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Aftershock-monitoring networks deployed in the epicentral area of a damaging earthquake play important roles in earthquake early warning and ShakeMap estimation, which contribute to hazard mitigation. Using distributed acoustic sensing (DAS) technology with dark fiber can significantly reduce deployment time and cost, and improve spatial sampling, both of which help capture more aftershocks. In this study, we used a 7.6 km dark fiber in Tangshan, China, to monitor seismicity after the 12 July 2020 Ms 5.1 earthquake. The DAS array detected dozens of earthquakes missed by the local permanent network that doubled the number of aftershocks. The relocated aftershocks are distributed mainly north of the DAS array, and the ground-motion pattern changes also hint small-scale features. Our successful results demonstrate the feasibility of using DAS and dark fiber for rapid postearthquake response.

SUMMARY Knowledge of attenuation structure is important for understanding subsurface material properties. We have developed a double-difference seismic attenuation (DDQ) tomography method for high-resolution imaging of 3-D attenuation structure. Our method includes two main elements, the inversion of event-pair differential ${t^*}$ ($d{t^*}$) data and 3-D attenuation tomography with the $d{t^*}$ data. We developed a new spectral ratio method that jointly inverts spectral ratio data from pairs of events observed at a common set of stations to determine the $d{t^*}$ data. The spectral ratio method cancels out instrument and site response terms, resulting in more accurate $d{t^*}$ data compared to absolute ${t^*}$ from traditional methods using individual spectra. Synthetic tests show that the inversion of $d{t^*}$ data using our spectral ratio method is robust to the choice of source model and a moderate degree of noise. We modified an existing velocity tomography code so that it can invert $d{t^*}$ data for 3-D attenuation structure. We applied the new method to The Geyser geothermal field, California, which has vapour-dominated reservoirs and a long history of water injection. A new Qp model at The Geysers is determined using P-wave data of earthquakes in 2011, using our updated earthquake locations and Vp model. By taking advantage of more accurate $d{t^*}$ data and the cancellation of model uncertainties along the common paths outside of the source region, the DDQ tomography method achieves higher resolution, especially in the earthquake source regions, compared to the standard tomography method using ${t^*}$ data. This is validated by both the real and synthetic data tests. Our Qp and Vp models show consistent variations in a normal temperature reservoir that can be explained by variations in fracturing, permeability and fluid saturation and/or steam pressure. A prominent low-Qp and Vp zone associated with very active seismicity is imaged within a high temperature reservoir at depths below 2 km. This anomalous zone is likely partially saturated with injected fluids.

Recently, distributed acoustic sensing (DAS) has been applied to shallow seismic structure imaging providing dense spatial sampling at a relatively low cost. DAS on a standard straight fiber-optic cable mostly records axial dynamic strain, which makes it difficult to separate the Rayleigh and Love wavefields. As a result, the mixed Rayleigh and Love wave signals cannot be used in the conventional surface-wave dispersion inversion method. Therefore, it is often ensured that the source and the cable are in the same line and only Rayleigh wave dispersion is used, which limits the constraints on structure and model resolution. We have inverted surface-wave dispersion spectra instead of dispersion curves. This inversion method can use mixed Rayleigh and Love waves recorded when the source and receiver array are not aligned. The multiple-channel records are transformed to the frequency domain, and a slant stack method is used to construct the dispersion spectra. The genetic algorithm method is used to obtain an optimal S-wave velocity model that minimizes the difference between theoretical and observed dispersion spectra. A series of synthetic tests are conducted to validate our method. The results suggest that our method not only improves the flexibility of the acquisition system design, but the Love wave data also provide additional constraints on the structure. Our method is applied to the active source and ambient noise data sets acquired at a geothermal site and provides consistent results for different data sets and acquisition geometries. The sensitivity of the dispersion spectra to layer thickness, density, and P-wave velocity is also discussed. With our method, the amount of usable data can be increased, helping deliver better subsurface images.

