JournalTOCs

Bulletin of the Seismological Society of America
Journal Prestige (SJR): 1.525

Citation Impact (citeScore): 2
Number of Followers: 26

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ISSN (Print) 0037-1106 - ISSN (Online) 1943-3573
Published by GeoScienceWorld

[17 journals]

Seismotectonic Analysis of the 7 October 2021 M w 5.9 Harnai
Earthquake, Pakistan
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  Abstract: ABSTRACTAn earthquake of moderate magnitude 5.9 occurred northeast of Harnai (Baluchistan), Pakistan, on 07 October 2021. This event caused several deaths and injuries due to the collapse of many mud houses in the epicentral area according to the Provincial Disaster Management Authority (PDMA). The event occurred along the Quetta syntaxis, which is one of the most seismically active zones of the country. The source mechanism of the mainshock based on the moment tensor inversion technique was determined using seismic data from the local network. Stress patterns and active tectonics within the Harnai area were investigated on the basis of recent event and previously available focal mechanism solutions. The 2021 earthquake source mechanism was oblique slip with 12 km focal depth and appears to be associated with the Harnai–Karahi fault zone. Interestingly, the nodal plane oriented in the east–west direction having a high dip corresponds to reverse faulting, whereas the low dip nodal plane trending northwest–southeast‐depicts a strike‐slip mechanism. The upper crustal stress field and current tectonic deformation in this region are governed by the oblique convergence between the Indian and Eurasian plates along the Chaman fault zone marking the transform plate boundary. The local tectonic stress pattern anomalously supports the low dip nodal plane of the determined fault plane solution representing transpressional environment in which originally produced reverse fault is being reactivated with a dominantly strike‐slip sense of movement. In this area, two stress regimes, strike‐slip and thrust mechanism, were identified based on homogenous stress patterns. The recent Harnai earthquake occurred in a region where thrust‐faulting environment due to horizontal principal compression (SHmax) directed in north‐northwest–south‐southeast act as first order stress owing to the collision of the Indian and Eurasian plates. The SHmax direction of other subset areas has a similar orientation but different stress ratio (R′), which depicts strike‐slip faulting. The presence of a rigid (Katawaz) block along the plate boundary, which impedes the Indian plate motion, causes the second‐order stresses that result in strike‐slip sense of movement in the pre‐existing reverse faults. The same stress seems to be responsible for bending of the existing faults to produce en echelon structures within the Sulaiman range and generates connecting faults in the form of a bookshelf model.
  PubDate: Fri, 27 Jan 2023 00:00:00 GMT
Improving Out of Network Earthquake Locations Using Prior Seismicity for
Use in Earthquake Early Warning
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  Abstract: ABSTRACTTimely alerts sent through earthquake early warning (EEW) programs allow those alerted to take protective actions to mitigate their risk from potentially damaging shaking. Over the past few years, ShakeAlert, the EEW program focused on the west coast of the contiguous United States, has grown, alerting communities within California, Oregon, and Washington about earthquakes where damaging shaking is expected. ShakeAlert uses a set of algorithms including the point‐source algorithm, earthquake point‐source integrated code (EPIC), to determine the location, magnitude, and origin time of potential earthquakes. Although EPIC produces low‐latency and low error solutions for many events originating within the seismic network on land, numerous recent small earthquakes rupturing offshore of northern California have EPIC location solutions with high error (>50 km compared to USGS locations). Because most events are occurring offshore, there is a limited number of stations that can trigger and contribute information in a timely manner for use in EEW. To better constrain location solutions in this region, we propose to include information about contemporary past seismicity into EPIC’s grid‐search algorithm through a Bayesian framework. This prior information layer downweights high error locations where EPIC’s proposed event location coincides with an area of low prior seismicity in preference for locations with a similar level of data fit that also have higher past seismicity. This addition to EPIC lowers the mean location error offshore northern California from 58 to 14 km.
  PubDate: Fri, 27 Jan 2023 00:00:00 GMT
A Bayesian Lasso Logistic Regression Model for Predicting the Probability
of Regional Seismic Phase Observation Using Sn in the Middle East and East
Asia as Examples
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  Abstract: ABSTRACTHigh‐frequency seismic wave blockage is often the result of strong attenuation, and the regional phase Sn is particularly prone to blockage in comparison with any of the other regional phases including Lg. As widespread blockage can lead to difficulty in the estimation of source parameters or path attenuation, accurate characterization of efficient regional wave propagation is necessary. We have applied two approaches to map Sn phase blockage: (1) the relatively standardized efficiency tomography and (2) a newly developed Bayesian logistic regression model that is able to predict the likelihood (probability) of phase blockage. As a byproduct of our Bayesian approach, we obtain measures of uncertainty for the probability of blockage. We applied both our methods on simulated efficiency data as well as real efficiency data obtained from earthquakes and stations from the middle east and eastern Asia. Our models successfully predict the probability of blockage zones with relatively high accuracy (>75%). In addition, we observe both low probability of Sn blockage and efficient Sn propagation in tectonically stable continental lithosphere, such as the Arabian plate, the Mediterranean Sea, northeastern Iran, the Ordos plateau, and the Sichuan basin. Regions with a high probability of Sn blockage or inefficient Sn propagation zones are in the tectonically active areas, such as the Tibetan and Iranian plateaus. Our probability of blockage model can also be used to image the regions where SnQ models are likely to be biased due to blocked data.
