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Following the development of unconventional oil and gas production in the Fort Worth Basin, Texas, a rapid increase in basin-wide seismicity began in 2008 that grew to include earthquakes affecting a substantial portion of the Dallas–Fort Worth metropolitan area. To assess and mitigate the seismic hazard, which in this region is impacted by the thickness of the sedimentary basin and accompanying soft soil layer, we estimate site effects at 22 seismic stations deployed to record these earthquakes. Site responses are derived using two different datasets and approaches: (1) a modified generalized inversion technique (GIT) based on the S-wave Fourier amplitude spectra from earthquakes; and (2) application of the quarter-wavelength approximation (QWA) using estimates of average shear-wave velocities in the upper 30 m, known as VS30. We find that site amplification estimates based on the two techniques are roughly consistent with one another (median of the amplification ratio = 0.92) over frequencies where a quarter-wavelength corresponds to ∼30 m depth. The site amplification factors from the two approaches are found on average to be about 3 at the quarter-wavelength frequency (QWF). These site amplification estimates are not well correlated with geologic characteristics including rock type and geologic age. Finally, QWA values at six sites do not match GIT site amplification at the QWF (outside of ± median absolute deviations boundary), which we attribute to a combination of the underlying assumptions of the QWA, uncertainty in VS30 estimates, and unmodeled site response complexity.

Earthquakes in the Fort Worth basin (FWB) have been induced by the disposal of recovered wastewater associated with extraction of unconventional gas since 2008. Four of the larger felt earthquakes, each on different faults, prompted deployment of local distance seismic stations and recordings from these four sequences are used to estimate the kinematic source characteristics. Source spectra and the associated source parameters, including corner frequency, seismic moment, and stress drop, are estimated using a modified generalized inversion technique (GIT). As an assessment of the validity of the modified GIT approach, corner frequencies and stress drops from the GIT are compared to estimates using the traditional empirical Green’s function (EGF) method for 14 target events. For these events, corner-frequency residuals (GIT−EGF) have a mean of −0.31 Hz, with a standard deviation of 1.30 Hz. We find consistent mean stress drops using the GIT and EGF methods, 9.56 and 11.50 MPa, respectively, for the common set of target events. The GIT mean stress drop for all 79 earthquakes is 5.33 MPa, similar to estimates for global intraplate earthquakes (1–10 MPa) as well as other estimates for induced earthquakes near the study area (1.7–9.5 MPa). Stress drops exhibit no spatial or temporal correlations or depth dependency. In addition, there are no time or space correlations between estimated FWB stress drops and modeled pore-pressure perturbations. We conclude that induced earthquakes in the FWB occurring on normal faults in the crystalline basement release pre-existing tectonic stresses and that stress drops on the four sequences targeted in this study do not directly reflect perturbations in pore-fluid pressure on the fault.

A generalized inversion technique (GIT) is applied to local seismic data from 90 induced earthquakes (ML 2.0–3.9) in the Fort Worth Basin (FWB) of north Texas to separate path, site, and source characteristics and to improve local seismic hazard assessment. Seismograms from three earthquake sequences on spatially separated basement faults are recorded on 66 temporary stations. Because of the lack of hard-rock recording sites within the sedimentary basin, we developed a site correction method for the appropriate GIT process. At about 30 km distance from the hypocenters, we observed a change in spectral attenuation and thus focus data analysis within this distance range. The estimated quality factors for S and P waves result in a QS that is larger than QP which we interpret as a result of concentrations of crustal pore fluids or partial fluid-saturated material along the path; an interpretation consistent with fluid-rich sedimentary rocks in the FWB. Strong site amplifications as much as five times on horizontal components reflect the thick sediments in the basin. A limited number of sites exhibit amplification or deamplification on the vertical component that limits the use of horizontal-to-vertical spectral ratio methods for characterizing the site effect relative to the site effects estimated by GIT. Stress drops for all earthquakes range from 1.18 and 21.73 MPa with a mean of 4.46 MPa, similar to values reported for tectonic intraplate events. The stress-drop values suggest that strong motion and seismic hazard from the injection-induced earthquake in the FWB are comparable to those for tectonic earthquakes. The strong site amplification and fluid effects on propagation attenuation may be crucial factors to take into account for estimating seismic hazards of induced earthquakes in sedimentary basins.

From 2006 through mid-2018, there have been 125 [Formula: see text] recorded earthquakes within the Fort Worth Basin and the Dallas-Fort Worth metropolitan area. There is general scientific consensus that this increase in seismicity has been induced by increases in pore-fluid pressure from wastewater injection and from cross-fault pore-pressure imbalance due to injection and production. Previous fault stress analyses indicate that many of the faults are critically stressed; therefore, careful consideration should be taken when injecting in close proximity to these structures. Understanding the structural characteristics that control geomechanical aspects of these earthquake-prone faults is vital in characterizing this known hazard. To improve understanding of faults in the system, we have developed a characterization using a new basin-wide fault interpretation and database that has been assembled through the integration of published data, 2D and 3D seismic surveys, outcrop mapping, earthquakes, and interpretations provided by operators resulting in a 3D structural framework of basement-rooting faults. Our results show that a primary fault system trends northeast–southwest, creating a system of elongate horsts and grabens. Fault architectures range from isolated faults to linked and cross-cutting relay systems with individual segments ranging in length from 0.5 to 80 km. The faults that have hosted earthquakes are generally less than 10 km long, trend toward the northeast, and exhibit more than 50 m of normal displacement. The intensity of faulting decreases to the west away from the Ouachita structural front. Statistical analysis of the fault length, spacing, throw, and linkage tendency enables a more complete characterization of faults in the basin, which can be used to mitigate the seismic hazard. Finally, we find that a significant percentage of the total population of faults may be susceptible to reactivation and seismicity as those that have slipped recently.