We present two new seismic velocity models for Alaska from joint inversions of body-wave and ambient-noise-derived surface-wave data, using two different methods. Our work takes advantage of data from many recent temporary seismic networks, including the Incorporated Research Institutions for Seismology Alaska Transportable Array, Southern Alaska Lithosphere and Mantle Observation Network, and onshore stations of the Alaska Amphibious Community Seismic Experiment. The first model primarily covers south-central Alaska and uses body-wave arrival times with Rayleigh-wave group-velocity maps accounting for their period-dependent lateral sensitivity. The second model results from direct inversion of body-wave arrival times and surface-wave phase travel times, and covers the entire state of Alaska. The two models provide 3D compressional- (VP) and shear-wave velocity (VS) information at depths ∼0–100 km. There are many similarities as well as differences between the two models. The first model provides a clear image of the high-velocity subducting plate and the low-velocity mantle wedge, in terms of the seismic velocities and the VP/VS ratio. The statewide model provides clearer images of many features such as sedimentary basins, a high-velocity anomaly in the mantle wedge under the Denali volcanic gap, low VP in the lower crust under Brooks Range, and low velocities at the eastern edge of Yakutat terrane under the Wrangell volcanic field. From simultaneously relocated earthquakes, we also find that the depth to the subducting Pacific plate beneath southern Alaska appears to be deeper than previous models.

SUMMARYAmbient seismic noise cross-correlation with three-component sensors yields a nine-component empirical Green's tensor, in which four components of the radial–vertical plane contain Rayleigh waves. We exploit the retrograde elliptical nature of particle motion of the fundamental mode Rayleigh wave to correct the phase of the four radial–vertical components and stack them to obtain an average fundamental mode Rayleigh-wave time-series. This technique can suppress incoherent noise and wave packets that do not follow the targeted elliptical particle motion. The same technique can be used to isolate the first higher mode Rayleigh wave that follows prograde elliptical particle motion. We first demonstrate the effectiveness of the method on synthetic waveforms and then apply it on noise cross-correlations computed in Central California. Using this method, we isolate 1st higher mode Rayleigh waves on noise cross-correlations in the Great Valley, California, which provides new phase velocity constraints for estimating velocity structure in the sedimentary basin. We also obtain improved estimates of fundamental mode Rayleigh-wave dispersion for surface-wave tomography. The waveforms stacked assuming retrograde particle motion return at least ∼20 per cent more group velocity dispersion measurements satisfying a minimum signal-to-noise ratio (SNR) criterion than the individual components for periods ∼4–18 s. For equivalent group velocity measurements, SNR for the stacked estimate of the fundamental mode Rayleigh wave is on average 40 per cent greater than that measured on the individual components at periods less than 10 s. The technique also provides an easy way to detect large errors in sensor orientation.

SUMMARYHigh seismicity rates in eastern Indonesia occur due to the complex interaction of several tectonic plates which resulted in two deadly, destructive earthquake sequences that occurred in Lombok Island and the city of Palu, Sulawesi in 2018. The first sequence began in July with an Mw 6.4 event near Lombok, culminating in an Mw 7.0 event 8 d later. This was then followed by a nearby Mw 6.9 event 12 d later. Approximately 1000 km to the northeast, a separate sequence began several weeks later near Palu where an Mw 7.5 event occurred that triggered a tsunami. In this study, we present hypocentre relocations for both earthquake sequences as well as all other regional earthquakes in eastern Indonesia. The relocations were performed using a teleseismic double-difference relocation method and arrival times for P and S waves from stations at local, regional, and teleseismic distances. The catalogue and phase data were taken from the Agency for Meteorology, Climatology and Geophysics (BMKG) of Indonesia and the International Seismological Centre (ISC) for the period of April 2009 through November 2018. The relocated catalogue provides an improved view of seismicity in eastern Indonesia over the study period, sharpening locations and interpretations of seismogenic features throughout the region. In the Lombok area, the relocated earthquakes clearly show a backarc thrust to the north of the Sunda-Banda Arc transition zone. The relocated aftershocks show that the destructive Mw 7.0 and Mw 6.9 earthquakes of the Lombok sequence ruptured two different regions: The Mw 7.0 earthquake propagated westward, whereas the Mw 6.9 earthquake propagated eastward. The entire sequence of Lombok earthquakes was most likely started by the Mw 6.4 event as the initial event or foreshock, which then triggered backarc thrusts on both sides. Several weeks later and far to the northeast, the Mw 7.5 Palu earthquake occurred along the Palu-Koro Fault, filling a seismic gap that had not ruptured in an Mw 6.0 event or larger since at least 1900. The distribution of aftershocks indicates that the northern part of the Palu-Koro Fault has lower relative seismicity rates than the southern part at shallow depths, and that off fault aftershocks are mostly located to the east of the Palu-Koro Fault.

