ALBERT

Description: We present distributed fiber-optic sensing data from an airplane landing near the EastGRIP ice core drilling site on the Northeast Greenland Ice Stream. The recordings of exceptional clarity contain at least 15 easily visible wave propagation modes corresponding to various Rayleigh, pseudoacoustic, and leaky waves. In the frequency range from 8 to 55 Hz, seven of the modes can be identified unambiguously. Based on an a priori firn and ice model that matches P-wave dispersion and the fundamental Rayleigh mode, a Backus–Gilbert inversion yields an S-wavespeed model with resolution lengths as low as a few meters and uncertainties in the range of only 10 m/s. An empirical scaling from S wavespeed to density leads to a depth estimate of the firn–ice transition between 65 and 71 m, in agreement with direct firn core measurements. This work underlines the potential of distributed fiber-optic sensing combined with strong unconventional seismic sources in studies of firn and ice properties, which are critical ingredients of ice core cli-matology, as well as ice sheet dynamics and mass balance calculations.

Description: 〈span〉〈div〉ABSTRACT〈/div〉The task of downloading comprehensive datasets of event‐based seismic waveforms has been made easier through the development of standardized webservices but is still highly nontrivial because the likelihood of temporary network failures or subtle data errors naturally increases when the amount of requested data is in the order of millions of relatively short segments. This is even more challenging because the typical workflow is not restricted to a single massive download but consists of fetching all possible available input data (e.g., with several repeated download executions) for a processing stage producing any desired user‐defined output. Here, we present stream2segment, a highly customizable Python 2+3 package helping the user in the entire workflow of downloading, inspecting, and processing event‐based seismic data by means of a relational database management system as archiving storage, which has clear performance and usability advantages, and an integrated processing subroutine requiring a configuration file and a single Python function to produce user‐defined output. Stream2segment can also produce diagnostic maps or user‐defined plots, which, unlike existing tools, do not require external software dependencies and are not static images but instead are interactive browser‐based applications ideally suited for data inspection or annotation tasks and subsequent training of classifiers in foreseen supervised machine‐learning applications.Stream2segment has already been used as a data quality tool for datasets within the European Integrated Data Archive and to create a weak‐motion database (in the form of a so‐called flat file) for the stable continental region of Europe in the context of the European Ground Shaking Intensity Model service, in turn an important building block for seismic hazard studies.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉By combining the declustered catalogs of 96 individual fault zones at various stages in their seismic cycles in the Chinese mainland into one normalized cycle, we greatly extend the seismic record beyond both the short historical and the modern instrumental observation periods compared with the much longer average recurrence interval, and thus create a complete seismicity picture during one seismic cycle. The temporal pattern of the integrated catalog demonstrates that the occurrence rate of M≥5.0 events near to the occurrence time of the maximum‐size earthquake of an individual fault zone is about maximum three times larger than at the middle stage, suggesting that it more closely follows a quadratic distribution, rather than a Poisson distribution, and strong foreshocks and aftershocks can last for several hundred years or even longer on intraplate faults, that is, about 20% of a seismic cycle. Besides, during one complete cycle, the number of events M≥5.0 is about seven, which increases with the growing maximum magnitude and tends to decreases with the increasing slip rate of an individual fault zone. The results derived from Monte Carlo simulations and statistical analyses for magnitude distributions show that most of individual fault zones have a significant magnitude gap between the maximum‐size event and the second largest event; typically the second largest earthquake magnitude is M∼6.0, implying that the gap increases with the maximum‐size earthquake magnitude; the magnitude gap between the maximum‐size earthquake and the maximal magnitude of other declustered events and the magnitude difference between the maximum‐size earthquake and its largest aftershock are similar.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We used a finite‐difference modeling method, developed by Olsen–Day–Cui, to simulate nonlinear‐viscoelastic basin effects in a spectral frequency range of 0.1–1 Hz in the Kinburn bedrock topographic basin, Ottawa, Canada, for large earthquakes. The geotechnical and geological features of the study area are unique: loose, postglacial sediments with very low shear‐wave velocities (〈200  m/s) overlying very firm bedrock with high shear‐wave velocities (〉2000  m/s). Comparing records and simulated velocity time series showed regular viscoelastic simulations could model the ground motions at the rock and soil sites in the Kinburn basin for the Ladysmith earthquake, a local earthquake occurred on 17 May 2013 with Mw 4.7 (MN 5.2). The Ladysmith earthquake was scaled to provide a strong level of shaking for investigating the nonlinear behavior of soil; therefore, a new nonlinear‐viscoelastic subroutine was introduced to the program. A modeled stress–strain relationship associated with ground‐motion modeling in the Kinburn basin using a scaled Ladysmith earthquake event of Mw 7.5 followed Masing’s rules. Using nonlinear‐viscoelastic ground‐motion simulations significantly reduced the amplitude of the horizontal component of the Fourier spectrum at low frequencies and the predicted peak ground acceleration and peak ground velocity values compared with regular linear viscoelastic simulations; hence, the lower soil amplification of seismic waves and the frequency and amplitude spectral content were altered by the nonlinear soil behavior. In addition, using a finite‐fault model to simulate an earthquake with Mw 7.5 was necessary to predict the higher levels of stresses and strains, which were generated in the basin. Using a finite‐fault source for the nonlinear‐viscoelastic simulation caused decreases in the horizontal components because of the shear modulus reduction and increase of damping.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Triplicated 〈span〉P〈/span〉 waveforms related to the 410‐km discontinuity from five intermediate‐depth earthquakes in the central Philippines are clearly recorded by the Chinese Digital Seismic Network, but some branches of the 〈span〉S〈/span〉‐wave triplications are obscure. Matching the observed 〈span〉P〈/span〉‐wave triplications with synthetics through a grid‐search technique, we obtain the best‐fit 1D 〈span〉P〈/span〉‐wave velocity model near the 410‐km discontinuity beneath the northeastern South China Sea. In such a model, a low‐velocity layer (LVL) is found to reside atop the mantle transition zone, and it is characterized by a thickness of 92.5±11.5  km and a 〈span〉P〈/span〉‐wave velocity decrement of 1.5%±0.1% compared with the IASP91 model. The relatively thick and weak LVL is possibly a response of a small amount of remnant hydrous partial melts after plume‐like upwelling.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Seismology has continuously recorded ground‐motion spanning up to decades. Blind, uninformed search for similar‐signal waveforms within this continuous data can detect small earthquakes missing from earthquake catalogs, yet doing so with naive approaches is computationally infeasible. We present results from an improved version of the Fingerprint And Similarity Thresholding (FAST) algorithm, an unsupervised data‐mining approach to earthquake detection, now available as open‐source software. We use FAST to search for small earthquakes in 6–11 yr of continuous data from 27 channels over an 11‐station local seismic network near the Diablo Canyon nuclear power plant in central California. FAST detected 4554 earthquakes in this data set, with a 7.5% false detection rate: 4134 of the detected events were previously cataloged earthquakes located across California, and 420 were new local earthquake detections with magnitudes −0.3≤ML≤2.4, of which 224 events were located near the seismic network. Although seismicity rates are low, this study confirms that nearby faults are active. This example shows how seismology can leverage recent advances in data‐mining algorithms, along with improved computing power, to extract useful additional earthquake information from long‐duration continuous data sets.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The aim of obtaining a single scale for earthquake magnitudes has led many studies in the past to either develop relationships among various existing scales or develop an altogether new scale to represent a wide range of magnitudes on a single scale. Although a reliable and standardized estimation of earthquake size is a basic requirement for all tectonophysical and engineering applications, different magnitude scales estimate different values for the same earthquake, thereby making such studies inadequate. The moment magnitude (Mw) scale has been referred to by various researchers as the best scale, one that matches well with the observed surface‐wave magnitudes with Ms≥7.5 at a global level. The formulation and validation of the Mw scale were carried out considering the southern California region for lower and intermediate earthquakes.In this study, an endeavor has been made to extend the moment magnitude scale to include lower and intermediate magnitudes in a global context emphasizing the use of body waves, particularly 〈span〉P〈/span〉 waves, in which data are abundant. We first investigate the degree of closeness of Mw values with other observed magnitudes (e.g., Ms and mb) for smaller and intermediate magnitude ranges considering global International Seismological Centre (ISC) and Global Centroid Moment Tensor (CMT) databases. To improve upon the consistency of the Mw scale for a wider range, a uniform generalized seismic moment magnitude scale Mwg=logM0/1.36−12.68, for magnitudes≥4.5, has been developed, considering 25,708 global earthquake events having mb and M0 values from ISC and Global CMT databases, respectively, during the period 1976–2006. The Mwg scale is also valid for 3.5≤mb≤7.0 because the relations between seismic moment and the magnitudes mb and Mwg are same.The greater accuracy of the Mwg scale over the Mw scale at different magnitudes (i.e., mb or Ms) is found to be statistically significant in the range including smaller and intermediate events. The similarity of the Mwg scale is also tested on 394 global seismic radiated energy values collected from 〈a href="https://pubs.geoscienceworld.org/bssa#rf6"〉Choy and Boatwright (1995)〈/a〉. It is observed that 76% of estimated radiated energy values obtained through the Mwg scale show closer agreement (than with Mw) to the observed radiated energy values. Mwg is computed from low‐ and high‐frequency spectra, and because it is consistent for small, intermediate, and large earthquake events, it will play a useful role as an earthquake magnitude estimator for all earthquake related studies.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Despite the theory for both Rayleigh and Love waves being well accepted and the theoretical predictions accurately matching observations, the direct observation of their quantifiable decay with depth has never been measured in the Earth’s crust. In this work, we present observations of the quantifiable decay with depth of surface‐wave eigenfunctions. This is done by making direct observations of both Rayleigh‐wave and Love‐wave eigenfunction amplitudes over a range of depths using data collected at the 3D Homestake array for a suite of nearby mine blasts. Observations of amplitudes over a range of frequencies from 0.4 to 1.2 Hz are consistent with theoretical eigenfunction predictions. They show a clear exponential decay of amplitudes with increasing depth and a reversal in sign of the radial‐component Rayleigh‐wave eigenfunction at large depths, as predicted for fundamental‐mode Rayleigh waves. Minor discrepancies between the observed eigenfunctions and those predicted using estimates of the local velocity structure suggest that the observed eigenfunctions could be used to improve the velocity model. Our results confirm that both Rayleigh and Love waves have the depth dependence that they have long been assumed to have. This is an important direct validation of a classic theoretical result in geophysics and provides new observational evidence that classical seismological surface‐wave theory can be used to accurately infer properties of Earth structure and earthquake sources.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Reference rock site conditions have a major role in site response analysis and therefore should be selected carefully. For the case of standard engineering applications, site response of soft soil sites is frequently evaluated with respect to a generic reference rock, which is commonly defined as part of the building standard.Amendment Five of the Israeli Standard for Design Provisions for Earthquake Resistance of Structures (SI‐413) uses the National Earthquake Hazards Reduction Program (NEHRP) site classification for different types of soils, defining the reference rock site as a site with average shear‐wave velocity of the upper 30 m (VS30) of 760  m/s. NEHRP classification is based on comprehensive studies conducted in the western United States, showing relatively good correlation to site response for that region but was never modified to fit Israeli local conditions.A new generic reference rock profile for Israel was assembled by compiling 43 local velocity profiles identified as rock profiles by their surface lithology, combined with an additional 141 deep velocity profiles that penetrate to depths of approximately 4 km. Following the formation of the generic rock profile, a continuous function representing the reference rock conditions for future seismic engineering applications in Israel was constructed. In addition, the amplification associated with the reference rock profile was calculated using multiple approaches.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉On 30 November 2018, three felt earthquakes occurred in the Septimus region of northeast British Columbia in an area where hydraulic fracturing was in progress. The proximity of oil and gas activities to populated areas and to critical infrastructure including major dams raises significant concern regarding the seismic hazard posed by moderate induced events and motivates study of their ground motions. Here, we analyze the ground‐motion amplitudes from these events recorded between 3 and 400 km. We use three‐component waveforms from 45 seismometer and accelerometer sensors to analyze the observed ground motions. The moment magnitude (Mw) of the first event is estimated as 4.6 using the vertical pseudoresponse spectral acceleration (PSA) based on the relations provided by 〈a href="https://pubs.geoscienceworld.org/srl#rf45"〉Novakovic 〈span〉et al.〈/span〉 (2018)〈/a〉. The Mw for the two smaller earthquakes are 3.5 and 4.0. The intensity of shaking from the Mw 4.6 and 4.0 events generally exceeded modified Mercalli intensity (MMI) VI at distances 〈6  km. The maximum duration above the MMI VI threshold at the closest station (3.5 km distance) from the mainshock is 1.6 s. The observed ground motions agree with the ground‐motion prediction equation (GMPE) of 〈a href="https://pubs.geoscienceworld.org/srl#rf45"〉Novakovic 〈span〉et al.〈/span〉 (2018)〈/a〉 for induced events in Oklahoma, with attenuation modified to match that for the study region, assuming typical regional site amplification. The inferred value of stress drop for the mainshock and the largest aftershock is approximately 50 bars based on the agreement of observed PSA values with the 〈a href="https://pubs.geoscienceworld.org/srl#rf45"〉Novakovic 〈span〉et al.〈/span〉 (2018)〈/a〉 GMPE.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉In November 2014, a temporary land and marine seismic network was deployed to monitor the drilling of an exploratory well in the Canary Channel (eastern Canary Islands). This region is characterized by low‐seismic activity; however, because of the increased awareness of the potential seismic hazard caused by hydrocarbon exploitation activities, the drilling operations were monitored with an unprecedented level of detail for an activity of this kind. According to the reported earthquakes, there was not a measurable increase in seismicity in the vicinity of the well. Overall seismic activity was low, which is consistent with the historical seismicity records. Harmonic tremor, explained here as resonances of the instrument‐seafloor system generated by bottom water currents in the area, was commonly detected on the ocean‐bottom seismometer (OBS) recordings. The marine network data also revealed dozens of nonseismic short‐duration signals per day that appear similar to other events on OBS recordings throughout the world. We suggest that they may be caused by direct perturbations on the OBS, mostly induced by ocean currents in the Canary Channel.