The rate of seismicity in the hydrocarbon‐producing Fort Worth Basin of north‐central Texas, which underlies the Dallas–Fort Worth metropolitan area, increased markedly from 2008 through 2015, coinciding spatiotemporally with injection of 2 billion barrels of wastewater into deep aquifers. Although the rate of seismicity has declined with injection rates, some earthquake sequences remained active in 2018 and new clusters have developed. Most of this seismicity occurred away from regionally mapped faults, challenging efforts to constrain the continuing hazards of injection‐induced seismicity in the basin. Here, we present detailed new models of potentially seismogenic faults and the stress field, which we use to build a probabilistic assessment of fault‐slip potential. Our new fault map, based on reflection seismic data, tens of thousands of well logs, and outcrop characterization, includes 251 basement‐rooted normal faults that strike dominantly north‐northeast, several of which extend under populated areas. The updated stress map indicates a relatively consistent north‐northeast–south‐southwest azimuth of the maximum horizontal principal stress over seismically active parts of the basin, with a transition from strike‐slip faulting in the north to normal faulting in the southeast. Based on these new data, our probabilistic analysis shows that a majority of the total trace length of the mapped faults have slip potential that is equal to or higher than that of the faults that have already hosted injection‐induced earthquake sequences. We conclude that most faults in the system are highly sensitive to reactivation, and we postulate that many faults are still unidentified. Ongoing injection operations in the region should be conducted with these understandings in mind.

Since 2008, earthquake sequences within the Fort Worth basin (FWB), north Texas, have been linked to wastewater disposal activities related to unconventional shale‐gas production. The North Texas Earthquake Study (NTXES) catalog (2008–2018), described and included herein, uses a combination of local and regional seismic networks to track significant seismic sequences in the basin. The FWB earthquakes occur along discrete faults that are relatively far apart (>30 km), allowing for more detailed study of individual sequence development. The three largest sequences (magnitude 3.6+) are monitored by local seismic networks (10 km) over time, implying that far‐field stress changes associated with fluid injection activities may be an important component to understanding the seismic hazard of induced seismicity sequences.

In recent years, numerous small earthquakes have occurred near the town of Pecos in West Texas; however, when this activity began and whether it was caused by increased petroleum industry activity has been uncertain because prior to 2017 there were few permanent seismograph stations in the region. We identify and locate earthquakes using data recorded since 2000 at TXAR, a sensitive 10‐station seismic array situated about 240 km south of Pecos. We thus show that in 2007 one earthquake occurred near Pecos, in 2009 several more occurred, and subsequently activity has increased considerably, with more than 2000 events identified in 2017. A time‐of‐day and year‐by‐year analysis identifies geographic areas in West Texas where events are likely to be natural earthquakes and quarry blasts. However, for the Pecos events, annual seismicity rates increase along with annual volumes of petroleum production and fluid waste disposal, suggesting a causal link. Analysis of seismograms collected by the EarthScope Transportable Array indicates the 2009 earthquakes had focal depths of 4.0‐5.2 km below sea level, within or just below strata where petroleum is produced and/or wastewater is injected. The largest earthquake to date had magnitude ML3.7, but the recent high activity rates suggest that greater magnitudes may be possible. For the years 2000‐2017, we provide a catalog of 10,753 epicenters of seismic events recorded at TXAR. Plain Language Summary Petroleum production in the Permian Basin of West Texas has been accelerating since 2007, and by 2023 it is anticipated Permian Basin production will exceed the production of every nation in the world other than Saudi Arabia. Developing this domestic source of energy has profound economic and political implications, especially since protecting vital foreign sources of energy has been a major factor affecting U.S. foreign policy. In recent years, numerous small earthquakes have occurred in the Delaware Basin (a subregion of the Permian Basin), but when this seismicity began has been uncertain because there were few seismographs in this region before 2017. We show that these anomalous earthquakes first occurred in 2009, and that many of them are probably induced by petroleum production in the Delaware Basin. Understanding the relationship between production and earthquake activity is a critical first step towards mitigating seismic hazards that could affect local populations and compromise the development of these vital petroleum resources.

On 7 May 2015, a MW 4.0 earthquake occurred near Venus, northeast Johnson County, Texas, in an area of the Bend Arch-Fort Worth Basin that reports long-term, high-volume wastewater disposal and that has hosted felt earthquakes since 2009. In the weeks following the MW 4.0 earthquake, we deployed a local seismic network and purchased nearby active-source seismic reflection data to capture additional events, characterize the causative fault, and explore potential links between ongoing industry activity and seismicity. Hypocenter relocations of the resulting local earthquake catalog span ~4-6 km depth and indicate a fault striking ~230°, dipping to the west, consistent with a nodal plane of the MW 4.0 regional moment tensor. Fault plane solutions indicate normal faulting, with B-axes striking parallel to maximum horizontal compressive stress. Seismic reflection data image the reactivated basement fault penetrating the Ordovician disposal layer and Mississippian production layer, but not displacing post-Lower Pennsylvanian units. Template matching at regional seismic stations indicates that low magnitude earthquakes with similar waveforms began in April 2008, with increasing magnitude over time. Pressure data from five saltwater disposal wells within 5 km of the active fault indicate a disposal formation that is 0.9-4.8 MPa above hydrostatic. We suggest that the injection of 28,000,000 m3 of wastewater between 2006 and 2015 at these wells led to an increase in subsurface pore fluid pressure that contributed to inducing this long-lived earthquake sequence. The 2015 MW 4.0 event represents the largest event in the continuing evolution of slip on the causative fault.