  PubDate: Wed, 25 Jan 2023 00:00:00 GMT
Dynamic Characteristics of TAIPEI 101 Skyscraper from Rotational and
Translation Seismometers
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  Abstract: ABSTRACTContinuous data streams from translational and rotational seismometers installed in TAIPEI 101 enable monitoring of the natural frequencies on different time scales. Using the 2014 seismic data recorded on the 90th floor of this high‐rise building and the meteorological data from a weather station located just 1 km away, we explored the characteristics and controlling factors of the ambient vibrations in TAIPEI 101. Using the random decrement technique, the three modal frequencies in translation were identified as 0.15 Hz (F1), 0.43 Hz (F2), and 0.78 Hz (F3). For rotation around the vertical axis, the modal frequencies were 0.23 Hz (R1), 0.59 Hz (R2), and 0.93 Hz (R3). In translation, TAIPEI 101 exhibits a trend of increasing modal frequency with increasing temperature but decreasing modal frequency with increasing wind velocity. The trend with temperature is reversed in rotation motion. The different frequency versus temperature relationship seen for rotational and translational motion demonstrates the importance of introducing rotational motion analysis into structural health monitoring. The change in modal frequencies were also found to decrease with growing amplitude. It is intriguing that F1 exhibits a weaker dependency with amplitude with respect to the higher modes, which may be associated with the suppression of F1 vibration caused by the damper installed in TAIPEI 101. Other than long‐term (seasonable) variation, we also highlight the hourly variation of the first‐mode amplitude throughout a day by comparing with weather and mobility data. Other than the atmospheric conditions that strongly influence the modal frequencies in long‐term behavior, we found that human activities may play an important role in the short‐term vibration characteristics of the building.
  PubDate: Fri, 20 Jan 2023 00:00:00 GMT
Performance and Next‐Generation Development of the Finite‐Fault
Rupture Detector (FinDer) within the United States West Coast ShakeAlert
Warning System
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  Abstract: ABSTRACTRapid fault rupture information is important to estimate seismic ground motions and damage in large earthquakes, and is, therefore, of great value for earthquake early warning (EEW) and rapid response. The Finite‐Fault Rupture Detector (FinDer) algorithm computes earthquake line‐source models by comparing spatial distributions of high‐frequency seismic amplitudes with precomputed template maps. FinDer is one of two seismic EEW algorithms currently adopted by the United States West Coast ShakeAlert EEW system. Between March 2018 and October 2022, FinDer detected 1048 earthquakes (2.3 ≤ M ≤ 7.1) inside the FinDer‐reporting region in California, Oregon, and Washington with a median detection time of 8.5 s (75th and 95th percentile: 11.5 s, 38.9 s) after event origin and median errors (first report) of 6.7 km (75th and 95th percentile: 10.5 km, 25.5 km) in location, −0.45 s (mean ± st. dev.: 0.1 ± 5.9 s) in origin time, and 0.33 units (mean ± st. dev.: 0.33 ± 0.31 m.u.) in magnitude. Ground motions estimated using FinDer source parameters are in excellent agreement with observed peak ground accelerations, and residuals are, on average, 30% smaller than if predicted from catalog source parameters. This suggests that FinDer’s simple source parameter terms are accounting for more complex high‐frequency source characteristics. This article summarizes the performance of FinDer in ShakeAlert and describes the recent improvements to the algorithm addressing issues encountered during real‐time operation. This includes the handling of latent seismic data, robust event detection in regions with sparse instrumentation, enabling faster magnitude convergence in large earthquakes, use of fault‐ and scenario‐specific earthquakes (e.g., along the Cascadia subduction zone or San Andreas fault), as well as increased robustness of FinDer in complex earthquake sequences. We demonstrate the performance of the new FinDer version 3 algorithm using waveform playbacks of selected events along the U.S. West Coast, Japan, and China, including both historic and synthetic earthquakes.