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The tsunami that followed the 1995 Mw 7.2 Nuweiba earthquake along the Dead Sea Transform in the Gulf of Elat–Aqaba (GOE) surprised the local population, who were unconcerned by seismogenic sea waves happening in a closed gulf, far away from the open ocean. Eyewitness reports, field observations, and a mareogram recorded near Elat demonstrated conclusively that tsunami hazard in the GOE deserves focused attention. Here we take up the challenge, adopting the GeoClaw package and investigating which of the available Nuweiba earthquake models are capable of better replicating the actual findings. In general, the simulated tsunamis that are based on both the seismological and Interferometric Synthetic Aperture Radar (InSAR) Nuweiba earthquake models are in line with the eyewitness descriptions of wave height, slight inundation, and limited damage. In addition, the simulations show a reasonable correlation with the amplitude and wave period derived from the analog mareogram, as expected from a tsunamigenic, mostly strike‐slip component earthquake. The InSAR inverse‐based modeling of the coseismic deformation, however, appears closer to the measured parameters of the recorded mareogram. The exception of 3–4 m high waves reported in the Nuweiba port may be the result of local perturbations in the harbor or the effect of a local tsunamigenic submarine landslide. The four countries of Israel, Jordan, Saudi Arabia, and Egypt that encircle the GOE seek to expand intensively their marine infrastructure, tourism, and population. The present study aims to warn the stakeholders around the GOE about the inevitable tsunamis.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Seismic waves that are recorded by near‐surface sensors are usually disturbed by strong noise. Hence, the recorded seismic data are sometimes of poor quality; this phenomenon can be characterized as a low signal‐to‐noise ratio (SNR). The low SNR of the seismic data may lower the quality of many subsequent seismological analyses, such as inversion and imaging. Thus, the removal of unwanted seismic noise has significant importance. In this article, we intend to improve the SNR of many seismological datasets by developing new denoising framework that is based on an unsupervised machine‐learning technique. We leverage the unsupervised learning philosophy of the autoencoding method to adaptively learn the seismic signals from the noisy observations. This could potentially enable us to better represent the true seismic‐wave components. To mitigate the influence of the seismic noise on the learned features and suppress the trivial components associated with low‐amplitude neurons in the hidden layer, we introduce a sparsity constraint to the autoencoder neural network. The sparse autoencoder method introduced in this article is effective in attenuating the seismic noise. More importantly, it is capable of preserving subtle features of the data, while removing the spatially incoherent random noise. We apply the proposed denoising framework to a reflection seismic image, depth‐domain receiver function gather, and an earthquake stack dataset. The purpose of this study is to demonstrate the framework’s potential in real‐world applications.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Seismic station data quality is commonly defined by metrics such as data completeness or background seismic noise levels in specific frequency bands. However, for temporary networks such as aftershock deployments or induced seismicity monitoring, the most critical metric is often how well the station performs when recording events of interest. A timely measure of station performance can be used for real‐time network maintenance and to help make decisions about which stations may need to be moved or are redundant. We develop new event‐based methods to quickly assess station and network performance, including estimating network magnitude of completeness, determining station signal‐to‐noise ratios as a function of earthquake magnitude, and computing relative station amplitudes. At times, a complete catalog of local seismic events may not exist such as in an aftershock deployment in which hundreds to thousands of small earthquakes may be happening and catalog generation efforts cannot keep up. To overcome this, we use an envelope of the average energy recorded by the network to identify events of interest. We find the log amplitude of events identified using this technique scales linearly with local earthquake magnitudes. We examine two U.S. Geological Survey aftershock networks in Oklahoma to demonstrate this approach can be used to identify poorly performing stations and determine network detection thresholds as early as one day following the deployment of a temporary network.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Interstation correlation is the basic operation in seismic noise and coda‐wave interferometry for signal extraction in imaging and monitoring applications. Conventional cross‐correlations evaluate the similarity between two signals along lag time, and they are efficiently computed by the fast Fourier transform (FFT), valuable to manage the large data volumes that ambient noise applications demand. The phase cross‐correlation (PCC) method contributes to increase convergence, a key issue in seismic ambient noise imaging and monitoring; however, it is much more computationally demanding. PCC evaluates similarity by subtracting the modulus of the sum and difference of the instantaneous phase of two signals. We introduce solutions to dramatically reduce the high‐computational cost of PCC. We show that PCC can be rewritten as a complex cross‐correlation and computed by the FFT when the moduli are raised to the power of 2, and we demonstrate PCC can improve waveform coherence and increase convergence compared with the default processing flow of 1‐bit amplitude normalization and standard cross‐correlation. Moreover, we develop a graphics processing unit implementation to accelerate computations when using powers other than 2 and particularly when using the power of 1. Finally, we extract Rayleigh‐ and body‐wave signals from many years of data from seismic stations distributed worldwide using PCC without a significant increase in computational cost compared with conventional cross‐correlation.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The foreshocks and aftershocks of large earthquakes provide valuable information for studying earthquake nucleation, fault rupture processes, and postseismic deformation. We investigated the spatiotemporal evolution of the seismicity 60 days before and 30 days after the 2018 Mw 6.4 Hualien earthquake, which occurred near the Milun fault (MF) in eastern Taiwan. By applying the matched‐filter technique to continuous waveform data, we identified approximately two times more earthquakes than listed in the standard Central Weather Bureau earthquake catalog. The spatial distributions of the foreshock and aftershock hypocenters are consistent with one of the mainshock nodal planes. We found foreshocks migrated to the southwest toward the mainshock hypocenter, while the aftershock area expanded immediately after the mainshock, exhibiting a northeastward step‐like logarithmic expansion. Most of the foreshocks occurred in the vicinity of the mainshock hypocenter, and some of them had high waveform similarities, implying they occurred virtually in the same region and were driven by aseismic slip. Our results suggest the existence of a blind southeast‐dipping fault cutting the MF. The step‐like extension of aftershocks is most likely due to fault segmentation.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉An outline of a Bayesian source location framework for using seismic and acoustic observations is developed and tested on synthetic and real data. Seismic and acoustic phenomena are both commonly used in detection and location of a variety of natural or man‐made events, such as volcanic eruptions, quarry blasts, and military exercises. Typically, seismic and acoustic observations have been utilized independently of each other. Here, we outline a Bayesian formulation for combining the two observations in a single estimate of the location and origin time. Using realistic estimates of uncertainty, we subsequently explore how combining the different observation types can benefit event location at local to near‐regional distances. We apply the method to synthetic data and to real observations from a mining blast in Bingham Mine in Utah. Our findings suggest that, for relatively sparse or azimuthally limited observations, the relative strengths of the two different phenomenologies enable more precise joint‐event localization than either seismic or infrasonic measurements alone.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Atmospheric processes are documented to modulate seismic noise in Fairfield Nodal three‐component geophones. Spectral analysis has shown high‐amplitude signals between 40 and 50 Hz in all waveforms inspected. The changes in spectral amplitudes and frequency are found to be modified by daily variations in wind velocity and temperature, which are temporally correlated for much of the study. The wind velocity is shown to affect a wide spectral band with peak amplitudes that depend on the distance from 〈span〉in situ〈/span〉 structures coupling wind energy into the shallow crust. The wind velocity increases the spectral amplitudes, most noticeably in the 40–50 Hz band; it produces a 15 Hz frequency modulation in the conditions of highest wind, with resonance frequencies up to 150 Hz. These signals likely reflect a superposition of multiple local and regional sources producing wind‐generated ground motions and nonlinear wave propagation in the shallow subsurface. During periods of temperatures below 0°C, a similar frequency modulation is observed, but the amplitudes are not as pronounced without the elevated wind velocity. A possible source of the continuous noise signal and the temperature‐dependent frequency modulation is the spike mount that is attached to the nodal housing. The noise signals modulated by the wind and temperature variations require installation procedures in order to mitigate the effects of the contaminating noise on the geophysical processes of interest.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The estimation of epicentral distance is a critical step in earthquake early warning systems (EEWSs) that is necessary to characterize the level of expected ground shaking. In this study, two rapid methodologies, that is, B‐Δ and C‐Δ, are evaluated to estimate the epicentral distance for use in the EEWSs around the Tehran region. Traditionally, the B and C coefficients are computed using acceleration records, however, in this study, we utilize both acceleration and velocity waveforms for obtaining a suitable B‐Δ and C‐Δ relationships for the Tehran region. In comparison with observations from Japan, our measurements fall within the range of scatter. However, our results show a lower trend, which can strongly depend on the few numbers of events and range of magnitude (small‐to‐moderate) of earthquakes used in the current research. To improve our result, we include some large earthquakes from Iran, Italy, and Japan with magnitude larger than 5.9. Although the optimal trend is finally obtained by fitting a line to the distance‐averaged points, we conclude that the same trend and relationship as Japan can be used in Tehran early warning system. We also found that B and C parameters are strongly compatible to each other. As time windows of 3.0 and 0.5 s after the 〈span〉P〈/span〉 onset are chosen respectively to compute the B and C values, so by selecting the C parameter as a proxy of B parameter to estimate the epicentral distance, we may save significant time in order of about 2.5 s in any earthquake early warning applications.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉We invert the shear‐wave displacement spectra obtained from 30 three‐component, broadband waveforms recorded within 300 km of the 6 November 2011 Mw 5.7 Prague, Oklahoma, earthquake to recover the site‐response contribution using an inversion method that simultaneously inverts for source, path, and site effects. Site‐response functions identify resonant frequencies within a range of 0.1–10 Hz that generally coincide with spectral peaks in horizontal‐to‐vertical ratio curves derived from the recorded waveforms. 〈span〉S〈/span〉‐wave velocity profiles available for several sites were also used to calculate theoretical 〈span〉SH〈/span〉 transfer functions that predict the site amplification due to the near‐surface soil structure down to depths of 30–50 m. The transfer functions do not provide resonance information below about 5–8 Hz, indicating that the spectral peaks in the site response obtained from the waveform analysis result from deeper velocity variations. A 0.3 Hz spectral peak observed at several stations, for example, coincides with the strong, surface‐wave amplitudes observed at 3 s periods for induced M≥3 earthquakes in Oklahoma and Kansas, suggesting that this resonant peak may be due to surface waves trapped in the upper ∼2  km sedimentary layer of the crust. Both shallow and deep contributions to the site response are important for the characterization of ground motion from central and eastern North America (CENA) earthquakes. We obtain a corner frequency of 0.229, consistent with independent observations of the size of the event. A frequency‐dependent attenuation relation of Q(f)=1107f0.398 consistent with prior CENA path measurements is also derived.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The waveform cross‐correlation technique is a popular tool for estimating the differential times of seismic phases in a fast and reliable manner. Differential times are used for a variety of methods, with the double‐difference relocation method HypoDD being the most popular. In this work, we analyzed the precision and possible error of cross‐correlated differential times by conducting a simple comparison with reference manual datasets. Our study was carried out on two well‐studied mainshock–aftershock datasets from the seismically active West Bohemia region (Czechia). We observed that the magnitude difference δML between two cross‐correlated earthquakes presents a significant bias, resulting in the over‐ or underestimation of the final differential time of both 〈span〉P〈/span〉 and 〈span〉S〈/span〉 waves. The earthquakes of differing magnitudes exhibit unequal first pulse durations in otherwise similar waveforms. As a result, the cross‐correlated differential time, which shifts seismograms to the position of maximum cross‐correlation, is different from the differential time between phase arrivals. Our test cases revealed that the resulting deviation from the true differential time depends on the actual δML and can reach values higher than 0.025 s when δML〉2. Hence, in standard differential time datasets, the error has a greater impact on the data related to strong events—mainshocks. In HypoDD applications, the error leads to mislocations of mainshocks, and at the same time, the locations of the weak events are improved. We demonstrate the mislocation potential of the error on relocated hypocenters of mainshock–aftershock sequences and earthquake swarms from West Bohemia, as well as on synthetic tests. The error cannot be avoided by changing the cross‐correlated window length or filtration. We propose a few suggestions to suppress the consequences of the magnitude difference data bias. Nonetheless, the differential times error and its effects cannot currently be completely suppressed using the mentioned methods.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉A typical seismic experiment involves installing 10–50 seismometers for 2–3 yr to record distant and local earthquakes, along with Earth’s ambient noise wavefield. The choice of the region is governed by scientific questions that may be addressed with newly recorded seismic data. In most experiments, not all stations record data for the full expected duration. Data loss may arise from defective equipment, improperly installed equipment, vandalism or theft, inadequate power sources, environmental disruptions (e.g., snow covering solar panels and causing power outage), and many other reasons. In remote regions of Alaska and northwestern Canada, bears are a particular threat to seismic stations. Here, we document three recent projects (Southern Alaska Lithosphere and Mantle Observation Network, Fault Locations and Alaska Tectonics from Seismicity, and Mackenzie Mountains EarthScope Project) in which bears were regular visitors to remote seismic stations. For these projects, there were documented bear encounters at 56 out of 88 remote stations and 6 out of 85 nonremote stations. Considering bear‐disrupted sites—such as dug‐up cables or outages—there were 29 cases at remote stations and one case at nonremote stations. We also compile bear encounters with permanent stations within the Alaska Seismic Network, as well as stations of the Alaska Transportable Array. For these two networks, the stations are designed with fiberglass huts that house and protect equipment. Data losses at these networks because of bears are minor (〈5%), though evidence suggests they are regularly visited by bears, and data disruptions are exclusively at remote stations. The primary goal of this study is to formally document the impacts of bears on seismic stations in Alaska and northwestern Canada. We propose that the threat of damage from bears to a station increases with the remoteness of the site and the density of bears, and it decreases with the strength and security of materials used. We suggest that low‐power electric fences be considered for seismic stations—especially for temporary experiments—to protect the equipment and to protect the bears. With the goal of 100% data returns, future seismic experiments in remote regions of bear country should carefully consider the impacts of bears.