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.

North Texas has experienced a roughly exponential increase in seismicity since 2008. This increase is primarily attributable to wastewater injection into the Ellenburger Formation—a carbonate formation located within and just above seismically active zones. To our knowledge, there has been no previous comprehensive ∼10 year analysis comparing regional seismicity with basin-wide injection and injection pressure of wastewater into the Ellenburger, even though monthly injection/pressure records have been made publically available for nearly a decade. Here we compile and evaluate more than 24,000 monthly injection volume and pressure measurements for the Ellenburger formation. We compare Ellenburger injection pressures and volumes to basin-wide injection pressures and volumes, and to earthquake locations and rates. The analysis shows where cumulative injection volumes are highest, where injection pressures and formation pressures are increasing, how injection volumes have changed regionally with time, and how Ellenburger injection volumes and pressures correlate in space and time with recent seismicity in North Texas. Results indicate that between 2005 and 2014 at least 270 million m 3 (∼1.7 billion barrels) of wastewater were injected into the Ellenburger formation. If we assume relative homogeneity for the Ellenburger and no significant fluid loss across the 63,000 km 2 basin, this volume of fluid would increase pore fluid pressure within the entire formation by 0.09 MPa (∼13 psi). Recent spot measurements of pressure in the Ellenburger confirm that elevated fluid pressures ranging from 1.7 to 4.5 MPa (250–650 psi) above hydrostatic exist in this formation, and this may promote failure on pre-existing faults in the Ellenburger and underlying basement. The analysis demonstrates a clear spatial and temporal correlation between seismic activity and wastewater injection volumes across the basin, with earthquakes generally occurring in the central and eastern half of the basin, where Ellenburger wastewater injection cumulative volumes and estimated pressure increases are highest. The increased seismicity correlates with increased fluid pressure, which is a potential cause for these earthquakes. Based on these results, we hypothesize it is plausible that the cumulative pressure increase across the basin may trigger earthquakes on faults located tens of kilometers or more from injection wells, and this process may have triggered the Irving-Dallas earthquake sequence. We use these results to develop preliminary forecasts for the region concerning where seismicity will likely continue or develop in the future, and assess what additional data are needed to better forecast and constrain seismic hazard.

In Texas, earthquakes have occurred in close association with activities accompanying petroleum production since 1925. Here we develop a five‐question test to categorize individual events as “tectonic,” “possibly induced,” “probably induced,” or “almost certainly induced.” In Texas, the probably induced and almost certainly induced earthquakes are broadly distributed geographically—in the Fort Worth basin of north Texas, the Haynesville Shale play area of east Texas, along the Gulf Coast in south Texas, and the Permian basin of west Texas. As the technologies applied to manage petroleum fields have evolved, induced earthquakes have been associated with different practices. In fields being driven by primary recovery prior to 1940, earthquakes occurred in fields extracting high volumes of petroleum from shallow strata. Subsequently, as field pressures decreased and secondary recovery technologies became common, earthquakes also occurred in association with waterflooding operations. Since 2008, the rate of earthquakes with magnitudes greater than 3 has increased from about 2 events/yr to 12 events/yr; much of this change is attributable to earthquakes occurring within a few kilometers of wastewater disposal wells injecting at high monthly rates. For three sequences monitored by temporary local seismograph networks, most hypocenters had focal depths at and deeper than the depth of injection and occurred along mapped faults situated within 2 km of injection sites. The record clearly demonstrates that induced earthquakes have been broadly distributed in several different geographic parts of Texas over the last 90 years.

Long-term fault slip history can diagnose natural versus induced earthquakes, independent of correlations with fluid injection., To assess whether recent seismicity is induced by human activity or is of natural origin, we analyze fault displacements on high-resolution seismic reflection profiles for two regions in the central United States (CUS): the Fort Worth Basin (FWB) of Texas and the northern Mississippi embayment (NME). Since 2009, earthquake activity in the CUS has increased markedly, and numerous publications suggest that this increase is primarily due to induced earthquakes caused by deep-well injection of wastewater, both flowback water from hydrofracturing operations and produced water accompanying hydrocarbon production. Alternatively, some argue that these earthquakes are natural and that the seismicity increase is a normal variation that occurs over millions of years. Our analysis shows that within the NME, faults deform both Quaternary alluvium and underlying sediments dating from Paleozoic through Tertiary, with displacement increasing with geologic unit age, documenting a long history of natural activity. In the FWB, a region of ongoing wastewater injection, basement faults show deformation of the Proterozoic and Paleozoic units, but little or no deformation of younger strata. Specifically, vertical displacements in the post-Pennsylvanian formations, if any, are below the resolution (~15 m) of the seismic data, far less than expected had these faults accumulated deformation over millions of years. Our results support the assertion that recent FWB earthquakes are of induced origin; this conclusion is entirely independent of analyses correlating seismicity and wastewater injection practices. To our knowledge, this is the first study to discriminate natural and induced seismicity using classical structural geology analysis techniques.