  PubDate: Wed, 18 Jan 2023 00:00:00 GMT
Earthquake Phase Association with Graph Neural Networks
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  Abstract: ABSTRACTSeismic phase association connects earthquake arrival‐time measurements to their causative sources. Effective association must determine the number of discrete events, their location, and origin times, and it must differentiate real arrivals from measurement artifacts. The advent of deep‐learning (DL) pickers, which provide high rates of picks from closely overlapping small‐magnitude earthquakes, motivates revisiting the phase association problem and approaching it using the methods of DL. We have developed a graph neural network associator that simultaneously predicts both source space–time localization, and discrete source‐arrival association likelihoods. The method is applicable to arbitrary geometry, time‐varying seismic networks of hundreds of stations, and is robust to high rates of sources and input picks with variable noise and quality. Our Graph Earthquake Neural Interpretation Engine (GENIE) uses one graph to represent the station set and another to represent the spatial source region. GENIE learns relationships from data in this combined representation that enable it to determine robust source and source‐arrival associations. We train on synthetic data, and test our method on real data from the northern California seismic network using input generated by the PhaseNet DL phase picker. We successfully re‐detect ∼96% of all events M >1 reported by the U.S. Geological Survey (USGS) during 500 random days between 2000 and 2022. Over a 100‐day continuous interval of processing in 2017–2018, we detect ∼4× the number of events reported by the USGS. Our new events have small‐magnitude estimates below the magnitude of completeness of the USGS catalog, and are located close to the active faults and quarries in the region. Our results demonstrate that GENIE can effectively solve the association problem under complex seismic monitoring conditions.
  PubDate: Fri, 13 Jan 2023 00:00:00 GMT
Rupture Process of the 2017 M w 5.5 Pohang, South Korea, Earthquake via
an Empirical Green’s Function Method
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  Abstract: ABSTRACTThe Mw 5.5 earthquake occurred in Pohang, South Korea on 15 November 2017, which is known as a “runaway earthquake,” and was triggered from a critical‐state fault as a result of fluid injection. As such earthquakes rarely occur, spatiotemporal slip distributions were investigated via the finite‐fault inversion based on the empirical Green’s function in this study. The rupture process can be divided into three steps: first, slip initiated and propagated only to the southwest from the hypocenter during the initial 0.6 s; in the second step from 0.6 to 2.4 s, the slip occurred to the southwest and northeast parts, in which the maximum seismic moment was released; in the third step from 2.4 to 6.0 s, slip occurred around the edge of the fault plane farther from the hypocenter, particularly in the deep part in the northeast direction. In each step, the seismic moment was released as approximately 6%, 59%, and 35%, respectively. The first step can be interpreted not as a part of the rupture process of the mainshock but as the immediate and distinct foreshock. Overall, most of the slip distributed southwest is consistent with the results of the directivity analysis using apparent source time functions. Although the average stress drop (~1 MPa) of the Pohang earthquake is considerably lower than that (~20 MPa) of the Mw 5.5 Gyeongju earthquake that naturally occurred in the vicinity of the Pohang, it is difficult to attribute it only to the fluid injection effect. Through this study, we improve our comprehension of the seismic source physics and mechanisms of the Pohang earthquake by analyzing the spatiotemporal slip history, the directivity of rupture process, and the spatial distribution of the stress drop on the fault plane.
  PubDate: Wed, 11 Jan 2023 00:00:00 GMT
Identification Protocols for Horizontal‐to‐Vertical Spectral
Ratio Peaks
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  Abstract: ABSTRACTPeaks in horizontal‐to‐vertical spectral ratios (HVSRs) of Fourier amplitudes from three‐component recordings of ground vibrations without undue local anthropogenic influence are used to identify site resonances, which are an important component of site response. We address two topics: (1) how should HVSR peaks be identified and (2) are there appreciable differences in HVSR derived using different instruments recording microtremors and seismic strong ground motions' We propose identifying peaks by considering peak amplitudes relative to neighboring ordinates and peak width. The procedure incorporates a regression tree algorithm that can be tuned to conform with user preferences toward relatively “conservative” or “liberal” peak identification (producing few or many sites with peaks, respectively). We then investigate the consistency of microtremor‐based HVSRs (mHVSRs) derived from seismometers and accelerometers, which show a high rate of false negatives (missed peaks) from accelerometers with a full scale of ± 2g or greater. In contrast, mHVSRs derived from collocated temporary and permanent seismometers (optimized to record teleseismic signals) have about 60%–80% consistency (with no apparent bias in peak attributes). This indicates that mHVSRs from accelerometers having a broad full scale are unreliable but that mHVSRs can be reliably obtained from temporary or permanent seismometers. Finally, we compare seismometer‐based HVSR from microtremor and earthquake (eHVSRs) sources. Results are consistent for 60%–70% of sites (i.e., both either do or do not have significant peaks, and when peaks are present, they occur at similar frequencies, <20% change). For sites with an mHVSR peak, the rate of corresponding eHVSR peaks is nearly 50%, whereas for sites without an mHVSR peak the eHVSR peak rate is low (about 20%). The mismatch rate for mHVSR peak sites is sufficiently high that the use of eHVSR to derive site response models is likely too optimistic (overestimates model effectiveness); mHVSR is preferred for consistency with information available in forward applications.