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Induced seismic events have been recorded recently in the southern midcontinent of the United States, including Texas. These events, associated with hydrocarbon exploration and the subsequent disposal of wastewater byproduct, have led to substantial public discussion regarding cause, public safety, and potential risks of damage to infrastructure. In an effort to better understand these events and to monitor earthquake activity in general, the 84th Texas Legislature funded creation of a statewide, seismic‐monitoring program known as the Texas Seismological Network (TexNet). The goal of TexNet is to provide authenticated data to evaluate the location, frequency, and likely causes of natural and induced earthquakes, so TexNet, through August 2018, deployed 58 new broadband seismic stations in the state of Texas. Of these, 25 are permanent and form, along with 18 existing broadband stations, an evenly spaced backbone, seismic network in the state. In addition to the permanent installations, 33 of the new stations are portable and have been deployed in four different areas of the state experiencing recent seismicity and having high‐socioeconomic importance. An earthquake‐management system (SeisComp3) is being used to detect, locate, and analyze earthquake events and earthquakes measuring ML 2 and above have been made available through various dissemination tools by the next working day. Depending on daily earthquake rate, events of magnitude down to 1.5 are publicly available in three business days from the time they are detected. The initial implementation of TexNet has reduced the magnitude of completeness (Mc) across Texas from 2.7 to less than 1.5 in specific areas and has played a role in a large decrease in uncertainties about earthquake‐source parameters.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉In December 2018, the National Aeronautics and Space Administration (NASA) Interior exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission deployed a seismometer on the surface of Mars. In preparation for the data analysis, in July 2017, the marsquake service initiated a blind test in which participants were asked to detect and characterize seismicity embedded in a one Earth year long synthetic data set of continuous waveforms. Synthetic data were computed for a single station, mimicking the streams that will be available from InSight as well as the expected tectonic and impact seismicity, and noise conditions on Mars (〈a href="https://pubs.geoscienceworld.org/srl#rf9"〉Clinton 〈span〉et al.〈/span〉, 2017〈/a〉). In total, 84 teams from 20 countries registered for the blind test and 11 of them submitted their results in early 2018. The collection of documentations, methods, ideas, and codes submitted by the participants exceeds 100 pages. The teams proposed well established as well as novel methods to tackle the challenging target of building a global seismicity catalog using a single station. This article summarizes the performance of the teams and highlights the most successful contributions.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Forecasting the full distribution of the number of earthquakes is revealed to be inherently superior to forecasting their mean. Forecasting the full distribution of earthquake numbers is also shown to yield robust projections in the presence of surprise large earthquakes, which in the past have strongly deteriorated the scores of existing models. We show this with pseudoprospective experiments on synthetic as well as real data from the Advanced National Seismic System database for California, with earthquakes with magnitude larger than 2.95 that occurred between the period 1971 and 2016. Our results call in question the testing methodology of the Collaboratory for the Study of Earthquake Predictability (CSEP), which amounts to assuming a Poisson distribution of earthquake numbers, which is known to be a poor representation of the heavy‐tailed distribution of earthquake numbers. Using a spatially varying epidemic‐type aftershock sequence (ETAS) model, we demonstrate a remarkable stability of the forecasting performance, when using the full distribution of earthquake numbers for the forecasts, even in the presence of large earthquakes such as Mw 7.1 Hector Mine, Mw 7.2 El Mayor–Cucapah, Mw 6.6 Sam Simeon earthquakes, or in the presence of intense swarm activity in northwest Nevada in 2014. Although our results have been derived for ETAS‐type models, we propose that all earthquake forecasting models of any type should embrace the full distribution of earthquake numbers, such that their true forecasting potential is revealed.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Nevada is a large western state in the United States with a seismic hazard that ranges from moderate to high, depending on location. This article identifies priorities to improve estimates of the seismic hazard in the most urbanized parts of the state, specifically the Reno–Carson City urban area of western Nevada and the Las Vegas urban region of southern Nevada. Collaborative task forces are needed to efficiently realize these priorities.For the Reno–Carson City region in western Nevada, the seismic hazard is high because of strain distributed across several active faults, including normal faults that dip beneath parts of the urban areas. The subsurface geometry and possible connections of these faults remain to be determined. The present large uncertainty in estimates of the slip rates can be reduced by future geological and geodetic studies, including trenching at more than one site per fault and increasing the density of geodetic stations to include multiple stations in the mountain ranges between faults to detect rotations. Adjustments to the ground‐motion models for the regional properties of western and southern Nevada could reduce ground‐motion uncertainties. Ground‐motion simulation research needs an improved 3D velocity model.The seismic hazard in Las Vegas is lower than in Reno. An expanded geodetic network and continued geological studies of the active faults are needed. Uncertainties in the geometry and activity of the Frenchman Mountain and Eglington faults particularly introduce significant uncertainties into the seismic hazard in the Las Vegas basin. The more distant Garlock and Death Valley faults in eastern California impact the hazard in Las Vegas because the Las Vegas basin amplifies long‐period ground motion and prolongs its duration, so reliable simulations from these sources are needed.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Reliable instrument recoverability and data quality rely on accurate estimates of instrument locations on the seafloor. However, freely available software for this estimation does not currently exist. We present OBSrange, an open‐source tool for robustly locating ocean‐bottom seismometers (OBSs) on the seafloor using acoustic transponder ranging data. Available in both MATLAB and Python (see 〈a href="https://pubs.geoscienceworld.org/srl#sc6Data%20and%20Resources"〉Data and Resources〈/a〉), the algorithm inverts two‐way acoustic ranging travel‐time data for instrument location, depth, and average water sound speed with the ability to accurately account for ship velocity, ray refraction through the water column specific to the region, and a known lateral offset between the ship’s Global Positioning System (GPS) receiver and acoustic transponder. The tool provides comprehensive estimates of model parameter uncertainty including bootstrap uncertainties for all four parameters as well as an F‐test grid search providing a 3D confidence ellipsoid around each station. We validate the tool using a synthetic travel‐time dataset and find average horizontal location errors on the order of ∼4  m for an instrument at 5000 m depth. An exploration of survey geometries shows significant variation in location precision depending on the pattern chosen. We explore the trade‐off between survey length and location uncertainty to quantitatively inform cruise planning strategies. The optimal survey radius for resolving instrument location depends on water depth and desired precision and nominally ranges from 0.75–1 nautical mile (NM) at 5000 m water depth to ∼0.25  NM at 500 m depth. Radial legs toward and away from the instrument are crucial for resolving the depth‐water velocity trade‐off, and thus circle surveys should be avoided. Line surveys, common for active source experiments, are unable to resolve the instrument location orthogonal to the survey line. We apply our tool to the 2018 Young Pacific OBS Research into Convecting Asthenosphere (ORCA) deployment in the south Pacific yielding an average root mean square data misfit of 1.96 ms with an average instrument drift of ∼170  m. Observed drifts reveal a clockwise rotation pattern of ∼500  km diameter that resembles a cyclonic mesoscale gyre observed in the geostrophic flow field, suggesting a potential application of accurate instrument drifts as a novel proxy for depth‐integrated flow through the water column.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉This article presents modified Mercalli intensity (MMI) data for the 22 February 2011 Mw 6.2 Christchurch, New Zealand, earthquake. These data include intensity levels above MMI 8 that have not been assigned previously. Two sources of data have been used in this research: GeoNet’s “Felt Classic” online questionnaires and felt reports gathered during a field study in Christchurch in February 2013. Taken together, these sets of data provided 331 valid (i.e., with all the needed information) felt reports in areas of MMI 8 or above, with 299 (90%) of the reports used to assign MMI levels above 8.This article presents a more detailed picture of the geographical damage distribution of this earthquake than has previously been available. The data differentiate damage in the center of Christchurch, with 8 communities assigned a community MMI (CMMI) of 9, 11 communities a CMMI of 10, and 8 communities a CMMI of 11, which is the maximum possible intensity in the New Zealand MMI scale, and a level of intensity not previously reported in New Zealand (〈a href="https://pubs.geoscienceworld.org/srl#rf6"〉Dowrick 〈span〉et al.〈/span〉, 2008〈/a〉).The geographical damage distribution for Christchurch has been updated for intensities below MMI 8. This was done using a recently developed method that groups intensity data and allows intensities to be aggregated for a community and a single value assigned. Comparisons between MMI and peak ground velocity using the CMMI data and two ground‐motion intensity correlation equations (GMICEs) indicate an underestimation of MMI when using the GMICEs and the need to review New Zealand’s GMICE.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Soil liquefaction causes significant damage to coastal infrastructure and buildings worldwide. Strong earthquake shaking can cause soil liquefaction in fully saturated sand deposits. Also, tsunamis can induce liquefaction, as well as enhanced sediment transport and scour, in coastal areas. To understand soil liquefaction potential during an earthquake–tsunami multi‐hazard, we develop a numerical model to predict the multi‐hazard induced excess pore water pressures. We calibrate and verify the numerical model by comparing results with laboratory experiments. Then, we perform numerical experiments using a recorded earthquake motion and hypothetical tsunami wave heights. The numerical experiments show that beach sand liquefies during earthquake loading. The sand then resediments during the quiescent period and the tsunami runup stage. Finally, during rapid tsunami drawdown, liquefaction can occur again, and liquefaction potential during tsunami drawdown primarily depends on the soil’s hydraulic conductivity, as well as the duration of the quiescent period. The results emphasize the need for predictions of earthquake–tsunami loading, as well as measurements of soil properties in coastal areas.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Limited data on strong earthquakes and their effect on structures pose challenges of making reliable risk assessments of tall buildings. For instance, although the collapse safety of tall buildings is likely controlled by large‐magnitude earthquakes with long durations and high low‐frequency content, there are few available recorded ground motions to evaluate these issues. The influence of geologic basins on amplifying ground‐motion effects raises additional questions. Absent recorded motions from past large magnitude earthquakes, physics‐based ground‐motion simulations provide a viable alternative. This article examines collapse risk and drift demands of a 20‐story archetype tall building using ground motions at four sites in the Los Angeles (LA) basin. Seismic demands of the building are calculated form nonlinear structural analyses using large datasets (∼500,000 ground motions per site) of unscaled, site‐specific simulated seismograms. Seismic hazard and building performance from direct analysis of Southern California Earthquake Center CyberShake motions are contrasted with values obtained based on conventional approaches that rely on recorded motions coupled with probabilistic seismic hazard assessments. At the LA downtown site, the two approaches yield similar estimates of mean annual frequency of collapse (λc), whereas nonlinear drift demands estimated with direct analysis are slightly larger primarily because of differences in hazard curves. Conversely, at the deep basin site, the CyberShake‐based analysis yields around seven times larger λc than the conventional approach, and both hazard and spectral shapes of the motions drive the differences. Deaggregation of collapse risk is used to identify the relative contributions of causal earthquakes, linking building responses with specific seismograms and contrasting collapse risk with hazard. A strong discriminative power of average spectral acceleration and significant duration for predicting collapse is observed.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We adapt the relative polarity method from 〈a href="https://pubs.geoscienceworld.org/bssa#rf29"〉Shelly 〈span〉et al.〈/span〉 (2016)〈/a〉 to compute focal mechanisms for microearthquakes associated with the 2014 hydroshearing stimulation at the Newberry volcano geothermal site. We focus the analysis on events relocated by 〈a href="https://pubs.geoscienceworld.org/bssa#rf2"〉Aguiar and Myers (2018)〈/a〉, who report that six event clusters predominantly comprise the 2014 sequence. Data quality allows focal mechanism analysis for four of the six event clusters. We use 〈a href="https://pubs.geoscienceworld.org/bssa#rf13"〉Hardebeck and Shearer (2002〈/a〉, 〈a href="https://pubs.geoscienceworld.org/bssa#rf14"〉2003〈/a〉; hereafter HASH) to compute focal mechanisms based on first‐motion polarities and 〈span〉S〈/span〉/〈span〉P〈/span〉 amplitude ratios. We manually determine 〈span〉P〈/span〉‐ and 〈span〉S〈/span〉‐wave polarities for a well‐recorded reference event in each cluster, then use waveform cross correlation to determine whether recordings of other events in the cluster are the same or reversed polarity at each network station. Most waveform polarities are consistent with the affiliated reference event, indicating similar focal mechanisms within each cluster. The deeper clusters are east–west‐striking normal faults, whereas the shallower clusters, close to the top of the open‐hole section of the borehole, are strike slip with east–west motion. Regional studies and prestimulation borehole breakouts find the maximum stress direction is vertical and maximum horizontal stress is approximately north–south. Fault geometry and focal mechanisms of microseismicity during the stimulation suggest that increased pressure from fluid injection predominantly caused changes in horizontal stress, consistent with predictions from numerical studies of stress change caused by fluid injection. At shallow depths, where previous studies suggest the difference between vertical and horizontal stress is lowest, injection appears to have rotated the direction of maximum stress from vertical to horizontal, resulting in strike‐slip motion. At greater depth, vertical stress continued to be the dominant direction during the stimulation, but fault orientation indicates either reactivation of pre‐existing fractures or rotation of the direction of maximum horizontal stress from approximately north–south to east–west.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Western Canada sedimentary basin (WCSB) has experienced an increase in seismicity during the last decade due primarily to hydraulic fracturing. Understanding the ground motions of these induced earthquakes is critical to characterize the increase in hazard. Stress drop is considered an important parameter in this context because it is a measure of the high‐frequency content of the shaking. We use the empirical Green’s function (EGF) method to determine 〈span〉S〈/span〉‐wave corner frequencies and stress drops of 87 earthquakes of moment magnitude (M) 2.3–4.4 in the WCSB. The EGF method is an effective technique to isolate earthquake source effects by dividing out the path and site components in the frequency domain, using a smaller collocated earthquake as an EGF. The corner frequency of the target event is determined for an assumed spectral ratio shape, from which the stress drop is computed.Assuming a fixed velocity, we find that the average stress drop for induced earthquakes in the WCSB for small‐to‐moderate events is 7.5±0.5  MPa, with a total range from 0.2 to 370 MPa. However, because of the dependence of stress drop on model conventions and constants, we consider the absolute stress‐drop value meaningful only for comparison with other results using the same underlying models. By contrast, corner frequency is a less‐ambiguous variable with which to characterize the source spectrum. The range of corner frequencies obtained in this study for events of M 4.0±0.5 is 1.1–5.8 Hz.Significant rupture directivity is observed for more than one‐third of the earthquakes, with station corner frequencies varying by about a factor of 4 with azimuth. This emphasizes the importance of having suitable station coverage to determine source parameters. We model directivity where evident using a Haskell source model and find that the rupture azimuths are primarily oriented approximately north–south throughout the region.