A phased array of 19 broadband seismometers was deployed from November 2009 to September 2011 in an effort to detect nonvolcanic tremor or tectonic tremor associated with the Reelfoot fault, northern Tennessee. An autodetection algorithm using broadband frequency–wavenumber analysis was used to search for the recurrence of signals first reported during an active source experiment in 2006. The original signals appeared as short duration, impulsive arrivals with a high phase velocity ranging from 3 to 25 km/s. We have identified thousands of similar signals on the 2‐year long array data. Two distinct detection peaks are observed with event azimuths from the west and northeast. The detections are most similar to the events seen in 2006 and are inferred to come from very small ( M L ∼−1) microearthquakes that occur in the shallow basement on faults adjacent to the Reelfoot fault. These include detections with coherent S ‐wave energy that reinforce the interpretation of very small local and regional events. Other signals detected show distinct changes in slowness and azimuth as a function of time. These events were interpreted as atmospheric acoustic sources. The high‐frequency content and impulsive arrivals of the nonacoustic arrivals are not consistent with tectonic tremor as seen in other parts of the world but do indicate seismic activity in the crust near the Reelfoot thrust fault that was previously unknown.

A three-dimensional (3-D), high-resolution P wave seismic attenuation model for the New Madrid Seismic Zone (NMSZ) is determined using P wave path attenuation (t*) values of small-magnitude earthquakes (MD

[1] Three-dimensional P and S wave velocity (VP and VS) models and high-resolution earthquake relocations are determined for the New Madrid Seismic Zone using double-difference local earthquake tomography. The data set consists of arrival times and differential times recorded by the Cooperative New Madrid Seismic Network (CNMSN) from 2000 to 2007 and the 1989–1992 Portable Array Network and Data Acquisition deployment. Waveform cross correlation–derived differential times for the CNMSN data are also incorporated. The velocity solutions are compatible with previous solutions centered on the active arms of seismicity and cover a broader area, including mafic intrusions along the margin of the Reelfoot rift. Major features include elevated VP and VS associated with the mafic plutons and reduced VP and VS along and southeast of the Axial fault (AF), a major arm of seismicity trending along the rift axis. Low VP extends to a depth of at least 20 km along the portion of the AF that extends south of the Missouri bootheel. A locally high VP/VS anomaly imaged along the central portion of the Reelfoot fault is spatially correlated with a significant change in fault trend and is interpreted as a region containing high pore pressure and/or water-filled microcracks.

[1] In 2009, a swarm of more than 30,000 earthquakes occurred beneath the Harrat Lunayyir lava field in northwest Saudi Arabia. This event was just one of several seismic swarms to occur in this region over the past decade. Surface deformation associated with the seismicity, modeled in previous studies using Interferometric Synthetic Aperture Radar (InSAR) data, is best attributed to the intrusion of a 10 km long dyke. However, little is known about the velocity structure beneath Harrat Lunayyir, making assessment of future seismic and volcanic hazards difficult. In this study, we use local double-difference tomography to generate a P wave velocity model beneath Harrat Lunayyir and to more precisely locate earthquakes from the 2009 seismic swarm. A pronounced fast velocity anomaly, centered at ~15 km depth with a shallower extension to the N-NW, is interpreted as an area of repeated magmatic intrusion. The crust surrounding the fast intrusion is slower than that suggested by broader-scale models for the Arabian Shield. The largest magnitude events occurred early in the swarm, concentrated at shallow depths (~2–8 km) beneath northern Harrat Lunayyir, and these events are associated with the dyke intrusion. Later, deep earthquakes (~15 km) beneath the southern end of the study region as well as a group of intermediate-depth events connecting the shallow and deep regions of seismicity occurred. These later events likely represent responses to the local stress conditions following the intrusion. Our results are unique since harrat magma systems are rarely imaged, and our observations, coupled with the seismic history in this region, suggest that future volcanic intrusions beneath Harrat Lunayyir are likely.

[1] Subduction zone earthquakes exhibit a wide spectrum of rupture times that reflect conditions on the megathrust fault. Tsunami earthquakes are examples of slower than expected ruptures that produce anomalously large tsunamis relative to the surface-wave magnitude. One model explaining tsunami earthquakes suggests slip within patches of low rigidity material at shallow depths. Heterogeneous fault conditions, such as having patches of low rigidity material surrounded by higher strength material, should produce heterogeneous earthquake rupture parameters. Here we investigate along-strike variation in rupture duration for 427 shallow thrust earthquakes (Mw = 5.0–7.0) in the Peru, Chile, Alaska, Tonga, Kuril, Izu, and Java-Sumatra subduction zones to explore how heterogeneous seismic and tectonic characteristics, such as differences in sediment type, thickness, and roughness of subducting bathymetry, affect earthquake properties. Earthquake source parameters, including rupture durations, are estimated using multi-station deconvolution of teleseismic P and SH waves to solve for earthquake source time functions, and all events are relocated using additional depth phase information. We classify events into shallow (≤26 km) and deep (>26 km and ≤61 km) groups based on the overall mean depth and focus on the longest duration events with moment normalized rupture durations of >1 standard deviation above the mean duration for each group. We find long-duration events at all depths within the study regions except Peru and Chile. We find no correlation with incoming sediment thickness or type, and limited spatial correlation with regions of past tsunami earthquakes, regions of observed afterslip, and subducting bathymetric features.