Seismic sensors and seismic imaging have been widely used to monitor the geophysical properties of the subsurface. As subsurface engineering techniques advance, more precise monitoring systems are ...

The 30 November 2018 magnitude 7.1 Anchorage earthquake occurred as a result of normal faulting within the lithosphere of subducted Yakutat slab. It was followed by a vigorous aftershock sequence with over 10,000 aftershocks reported through the end of July 2019. The Alaska Earthquake Center produced a reviewed aftershock catalog with a magnitude of completeness of 1.3. This well‐recorded dataset provides a rare opportunity to study the relationship between the aftershocks and fault rupture of a major intraslab event. We use tomoDD algorithm to relocate 2038 M≥2 aftershocks with a regional 3D velocity model. The relocated aftershocks extend over a 20 km long zone between 47 and 57 km depth and are primarily confined to a high VP/VS region. Aftershocks form two clusters, a diffuse southern cluster and a steeply west‐dipping northern cluster with a gap in between where maximum slip has been inferred. We compute moment tensors for the Mw>4 aftershocks using a cut‐and‐paste method and careful selection of regional broadband stations. The moment tensor solutions do not exhibit significant variability or systematic differences between the northern and southern clusters and, on average, agree well with the mainshock fault‐plane parameters. We propose that the mainshock rupture initiated in the Yakutat lower crust or uppermost mantle and propagated both upward into the crust to near its top and downward into the mantle. The majority of the aftershocks are confined to the seismically active Yakutat crust and located both on and in the hanging wall of the mainshock fault rupture.

Knowledge of the shear wave velocity structure in the Parkfield, California region is a key factor for obtaining accurate earthquake locations and improving wave propagation simulations, but it has been poorly resolved in previous studies. We have conducted an ambient noise tomography study using continuous waveform data from available seismic stations, and higher resolution was achieved. The shear wave velocity structure in the crust was inverted from Rayleigh wave dispersion curves between 1 and 16 s. The new shear wave velocity structure correlates well with an existing compressional wave velocity model obtained from body-wave arrival time tomography. A low velocity zone (LVZ) in the lower crust was revealed on the southwest side of the San Andreas Fault, which is opposite to the pattern in the shallow crust. Two LVZs in the mid- and lower crust were revealed whose locations correlate with a high conductivity body, which supports the existence of fluid channels from depth into the SAF. Velocity variations at depth near the SAF also suggest a correlation with changes of the amplitude of low frequency earthquakes and with the resistivity structure.

National Science Foundation (NSF) EAR-1411779 National Sciences and Engineering Research Council of Canada (NSERC) through a PGS-D scholarship

The important task of tracking seismic activity requires both sensitive detection and accurate earthquake location. Approximate earthquake locations can be estimated promptly and automatically; how...

We derive high-resolution P and S seismic velocities (VP and VS) within the South-Central Transverse Ranges section of the San Andreas Fault (SAF), using a new double-difference tomography algorithm incorporating both event-pair and station-pair differential times. The addition of station-pair data allows for better absolute event locations and higher model resolution at shallow depths. Velocities within a 222 km × 164 km region are inverted using > 1,000,000 P and S arrival (picked with an automatic detection algorithm) and differential times from > 10,000 local events recorded by > 250 stations. Similarly large P and S datasets lead to high-quality VP/VS estimates of the region. The resulting models include low velocities along major fault segments and across-fault velocity contrasts. They also show very high VP/VS anomalies near shallow damaged rock, whereas fault zones exhibit either low ( 1.73) VP/VS characteristics at greater depth. The variations in amplitude of these anomalies along the SAF through San Gorgonio Pass (SGP) suggest abrupt west-to-east changes in elastic properties. Moreover, their geometries imply near-vertical SAF segments northwest of SGP and northeast dipping faults southeast of that area. The SAF near Coachella Valley is estimated to dip by 57°. Regional-scale low and high VP/VS values are related to relative abundances of crystalline or metamorphic rocks. Near-fault VP/VS anomalies at depth are likely associated with changes in wet crack geometries. The obtained results can improve future calculations of seismic motion from large earthquakes in the area and related seismic hazard estimates.