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The Montello thrust is part of a complex system of the active southeastern front of the Alps in northeastern Italy. An underground gas‐storage facility operates in the anticline at the hanging wall of this thrust system. The Collalto Seismic Network was designed to monitor the induced microseismicity and natural earthquakes around this storage. We analyzed the first 6 yr seismic catalog, containing 1635 earthquakes with −0.8≤ML≤4.5 localized with an enhanced 1D velocity model. The clearly aligned seismicity pattern depicts the Montello thrust as an ∼1000  km2 plane, gently dipping to the northwest and locally interrupted by high‐angle faults, which are nearly perpendicular to the main plane. The observed seismicity suggests a creeping behavior of the main thrust at seismogenic depth (5–13 km). However, we cannot exclude the possibility some parts of the system not microseismically active might be locked and under loading stress condition. Finally, we did not observe a space–time correlation between the microseismicity and the anthropogenic activity occurring in the reservoir.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉A wealth of data collected over the past three decades has demonstrated that volcanic unrest is often associated with elevated levels of seismicity. Volcano seismic networks commonly record intense swarms of earthquakes in the weeks to months before eruptions; peak rates of more than one event per minute are common. The ability to readily detect and classify these signals is crucial to effective monitoring operations and hazard assessment. The sheer volume of information collected, however, poses a challenge to volcano observatories because of the unrealistically large number of staffs required for manual inspection of these data. Here, we present Recursive Entropy Method of Segmentation (REMOS), a computationally efficient Python workflow used to detect, extract, and classify volcanic earthquakes starting from raw continuous waveform data. Within REMOS, seismograms are first analyzed using the well‐established short‐term average/long‐term average method to identify trigger times of candidate earthquakes. A new algorithm based on measurements of seismic energy and minimum entropy is then used to investigate large amounts of earthquake triggers and to discriminate and parse events into individual waveforms for further analyses. REMOS also includes a facility for classification of the extracted waveforms based on simple frequency‐domain metrics. Finally, the results can be visualized using t‐distributed stochastic neighbor embedding, a technique for dimensionality reduction that is particularly well suited to inspection of high‐dimensional datasets. In this work, we demonstrate the use of REMOS with seismic data recorded in 2007 during a period of unrest and eruption at Bezyminany Volcano. Our results show that REMOS can efficiently detect, segment, and classify earthquakes at scale and at very low computational cost.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Oklahoma is one of the most seismically active places in the United States as a result of industry activities. To characterize the fault networks responsible for these earthquakes in Oklahoma, we relocated a large‐scale template‐matching catalog between 2010 and 2016 using the GrowClust algorithm. This relocated catalog is currently the most complete statewide catalog for Oklahoma during this seven year window. Using this relocated catalog, we identified seismogenic fault segments by developing an algorithm (FaultID) that clusters earthquakes and then identifies linear trends within each cluster. Considering the large number of earthquakes in Oklahoma, this algorithm made the process of identifying previously unmapped seismogenic faults more approachable and objective. We identify approximately 2500 seismogenic fault segments that are in general agreement with focal mechanisms and optimally oriented relative to maximum principle stress measurements. We demonstrate that these fault orientations can be used to approximate the maximum principle stress orientations.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The USArray ground‐motion visualization (GMV) is an Incorporated Research Institutions for Seismology (IRIS) video product that illustrates how seismic waves travel away from an earthquake by depicting seismometers as symbols that vary in color according to the recorded amplitudes. GMVs are typically the most popular product the IRIS produces following an earthquake (e.g., ∼10,000 unique views for a recent Oklahoma earthquake). Many instructors feel that dynamic visualizations offer learning advantages over static media when demonstrating dynamic processes, but research indicated they can impede learning by placing greater information processing requirements on the learner. We sought to evaluate changes in student understanding of seismic waves from GMVs by collecting data from three different college‐level settings: general student population in a psychology laboratory (novices), students in middle‐ and upper‐level geoscience courses (geoscience majors), and a seismology research group. A seven‐question multiple‐choice assessment was developed for use in all three settings and then administered in the laboratory and classroom. Using a similar question before and after the GMV viewing, we found that most geoscience majors understood seismic‐wave concepts prior to the GMV and the GMV improved their understanding. Only about half of the novices appeared to understand seismic‐wave concepts prior to the GMV and performance decreased after the GMV. Performance decreases were larger when students watched an alternative tutorial GMV developed to further illustrate what a GMV represents. An increase in the breadth of incorrect answer selections by novices indicates they became more confused about what happens to energy from an earthquake when shown a GMV. Lower performance on other post‐GMV questions by novices suggests that the current style of GMVs are unable to teach basic seismological concepts to people who do not have some formal geoscience training. Although web traffic to GMVs indicates people’s interest in watching the videos, watching GMVs does not appear to translate to improved understanding of seismic waves for novices. Future development of dynamic visualizations such as GMVs should consider the cognitive load these learning materials impose on the learner and seek to further implement principles of multimedia instructional design that minimize cognitive processing demands.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉When an earthquake occurs, a key parameter in the emergency’s management is the knowledge of the most stressed areas by the ground motion. The focal mechanism is an essential source parameter for producing realistic shake maps. Although the approaches for estimating earthquake location and magnitude are now consolidated, automatic solutions for the focal mechanism are not always provided by the agencies or available at later times after inversion of waveforms for the determination of moment tensor components. We introduce a new approach for the automatic determination of the earthquake focal mechanism, using the spatial distribution of observed absolute initial 〈span〉P〈/span〉‐wave peak amplitudes, corrected for the geometrical attenuation effect, in an evolutionary, Bayesian framework. We applied the proposed methodology to the main earthquakes of the 2016–2017 central Italy seismic sequence finding that our rapid automatic estimates of the focal mechanism are in good agreement with those of the reference solutions.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Modeling the seismic potential of active faults and their associated epistemic uncertainties is a fundamental step of probabilistic seismic‐hazard assessment (PSHA). Seismic hazard and earthquake rate in fault systems (SHERIFS) is an open‐source python code that builds hazard models including earthquake ruptures involving several fault sections or fault‐to‐fault (FtF) ruptures. It contains user‐friendly tools to calculate the annual rate of FtF ruptures in a fault system based on the slip‐rate estimates and accounting for associated background seismicity.SHERIFS applies a forward incremental approach following three rules: (1) the FtF ruptures allowed in the fault system are defined as input by the user and explored randomly, (2) the magnitude–frequency distribution of the modeled seismicity in the fault system must follow an imposed shape, and (3) the slip‐rate budget attributed to each fault section must be preserved in the calculation if the first two rules allow it. Indeed, in some cases, a fraction of the slip‐rate budget must be considered as being spent in non‐mainshock events such as creep or postseismic slip. Background seismicity rates are defined by the hazard modeler as the ratio of seismicity occurring on the modelled faults for different ranges of magnitude.Given a coherent set of input hypotheses, SHERIFS allows end users to build the seismic‐hazard fault model thanks to an interactive user‐friendly interface. It aims to help interactions between field data collectors and hazard modelers to explore and weight epistemic uncertainties affecting the input hypotheses. To do so, SHERIFS includes tools to compare modeled earthquake rates with the available local data (earthquake catalog and paleoseismological data). This comparison can be used to weigh different hypotheses explored in a logic tree and discard the hypotheses that are not in agreement with the data. SHERIFS’s outputs are in a format that can be used directly as inputs for PSHA in the OpenQuake engine (〈a href="https://pubs.geoscienceworld.org/srl#rf13"〉Pagani 〈span〉et al.〈/span〉, 2014〈/a〉).〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉A procedure for removing noise or signal from seismic time series using the continuous wavelet transform (CWT) is developed through the common assumption of noise stationarity for pre‐event or postevent estimates of the noise. Noise and signal are efficiently separated using nonlinear thresholding of the CWT avoiding computationally intensive block thresholding algorithms on the wavelet scale‐time plane. Efficiency is gained by estimating the characteristic statistics of pre‐event noise using empirical cumulative distribution functions and then using these characteristics to threshold the entire time series using hard or soft nonlinear thresholding. In addition, scale‐time windowing of the CWT scalogram and inverse transforming into the time domain allows unprecedented control in partitioning a seismogram into component wave types that can subsequently be used to infer characteristics of Earth structure and source excitation. Noise can be separated from signal and signals decomposed into discrete wave groups. CWT techniques offer unique and intuitive alternatives to traditional Fourier methods for analyzing noise and signal useful for structure and source studies, event detection, and ambient‐noise interferometry.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉We present the first study of seismicity in the region of the Jalisco Block using data recorded by the Jalisco Seismic Accelerometric Telemetric Network between June and December 2015. During this period, 683 local earthquakes with magnitudes between 1.0〈ML≤4.0 were identified and relocated with Hypo71PC. From this catalog, we identify a heterogeneous hypocentral distribution with six continental crustal seismogenic areas. We also observed seismicity associated with the subduction process that extends 180 km from the Mesoamerican trench, which suggests an estimated dip angle of the slab between 22° and 31°. A subtle dip also suggests oblique subduction toward the Colima rift zone and bending of the Rivera plate. These observations are in agreement with previous partial regional studies using local seismic networks. Two seismic swarms were observed in this period, one in the Bahia de Banderas seismogenic zone, and a second in the Guadalajara Metropolitan zone. We note two areas on the northern coast of Jalisco with meager rates of seismicity.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉In 2016, the U.S. Geological Survey deployed 〉1800 vertical-component nodal seismometers in Grant County, Oklahoma, to study induced seismic activity associated with production of the Mississippi limestone play. The LArge‐n Seismic Survey in Oklahoma (LASSO) array operated for approximately one month, covering a 25 km by 32 km region with a nominal station spacing of ∼400  m. Primary goals of the deployment were to detect microseismic events not captured by the sparser regional network stations and to provide nearly unaliased records of the seismic wavefield. A more complete record of earthquakes allows us to map the spatiotemporal evolution of induced event sequences and illuminates the structures on which the events occur. Dense records of the seismic wavefield also provide improved measurements of the earthquake source, including focal mechanisms and stress drops. Taken together, we can use these findings to glean insights into the processes that induce earthquakes. Here, we describe the array layout, features of the nodal sensors, data recording configurations, and the field deployment. We also provide examples of earthquake waveforms recorded by the array to illustrate data quality and initial observations. LASSO array data provide a significant resource for understanding the occurrence of earthquakes induced by wastewater disposal.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The prediction of accurate source‐to‐receiver travel times and wave paths through heterogeneous media is of major interest in global seismology and microseismic communities. Many algorithms have been proposed to address this problem, among which eikonal solvers have the best accuracy but lack computational efficiency. To facilitate the use of eikonal solvers with a high performance and visualization ability, this article presents a free, open‐source, graphical package named 3DMRT. 3DMRT propagates wavefronts through a 3D heterogeneous medium. Starting with a geologic model, the package first constructs a gridded velocity model. The application implements nine eikonal solvers and provides one‐point and multipoint raytracing functionality. In addition, 3DMRT is complemented with an additional command line tool that allows integration into other programs for further applications.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉On the afternoon of 15 November 2017, the coastal city of Pohang, Korea, was rocked by a magnitude 5.5 earthquake (Mw, U.S. Geological Survey). Questions soon arose about the possible involvement in the earthquake of the Republic of Korea’s first enhanced geothermal system (EGS) project because the epicenter of the earthquake was located near the project’s drill site. The Pohang EGS project was intended to create an artificial geothermal reservoir within low‐permeability crystalline basement by hydraulically stimulating the rock to form a connected network of fractures between two wells, PX‐1 and PX‐2, at a depth of ∼4  km. Forensic examination of the tectonic stress conditions, local geology, well drilling data, the five high‐pressure well stimulations undertaken to create the EGS reservoir, and the seismicity induced by injection produced definitive evidence that earthquakes induced by high‐pressure injection into the PX‐2 well activated a previously unmapped fault that triggered the Mw 5.5 earthquake. Important lessons of a general nature can be learned from the Pohang experience and can serve to increase the safety of future EGS projects in Korea and elsewhere.〈/span〉

Description: 〈span〉We thank 〈a href="https://pubs.geoscienceworld.org/bssa#rf2"〉Guidotti 〈span〉et al.〈/span〉 (2019)〈/a〉 for their comment on our work, and apologize for the incorrect statement, as written, that their paper did not contain quantitative analysis. The cited phrase on page 2131 in 〈a href="https://pubs.geoscienceworld.org/bssa#rf4"〉Razafindrakoto 〈span〉et al.〈/span〉 (2018)〈/a〉 was intended to more specifically note that 〈a href="https://pubs.geoscienceworld.org/bssa#rf3"〉Guidotti 〈span〉et al.〈/span〉 (2011)〈/a〉 did not quantitatively compare their simulation‐observation misfit with alternative predictions using conventional empirical ground‐motion models. The 〈a href="https://pubs.geoscienceworld.org/bssa#rf1"〉Anderson (2004)〈/a〉 criteria provide a quantitative measure of misfit in an absolute sense, but are not able to provide a relative comparison with widely adopted empirical models used in ground‐motion prediction and seismic hazard analysis.〈/span〉