We present a new tomographic model of the mantle in the area of the 2010 M8.8 Maule earthquake and surrounding regions. Increased ray coverage provided by the aftershock data allows us to image the detailed subducting slab structure in the mantle, from the region of flat slab subduction north of the Maule rupture to the area of overlapping rupture between the 1960 M9.5 and the 2010 M8.8 events to the south. We have combined teleseismic primary and depth phase arrivals with available local arrivals to better constrain the teleseismic earthquake locations in the region, which we use to conduct nested regional–global tomography. The new model reveals the detailed structure of the flat slab and its transition to a more moderately dipping slab in the Maule region. South of the Maule region, a steeply dipping relic slab is imaged from ∼200 to 1000 km depth that is distinct from the moderately dipping slab above it and from the more northerly slab at similar depths. We interpret the images as revealing both horizontal and vertical tearing of the slab at ∼38°S to explain the imaged pattern of slab anomalies in the southern portion of the model. In contrast, the transition from a horizontal to moderately subducting slab in the northern portion of the model is imaged as a continuous slab bend. We speculate that the tearing was most likely facilitated by a fracture zone in the downgoing plate or alternatively by a continental scale terrane boundary in the overriding plate.

SUMMARY We use measurements at 35 GPS stations in northern Central America and 25 seismometers at teleseismic distances to estimate the distribution of slip, source time function and Coulomb stress changes of the Mw = 7.3 2009 May 28, Swan Islands fault earthquake. This event, the largest in the region for several decades, ruptured the offshore continuation of the seismically hazardous Motagua fault of Guatemala, the site of the destructive Ms = 7.5 earthquake in 1976. Measured GPS offsets range from 308 millimetres at a campaign site in northern Honduras to 6 millimetres at five continuous sites in El Salvador. Separate inversions of geodetic and seismic data both indicate that up to ∼1 m of coseismic slip occurred along a ∼250-km-long rupture zone between the island of Roatan and the eastern limit of the 1976 M = 7.5 Motagua fault earthquake in Guatemala. Evidence for slip ∼250 km west of the epicentre is corroborated independently by aftershocks recorded by a local seismic network and by the high concentration of damage to structures in areas of northern Honduras adjacent to the western limit of the rupture zone. Coulomb stresses determined from the coseismic slip distribution resolve a maximum of 1 bar of stress transferred to the seismically hazardous Motagua fault and further indicate unclamping of normal faults along the northern shore of Honduras, where two M > 5 normal-faulting earthquakes and numerous small earthquakes were triggered by the main shock.

We analyze event archives and continuous waveform data recorded by the Cooperative New Madrid Seismic Network from 1995 to 2008 in conjunction with waveform cross‐correlation techniques to investigate the spatiotemporal distribution of small‐magnitude ( M D

On 18 April 2008, a moderate-sized earthquake ( Mw 5.2) occurred near the Indiana-Illinois state border within 3 km of the Mt. Carmel–New Harmony fault at a depth of 16 km near the northern termination of the Wabash Valley fault system (WVFS). A total of 257 aftershocks were recorded over the next month by a fourteen-station temporary network deployed by Indiana University and University of Memphis/Center for Earthquake Research and Information (CERI). The number of recorded aftershocks is greater than aftershocks recorded from previous earthquakes in the WVFS of similar magnitude within the last 50 years. The number and density of local stations allowed the generation of precise hypocentral relocations using the combination of waveform cross-correlation and joint hypocentral techniques. The relocated hypocenters indicate a well-defined near-vertical fault plane striking 295° and dipping 85° to the north. Analysis of the temporal distribution of aftershocks reveals an anomalously productive aftershock sequence relative to other earthquakes in the region, including four aftershocks with M > 4.0, the largest of which reached 0.6 units of the mainshock, an unusually low b value of 0.579, and Omori law decay parameters of a = –0.662 and p = 1.152. The inferred fault orientation is consistent with the focal mechanism of the mainshock and nearly orthogonal with respect to the trace of the neighboring Mt. Carmel–New Harmony fault. The interpreted fault orientation suggests that the aftershock sequence occurred on a transfer structure near the fault termination. The structure may be related to the change in deformation styles suggested by the transition from the northeast-trending WVFS to the northwest-trending La Salle anticlinorium.

We present high-resolution earthquake locations for the New Madrid seismic zone (NMSZ) using the double-difference location method. The NMSZ consists of four major arms of seismicity centered in the central United States and is one of the few places of concentrated earthquake activity far from a major plate boundary. The zone generates approximately 200 earthquakes per year. Double-difference relocation techniques prove well suited for this region because the distance between neighboring events is small and station coverage is relatively dense. The initial data set consists of 1,394 earthquakes recorded between 2000 and 2006 by 197 stations. The catalog contains approximately 67,000 P -wave and 54,000 S -wave observations, which yields 480,000 differential times. Waveform cross-correlation of P and S waves provides an additional 135,000 high-precision differential times. Relocated hypocenters align along individual segments of the seismic zone, providing a sharper image of the NMSZ faults.