© 2017 The Author(s) At Brady Hot Springs, a geothermal field in Nevada, heated fluids have been extracted, cooled, and re-injected to produce electrical power since 1992. Analysis of daily pumping records and catalogs of microseismicity between 2010 and 2015 indicates a statistically significant correlation between days when the daily volume of production was at or above its long-term average rate and days when no seismic event was detected. Conversely, shutdowns in pumping for plant maintenance correlate with increased microseismicity. We hypothesize that the effective stress in the subsurface has adapted to the long-term normal operations (deep extraction) at the site. Under this hypothesis, extraction of fluids inhibits fault slip by increasing the effective stress on faults; in contrast, brief pumping cessations represent times when effective stress is decreased below its long-term average, increasing the likelihood of microseismicity.

The states of Jalisco, Colima, and Michoacan in western Mexico overlie the boundary of the subducting Rivera and Cocos plates, presenting an ideal target for seismological inquiry to better understand the resulting mantle flow and regional volcanism. The different dips between the two subducting plates are thought to provide a mantle conduit that has contributed to the Colima Volcanic Complex (CVC), but there is considerable debate on the depth of the Rivera plate and width of the resulting conduit. With data from the Mapping the Rivera Subduction Zone (MARS) and Colima Deep Seismic Experiment (CODEX) networks, two temporary broadband arrays deployed in the region between 2006 and 2008, we inverted for three-dimensional P- and S-wave velocity as well as Vp/Vs structure of the upper ~ 70 km of the crust and mantle in the Jalisco region. Using a newly-developed automatic P- and S-wave picker, we increased P picks by ~ 74% and S picks by more than a factor of four compared to a database of manual picks for the 803 earthquakes used in the inversion. Additional relocated earthquakes extending to the trench are consistent with previous interpretations of the Rivera and Cocos plate interfaces. Areas of high Vp/Vs above both subducting slabs suggest the presence of fluids resulting from dehydration of subducted material. Extensive crustal seismicity occurs near these anomalies. A zone of high Vp/Vs is also present under the CVC. We also compare the results of different methods for obtaining Vp/Vs: a direct inversion for Vp/Vs from S minus P times versus simply dividing the Vp model by the Vs model. We find direct inversions of S minus P times to be more reliable.

We present an ambient noise tomographic study of the mid-crustal magmatic system in the Katmai area, Alaska. Using multi-component (vertical and radial) seismic records at 70 stations deployed at various times over 20 years, we construct corresponding cross-correlograms and extract ~1010 Rayleigh-wave dispersion curves in the periods between 2 and 30 s. These enable us to obtain corresponding group velocity maps and finally a 3-D shear-wave velocity model to a depth of 25 km. The most striking feature is a ~25 km wide low-velocity anomaly beneath the Katmai area at depths of approximately 10 to 20 km, which we interpret as a mid-crustal magma storage zone. Our study complements previous local-scale tomography studies that unveil small upper crustal reservoirs, revealing the likely deep magma source ultimately responsible for the 1912 Novarupta-Katmai eruption.

The Deep Fault Drilling Project (DFDP) is a multinational scientific drilling effort to study the evolution, structure, and seismogenesis of the Alpine fault, New Zealand, via in situ measurements of fault rock properties. The second phase of drilling (DFDP‐2), undertaken in the Whataroa Valley in late 2014, was intended to intersect the Alpine fault at a depth of around 1 km. In conjunction with the drilling and on‐site science activities, a real‐time seismic monitoring scheme and traffic‐light response protocol were established to detect, locate, and if necessary respond to seismicity within 30 km of the drill site. This network was operated around the clock between late August 2014 and early January 2015, and we detected and located 493 earthquakes of M L 0.6–4.2. None of these earthquakes occurred within 3 km of the drill site, and nor did any of the seismicity detected require changes to drilling operations. The monitoring was undertaken using open‐source software operated by an international team of 16 seismologists (including eight postgraduate students) working in 7 institutions and 3 countries to provide rapid on‐ and off‐site manual checking and relocating of events. The team’s standard response time between detection and final location was less than 30 min under normal background seismicity conditions and up to 1 hr during swarm activity and for low‐priority, distant (≥30 km epicentrally from the drill site) earthquakes. This article documents the methodology, infrastructure, protocols, outcomes, and key lessons of this monitoring.