Description: 〈span〉Since its development in the late 1960s and 1970s, plate tectonics has caught hold and been used to explain many fundamental processes on Earth (〈a href="https://pubs.geoscienceworld.org/srl#rf19"〉Vine, 1966〈/a〉; 〈a href="https://pubs.geoscienceworld.org/srl#rf14"〉Oliver and Isacks, 1967〈/a〉; 〈a href="https://pubs.geoscienceworld.org/srl#rf17"〉Sykes, 1967〈/a〉; 〈a href="https://pubs.geoscienceworld.org/srl#rf7"〉Isacks 〈span〉et al.〈/span〉, 1968〈/a〉; 〈a href="https://pubs.geoscienceworld.org/srl#rf11"〉McKenzie, 1972〈/a〉). Plate tectonics gave rise to the concept of subduction: places where oceanic lithosphere is recycled into the mantle and the site of the largest earthquakes, devastating tsunamis, volcanic eruptions, and large amounts of deformation. Although data were often sparse in the early years of plate tectonics, there was a framework in which to understand subduction. In the last two decades, there has been an explosion of data related to subduction processes resulting in literally thousands of papers, based on seismological observations, geodesy, numerical models, geochemistry, and other geophysical datasets. Hence, having consolidated a general vision for subduction, the most recent research efforts focus on identifying and explaining the range of slip behaviors, tsunamic generation, subduction channel properties, anisotropy, seismic triggering, and nonvolcanic (tectonic) tremors at a range of scales that are observed in subduction zones.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Recent efforts have been made to model the rupture process of large earthquakes in near‐real time (NRT) in Chile. In this study, we propose an automated procedure using strong‐motion data in an integrated system, which can characterize large earthquakes with a finite‐fault model (FFM) in NRT. We developed several heuristic rules using the preliminary 〈span〉W〈/span〉‐phase solutions to automatically set up the search ranges of the finite‐fault inversions. The results show using strong‐motion data and a 〈span〉W〈/span〉‐phase magnitude, it is possible to obtain a rapid kinematic FFM in just a few minutes after the earthquake origin time.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉In preparation for the next phase of the Source Physics Experiments, we acquired an active‐source seismic dataset along two transects totaling more than 30 km in length at Yucca Flat, Nevada, on the Nevada National Security Site. Yucca Flat is a sedimentary basin which has hosted more than 650 underground nuclear tests (UGTs). The survey source was a novel 13,000 kg modified industrial pile driver. This weight drop source proved to be broadband and repeatable, richer in low frequencies (1–3 Hz) than traditional vibrator sources and capable of producing peak particle velocities similar to those produced by a 50 kg explosive charge. In this study, we performed a joint inversion of 〈span〉P〈/span〉‐wave refraction travel times and Rayleigh‐wave phase‐velocity dispersion curves for the 〈span〉P〈/span〉‐ and 〈span〉S〈/span〉‐wave velocity structure of Yucca Flat. Phase‐velocity surface‐wave dispersion measurements were obtained via the refraction microtremor method on 1 km arrays, with 80% overlap. Our 〈span〉P〈/span〉‐wave velocity models verify and expand the current understanding of Yucca Flat’s subsurface geometry and bulk properties such as depth to Paleozoic basement and shallow alluvium velocity. Areas of disagreement between this study and the current geologic model of Yucca Flat (derived from borehole studies) generally correlate with areas of widely spaced borehole control points. This provides an opportunity to update the existing model, which is used for modeling groundwater flow and radionuclide transport. Scattering caused by UGT‐related high‐contrast velocity anomalies substantially reduced the number and frequency bandwidth of usable dispersion picks. The 〈span〉S〈/span〉‐wave velocity models presented in this study agree with existing basin‐wide studies of Yucca Flat, but are compromised by diminished surface‐wave coherence as a product of this scattering. As nuclear nonproliferation monitoring moves from teleseismic to regional or even local distances, such high‐frequency (〉5  Hz) scattering could prove challenging when attempting to discriminate events in areas of previous testing.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Atlantic and Gulf Coastal Plain in the southern and southeastern United States contains extensive Cretaceous and Cenozoic sedimentary sequences of variable thickness. We investigated the difference in response of sites in the Coastal Plain relative to sites outside that region using Fourier spectral ratios from 17 regional earthquakes occurring in 2010–2018 recorded by the Earthscope transportable array and other stations. We used mean coda and 〈span〉Lg〈/span〉 spectra for sites outside the Coastal Plain as a reference. We found that Coastal Plain sites experience amplification of low‐frequency ground motions and attenuation at high‐frequencies relative to average site conditions outside the Coastal Plain. The spectral ratios at high frequencies gave estimates of the difference between kappa at Coastal Plain sites and the reference condition. Differential kappa values determined from the coda are correlated with the thickness of the sediment section and agree with previous estimates determined from 〈span〉Lg〈/span〉 waves. Averaged estimates of kappa reach ∼120  ms at Gulf coast stations overlying ∼12  km of sediments. Relations between 〈span〉Lg〈/span〉 spectral ratio amplitudes versus sediment thickness in successive frequency bins exhibit consistent patterns, which were modeled using piecewise linear functions at frequencies ranging from 0.1 to 2.8 Hz. For sediment thickness greater than ∼0.5  km, the spectral amplitude ratio at frequencies higher than approximately ∼3  Hz is controlled by the value of kappa. The peak frequency and maximum relative amplification at frequencies less than ∼1.0  Hz depend on sediment thickness. At 0.1 Hz, the mean Fourier amplitude ratio (Coastal Plain/reference) is about 2.7 for sediment of 12 km thickness. Analysis of residuals between observed and predicted ground motions suggests that incorporating the amplification and attenuation as functions of sediment thickness may improve ground‐motion prediction models for the Coastal Plain region.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉On 20 January 2019, the Chilean cities of Coquimbo and La Serena were shaken by an intraplate earthquake of Mw 6.7 located at 70 km depth. High peak ground acceleration values and macroseismic intensities were reported. The mainshock was followed by more than 150 aftershocks higher than ML 2.5, a seismic sequence completely recorded by local stations. Using a 3D velocity model, we precisely located the seismicity. The aftershocks were located some 20 km above and shifted from the mainshock but still inside the Nazca plate. We also performed moment tensor inversion of nine events obtaining mostly normal‐fault focal mechanisms and kinematic inversions using the elliptical‐patch approach. We found that the mainshock broke an approximated zone of 6 km by 8 km, propagated upward in the northwest direction and away from the aftershock area. The rupture inverted from accelerograms containing up to 1 Hz was characterized with a high stress drop of 7.51 MPa and a short seismic source time function of only 3 s duration.〈/span〉

Description: 〈span〉Figures 9 and 10 of 〈a href="https://pubs.geoscienceworld.org/srl#rf1"〉Lei 〈span〉et al.〈/span〉 (2019)〈/a〉 contained an erroneous color bar for Coulomb failure stress. Corrected figures are shown here. No changes are required for the related descriptions in the text.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We define the Wilmington blind‐thrust as a tectonically active fault capable of generating large damaging earthquakes, through analysis of 2D and 3D seismic reflection surveys, petroleum and water wells, and recent mapping of groundwater aquifers in the southwestern Los Angeles basin. This overturns the long‐held view that the fault became dormant in the Late Pliocene, barring its inclusion in state‐of‐the‐art regional earthquake hazard assessments. The size of the fault suggests that it is capable of generating moderate‐magnitude earthquakes (Mw 6.3–6.4), whereas potential linkages with other nearby faults (e.g., Huntington Beach, Torrance, and Compton) pose the threat of larger multisegment events (Mw〉7). These earthquakes would directly impact the overlying Ports of Los Angeles and Long Beach, as well as the broader Los Angeles metropolitan area.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We mapped a poorly documented 35‐km‐long section of the northern San Andreas fault (NSAF) zone between Tomales Point and Fort Ross, California. Mapping is largely based on high‐resolution seismic‐reflection profiles (38 fault crossings), multibeam bathymetry, and onshore geology. NSAF strike in this section is nearly parallel to plate motion, characterized by a slight (∼2°) northerly (transtensional) bend in the south between Tomales Bay and the Bodega isthmus, and a northwesterly (transpressional) ∼5° bend in the north between the Bodega isthmus and Fort Ross. The southern transtensional bend is the northern part of the now‐submerged, linear, ∼50‐km‐long and 1–2‐km‐wide, Tomales–Bodega valley. The valley floor is cut by a complex zone of subparallel, variably continuous fault strands, and the deformed valley fill is an inferred mix of late Quaternary marine and nonmarine strata. In the northern part of this elongate valley, Holocene fault offset occurred on two fault strands about 740 m apart. The northern transpressional bend is characterized by narrow, elongate, asymmetric basins containing as much as 56 m of inferred latest Pleistocene to Holocene sediment.Between Bodega Head and Fort Ross, the gently dipping (∼0.8°) shelf includes two large (4.8 and 5.9  km2) zones of sediment failure that we speculatively correlate with the 1906 San Francisco NSAF earthquake. Similar sediment‐failure zones should be common along offshore reaches of the NSAF and other nearshore fault zones but have apparent limited preservation potential. Onland geomorphic impacts of the mainly offshore NSAF include: (1) northward upwarping of uplifted marine terraces in the transpressional zone north of Bodega Bay; and (2) blocking of littoral sediment transport by uplifts on the west flank of the NSAF at Bodega Head and Tomales Point, resulting in rapidly accreting beaches and large coastal sand dune complexes.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉In different subsurface energy technologies, traffic light systems (TLSs) have been implemented for limiting the strength of induced seismicity. Despite their widespread application, fundamental assumptions regarding the controllability of induced seismicity were usually not reviewed. This is the focus of the current article, in which we discuss limitations of a TLS in the context of seismicity induced by fluid injection and gas production.Most existing TLSs are based on a critical earthquake magnitude or vibration level that should be prevented to occur. Operational measures are defined to be taken after an induced earthquake exceeds predefined threshold values. This concept rests on the tacit assumptions that induced earthquakes of a critical strength announce themselves by precursory events of smaller strength and that future earthquakes of a critical strength can be prevented by modifying or stopping subsurface operations. We investigate to what extent these assumptions can be justified by studying observation data from a dozen fluid‐injection operations in geothermal reservoirs as well as from gas production in 26 gas fields in The Netherlands.In our case studies, whereas fluid injection–induced seismicity generally starts at a low‐magnitude level and exhibits a gradual temporal increase of the maximum earthquake magnitude with the duration of the injection, the largest magnitude event frequently occurs postinjection. The temporal evolution of the seismicity induced by gas production in The Netherlands is less systematic. In some gas fields, seismicity started at a comparatively large‐magnitude level (ML≥2.7) without detectable precursors. A correlation between seismic activity and the gas production rate is only observed in the largest gas field.Our findings indicate that the precision to what an earthquake of a given strength can be prevented by a TLS has more limitations than typically assumed.〈/span〉

Description: 〈span〉〈span〉Seismological Research Letters〈/span〉 is retracting the article “Preliminary Analysis of Strong Ground Motions in the Heavily Damaged Zone in Mashiki Town, Kumamoto, Japan, during the Mainshock of the 2016 Kumamoto Earthquake (Mw 7.0) Observed by a Dense Seismic Array” by Yoshiya Hata, Hiroyuki Goto, and Masayuki Yoshimi (〈span〉Seismological Research Letters〈/span〉 (2016) 87 (5): 1044–1049. https://doi.org/10.1785/0220160107) published on 10 August 2016. This retraction is in response to the results of an investigation into allegations of specific research misconduct that occurred at Osaka University. The Investigation Committee set up by Osaka University determined that the lead author, Hata, fabricated the data presented in the article by manipulating data observed by seismographs installed by other institutes and by other means.〈/span〉

Description: 〈span〉At the request of the authors, 〈span〉Bulletin of the Seismological Society of America〈/span〉 is retracting the article “Nonlinear Site Response at KiK‐net KMMH16 (Mashiki) and Heavily Damaged Sites during the 2016 Mw 7.1 Kumamoto Earthquake, Japan” by Hiroyuki Goto, Yoshiya Hata, Masayuki Yoshimi, and Nozuma Yoshida (〈span〉Bulletin of the Seismological Society of America〈/span〉 (2017) 107 (4): 1802–1816. https://doi.org/10.1785/0120160312) published on 4 July 2017. An investigation by Osaka University confirmed that one of the authors, Yoshiya Hata, fabricated data in his research. The co‐authors notified the journal that this article included the fabricated data in the validation process.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We present new results from paleoseismic trenches excavated across the main trace of the Rose Canyon fault zone (RCFZ) in Old Town‐San Diego, California, to determine the timing of late‐Holocene earthquakes. There is evidence for four large surface‐rupturing events, as well as two smaller events, the youngest of which cuts the early historical living surface that contains glass, ceramics, and a historical era foundation. This youngest event is likely related to the 1862 San Diego earthquake, which had an estimated magnitude close to 〈strong〉M〈/strong〉 6. The age of older ruptures is constrained by 36 radiocarbon dates that exhibit good stratigraphic order. The four larger events produced substantially more ground deformation, and over a broader width of the fault zone, than the 1862 event. The youngest of the four larger events is found immediately below the historical horizon and likely correlates with the most recent event recognized at multiple trench sites along the RCFZ in San Diego and dates to the mid‐1700s. The three older events have all occurred in the past 3300 yr, with the penultimate large event dated to about A.D. 1300.The results of this paleoseismic study indicate that the RCFZ has sustained activity throughout the late‐Holocene and into the historical period. These results also suggest that the RCFZ has a late‐Holocene recurrence interval of ∼700  yr, which is several hundred years shorter than previous estimates. Comparison of RCFZ paleoseismic results with paleoseismic data from the Newport–Inglewood fault zone (NIFZ) shows that some RCFZ earthquakes have similar timing with NIFZ events, most likely indicating the occurrence of a sequence or cluster of events on the coastal system of strike‐slip faults. The alternative explanation—very large earthquakes rupturing both faults simultaneously—is unlikely when both the slip rate and recurrence intervals for these faults are considered.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉A robust algorithm has been developed for the automatic picking of 〈span〉P〈/span〉‐wave arrival times. Owing to the properties of the local extrema scalogram (LES), this algorithm finds all significant quasi‐periodic peaks and valleys without selecting a specific frequency. Consequently, the 〈span〉P〈/span〉‐wave arrival times can be accurately derived from the peaks and valleys of the seismic signal. A comparison of the proposed algorithm with the common short‐term average/long‐term average (STA/LTA) method and the Akaike information criterion (AIC) method is conducted using real data. The results show that our method consistently outperforms both methods, especially when substantial noise is present.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Studies of strong ground motion of the 2008 great Mw 7.9 Wenchuan earthquake indicate that the horizontal‐component peak ground acceleration (PGA) is significantly enhanced within the 100‐ to 300‐km fault distance range. In this article, through analysis of waveform recordings of Wenchuan aftershocks of single earthquakes at multiple stations and multiple earthquakes at single stations, we confirm the existence of large‐amplitude 〈span〉SmS〈/span〉 within the critical epicentral region (∼100–300  km) in this area, with amplitude consistently 3–5 times stronger than direct 〈span〉S〈/span〉. Based on synthetic modeling of the horizontal‐component peak ground velocity (PGV) and PGA attenuation relationships, it is found that in the same epicentral distance range, the postcritical 〈span〉SmS〈/span〉 arrivals clearly enlarge the amplitude, which also explains the increased PGA values of the Wenchuan mainshock within the critical distance range. We also quantify the influence of 〈span〉SmS〈/span〉 on the ground‐motion amplitudes and modify an existing model to obtain a new hinged‐trilinear attenuation model of this area, which provides a better fit to the recorded PGA amplitudes of the Wenchuan mainshock.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉After the 2011 Mineral, Virginia, earthquake, a temporary dense array (aftershock imaging with dense arrays [AIDA]) consisting of ∼200 stations was deployed at 200–400 m spacing near the epicenter for 12 days. Backprojection of the data was used to automatically detect and locate aftershocks. The co‐deployment of a traditional aftershock network of 36 stations at ∼2–10  km spacing enables a quantitative comparison. The AIDA backprojection aftershock catalog is complete to 〈span〉M〈/span〉−0.5 and includes 1673 events. For comparison, the traditional network was complete to M−0.1 with 813 events within the same time period and spatial volume. Only 494 of the traditional network events were of sufficient quality to compute improved double‐difference locations, for a completeness of 〈span〉M〈/span〉+0.2. The AIDA backprojection catalog observes the same major patterns of seismicity in the epicentral region, but additional details are illuminated, and absolute uncertainty was reduced. The primary zone of seismicity is not a single fault but is a tabular zone of multiple small faults with no resolvable internal structures. This zone has a subtle concave shape along strike and with depth, and a broader zone of newly detected events is observed at shallow depth. In addition, a shallow cluster was detected and located to the east of the main aftershock zone. The addition of smaller events to the catalog did not change the b‐value but illuminated spatial and temporal patterns. The b‐value is different at less than about 3 km depth than at greater depth. Very low b‐value, especially at greater depth, is consistent with observed very high stress drops. The results indicate the benefits of dense arrays and autodetection by backprojection for aftershock studies. The reduced detection threshold and higher spatial resolution enabled the study of earthquake mechanisms and strain transfer at a smaller scale.〈/span〉