SUMMARY Families of similar earthquakes at shallow depths occurring over multiple timescales have been identified prior to and during the 2005–2006 eruption of Augustine Volcano. The use of conventional and double-difference location methods failed to result in stable locations, due primarily to noisy site conditions as well as unfavourable station geometry for the Alaska Volcano Observatory stations. Previous work found that relatively robust estimates of the median arrival time difference between station pairs could be determined and used a constant velocity half-space model, true station elevations and a variant of the method of hyperbolas to estimate locations for four of the clusters. We have carried out a more extensive location analysis, including data from nine clusters and incorporating a linear gradient velocity model obtained from preliminary forward modelling of the data. Our location results show a progressive shallowing of the clusters with time. Clusters prior to 2005 December lie near sea level, whereas those in 2005 December and 2006 January concentrate about 500 m above sea level (asl) prior to the explosive eruptions and about 500 and 1200 m asl, respectively, (essentially at the surface) afterwards. These results are consistent with geodetic observations and modelling.

Waveform cross-correlation with bispectrum verification is combined with double-difference tomography to increase the precision of earthquake locations and constrain regional 3D P -wave velocity heterogeneity at Great Sitkin volcano, Alaska. From 1999 through 2005, the Alaska Volcano Observatory (AVO) recorded ∼1700 earthquakes in the vicinity of Great Sitkin, including two M L 4.3 earthquakes that are among the largest events in the AVO catalog. The majority of earthquakes occurred during 2002 and formed two temporally and spatially separate event sequences. The first sequence began on 17 March 2002 and was centered ∼20 km west of the volcano. The second sequence occurred on the southeast flank of Great Sitkin and began 28 May 2002. It was preceded by two episodes of volcanic tremor. Earthquake relocations of this activity on the southeast flank define a vertical planar feature oriented radially from the summit and in the direction of the assumed regional maximum compressive stress due to convergence along the Alaska subduction zone. This swarm may have been caused or accompanied by the emplacement of a dike. Relocations of the mainshock–aftershock sequence occurring west of Great Sitkin are consistent with rupture on a strike-slip fault. Tomographic images support the presence of a vertically dipping fault striking parallel to the direction of convergence in this region. The remaining catalog hypocenters relocate along discrete features beneath the volcano summit; here, low P -wave velocities possibly indicate the presence of magma beneath the volcano.

The Mw 9.0 2004 Sumatra–Andaman Islands and Mw 8.6 Nias Island great earthquake sequences have generated over 5000 catalog-reported earthquakes along ∼1700 km of the Sumatra–Andaman and western Sunda regions. Studies of prior regional seismicity have been limited to global catalog locations that often have poorly constrained epicenters and depths. Approximately 3650 teleseismically well- recorded earthquakes occurring in this region during the period 1918–2005 are relocated with special attention to focal depth. Reduced uncertainties of epicenters and depths in the region (on the order of 15 and 10 km, respectively) foster interpretation of focal mechanism data and provide additional details about the subducting Indian and Australian plates. The revised earthquake dataset reveals a sharp delineation between aftershocks of the 2004 and 2005 earthquakes near Simeulue Island and a steepening in slab dip from south to north. The downdip width of the aftershock zone of the 2004 Mw 9.0 earthquake varies from ∼200 km at its northern end to ∼275 km at its southern end, and events located between 35 and 70 km focal depth occur more frequently in the southernmost section of this aftershock zone. Outer- rise and near-trench normal and strike-slip faulting earthquakes also increase in frequency following the 2004 and 2005 earthquakes. Earthquake swarms triggered along the Andaman backarc spreading center both north of Sumatra and near Siberut Island, 100 km south of the Nias Island aftershock sequence, illustrate the complex and variable nature of seismicity following these great earthquakes.

In November 2013, a series of earthquakes began along a mapped ancient fault system near Azle, Texas. Here we assess whether it is plausible that human activity caused these earthquakes. Analysis of both lake and groundwater variations near Azle shows that no significant stress changes were associated with the shallow water table before or during the earthquake sequence. In contrast, pore-pressure models demonstrate that a combination of brine production and wastewater injection near the fault generated subsurface pressures sufficient to induce earthquakes on near-critically stressed faults. On the basis of modelling results and the absence of historical earthquakes near Azle, brine production combined with wastewater disposal represent the most likely cause of recent seismicity near Azle. For assessing the earthquake cause, our research underscores the necessity of monitoring subsurface wastewater formation pressures and monitoring earthquakes having magnitudes of ∼M2 and greater. Currently, monitoring at these levels is not standard across Texas or the United States., Whether exploration causes earthquakes has been a matter of recent contention particularly regarding shale gas exploration. Here, the authors use hydraulic modelling and earthquake locations to show that brine production and wastewater injection in the Azle area are likely causing earthquakes.

The Nicoya Peninsula in Costa Rica directly overlies the seismogenic zone of the Middle America Trench, making it an ideal location for geophysical investigations of shallow subduction zone earthquake processes. As part of the collaborative Costa Rica Seismogenic Zone Experiment (crseize), a seismic network consisting of 20 land and 14 ocean-bottom seismometers recorded small magnitude local earthquakes along the Nicoya Peninsula from December 1999 to June 2001. Previous studies have used these data to compute local earthquake locations and 3D velocity structure to identify plate boundary seismicity and to investigate seismogenic behavior. Here we utilize waveform cross-correlation and clustering techniques in an attempt to improve earthquake relocations and determine first-motion focal mechanisms to validate, refine, and expand on existing models. Due to the high quality of the original locations and the small cross-correlation P -wave arrival time adjustments, large differences between the previously determined and the cross-correlated earthquake locations are not observed. However, focal mechanism determinations using cross-correlated P waveforms are significantly enhanced. Approximately 90% of the focal mechanisms computed for events previously identified as interplate earthquakes are consistent with underthrusting. Focal mechanisms for continental intraplate events indicate dextral strike-slip motion in the central region and normal faulting at the southern tip of the peninsula. These motions may be associated with oblique convergence and seamount subduction, respectively. Within the subducting plate, steep P and T axes of earthquakes below 50 km depth are consistent with unbending of the slab.