  PubDate: Tue, 10 Jan 2023 00:00:00 GMT
Incorporating Full Elastodynamic Effects and Dipping Fault Geometries in
Community Code Verification Exercises for Simulations of Earthquake
Sequences and Aseismic Slip (SEAS)
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  Abstract: ABSTRACTNumerical modeling of earthquake dynamics and derived insight for seismic hazard relies on credible, reproducible model results. The sequences of earthquakes and aseismic slip (SEAS) initiative has set out to facilitate community code comparisons, and verify and advance the next generation of physics‐based earthquake models that reproduce all phases of the seismic cycle. With the goal of advancing SEAS models to robustly incorporate physical and geometrical complexities, here we present code comparison results from two new benchmark problems: BP1‐FD considers full elastodynamic effects, and BP3‐QD considers dipping fault geometries. Seven and eight modeling groups participated in BP1‐FD and BP3‐QD, respectively, allowing us to explore these physical ingredients across multiple codes and better understand associated numerical considerations. With new comparison metrics, we find that numerical resolution and computational domain size are critical parameters to obtain matching results. Codes for BP1‐FD implement different criteria for switching between quasi‐static and dynamic solvers, which require tuning to obtain matching results. In BP3‐QD, proper remote boundary conditions consistent with specified rigid body translation are required to obtain matching surface displacements. With these numerical and mathematical issues resolved, we obtain excellent quantitative agreements among codes in earthquake interevent times, event moments, and coseismic slip, with reasonable agreements made in peak slip rates and rupture arrival time. We find that including full inertial effects generates events with larger slip rates and rupture speeds compared to the quasi‐dynamic counterpart. For BP3‐QD, both dip angle and sense of motion (thrust versus normal faulting) alter ground motion on the hanging and foot walls, and influence event patterns, with some sequences exhibiting similar‐size characteristic earthquakes, and others exhibiting different‐size events. These findings underscore the importance of considering full elastodynamics and nonvertical dip angles in SEAS models, as both influence short‐ and long‐term earthquake behavior and are relevant to seismic hazard.
  PubDate: Tue, 10 Jan 2023 00:00:00 GMT
Modulation of Seismic Radiation by Fault‐Scale Geology of the 2016 M w
6.0 Shallow Petermann Ranges Earthquake (PRE) in Central Australia
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  Abstract: ABSTRACTUnderstanding the modulatory influence of fault‐scale geology on seismic behavior of earthquake faults is central to determining the physics of faulting and seismic hazard analysis. Although laboratory experiments predict that seismic parameters can be modulated by fault‐scale geology, there is scant empirical evidence of this process at field scale due largely to a lack of shallow earthquakes of which causative faults can be mapped to known bedrock structure. The 20 May 2016 Mw 6 Petermann Ranges earthquake (PRE) is the best‐recorded continental event in Australia to date, and it is an excellent candidate to investigate the possible link between seismic parameters and fault‐scale geology as its causative fault has previously been linked to known bedrock structure using distributions of aftershocks, surface observations, and geophysical mapping. In this study, we analyze strain energy partitioning of PRE by determining seismic radiation efficiency (0.31) and apparent stress (0.34 MPa) together with previously estimated stress drop (2.2 MPa) and find that the combination of these macroseismic parameters deviates from that expected of a shallow immature fault in intraplate continental regions typically characterized by large recurrence intervals. It instead appears to have mimicked a mature fault, which we attribute to the characteristics of the causative fault confined to mechanically weaker, phyllosilicate‐rich foliations of the bedrock that have anomalously lower fracture energy. Therefore, PRE rupture suggests the presence of a spectrum of shallow (<20 km) fault slip behavior modulated by fault‐scale geology.
  PubDate: Tue, 10 Jan 2023 00:00:00 GMT
Active Faults Revealed and New Constraints on Their Seismogenic Depth from
a High‐Resolution Regional Focal Mechanism Catalog in Myanmar
(2016–2021)
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  Abstract: ABSTRACTWe derive a new earthquake focal mechanism catalog for 86 Mw>4.0 earthquakes that occurred in the Myanmar region from 2016 to 2021. We apply the generalized Cut‐and‐Paste inversion method to a new set of regional broadband waveform data to obtain the earthquake focal mechanism and centroid depth with uncertainties estimated in a bootstrapping manner. Compared with global earthquake catalogs, our results are better aligned with mapped, active faults and reveal seismic activity along unmapped, blind faults. Our new catalog shows that the Sagaing Fault is more active in its northern segment with deeper seismogenic zone (∼27 km) compared to its southern segment that has a shallower seismogenic zone (∼10 km), sandwiching a seismic gap in its central segment. Earthquakes that occurred on the unmapped, blind faults beneath the Central Myanmar Basin at shallow depths (3–12 km) suggest a dominating northeast–southwest compressional stress field. Shallow earthquakes beneath the Indo‐Myanmar Range (IMR) are rare, instead, north–south‐oriented strike‐slip faults are active within the deep accretionary wedge or lower crust of the Myanmar plate between depths of 20 and 40 km. At the eastern edge of the IMR, earthquakes with high‐angle thrust mechanisms occurred between depths of 30 and 48 km, likely along steep faults separating the accretionary wedge from the Myanmar forearc crust. High‐resolution intraslab focal mechanisms show that to the north of 22° N, slab deformation is dominated by strike‐slip earthquakes with subvertical fault planes down to a depth of ∼25 km beneath the slab, suggesting lateral shear within the slab due to the northward motion of the Indian plate. To the south, more normal‐faulting earthquakes suggest a stronger role of plate‐bending processes in the slab deformation.