Description: 〈span〉〈span〉SRL〈/span〉 is indebted to the dedicated people who provide peer reviews of submitted papers. Providing a conscientious and timely review is a vital service, both to authors and to readers. The Editorial Board would like to express sincere gratitude to the following people, who completed one or more reviews between November 2017 and October 2018. We apologize for errors or omissions.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Monitoring of seismic signals generated by slow deformation at convergent and transform plate boundaries worldwide, known as tectonic tremor, might provide insights into deformation processes in the source regions of megathrust earthquakes. Tremor signals occur dominantly in the 2–8 Hz frequency band and can last for tens of seconds to several minutes, in contrast to typical earthquakes that produce seismic signals at frequencies up to several tens of hertz and last less than a minute. Because tremor is caused by stochastic processes, the resultant waveforms are represented by a stochastic function and construction of deterministic measures to discriminate tremor signals from earthquakes is very difficult. In this study, we used a convolutional neural network (CNN) to discriminate the signals of tectonic tremor from those of local earthquakes in running spectral images of these signals. We developed a method (seismic running spectra‐CNN [SRSpec‐CNN]) that is sensitive to the absolute frequency of signal appearance, which reflects the physical properties of the signal source, but is insensitive to the time of signal onset. SRSpec‐CNN has 130,211 parameters that were trained by 17,213 images of 64×64  pixels. Based on simultaneous analyses of the frequency contents and durations of the signals, we achieved 99.5% accuracy for our identifications of signals from tectonic tremor, local earthquakes, and noise. Because running spectra clearly differentiate the characteristic features of these signals, we were able to achieve this high accuracy by using a CNN of simple architecture.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉I review the unusual swarm of 50 earthquakes, Mw 5.2–5.4, which occurred at Kīlauea caldera between May and August 2018. Given the strong similarity in published source mechanisms derived by centroid moment tensor (CMT) methods, I stacked the 〈span〉P〈/span〉 waves from a global distribution of stations from local to antipodal distances. The composite source mechanism is consistent with a normal planar fault striking 335° NNW, dipping 78° W, with a rake nearly −90°. The differences between the composite source and waveform‐derived sources are due in part to the choice of source depth, seismic velocity, and rigidity beneath Kīlauea summit. Antipodal 〈span〉PKIKP〈/span〉 observations of the bottom of the source are derived from stacked data from three Global Seismographic Network (GSN) stations at ∼175°Δ in southern Africa. The stacked 〈span〉PKIKP〈/span〉 displacement data show dilatational first motions, apparently inconsistent with proposed piston models of the volcano–earthquake activity for comparable swarms observed in the Galapagos, Miyakejima, and Iceland. As recognized in these prior studies, the vertical‐〈span〉P〈/span〉 compensated linear vector dipole (CLVD) moment tensor solutions likely result from arcuate faults, which collectively circumscribe the caldera. The interevent times of the swarm sequence are not random; rather, the time interval between events increases by ∼1/2  hr for each earthquake.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The deployment of oceanic seismic arrays facilitated unique data sets for the science community in imaging the seismic structures and understanding the lithosphere and mantle dynamics at subduction zone systems and other tectonic settings. The data quality is fundamental to ensure reliable seismic results using records from ocean‐bottom seismometers. In this study, we conduct a comprehensive analysis of factors that may affect the signal‐to‐noise ratio (SNR) of the fundamental‐mode Rayleigh waves, as a proxy for the waveform quality, within the Cascadia subduction zone. We use stations from Cascadia Initiative, Gorda deformation zone experiment, Blanco transform fault experiment, and Neptune Canada array. The empirical Green’s functions (EGFs) of Rayleigh waves are extracted from ambient‐noise seismic waveforms and filtered at 10‐ to 35‐s periods. In general, the SNR of the EGFs decreases with increasing interstation distance and increasing sediment thickness. A portion of stations, mainly located within the Gorda plate and along the trench, demonstrates temporal variations of the data quality, with the highest SNR observed during the fall and winter seasons. The SNR demonstrates a complicated pattern in terms of the length of the time series used to extract EGFs. Most stations within the Juan de Fuca (JDF) plate show improvement of data quality with increasing length. However, for many stations located within the accretionary wedge and the Gorda plate, the ratio does not increase much by stacking more data. The distinctly different patterns of the SNR between the Gorda and JDF plates indicate possible impacts of lithosphere properties on data quality.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Rapid association of seismic phases and event location are crucial for real‐time seismic monitoring. We propose a new method, named rapid earthquake association and location (REAL), for associating seismic phases and locating seismic events rapidly, simultaneously, and automatically. REAL combines the advantages of both pick‐based and waveform‐based detection and location methods. It associates arrivals of different seismic phases and locates seismic events primarily through counting the number of 〈span〉P〈/span〉 and 〈span〉S〈/span〉 picks and secondarily from travel‐time residuals. A group of picks are associated with a particular earthquake if there are enough picks within the theoretical travel‐time windows. The location is determined to be at the grid point with the most picks, and if multiple locations have the same maximum number of picks, the grid point among them with smallest travel‐time residuals. We refine seismic locations using a least‐squares location method (VELEST) and a high‐precision relative location method (hypoDD). REAL can be used for rapid seismic characterization due to its computational efficiency. As an example application, we apply REAL to earthquakes in the 2016 central Apennines, Italy, earthquake sequence occurring during a five‐day period in October 2016, midway in time between the two largest earthquakes. We associate and locate more than three times as many events (3341) as are in Italy's National Institute of Geophysics and Volcanology routine catalog (862). The spatial distribution of these relocated earthquakes shows a similar but more concentrated pattern relative to the cataloged events. Our study demonstrates that it is possible to characterize seismicity automatically and quickly using REAL and seismic picks.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S‐net) is a novel cabled ocean‐bottom station network covering a broad offshore region east of northeastern Japan. To best use the S‐net data, we estimated sensor orientations of all 150 S‐net stations, because without this information the orientations of measurements in geodetical coordinates cannot be specified. We determined three parameters of the sensor orientation at each station: the tilt angle of the long axis of the cable, the rotation angle around the long axis, and the azimuth of the long axis. We estimated the tilt and rotation angles by using the direct current components of accelerometers recording the gravitational acceleration. The tilt and rotation angles slightly varied within the range of 0.001°–0.1° for most stations during the period from 2016 to 2018 except for coseismic steps of rotation angles greater than 1° because of the 20 August 2016 Mw 6.0 off Sanriku and 20 November 2016 Mw 6.9 off Fukushima earthquakes. The long‐axis azimuths were estimated by the particle motions of long‐period Rayleigh waves. We used the accelerometer records in 0.01–0.03 Hz of 7–14 teleseismic earthquakes with Mw 7.0–8.2. The azimuths were constrained with 95% confidence intervals of ±3°–12°. After correcting original waveforms based on the estimated sensor orientation, we confirmed coherent waveforms within the whole S‐net stations and separation of Rayleigh and Love waves in radial and transverse components. The waveforms were also coherent with those of on‐land broadband stations. We provide the estimated sensor orientations and rotation matrix for conversion from the XYZ to east, north, and up components. The estimated orientation can be a fundamental resource for further seismic and geodetic explorations based on S‐net data.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉A new computational platform for seismic hazard assessment is presented. The platform, named SeismicHazard, allows characterizing the intensity, uncertainty, and likelihood of ground motions from subduction‐zone (shallow interface and intraslab) and crustal‐zone earthquakes, considering site‐specific as well as regional‐based assessments. The platform is developed as an object‐oriented MATLAB graphical user interface, and it features several state‐of‐the‐art capabilities for probabilistic and deterministic (scenario‐based) seismic hazard assessment. The platform integrates the latest developments in performance‐based earthquake engineering for seismic hazard assessment, including seismic zonation models, ground‐motion models (GMMs), ground‐motion correlation structures, and the estimation of design spectra (uniform hazard spectra, classical conditional mean spectrum (CMS) for a unique tectonic setting). In addition to these standard capabilities, the platform supports advanced features, not commonly found in existing seismic hazard codes, such as (a) computation of source parameters from earthquake catalogs, (b) vector‐probabilistic seismic hazard assessment, (c) hazard evaluation based on conditional GMMs and user‐defined GMMs, (d) uncertainty treatment in the median ground motions through continuous GMM distributions, (e) regional shaking fields, and (f) estimation of CMS considering multiple GMMs and multiple tectonic settings. The results from the platform have been validated against accepted and well‐documented benchmark solutions.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The Mw 7.1 47 km deep earthquake that occurred on 30 November 2018 had deep societal impacts across southcentral Alaska and exhibited phenomena of broad scientific interest. We document observations that point to future directions of research and hazard mitigation. The rupture mechanism, aftershocks, and deformation of the mainshock are consistent with extension inside the Pacific plate near the down‐dip limit of flat‐slab subduction. Peak ground motions 〉25%g were observed across more than 8000  km2, though the most violent near‐fault shaking was avoided because the hypocenter was nearly 50 km below the surface. The ground motions show substantial variation, highlighting the influence of regional geology and near‐surface soil conditions. Aftershock activity was vigorous with roughly 300 felt events in the first six months, including two dozen aftershocks exceeding 〈span〉M〈/span〉 4.5. Broad subsidence of up to 5 cm across the region is consistent with the rupture mechanism. The passage of seismic waves and possibly the coseismic subsidence mobilized ground waters, resulting in temporary increases in stream flow. Although there were many failures of natural slopes and soils, the shaking was insufficient to reactivate many of the failures observed during the 1964 〈span〉M〈/span〉 9.2 earthquake. This is explained by the much shorter duration of shaking as well as the lower amplitude long‐period motions in 2018. The majority of observed soil failures were in anthropogenically placed fill soils. Structural damage is attributed to both the failure of these emplaced soils as well as to the ground motion, which shows some spatial correlation to damage. However, the paucity of instrumental ground‐motion recordings outside of downtown Anchorage makes these comparisons challenging. The earthquake demonstrated the challenge of issuing tsunami warnings in complex coastal geographies and highlights the need for a targeted tsunami hazard evaluation of the region. The event also demonstrates the challenge of estimating the probabilistic hazard posed by intraslab earthquakes.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉We present a fundamental solution‐based finite‐element (FE) method to homogenize heterogeneous elastic medium, that is, fault zone, under static, and dynamic loading. This method incorporates Eshelby’s strain perturbation into FE weak forms. The resulting numerical model implicitly considers the existence of inhomogeneity bodies within each element, without introducing additional degrees of freedom. The new method is implemented within an open‐source FE package that is applicable to alternating seismic and aseismic cycles. To demonstrate this method, we modify a dynamic fault‐slip problem, hosted at Southern California Earthquake Center (SCEC), by introducing a fault zone that contains different microstructures than the host matrix. The preliminary results suggest that the fault‐zone microstructure orientation has effects on fault slip, seismic arrivals and waveform frequency contents.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The intraplate Gujarat region located at the trijunction of three failed rifts, Kachchh, Narmada, and Cambay, is one of the most seismically active intraplate regions of the world. Among these three, the Cambay basin has been investigated thoroughly for petroleum. However, the basin has not been studied from a seismotectonic perspective. For the past few years, the northern part of the Cambay basin is becoming active with reasonably frequent earthquake occurrences. In the past 10 yr, ∼995 earthquakes have been recorded from the region with a maximum magnitude up to 4.2. Most of the earthquakes are in the magnitude range 1–3. Since 2009, four Global Positioning System (GPS) stations have been in operation in the vicinity of the Cambay basin, and a maximum deformation of 1.8±0.1  mm/yr has been estimated. The GPS‐derived strain rates of ∼0.02–0.03  microstrain/yr are prevalent in the region. An average strain rate of 0.02  microstrain/yr in the region can generate an earthquake of magnitude 6.4. The focal mechanisms of the earthquakes have been mostly normal with strike‐slip component and corroborated by the geodetic strain tensors. Most of the seismicity is clustered in the basement ridges, striking along pre‐existing Precambrian trends that cross the Cambay basin. Complex geodynamics have developed around the northern part of the Cambay rift because of the various movements along several faults, presence of basement ridges, and subsurface plutonic bodies in a failed rift, which are creating stresses and causing earthquakes in this part of the rift. We postulated that the highly heterogeneous subsurface structure beneath the northern part of the Cambay rift is creating additional stress, which is superimposing on the regional stress field substantially, and this mechanism is plausibly facilitating the localized extensional tectonics in the region where compression is expected.