuming an interplate boundary of variable dip and striking 195°, yields a compact slip maximum of about 33 m located 200 km east of Sendai. The geodetic moment is 4.06 × 1022 N m, corresponding to Mw = 9.0. The area of maximum slip is concentrated at a depth of about 10 km, is updip of the rupture areas of the M ≳ 7 Miyagi-oki earthquakes of 1933, 1936, 1937, and 1978, and roughly coincides with the rupture area of the M7.1 1981 Miyagi-oki earthquake. The overlap of the 2011 slip area with several preceding ruptures suggests that the same asperities may rupture repeatedly with M ≳ 7 events within several decades of one another.
- Published
- 2011
9. Direct test of static stress versus dynamic stress triggering of aftershocks
- Author
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Malcolm J. S. Johnston and Fred F. Pollitz
- Subjects
Geophysics ,Source area ,Direct test ,Static stress ,General Earth and Planetary Sciences ,Aftershock ,Seismology ,Seismic wave ,Geology ,Dynamic stress ,Shock (mechanics) - Abstract
[1] Aftershocks observed over time scales of minutes to months following a main shock are plausibly triggered by the static stress change imparted by the main shock, dynamic shaking effects associated with passage of seismic waves from the main shock, or a combination of the two. We design a direct test of static versus dynamic triggering of aftershocks by comparing the near-field temporal aftershock patterns generated by aseismic and impulsive events occurring in the same source area. The San Juan Bautista, California, area is ideally suited for this purpose because several events of both types of M ∼ 5 have occurred since 1974. We find that aftershock rates observed after impulsive events are much higher than those observed after aseismic events, and this pattern persists for several weeks after the event. This suggests that, at least in the near field, dynamic triggering is the dominant cause of aftershocks, and that it generates both immediate and delayed aftershock activity.
- Published
- 2006
10. Stress changes along the Sunda trench following the 26 December 2004 Sumatra-Andaman and 28 March 2005 Nias earthquakes
- Author
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Nithiwatthn Choosakul, Fred F. Pollitz, Paramesh Banerjee, Roland Bürgmann, and Manabu Hashimoto
- Subjects
Geophysics ,Asthenosphere ,Trench ,Coulomb failure stress ,General Earth and Planetary Sciences ,Far East ,Seismic cycle ,Seismology ,Geology - Abstract
[1] The 26 December 2004 Mw = 9.2 and 28 March 2005 Mw = 8.7 earthquakes on the Sumatra megathrust altered the state of stress over a large region surrounding the earthquakes. We evaluate the stress changes associated with coseismic and postseismic deformation following these two large events, focusing on postseismic deformation that is driven by viscoelastic relaxation of a low-viscosity asthenosphere. Under Coulomb failure stress (CFS) theory, the December 2004 event increased CFS on the future hypocentral zone of the March 2005 event by about 0.25 bar, with little or no contribution from viscous relaxation. Coseismic stresses around the rupture zones of the 1797 and 1833 Sunda trench events are negligible, but postseismic stress perturbations since December 2004 are predicted to result in CFS increases of 0.1 to 0.2 bar around these rupture zones between 2 and 8 years after the December 2004 event. These are considerable stress perturbations given that the 1797 and 1833 rupture zones are likely approaching the end of a complete seismic cycle.
- Published
- 2006
11. Postseismic deformation and stress changes following the 1819 Rann of Kachchh, India earthquake: Was the 2001 Bhuj earthquake a triggered event?