  PubDate: Wed, 04 Jan 2023 00:00:00 GMT
Characterizing Multisubevent Earthquakes Using the Brune Source Model
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  Abstract: ABSTRACTAlthough the Brune source model describes earthquake moment release as a single pulse, it is widely used in studies of complex earthquakes with multiple episodes of high moment release (i.e., multiple subevents). In this study, we investigate how corner frequency estimates of earthquakes with multiple subevents are biased if they are based on the Brune source model. By assuming complex sources as a sum of multiple Brune sources, we analyze 1640 source time functions of Mw 5.5–8.0 earthquakes in the seismic source characteristic retrieved from deconvolving teleseismic body waves catalog to estimate the corner frequencies, onset times, and seismic moments of subevents. We identify more subevents for strike‐slip earthquakes than dip‐slip earthquakes, and the number of resolvable subevents increases with magnitude. We find that earthquake corner frequency correlates best with the corner frequency of the subevent with the highest moment release (i.e., the largest subsevent). This suggests that, when the Brune model is used, the estimated corner frequency and, therefore, the stress drop of a complex earthquake is determined primarily by the largest subevent rather than the total rupture area. Our results imply that, in addition to the simplified assumption of a radial rupture area with a constant rupture velocity, the stress variation of asperities, rather than the average stress change of the whole fault, contributes to the large variance of stress‐drop estimates.
  PubDate: Wed, 04 Jan 2023 00:00:00 GMT
Interseismic Strain Accumulation between the Colorado Plateau and the
Eastern California Shear Zone: Implications for the Seismic Hazard near
Las Vegas, Nevada
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  Abstract: ABSTRACTCrustal deformation in the central Basin and Range between the Colorado plateau and the Eastern California Shear Zone is active but slow, making it a challenge to assess how strain is distributed and crustal motion transferred. However, knowledge of strain rates is very important, particularly for addressing the seismic hazard for both the Las Vegas urban area and the site of the proposed Yucca Mountain nuclear waste repository, in southern Nevada. Global Positioning System (GPS) data provide important constraints, particularly now that the GPS network in the area has substantially expanded in recent years. However, because deformation is slow, it is important to mitigate any transient tectonic and nontectonic signals to obtain the most accurate long‐term interseismic motion and robust estimation of strain rates. We use data from all GPS stations in the region including both long‐running continuous and semicontinuous stations. We model and remove postseismic displacements at these stations using source parameters for 41 events, dating back to the 1700 Cascadia megathrust earthquake, which contribute significantly to the deformation field within the central Basin and Range. We also remove correlated noise from the time series with the common‐mode component imaging technique. We find that removal of both the postseismic transients and common‐mode noise substantially reduces the uncertainties and spatial variation in the velocities. We find east–west extension across the Las Vegas Valley of 0.5–0.6 mm/yr. The interseismic strain rate field, calculated with the final velocities, reveals higher strain rates through southern Nevada than in previous studies, with rates within Las Vegas Valley of 8.5±2.4×10−9 yr−1. Our results also confirm shear along the Pahranagat shear zone, but the estimated amplitude is strongly affected by postseismic relaxation.