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The first seismic stations of the Los Alamos Seismic Network (LASN) were installed in 1973, as a part of research on monitoring of nuclear testing. The extent of the network rapidly expanded by the late 1970s. By the middle 1980s, spatial coverage of the network was drastically reduced, due to a loss of funding. Since then, however, it has been possible to expand the coverage of the network with additional stations and to slowly upgrade network equipment to match contemporary instrument standards. These improvements will make it possible to keep recording the network’s data and to initiate real‐time exchange of data with other institutions to improve earthquake monitoring throughout New Mexico and neighboring states. During more than 40 yr of operation, the network has provided a slow but steady increase in the volume of earthquake seismograms available to study seismicity and tectonics in north‐central New Mexico. The current network covers an area from the Valles Caldera on the west, to within the Rio Grande rift on the east. LASN has yielded locations for about 900 earthquakes in north‐central New Mexico between 1973 and 2013 (the most recent locations available). Epicenters of these have a complex pattern, with some that can be attributed to the deformation of the rift, though most are spread in areas west of the rift. A lack of seismicity in and near the Valles Caldera reinforces an earlier observation, based on far fewer earthquakes, which first called attention to this paucity of seismicity and attributed it to ductile deformation resulting from elevated crustal temperatures.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The association of seismic‐wave arrivals with causative earthquakes becomes progressively more challenging as arrival detection methods become more sensitive, and particularly when earthquake rates are high. For instance, seismic waves arriving across a monitoring network from several sources may overlap in time, false arrivals may be detected, and some arrivals may be of unknown phase (e.g., 〈span〉P〈/span〉 or 〈span〉S〈/span〉 waves). We propose an automated method to associate arrivals with earthquake sources and obtain source locations applicable to such situations. To do so, we use a pattern detection metric based on the principle of backprojection to reveal candidate sources followed by graph‐theory‐based clustering and an integer linear optimization routine to associate arrivals with the minimum number of sources necessary to explain the data. This method solves for all sources and phase assignments simultaneously, rather than in a sequential greedy procedure as is common in other association routines. We demonstrate our method on both synthetic and real data from the Integrated Plate Boundary Observatory Chile seismic network of northern Chile. For the synthetic tests, we report results for cases with varying complexity, including rates of 500 earthquakes/day and 500 false arrivals/station/day, for which we measure true positive detection accuracy of 〉95%. For the real data, we develop a new catalog between 1 January 2010 and 31 December 2017 containing 817,548 earthquakes, with detection rates on average 279 earthquakes/day and a magnitude‐of‐completion of M∼1.8. A subset of detections are identified as sources related to quarry and industrial site activity, and we also detect thousands of foreshocks and aftershocks of the 1 April 2014 Mw 8.2 Iquique earthquake. During the highest rate of aftershock activity, 〉600 earthquakes/day are detected in the vicinity of the Iquique earthquake rupture zone.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉When studying the performance of distributed infrastructure in earthquakes, spatial variations in strong ground motion have a significant impact. Currently, prediction models for spatial ground‐motion variations in future earthquakes are calibrated using ground‐motion observations from densely recorded earthquakes. Although useful, that calibration process requires strong assumptions about stationarity and isotropy of correlations. This article reports results from conducting analogous spatial variation estimation using physics‐based simulations from the CyberShake platform. This platform contains simulated ground motions from hundreds of thousands of rupture realizations, at locations throughout southern California, providing a synthetic ground‐motion catalog that is much richer than we could ever hope to achieve from recordings. That richness allows significant relaxation of stationarity and isotropy assumptions, and provides new insights regarding the role of source and path heterogeneity on the spatial correlation of ground‐motion amplitudes. The results suggest that geological conditions, source effects, and path effects have significant impacts on spatial correlations. In addition, this work serves as a new dimension of ground‐motion simulation validation, because the estimated correlations can be compared to results from past earthquakes.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We present the results from an onshore seismic refraction and wide‐angle reflection profile, conducted in 2015, across the coastal plain and eastern Piedmont provinces of North Carolina. We use forward modeling to create 1D synthetic seismogram models and then invert first break picks to create 2D 〈span〉P〈/span〉‐ and 〈span〉S〈/span〉‐wave velocity models. The crustal thickness is 38 km beneath the Piedmont and central coastal plain, but it thins to 32 km at the coastline. The average thickness of the upper crust is 11 km with an average 〈span〉P〈/span〉‐wave velocity (VP) of 6.0  km/s and 〈span〉S〈/span〉‐wave velocity (VS) of 3.5  km/s. A prominent seismic low‐velocity zone (LVZ) (VP〈6.0 and VS〈3.6  km/s) exists between the depths of 6 and 11 km, beneath the western third of the seismic profile. The middle crust varies greatly in thickness, increasing from 3 km in the west (eastern Piedmont) to 13 km in the east (coastal plain), with seismic velocities of 6.5  km/s for VP and 3.8  km/s for VS. The lower crust thins significantly toward the rifted Atlantic margin, decreasing from 24 km thick in the west (Piedmont) to 8 km at the coastline, with velocities of approximately 6.9  km/s for VP and 3.9  km/s for VS. We estimate the composition of the crust by comparing the measured values of VP and Poisson’s ratio with laboratory measurements. The upper and middle crusts are in agreement with a felsic composition, while the lower crustal composition is predominately felsic to intermediate. The LVZ in the upper crust is associated with thin layers of the mylonitic rocks involved in the top and the bottom of thrusting, and the top of the lower crust could be the master detachment fault during the thin‐skinned Alleghanian orogeny. The eastward thinning of the lower crust is consistent with crustal extension during the Mesozoic rifting of the Atlantic margin.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Major structures within the mantle wedge are often revealed from seismic velocity anomalies, such as low‐velocity zones at magma reservoirs, partial melting regions, or the upwelling asthenosphere. However, no significant seismic boundaries have been reported in the shallow mantle wedge beneath volcanic arcs. Here, we present evidence for a strong seismic reflector dipping in the opposite direction of the subducting slab in the mantle wedge beneath northern Taiwan in the western end of the Ryukyu subduction system. We find that two unambiguous 〈span〉P〈/span〉 waves generated by a deep earthquake (ML 5.1) at a depth of 132.5 km were clearly recorded by the dense seismic array (Formosa Array), composed of 140 broadband seismic stations with a station spacing of approximately 5 km in northern Taiwan. Forward modeling using both raytracing and travel times shows that a seismic reflector exists beneath the Tatun volcano group (TVG) around depths of 80–110 km. The reflector dips in the opposite direction of the subducting slab and is unlikely to be associated with mantle wedge corner flow. Instead, it probably belonged to parts of possible structures such as the asthenospheric flow, the mantle diapir, or other undiscovered structures above the subducting slab. No matter what the seismic boundary is exactly, it might be associated with the active volcanism in the TVG. The detailed geometry and mechanism of the seismic boundary in the mantle wedge will be obtained as the Formosa Array collects more seismic data in the near future.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Magnitude‐rupture scaling relations describe how the length, width, and area of fault rupture vary with earthquake magnitude. These parameters are required in seismic hazard models to fit the models’ earthquakes onto faults and to define the site‐rupture distances needed in ground‐motion prediction equations. We collected the magnitude and rupture parameters of 91 earthquakes in Mainland China and nearby regions to study magnitude‐rupture scaling relations. We find no systematic deviations for the subsurface rupture length (RLD) obtained from different methods versus earthquake magnitude. We performed regressions of RLD versus magnitude and versus rupture width using general orthogonal regression. Then, we derived the relations between rupture area and magnitude. Our relations are not statistically different from the results derived by others using global datasets, if the parameters of the five pre‐1900 great earthquakes in eastern China are not used. However, if the five earthquakes are used, the magnitude‐rupture length scaling relation for large strike‐slip earthquakes in eastern China gives shorter rupture lengths than earthquakes in western China and other plate boundary regions in the world.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Juan Fernandez Islands (JFI) are located in the Pacific Ocean 675 km west of the Chilean coast. This archipelago has historically been affected by large tsunamis. Robinson Crusoe Island (RCI), the main island of the JFI, was first inhabited in 1749. Since then, several tsunamis have destroyed RCI port structures and sometimes caused deaths. Ground shaking perceived by the inhabitants has preceded some tsunami arrivals. Seismological instrumentation was temporarily deployed on RCI in 1999, and a permanent station has been operating since 2014. Here, we use these data to characterize the seismic waves that arrive at the JFI and to determine whether shaking perception could be used as a tsunami early warning system. We compute peak ground accelerations (PGAs) from 〈span〉P〈/span〉, 〈span〉S〈/span〉, and 〈span〉T〈/span〉 waves generated by Peruvian and Chilean earthquakes and find that the largest ground shakings are mostly related to 〈span〉T〈/span〉‐wave arrivals, which correlate with macroseismic modified Mercalli intensities lower than III. From the analysis of PGAs and macroseismic intensities, we conclude that shaking perception can be associated with large megathrust earthquakes, subduction events generated in the deep zone of seismogenic contact, and local seismicity. Unfortunately, potential tsunami earthquakes that occur on the Chilean coast will not be felt on RCI. Consequently, ground shaking in the JFI would not be a good proxy for tsunami warning, and a robust tsunami early warning system is necessary for RCI.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We present a joint inversion for empirical Green’s functions (EGFs) and high‐resolution non‐double‐couple (non‐DC) moment tensors. First, the EGFs are constructed using known moment tensors of earthquakes occurring in a small focal zone. Second, the estimated EGFs are applied to refine the original moment tensors used for constructing the EGFs. Because the EGFs describe the velocity model better than the standard GFs, the refined moment tensors are more accurate. The method is applied to real observations of earthquakes of the 2008 swarm in West Bohemia, Czech Republic, where tiny details in fracturing in the focal zone are revealed. Refined moment tensors indicate fault closing caused by compaction of fault gouge during fracturing process related to fault weakening by fluids in the focal zone. The application of the proposed inversion can improve moment tensors reported in existing local, regional, or global catalogs for areas with a concentrated seismicity.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Accelerograms are the primary source for characterizing strong ground motion. It is therefore of paramount interest to have high‐quality recordings free from any nonphysical contamination. Frequently, accelerograms are affected by baseline jumps and drifts, either related to the instrument and/or a major earthquake. In this work, I propose a correction method for these undesired baseline drifts based on segmented linear least squares. The algorithm operates on the integrated waveforms and combines all three instrument components to estimate a model that modifies the baseline to be at zero continuously. The procedure consists of two steps: first a suite of models with variable numbers of discontinuities is derived for all three instrument components. During this process, the number of discontinuities is reduced in a parsimonious way, for example, two very close discontinuities are merged into a single one. In the second step, the optimal model is selected on the basis of the Bayesian information criterion. I exemplify the application on synthetic waveforms with known discontinuities and on observed waveforms from a unified strong‐motion database of the Japan Meteorological Agency (JMA) and the National Research Institute for Earth Science and Disaster Prevention (NIED, Japan) networks for the major events of the 2016 Kumamoto earthquakes. After the baseline jump correction, the waveforms are furthermore corrected for displacement according to 〈a href="https://pubs.geoscienceworld.org/srl#rf21"〉Wang 〈span〉et al.〈/span〉 (2011)〈/a〉. The resulting displacements are comparable to the Interferometric Synthetic Aperture Radar‐derived displacement estimates for the Kumamoto earthquake sequence.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Communicating probabilities of natural hazards to varied audiences is a notoriously difficult task. Many of these challenges were encountered during the 2016 Bombay Beach, California, swarm of ~100 2≤M≤4.3 earthquakes, which began on 26 September 2016 and lasted for several days. The swarm’s proximity to the southern end of the San Andreas fault caused concern that a larger earthquake could be triggered. Within 1–2 days, different forecast models were used to evaluate the likelihood of a larger event with two agencies (the U.S. Geological Survey [USGS] and the California Governor’s Office of Emergency Services) releasing probabilities and forecasts for larger earthquakes. Our research explores communication and news media efforts, as well as how people on a microblogging social media site (Twitter) responded to these forecasts. Our findings suggest that news media used a combination of information sources, basing their articles on what they learned from social media, as well as using information provided by government agencies. As the swarm slowed down, there is evidence of the continued interplay between news media and social media, with the USGS issuing revised probability reports and scientists from the USGS and other institutions participating in media interviews. In reporting on the swarm, news media often used language more generally than the scientists; terms such as probability, likelihood, chance, and possibility were used interchangeably. Knowledge of how news media used scientific information from the 2016 Bombay Beach forecasts can assist local, state, and federal agencies in developing effective communication strategies to respond to future earthquakes.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The border between Georgia and South Carolina has a moderate amount of seismicity typical of the Piedmont Province of the eastern United States and greater than most other intraplate regions. Historical records suggest on average a Mw 4.5 earthquake every 50 yr in the region of the J. Strom Thurmond Reservoir, which is located on the border between Georgia and South Carolina. The Mw 4.1 earthquake on 15 February 2014 near Edgefield, South Carolina, was one of the largest events in this region recorded by nearby modern seismometers, providing an opportunity to study its source properties and aftershock productivity. Using the waveforms of the Mw 4.1 mainshock and the only cataloged Mw 3.0 aftershock as templates, we apply a matched‐filter technique to search for additional events between 8 and 22 February 2014. The resulting six new detections are further employed as new templates to scan for more events. Repeating the waveform‐matching method with new templates yields 13 additional events, for a total of 19 previously unidentified events with magnitude 0.06 and larger. The low number of events suggests that this sequence is deficient in aftershock production, as compared with expected aftershock productivities for other mainshocks of similar magnitudes. Hypocentral depths of the Mw 4.1 mainshock and Mw 3.0 aftershock are estimated by examining the differential time between a depth phase called 〈span〉sPL〈/span〉 and 〈span〉P〈/span〉‐wave arrivals, as well as by modeling the depth phase of body waves at shorter periods. The best‐fitting depths for both events are around 3–4 km. The obtained stress drops for the Mw 4.1 mainshock and Mw 3.0 aftershock are 3.75 and 4.44 MPa, respectively. The corresponding updated moment magnitude for the aftershock is 2.91.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Mackenzie Mountains EarthScope Project—a collaboration between Colorado State University, the University of Alaska, Michigan State University, and Yukon College—deployed a roughly linear, 40‐station broadband seismographic network. This network crossed the actively deforming Northern Canadian Cordillera and the Mackenzie Mountains in Yukon, California; it also extended into the Canadian Shield in Northwest Territories, California. The array was deployed between July 2016 and August 2018 (with four pilot stations installed in July 2015 and three extended stations operating through August 2019) coinciding with and complementing the deployment of the EarthScope Transportable Array to Alaska and western Canada. In this article, we present an overview of project scientific objectives, station configurations, and site conditions; discuss environmental challenges, including those that resulted in station downtime (e.g., spring flooding and encounters with bears); and suggest potential solutions to such subarctic challenges for the benefit of future deployments in comparable regions. We also include an initial characterization of seasonal and geographic variations in ambient seismic noise for the northwestern Canadian Cordillera.