- Author
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Roland Bürgmann, A. To, and Fred F. Pollitz
- Subjects
Geophysics ,Tectonophysics ,Intraplate earthquake ,General Earth and Planetary Sciences ,Earthquake rupture ,Crust ,Induced seismicity ,2008 California earthquake study ,Indian Shield ,Seismology ,Geology ,Foreshock - Abstract
[1] The 2001 Mw 7.6 Bhuj earthquake occurred in an intra-plate region with rather unusual active seismicity, including an earlier major earthquake, the 1819 Rann of Kachchh earthquake (M7.7). We examine if static coseismic and transient postseismic deformation following the 1819 earthquake contributed to the enhanced seismicity in the region and the occurrence of the 2001 Bhuj earthquake, ∼100 km away and almost two centuries later. Based on the Indian shield setting, great rupture depth of the 2001 event and lack of significant early postseismic deformation measured following the 2001 event, we infer that little viscous relaxation occurs in the lower crust and choose an upper mantle effective viscosity of 1019 Pas. The predicted Coulomb failure stress (DCFS) on the rupture plane of the 2001 event increased by more than 0.1 bar at 20 km depth, which is a small but possibly significant amount. Stress change from the 1819 event may have also affected the occurrence of other historic earthquakes in this region. We also evaluate the postseismic deformation and ΔCFS in this region due to the 2001 event. Positive ΔCFS from the 2001 event occur to the NW and SE of the Bhuj earthquake rupture.
- Published
- 2004
12. Observations of free oscillation amplitude anomalies
- Author
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Fred F. Pollitz, F. A. Dahlen, and Jeffrey Park
- Subjects
Physics ,Geometrical optics ,Earth structure ,engineering.material ,Geodesy ,Computational physics ,Geophysics ,Amplitude ,Frequency domain ,Epicenter ,engineering ,General Earth and Planetary Sciences ,Group velocity ,Seismogram ,Spherical Earth - Abstract
In a frequency domain analysis of long-period seismic data, observations of free oscillation excitation amplitudes are required in addition to frequency shifts to constrain odd-degree aspherical earth structure. To determine the feasibility of incorporating such data into large-scale inversions, we have undertaken a preliminary analysis of the observed amplitudes of fundamental spheroidal mode multiplets oSl with 26 ≤ l ≤ 43 using seismograms recorded by the IDA network. For many records, the amplitudes may be significantly better fit by allowing for a systematic shift of the theoretical spherical earth nodal lines either toward or away from the epicenter. In the great-circular geometrical optics approximation, this spatial phase shift of the amplitude pattern can be asymptotically represented in terms of the minor arc integral of δωlocal — along the source-receiver path. Results from common source-receiver paths indicate consistent sampling of the structure underlying these paths. For a number of paths, the results differ significantly from the theoretical predictions of model M84A of Woodhouse and Dziewonski.
- Published
- 1987
13. Postseismic gravity change after the 2006-2007 great earthquake doublet and constraints on the asthenosphere structure in the central Kuril Islands.
- Author
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Han SC, Sauber J, and Pollitz F
- 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 M
w 8.3 thrust and 2007 Mw 8.1 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 ~4 µGal, observed consistently from various GRACE solutions around the epicentral area during 2007-2015, is interpreted as resulting from gradual seafloor uplift by ~6 cm produced by postseismic relaxation. The GRACE data are best fit with a model of 25-35 km for the elastic thickness and ~1018 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
- Full Text
- View/download PDF
14. Broadscale postseismic gravity change following the 2011 Tohoku-Oki earthquake and implication for deformation by viscoelastic relaxation and afterslip.
- Author
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Han SC, Sauber J, and Pollitz F
- Abstract
The analysis of GRACE gravity data revealed postseismic gravity increase by 6 μGal over a 500 km scale within a couple of years after the 2011 Tohoku-Oki earthquake, which is nearly 40-50% of the coseismic gravity change. It originates mostly from changes in the isotropic component corresponding to the M
rr moment tensor element. The exponential decay with rapid change in a year and gradual change afterward is a characteristic temporal pattern. Both viscoelastic relaxation and afterslip models produce reasonable agreement with the GRACE free-air gravity observation, while their Bouguer gravity patterns and seafloor vertical deformations are distinctly different. The postseismic gravity variation is best modeled by the biviscous relaxation with a transient and steady state viscosity of 1018 and 1019 Pa s, respectively, for the asthenosphere. Our calculated higher-resolution viscoelastic relaxation model, underlying the partially ruptured elastic lithosphere, yields the localized postseismic subsidence above the hypocenter reported from the GPS-acoustic seafloor surveying.- Published
- 2014
- Full Text
- View/download PDF
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