  PubDate: Tue, 03 Jan 2023 00:00:00 GMT
Channel Incision Ages to the Rescue: An Improved Age for the Penultimate
Earthquake That Ruptured the Carrizo Section of the South‐Central San
Andreas Fault
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  Abstract: ABSTRACTA primary step toward assessing the time and size of future earthquakes is identifying earthquake recurrence patterns in the seismic record. The San Andreas fault (SAF) is one of the most studied active faults in the world. However, there is no unequivocal interpretation of paleoseismic data to determine the timing and rupture extent of the earthquakes that occurred prior to the historical 1857 and 1906 ruptures. The penultimate earthquake is the least well‐dated earthquake along the Cholame, Carrizo, and Big Bend sections of the SAF. The main reason for this poor determination is because the past few hundred years have seen large natural fluctuations in atmospheric C14 concentration. These fluctuations mean that a single radiocarbon date may yield a calibrated age consisting of several possible age ranges. At sites along frequently rupturing faults with historical ruptures, such as the SAF and the North Anatolian fault in Turkey, determining the incision age of channels displaced only by the most recent earthquake can place a tighter minimum limit on the possible age range of the penultimate earthquake. In our study, we dated five sandy fill units with the post‐infrared infrared‐stimulated luminescence method on single feldspar grains of channel Sieh 31 in the Carrizo Plain. The data indicate the channel Sieh 31, offset ∼6 m during the 1857 earthquake, incised before ∼1740 ± 30 C.E. (1σ). This new result trims the age constraint of the penultimate earthquake that ruptured the Carrizo section of the SAF determined at the nearby Bidart Fan site from 1640–1857 to 1631–1745 C.E., tightening the age constraint by nearly 80 yr. The revised mean recurrence interval for surface rupturing earthquakes along the Carrizo section of the south‐central SAF is 117 yr (95% confidence interval 62–255 yr). This approach can improve paleoearthquake age and slip‐per‐earthquake constraints.
  PubDate: Tue, 03 Jan 2023 00:00:00 GMT
Resonance versus Shape of Sedimentary Basins
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  Abstract: ABSTRACTSediment‐filled basins are of interest both in exploration geophysics and in earthquake seismology in assessing their potential to amplify ground shaking. By means of numerical models, we present a number of dimensionless ratios between the main resonance frequencies that can be measured on the top of the basins and their geometry and mechanical properties, under the hypothesis of rigid bedrock. These resonance frequency ratios depend largely on the aspect ratio of the basins, whereas the mechanical properties of the sediments (specifically, the Poisson’s coefficient) do not substantially affect the horizontal resonance frequency ratios. We discuss whether the modal frequencies measured on real basins can be used to extract information about the basins themselves and whether the theoretical expected modal sequence can be used to design experimental surveys aimed at assessing the dynamic behavior of basins.
  PubDate: Tue, 27 Dec 2022 00:00:00 GMT
A More Realistic Earthquake Probability Model Using Long‐Term Fault
Memory
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  Abstract: ABSTRACTForecasts of the probability of a large earthquake occurring on a fault during a specific time interval assume that a probability distribution describes the interevent times between large earthquakes. However, current models have features that we consider unrealistic. In these models, earthquake probabilities remain constant or even decrease after the expected mean recurrence interval, implying that additional accumulated strain does not make an earthquake more likely. Moreover, these models assume that large earthquakes release all accumulated strain, despite evidence for partial strain release in earthquake histories showing clusters and gaps. As an alternative, we derive the necessary equations to calculate earthquake probabilities using the long‐term fault memory (LTFM) model. By accounting for partial strain release, LTFM incorporates the specific timing of past earthquakes, which commonly used probability models cannot do, so it can forecast gaps and clusters. We apply LTFM to the southern San Andreas fault as an example and show how LTFM can produce better forecasts when clusters and gaps are present. LTFM better forecasts the exceptionally short interevent time before the 1857 Fort Tejon earthquake. Although LTFM is more complex than existing models, it is more powerful because (unlike current models) it incorporates fundamental aspects of the strain accumulation and release processes causing earthquakes.
  PubDate: Tue, 27 Dec 2022 00:00:00 GMT
H/V Analysis in Juchitán de Zaragoza, Oaxaca, Following the 2017 M 8.2
Tehuantepec, México, Earthquake
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  Abstract: ABSTRACTIn September 2017, over 450 lives were lost in Mexico as a result of two unusual, large‐magnitude, normal earthquakes. On 7 September, an M 8.2 earthquake occurred offshore of the State of Oaxaca in the Gulf of Tehuantepec, one of the largest extensional earthquakes to have occurred in a subduction zone. Twelve days later on 19 September an M 7.1 damaging earthquake struck near Puebla and Morelos, over 600 km away. Both earthquakes occurred in the downgoing Cocos plate, which is subducting beneath the North American plate. The first large event was followed on 23 September by a shallow M 6.1 extensional earthquake near Juchitán de Zaragoza, Oaxaca. Researchers from Mexico and the United States collaborated to deploy a temporary seismic network to study the aftershocks of the M 8.2 Tehuantepec, Mexico, earthquake, which included a three‐week deployment of 51 Magseis Fairfield Z‐Land 5‐Hz three‐component nodal seismometers (“nodes”) near Juchitán and a 6‐month deployment of 10 Nanometrics Trillium 120PA broadband seismometers with Reftek RT130 dataloggers for 6 months. In this article, we analyze the capabilities of the nodes to calculate the horizontal/vertical (H/V) spectral ratio and relative amplification using both microtremors and earthquakes and validate the results calculated with the nodes using data from broadband stations from this and previous deployments in the area. We create maps showing a correlation of the distribution of the fundamental frequency and relative amplification of the soil and compare them with the geology and the damage caused by the September 2017 earthquakes. There is a lack of public awareness and discrepancies in the construction procedures in the region, and we find that the majority of damaged houses in the area of study followed the location of river beds and tended to be in places with low resonance frequencies despite being in a low amplification zone.