The Deep Fault Drilling Project DFDP) on the central Alpine Fault, South Island, NewZealand, has motivated a broad range of geophysical and geological studies intended to characterize the fault system in the locality of the drill site at various scales. In order to better understand the structural features of the central Alpine Fault, we have developed 3-D P-and S-wave velocity VP and VS) models of the region by double-difference tomography using data sets from multiple seismic networks. In previous work, the quality of the S-wave model has been poor due to the small number of available S-wave picks. We have utilized a new high-accuracy automatic S-wave picker to increase the number of usable S-wave arrivals by more than a factor of two, thereby substantially improving the VS model. Compared to previous studies, our new higher-resolution VP model based on more observations shows a clear VP contrast higher VP on the southeast hanging wall side) at depths of 5-10 km near the DFDP drill sites. With our better resolved VS model, in the same region, we detect a sharply defined high VS body VS > 3.7 km s(-1)) within the hanging wall. Our earthquake relocations reveal the presence of clusters within and around low-velocity zones in the hanging wall southeast of the Alpine Fault. Together with the improved earthquake locations, the P-and S-wave tomography results reveal the Alpine Fault to be marked by a velocity contrast throughout most of the study region. The fault dips southeastwards at about 50. from 5 to 15 km depth, as inferred from the velocity structure, seismicity and observations of fault zone guided waves.

A field test that was conducted at Garner Valley, California, on 11 and 12 September 2013 using distributed acoustic sensing (DAS) to sense ground vibrations provided a continuous overnight record of ambient noise. The energy of ambient noise was concentrated between 5 and 25 Hz, which falls into the typical traffic noise frequency band. A standard procedure (Bensen et al. , 2007) was adopted to calculate noise cross‐correlation functions (NCFs) for 1‐min intervals. The 1‐min‐long NCFs were stacked using the time–frequency domain phase‐weighted‐stacking method, which significantly improves signal quality. The obtained NCFs were asymmetrical, which was a result of the nonuniform distributed noise sources. A precursor appeared on NCFs along one segment, which was traced to a strong localized noise source or a scatterer at a nearby road intersection. NCF for the radial component of two surface accelerometers along a DAS profile gave similar results to those from DAS channels. We calculated the phase velocity dispersion from DAS NCFs using the multichannel analysis of surface waves technique, and the result agrees with active‐source results. We conclude that ambient noise sources and the high spatial sampling of DAS can provide the same subsurface information as traditional active‐source methods.

Synopsis A brief review is given of the topics in classical probability and statistics that are used throughout the book. Connections between probability theory and its application to the analysis of data with random measurement errors are highlighted. Note that some very different philosophical interpretations of probability theory are discussed in Chapter 11 .

The characteristics of rank-deficient and ill-conditioned linear systems of equations are explored using the singular value decomposition. The connection between model and data null spaces and solution uniqueness and ability to fit data is examined. Model and data resolution matrices are defined. The relationship between singular value size and singular vector roughness and its connection to the effect of noise on solutions are discussed in the context of the fundamental trade-off between model resolution and instability. Specific manifestations of these issues in rank-deficient and ill-conditioned discrete problems are shown in several examples.

The great 2012 Mw8.6 strike‐slip earthquake beneath the Wharton Basin generated acomplex aftershock sequence that maps onto a system of conjugate faults. Analysis of high‐precisionaftershock locations with improved depth constraint is used here to characterize the seismogenic limits ofthe oceanic lithosphere. The study presents teleseismic double‐difference earthquake relocation results for695 events in and around the 2012 Wharton Basin intraplate earthquake sequence. We highlight sevenmajor clusters of seismicity and show that the 2012 earthquake sequence ruptured in the oceanic crust andupper mantle. The refined aftershock locations projected onto available mainshockfinite‐fault modelsshow that aftershocks occur outside the largest coseismic slip region and tend to cluster in low slip areas, apattern commonly seen for large continental and megathrust sequences. For events with depth phases, therelocated focal depths generally correspond to predicted depths from pP‐P time observations. ReportedpP‐P observations for intraplate events correspond to depths ranging from ~5 to 35–40 km, such that thedeepest events occur within the expected limit of brittle seismic failure at 600 °C, here defined by ahalf‐space cooling model of the region. The 74 low magnitude events in our catalog that locate below the600 °C isotherm do not have consistent depth phase observations and cannot be interpreted as strongevidence of rupture into the ductile regime. The refined double‐difference catalog supports that, along withdeep coseismic rupture, moderate‐sized earthquakes ruptured across the full extent of the elastic oceaniclithosphere in the Wharton Basin.