OVSICORI The subduction plate interface along the Nicoya Peninsula, Costa Rica, generates damaging large (Mw > 7.5) earthquakes. We present hypocenters and 3-D seismic velocity models (VP and VP/VS) calculated using simultaneous inversion of P- and S-wave arrival time data recorded from small magnitude, local earthquakes to elucidate seismogenic zone structure. In this region, interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes. Plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern Nicoya Peninsula, respectively. Microseismicity offset across the plate suture of East Pacific Rise-derived and Cocos-Nazca Spreading Center derived oceanic lithosphere is ∼5 km, revising earlier estimates suggesting ∼10 km of offset. Interplate seismicity begins downdip of increased locking along the plate interface imaged using GPS and a region of low VP along the plate interface. The downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental Moho intersection, possibly due to the onset of ductile behaviour. Slow forearc mantle wedge Pwave velocities suggest 20–30 per cent serpentinization across the Nicoya Peninsula region while calculated VP/VS values suggest 0–10 per cent serpentinization. Interpretation of VP/VS resolution at depth is complicated however due to ray path distribution.We posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes. La interfaz de placas de subducción a lo largo de la Península de Nicoya, Costa Rica, genera grandes terremotos dañinos (Mw > 7,5). Presentamos hipocentros y modelos tridimensionales de velocidad sísmica (VP y VP/VS) calculados utilizando la inversión simultánea de datos de tiempos de llegada de ondas P y S registrados en terremotos locales de pequeña magnitud para dilucidar la estructura de la zona sismogénica. En esta región, la microsismicidad del ciclo intersísmico no define únicamente el alcance potencial de ruptura de los grandes terremotos. La microsismicidad de la interfaz de placas se extiende de 12 a 26 y de 17 a 28 km bajo el nivel del mar debajo del sur y norte de la península de Nicoya, respectivamente. El desplazamiento de la microsismicidad a través de la sutura de la placa de la litosfera oceánica derivada del East Pacific Rise y del Cocos-Nazca Spreading Center es de ~5 km, lo que revisa estimaciones anteriores que sugieren ~10 km de desplazamiento. La sismicidad entre placas comienza echado abajo del aumento del bloqueo a lo largo de la interfaz de la placa fotografiada con GPS y una región de baja VP a lo largo de la interfaz de la placa. El borde echado hacia abajo de la microsismicidad de la interfaz de la placa ocurre echado hacia arriba de la losa oceánica y la intersección del Moho continental, posiblemente debido al inicio del comportamiento dúctil. Las velocidades lentas de la onda P de la cuña del manto del antearco sugieren una serpentinización del 20 al 30 por ciento en toda la región de la península de Nicoya, mientras que los valores calculados de VP/VS sugieren una serpentinización del 0 al 10 por ciento. Sin embargo, la interpretación de la resolución VP/VS en profundidad es complicada debido a la distribución de la trayectoria de los rayos. Postulamos que la cuña del manto del antearco está serpentinizada regionalmente, pero aún puede sostener la ruptura durante los terremotos más grandes de la zona sismogénica. Universidad Nacional, Costa Rica University of California-Santa Cruz, USA Georgia Institute of Technology, USA University of California-San Diego, USA University of Miami, USA IFM-GEOMAR, Alemania Observatorio Vulcanológico y Sismológico de Costa Rica

Continuous measurements of fluid flow were made over a six month period across the Nicoya Peninsula, Costa Rica (Pacific), convergent margin utilizing osmotically-driven fluid flow meters designed to quantify both inflow and outflow rates on the order of ~10 � 5 to 3 cm/d. Significant transience in flow was observed through the surface of the forearc. Three periods of correlated flow signals were seen on the subduction forearc among three instruments located in the out-of-sequence thrust (OOST) zone over along-margin strike distances of ~30 km. Amplitudes of ground velocity recorded on collocated ocean bottom seismometers (OBS) increase during the three correlated flow events. The seismic signal has frequency characteristics that resemble volcanic and non-volcanic tremor. We hypothesize that repeated plate boundary slow slip events, potentially originating at the up dip limit of the seismogenic zone, generate the observed signals within the toe of the forearc. We propose a model in which the poro-elastic stress/strain field around a series of creep dislocations simultaneously forces flow through fracture networks in the forearc and oceanic basement rocks and induces diffuse flow through the shallow sediments. The former generates the seismic tremor-like noise recorded by the OBSs and the latter generates the flow transients recorded by the fluid flow meters. We suggest that high sensitivity fluid flow meters can be utilized to detect transient tectonic strain events in offshore environments where traditional geodetic techniques lack resolution or are not possible. D 2005 Elsevier B.V. All rights reserved.