  PubDate: Fri, 23 Dec 2022 00:00:00 GMT
Comparative Study of the Performance of Seismic Waveform Denoising Methods
Using Local and Near‐Regional Data
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  Abstract: ABSTRACTSeismic waveform data are generally contaminated by noise from various sources, which interfere with the signals of interest. In this study, we implemented and applied several noise suppression methods using data recorded by the regional network of the University of Utah Seismograph stations. The denoising methods, consisting of approaches based on nonlinear thresholding of continuous wavelet transforms (CWTs, e.g., Langston and Mousavi, 2019), convolutional neural network (CNN) denoising (Tibi et al., 2021), and frequency filtering, were all subjected to the same analyses and level of scrutiny. We found that for frequency filtering, the improvement in signal‐to‐noise ratio (SNR) decreases quickly with decreasing SNR of the input waveform, and that below an input SNR of about 32 dB the improvement is relatively marginal and nearly constant. In contrast, the SNR gains are low at high‐input SNR and increase with decreasing input SNR to reach the top of the plateaus corresponding to gains of about 18 and 23 dB, respectively, for CWT and CNN denoising. The low gains at high‐input SNRs for these methods can be explained by the fact that for an input waveform with already high SNR (low noise), only very little improvement can be achieved by denoising, if at all. Results involving 4780 constructed waveforms suggest that in terms of degree of fidelity for the denoised waveforms with respect to the ground truth seismograms, CNN denoising outperforms both CWT denoising and frequency filtering. Onset time picking analyses by an experienced expert analyst suggest that CNN denoising allows more picks to be made compared with frequency filtering or CWT denoising and is on par with the expert analyst’s processing that follows current operational procedure. The CWT techniques are more likely to introduce artifacts that made the waveforms unusable.
  PubDate: Fri, 23 Dec 2022 00:00:00 GMT
Deriving Site Effect‐Free Hard‐Rock Time Histories in Japan from the
Generalized Inversion Technique
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  Abstract: ABSTRACTA key component in seismic hazard assessment is the determination of time histories for hard‐rock site conditions, either as input motion for site response computations or for applications to installations built on this site type. The state of the practice is to apply physics‐based corrections for removing site effects from surface recordings to obtain the underlying bedrock motion. Here, we propose and test the use of the generalized inversion technique (GIT) for deconvolving surface recordings to hard‐rock time series at the amplification‐free seismic bedrock. As part of the proposed procedure, an event‐specific phase scaling method is presented, which allows changes in signal duration to be considered. For validation purposes, we select a total of 90 Kiban–Kyoshin network (KiK‐net) surface‐downhole sites having no significant velocity contrasts below their downhole sensors and with the latter being located at sufficient depth so that they are not impaired significantly by downgoing waves. We evaluate the effectiveness of the empirical predictions by comparison with recorded time series at the downhole sensors. We find quite high correlations and small variations in both spectral shape and amplitude over the entire frequency range for the GIT deconvolution at the majority of the 90 KiK‐net sites with all mean residuals less than 0.25 in contrast to empirical and 1D modeling approaches, which significantly overestimate the level of hard‐rock ground motion for frequencies larger than a few Hertz.
  PubDate: Thu, 22 Dec 2022 00:00:00 GMT
A Real‐Time and Data‐Driven Ground‐Motion Prediction Framework for
Earthquake Early Warning
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  Abstract: ABSTRACTThe ShakeAlert earthquake early warning system in the western United States characterizes earthquake source locations and magnitudes in real time, issuing public alerts for areas where predicted ground‐motion intensities exceed a threshold value. Although rapid source characterization methods have attracted significant scientific attention in recent years, the ground‐motion models used by ShakeAlert have received notably less. This study develops a data‐driven framework for earthquake early warning‐specific ground‐motion models by precomputing and incorporating site‐specific corrections, while using a Bayesian approach to estimate event‐specific corrections in real time. The study involves analyzing a quality‐controlled set of more than 420,000 seismic recordings from 1389 M 3–7 events in the state of California, from 2011 to 2022. We first compare the observed ground motions to predictions from existing ground‐motion models, namely the modified Boore and Atkinson (2008) and active crustal Next Generation Attenuation (NGA)‐West2 ground‐motion prediction equations, before implementing a new Bayesian model optimized for a real‐time setting. Residual analysis of peak ground acceleration and peak ground velocity metrics across a host of earthquake rupture scenarios from the two ground‐motion models show that the active crustal NGA‐West2 model is better suited for ShakeAlert in California. In addition, the event‐terms calculated using our Bayesian approach rapidly converge such that errors from earthquake magnitude estimation can be corrected for when forecasting shaking intensity in real time. Equipped with these improved ground‐shaking predictions, we show that refined ShakeAlert warnings could be issued to the public within as soon as 5 s following ShakeAlert’s initial warning. This approach could be used both to reduce prediction uncertainties and thus improve ShakeAlert’s alerting decision.