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉During the past 150 yr, the city of Almaty (formerly Verny) in Kazakhstan has suffered significant damage due to several large earthquakes. The 9 June 1887 Mw 7.3 Verny earthquake occurred at a time when the city mainly consisted of adobe buildings with a population of 30,000, with it being nearly totally destroyed with 300 deaths. The 3 January 1911 Mw 7.8 Kemin earthquake caused 390 deaths, with 44 in Verny itself. Remarkably, this earthquake, which occurred around 40 km from Verny, caused significant soil deformation and ground failure in the city. A crucial step toward preparing for future events, mitigating against earthquake risk, and defining optimal engineering designs, involves undertaking site response studies. With regard to this, we investigate the possibility that the extreme ground failure observed after the 1911 Kemin earthquake could have been enhanced by the presence of a shallow frozen ground layer that may have inhibited the drainage of pore pressure excess through the surface, therefore inducing liquefaction at depth. We make use of information collected regarding the soil conditions around the city at the time of the earthquakes, the results from seismic noise analysis, borehole data, and surface temperature data. From these datasets, we estimated the necessary parameters for evaluating the dynamic properties of the soil in this area. We successively characterize the corresponding sediment layers at the sites of the observed liquefaction. Although the estimated soil parameters are not optimally constrained, the dynamic analysis, carried out using selected strong‐motion recordings that are expected to be compatible with the two considered events, indicated that the extensive ground failure that occurred during the Kemin event could be due to the presence of a superficial frozen soil layer. Our results indicate that for this region, possible seasonal effects should, therefore, be considered when undertaking site effect studies.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Historical reports of earthquakes occurring before the twentieth century along the Dead Sea Transform (DST) are available for the past 3000 yr. Most of them are organized in various catalogs, reappraisals, and lists. Using a comprehensive and consistent compilation of these reports, the historical seismicity associated with the DST as a complete tectonic unit was examined. The compilation, supported by paleoseismic and archeoseismic evidence, resulted in 174 reliable historical earthquakes and 112 doubtful ones. The reliable earthquakes, along with 42 post‐nineteenth century instrumental earthquakes, are an up‐to‐date evaluation of the DST seismicity starting from the mid‐eighth century B.C.E. until 2015 C.E. Additionally, the scenario of historical earthquakes such as the 363 C.E. and 1033 C.E. events was resolved. The characterization of temporal and spatial patterns of DST seismicity, classifying them into four geographical zones, raised that most of the northern destructive earthquakes are clustered while clustering at the central and southern zones is less abundant.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Focal depths of earthquakes help reveal the seismogenic structure and crustal rheology, as well as the rupture nucleation process and coseismic rupture propagation process. In regions with dense seismic network, focal depth can be usually estimated precisely with travel time of 〈span〉Pg〈/span〉 and 〈span〉Sg〈/span〉, which is challenging in the sparse network. In this article, we model the recently proposed local depth phase 〈span〉sPL〈/span〉 to determine the focal depths of the aftershock sequence of the 2011 Virginia earthquake. The depth solutions of 10 〈strong〉M〈/strong〉 2.5+ events are consistent with results determined with dense temporary stations, with a maximum difference less than 1 km and average deviation less than 0.5 km. This study indicates that reliable focal depth can be obtained via modeling 〈span〉sPL〈/span〉 with waveform records from one or a few seismic stations, implying the applicability in sparse network.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉In the last decade, research on earthquake early warning systems (EEWSs) has undergone rapid development in terms of theoretical and methodological advances. These include advances in real‐time data analysis, improved telemetry, and computer technology; they are becoming useful tools for practical real‐time seismic hazard mitigation. The main focus of this project is to undertake a feasibility study of an EEWS for the New Madrid seismic zone (NMSZ) from the standpoint of source location. Magnitude determination is addressed in a separate paper. The NMSZ covers a wide area with several heavily populated cities, vital infrastructures, and facilities located within a radius of less than 70 km from the epicenters of the 1811–1812 earthquakes. One of the challenges associated with the NMSZ is that whereas low‐to‐moderate levels of seismic activity are common, larger earthquakes are rare (i.e., there are no instrumentally recorded data for earthquakes with magnitudes greater than M 5.5 in the NMSZ). We also recognize that it may not be realistic to provide early warnings for all possible sources, as is done on the west coast of the United States; as such, we focus on a specific source zone. We examine the stations within the NMSZ to answer the following question: “What changes should be applied to the NMSZ network to make it suitable for EEW?” We also explore needed changes to the Advanced National Seismic System (ANSS) earthquake monitoring system real time (AQMS RT) data acquisition system (DAS) to make it useful for EEW. Our results show that EEW is feasible, though several technical challenges remain in incorporating its use with the present network.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Recent work shows that machine learning (ML) can predict failure time and other aspects of laboratory earthquakes using the acoustic signal emanating from the fault zone. These approaches use supervised ML to construct a mapping between features of the acoustic signal and fault properties such as the instantaneous frictional state and time to failure. We build on this work by investigating the potential for unsupervised ML to identify patterns in the acoustic signal during the laboratory seismic cycle and precursors to labquakes. We use data from friction experiments showing repetitive stick‐slip failure (the lab equivalent of earthquakes) conducted at constant normal stress (2.0 MPa) and constant shearing velocity (10  μm/s). Acoustic emission signals are recorded continuously throughout the experiment at 4 MHz using broadband piezoceramic sensors. Statistical features of the acoustic signal are used with unsupervised ML clustering algorithms to identify patterns (clusters) within the data. We find consistent trends and systematic transitions in the ML clusters throughout the seismic cycle, including some evidence for precursors to labquakes. Further work is needed to connect the ML clustering patterns to physical mechanisms of failure and estimates of the time to failure.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉The April 2016 Pedernales earthquake ruptured a 100 km by 40 km segment of the subduction zone along the coast of Ecuador in an Mw 7.8 megathrust event east of the intersection of the Carnegie ridge with the trench. This portion of the subduction zone has ruptured on decadal time scales in similar size and larger earthquakes, and exhibits a range of slip behaviors, variations in segmentation, and degree of plate coupling along strike. Immediately after the earthquake, an international rapid response effort coordinated by the Instituto Geofísico at the Escuela Politécnica Nacional in Quito deployed 55 seismometers and 10 ocean‐bottom seismometers above the rupture zone and adjacent areas to record aftershocks. In this article, we describe the details of the U.S. portion of the rapid response and present an earthquake catalog from May 2016 to May 2017 produced using data recorded by these stations. Aftershocks focus in distinct clusters within and around the rupture area and match spatial patterns observed in long‐term seismicity. For the first two and a half months, aftershocks exhibit a relatively sharp cutoff to the north of the mainshock rupture. In early July, an earthquake swarm occurred ∼100  km to the northeast of the mainshock in the epicentral region of an Mw 7.8 earthquake in 1958. In December, an increase in seismicity occurred ∼70  km to the northeast of the mainshock in the epicentral region of the 1906 earthquake. Data from the Pedernales earthquake and aftershock sequence recorded by permanent seismic and geodetic networks in Ecuador and the dense aftershock deployment provide an opportunity to examine the persistence of asperities for large to great earthquakes over multiple seismic cycles, the role of asperities and slow slip in subduction‐zone megathrust rupture, and the relationship between locked and creeping parts of the subduction interface.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉To see any change in seismic velocities that may be associated with an abrupt change in the regional geology (granitic rock in contact with sediments), we conducted a refraction seismic study in the Peninsular Ranges of Baja California, which is in the Mexico–southwestern Laguna Salada (LS) region. We installed 30 three‐component portable seismic stations, supplemented with two permanent six‐component stations of the Northwest Mexico Seismic Network (RESNOM). The stations, spaced ∼6  km along a refraction profile, recorded two blasts; these were the direct shot located to the south of the city of Ensenada and the reverse shot in the southwestern LS (southwest–northeast direction). Record sections show seismograms with impulsive 〈span〉P〈/span〉 arrivals at nearby stations. Rays from the two blasts were modeled (using asymptotic ray theory) to obtain a 〈span〉P〈/span〉‐wave velocity model from 0 to ∼15  km depth along the refraction profile. Our modeling results are as follows: in the southwestern part of the profile (0–25 km distance), a low‐velocity zone of ∼2  km/s exists between the depths of 0 and 3.5 km; in Sierra Juárez, the mean 〈span〉P〈/span〉‐wave velocity is ∼5.6  km/s between the depths of 0 and 5 km; and in southwestern LS, a low‐velocity layer of ∼2.5  km/s exists between the depths of 0 and ∼3  km. We also modeled a layer of ∼6.5  km/s between 4 and 12 km in the Ensenada–Ojos Negros region, and between the depths of 4 and 8 km below the southwestern LS. From a profile distance of 0 to 50 km, a velocity zone of ∼6.7  km/s appears between the depths of 12 and 15 km.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We estimated the site amplification of regional high‐frequency 〈span〉Lg〈/span〉 seismic phases by a reverse two‐station (RTS) method using seismic events (Mw 4–6) recorded by Earthscope’s Transportable Array from 2010 to 2013. We compare regional site amplification estimates (horizontal and vertical) from the RTS technique with horizontal‐to‐vertical spectral ratio (HVSR) estimates derived from ambient noise and earthquake records. We compare the RTS results with (1) shallow shear‐wave velocity estimates from near‐surface (horizontal/vertical) ratios of the local body‐wave (initial 〈span〉P〈/span〉‐wave) method, and (2) high topography, basins, and sediment thicknesses. Our RTS results show a strong positive correlation between regional site amplification and basins such as the Michigan basin, the Illinois basin, and the Mississippi embayment. In the case of the Illinois and Michigan basins, the higher the frequency, the higher the horizontal and vertical amplification. Waves passing through the Appalachian and Ozark plateaus are deamplified on both vertical and horizontal ground components; however, the variation in amplification with frequency is larger for horizontal motion than vertical motion. In some regions, such as the western edge of the Appalachian basin and southern Illinois basin, vertical amplification decreases with frequency but horizontal amplification is essentially invariant with respect to frequency. Topography and sediment thickness are likely to affect amplification and both factors likely frequency dependent. There is a negative correlation between the RTS‐measured amplification and shallow shear‐wave velocity, whereas HVSR shows a negative correlation only for low frequencies 〈2.0  Hz. We conclude that regional ground‐motion amplification is clearly a function of more than one variable. In general, it appears that both regional topography (i.e., long‐wavelength topography) and deeper subsurface seismic structures (basins and sediments) have a large impact on site amplification.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Investigations of earthquake‐induced liquefaction features, including sand blows and their feeder dikes, in the New Madrid seismic zone have led to the discovery of a previously unrecognized earthquake that occurred in A.D. 0±200  yr. In addition, new findings support another New Madrid earthquake sequence in 1050±250  yr B.C. The studies, which consisted of geological and geophysical reconnaissance, archaeological surveys and excavations, as well as paleoseismic trenching and logging, focused on two sites located in northeastern Arkansas. Characteristics of the liquefaction features, their stratigraphic relations with cultural horizons and artifacts, and radiometric dating support the timing of the liquefaction events. With the addition of the A.D. 0 and 1050 B.C. events to the New Madrid earthquake chronology (A.D. 1811–1812, A.D. 1450, A.D. 900, A.D. 0, 1050 B.C., and 2350 B.C.), a recurrence time of approximately 1100 yrs is estimated for the period between 2350 B.C. and A.D. 900, whereas a previously established recurrence time of ∼500  yrs remains unchanged for the period between A.D. 900 and 1811. The findings of sand blows indicative of additional New Madrid earthquakes between 2350 B.C. and A.D. 900 suggest that the New Madrid earthquake chronology may still be incomplete, and if so, an 1100‐yr recurrence time may be an overestimate for this time period.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉We present a new near‐real‐time method for converting earthquake source parameters into ground motion (GM) at locations across the west coast of the United States. This method, called earthquake information to ground motion (eqInfo2GM), has been implemented as part of the ShakeAlert earthquake early warning (EEW) system and makes estimated GM accessible to users on the EEW timeframe of seconds, as the U.S. Geological Survey ShakeMap does at higher resolution and accuracy in the minutes following a seismic event. Whereas the higher fidelity ShakeMap comes at the cost of longer processing times, eqInfo2GM effectively provides a predicted ShakeMap before shaking arrives at many locations. We describe key design details, including ground‐motion prediction equations (GMPEs) implemented, modifications made to optimize for speed, and formats created for conveying GM severity. The GM output format determines added latency and reflects a trade‐off between speed and accuracy; for our test earthquake data set, added latency is in the 0.01–1.5 s range after earthquake source parameters have been generated. GMPE implementations are validated against predicted ShakeMaps (without observations), with almost all events showing minimal mean shaking intensity level differences, reflecting variations only in treatment of source distances and VS30 data. Comparison against ShakeMaps computed with observations (a proxy for true GM) show larger differences, demonstrating the challenges of working in the EEW timeframe, when full source characterization and peak ground motion observations are both unavailable. Although specific configurations and features of the method will evolve as the needs of the EEW user community become evident, eqInfo2GM is expected to improve the overall utility of EEW alerts by providing end users with estimates of predicted local GM hazard. Such near‐real‐time estimates will enable users to decide more accurately what action to take to reduce the impact of imminent, potentially damaging shaking.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉An important predictive variable for site amplification is the site dominant frequency (fd). At seismic monitoring stations, fd can be calculated from the peak of the horizontal‐to‐vertical spectral ratios (HVSRs) obtained from earthquake recordings (eHVSR). For other sites, fd can be estimated from microseismic (mHVSR) observations. We compare the fd values derived from eHVSR (5% damped response spectra from the Next Generation Attenuation‐West2 [NGA‐West2] database; 〈a href="https://pubs.geoscienceworld.org/bssa#rf2"〉Ancheta 〈span〉et al.〈/span〉, 2014〈/a〉) with those derived from mHVSR (Fourier spectra from 〈a href="https://pubs.geoscienceworld.org/bssa#rf50"〉Yong 〈span〉et al.〈/span〉, 2013〈/a〉) for seismic stations in California. We show that the logarithm of eHVSR fd scales linearly with the logarithm of mHVSR fd, with a standard deviation of 0.14log10 units for mHVSR fd larger than 0.2 Hz. The relationship holds for microseismic surveys at distances up to 300 m away from the seismic stations. The results of this study have beneficial implications for the characterization of site response in modern ground‐motion models as well as in building codes.〈/span〉