The two largest earthquakes of the past 40 years ruptured a 1600-kilometer-long portion of the fault boundary between the Indo-Australian and southeastern Eurasian plates on 26 December 2004 [seismic moment magnitude ( M w ) = 9.1 to 9.3] and 28 March 2005 ( M w = 8.6). The first event generated a tsunami that caused more than 283,000 deaths. Fault slip of up to 15 meters occurred near Banda Aceh, Sumatra, but to the north, along the Nicobar and Andaman Islands, rapid slip was much smaller. Tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer.

Geologic mapping of southern Rusalka Planitia, Venus, reveals interactions of volcanism, tectonism, and topography. We recognize three regional plains units (prR1, prR2, and prR3) based on crosscutting structural relations, embayment patterns, radar brightness, and surface roughness data. Delineation of secondary (tectonic) structures allows us to constrain the relative temporal relations between the three material units. Unit prRl, a radar dark smooth unit exposed in local topographic highs, hosts NE trending extension fractures. Low-viscosity lava flows of prR2, the most areally extensive unit, fill local topographic lows and the NE trending fractures. A shield-sourced lava unit, prR3, overlies prR2 on the basis of embayment relations and radar brightness. NW trending wrinkle ridges deform all three plains units and record regional contraction. Locally, flood lava flows that fill NE trending fractures are structurally inverted to form short, stepped NE trending wrinkle ridges. Map patterns indicate that prR2 comprises a thin layer (

We have integrated waveform and arrival‐onset data collected in Costa Rica as part of the National Science Foundation (NSF)‐sponsored Costa Rica Seismogenic Zone Experiment (CRSEIZE) and along central Costa Rica and Nicaragua as part of the German SFB 574 program. The five arrays, composed of different sensor types (one‐ and three‐component land and ocean bottom seismometers and hydrophones), were archived using different software packages (Antelope and SEISAN) and were automatically and manually picked using various quality criteria resulting in a disparate set of pick weights. We evaluate pick quality using automated arrival detection and picking algorithm based on the wavelet transform and Akaike information criterion picker. The consistency of the arrival information over various scales provides a basis for assigning a quality to the analyst pick. Approximately 31% of P arrival times and 26% of S times have been classified as high‐quality picks (quality 0–1). An additional 21% of P times and 27% of S arrivals are good quality (quality 2–3). The revised quality picks are mapped directly into new pick weights for inversion studies. We explore the effect of new weighting and removal of poor data by relocating hypocenters through a minimum 1D velocity model and conducting double‐difference local earthquake tomography (LET). Analysis of the hypocenter relocation and seismic velocity tomography results suggest that using the improved quality determinations have a greater effect on improving sharpness in the velocity images than on the magnitude of hypocentral movement. Online Material: Figures of waveforms, event statistics, and tomography; and tables of station and event parameters, station qualities, velocity model, and hypocentral parameters.

[1] The great Mw = 9.0 2011 Tohoku earthquake appears to have complex rupture characteristics, with slower rupture velocity during the early portion of the rupture and spatial variations in the radiation frequency content. These spatial and temporal variations suggest that the subduction zone fault has spatially varying friction conditions that led to differences in the 2011 rupture characteristics, conditions that might also affect other earthquakes within the rupture zone. We find spatial variations for source parameters of 90 relocated earthquakes between 1992 and 2011 along northern Japan, with longer durations observed in shallow near trench events relative to shorter duration deeper events. A majority of these events do not lie within the high slip zone of 2011, however, and occur instead in the region of the 1896 tsunami earthquake to the north. We also find correlation between the longest duration event locations and low seismic velocities based on recent tomography models.

[1] Various models for the generation of tsunami earthquakes have been proposed, including shallow earthquake slip through low strength materials. Because these physical fault conditions would likely affect other earthquakes in the same rupture zone, source properties of other events may provide a guide to locations of tsunami earthquakes. The 25 October 2010 Mw = 7.8 Mentawai tsunami earthquake and surrounding events provide a test of this hypothesis. We determine slip patterns for the mainshock and relocate aftershocks, with the majority occurring in the near trench region. The two largest magnitude aftershocks occurred within the downdip end of the mainshock rupture area and have long moment-normalized rupture duration, likely related to fault zone conditions. Several older relocated earthquakes at the northern edge of the 2010 rupture area also have long duration character, suggesting both spatial and temporal consistency in the conditions needed to produce slow seismic processes along this margin.

[1] We have extended the double-difference seismic tomography method to teleseismic distances with 3-D ray tracing conducted through nested regional-global velocity models and applied the method to relocate seismicity from the Sumatra-Andaman region before and after the great earthquakes of 2004 and 2005. We tested the algorithm's accuracy using both independent local data and an alternate relocation method and found good agreement between the results. The use of depth phases, differential times, and a realistic 3-D velocity model improves the accuracy and precision of epicenters and focal depths, systematically shifting them perpendicular to the trench and shallower, respectively. The relocations refine the location of the megathrust and other faults, the patterns of aftershocks, and their relation to slip during the two great earthquakes. In addition, the relocations reveal several discrete features not readily discernible in the scatter of teleseismic catalogs, including an arcuate, narrow band of earthquakes presumed to define the updip rupture limit in the 2005 event and a lineation at depth tracing the subduction of the Investigator Fracture Zone. When viewed in conjunction with tomography results, the geometry and structural features of the subduction zone are revealed in unprecedented detail.