  PubDate: Tue, 20 Dec 2022 00:00:00 GMT
Seismic Response of Nenana Sedimentary Basin, Central Alaska
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  Abstract: ABSTRACTNenana basin in central Alaska is a long (90 km), narrow (12 km), and deep (7 km) sedimentary basin aligned with an active fault zone producing Mw≥6 earthquakes. From 2015 to 2019, 13 broadband seismic stations were deployed in the region as part of the Fault Locations and Alaska Tectonics from Seismicity project. These stations recorded a wide range of earthquakes, including Mw 3–4 directly below the basin as well as several regional earthquakes Mw>6. These 43 local and regional earthquakes, in addition to five teleseismic events and continuously recorded ambient noise, provide a data set that we use to quantify the response of Nenana basin to the seismic wavefield. We calculate spectral ratios between each station and a bedrock reference station for 48 earthquakes. We find amplification of 11–14 dB (amplification ratio 3.5–5.0) for low frequencies (0.1–0.5 Hz), and 8–15 dB (amplification ratio 2.5–5.6) for high frequencies (0.5–4.0 Hz) on the vertical component. At low frequencies, amplification of the earthquake wavefield agrees well with amplification of seismic noise, with both data sets exhibiting stronger amplification on the horizontal components, in comparison with the vertical component. Furthermore, stations overlying the deeper part of the basin exhibit stronger amplification, whereas stations at the margin of the basin exhibit minimal low‐frequency amplification. At higher frequencies, amplification occurs at both deeper basin stations and also marginal basin stations. Our study establishes a catalog of diverse events for future theoretical and numerical studies that can use Nenana basin to better understand the complex influence of sedimentary basins on the seismic wavefield.
  PubDate: Fri, 16 Dec 2022 00:00:00 GMT
Ranking Earthquake Sources Using Spatial Residuals of Seismic Scenarios:
Methodology Application to the 1909 Benavente Earthquake
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  Abstract: ABSTRACTHistorical earthquakes are of major importance in the analysis of seismic hazards, in particular for stable continental regions. In this article, we propose a methodology that uses seismic scenarios to provide constraints on the location of the seismic source of historical earthquakes. Our methodology involves generating seismic scenarios for the proposed seismic sources and comparing the results to the observed intensity field of the earthquake. To avoid the bias related to strongly heterogeneously distributed datasets, we focus on data points that are useful in discriminating between competing ruptures. These data are identified by the spatial patterns of residuals between seismic scenarios produced for each source. We apply this methodology to a test event—the 1999 Athens earthquake—for which both the magnitude and location are constrained by independent data, and to the 1909 Benavente earthquake, for which the magnitude is constrained by seismological studies, but the location is uncertain due to the very poor azimuthal coverage available. Within its application limits, the proposed methodology was capable of identifying the source of the Athens earthquake amongst different ruptures located few kilometers apart. The analysis performed for the 1909 Benavente earthquake suggests that the eastern strand of the lower Tagus Valley fault zone is the most likely seismic source for earthquake, amongst those proposed in the literature.
  PubDate: Thu, 15 Dec 2022 00:00:00 GMT
Estimates of κ 0 and Effects on Ground Motions in the San Francisco
Bay Area
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  Abstract: ABSTRACTGround‐motion studies are a key component of seismic hazard analyses and often rely on information of the source, path, and site. Extensive research has been done on each of these parameters; however, site‐specific studies are of particular interest to seismic hazard studies, especially in the field of earthquake engineering, as near‐site conditions can have a significant impact on the resulting ground motion at a site. There has been much focus on the constraint of site parameters and their application to seismic hazard studies, especially in the development of ground‐motion models (GMMs). Kappa is an observational parameter describing the high‐frequency attenuation of spectra, and its site contribution (κ0) has shown to be a good predictor of high‐frequency ground motions; however, measurements are often limited. In this study, we develop a κ0 dataset for the San Francisco Bay area (SFBA) by estimating κ0 for 228 stations, and we produce a continuous regional map of κ0. We find κ0 to range between 0.003 and 0.072 s, with larger values concentrating on the east, north, and south sides of the bay, and lower values concentrating on the west side. We also evaluate the robustness of κ0 as a site parameter and find it to correlate with peak ground acceleration. These estimates of κ0 can add predictive power to GMMs, thus increasing the accuracy of predicted ground motion and improving the robustness of ground‐motion studies in the SFBA.
  PubDate: Tue, 13 Dec 2022 00:00:00 GMT