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At least 685 records · Page 38Linked to original sources

USGS2020

Evidence of previous faulting along the 2019 Ridgecrest, California earthquake ruptures

The July 2019 Ridgecrest earthquake sequence in southeastern California was characterized as surprising because only ~35% of the rupture occurred on previously mapped faults. Employing more detailed inspection of pre-event high-resolution topography and imagery in combination with field observations, we document evidence of active faulting in the landscape along the entire fault system. Scarps, deflected drainages, and lineaments and contrasts in topography, vegetation, and ground color demonstrate previous slip on a dense network of orthogonal faults, consistent with patterns of surface rupture observed in 2019. Not all of these newly mapped fault strands ruptured in 2019. Outcrop-scale field observations additionally reveal tufa lineaments and sheared Quaternary deposits. Neotectonic features are commonly short (<2 km), discontinuous, and display en echelon patterns along both the M 6.4 and M 7.1 ruptures. These features are generally more prominent and better preserved outside the late Pleistocene lake basins. Fault expression may also be related to deformation style: scarps and topographic lineaments are more prevalent in areas where substantial vertical motion occurred in 2019. Where strike-slip displacement dominated in 2019, the faults are mainly expressed by less prominent tonal and vegetation features. Both the NE- and NW-trending active fault systems are subparallel to regional bedrock fabrics that were established as early as ~150 Ma, and may be reactivating these older structures. Overall, we estimate that 50-70% (i.e., an additional 15-35%) of the 2019 surface ruptures could have been recognized as active faults with detailed inspection of pre-event data. Similar detailed mapping of potential neotectonic features could help improve seismic hazard analyses in other regions of eastern California and elsewhere that have distributed faulting or incompletely mapped faults. In areas where faults cannot be resolved as single thoroughgoing structures, a zone of potential faulting should be used as a hazard model input.

California↗

USGS2020

Surface displacement distributions for the July 2019 Ridgecrest, California earthquake ruptures

Surface rupture in the 2019 Ridgecrest, California, earthquake sequence occurred along two orthogonal cross faults and includes dominantly left‐lateral and northeast‐striking rupture in the M w "> M w 6.4 foreshock and dominantly right‐lateral and northwest‐striking rupture in the M w "> M w 7.1 mainshock. We present > 650 "> > 650 field‐based, surface‐displacement observations for these ruptures and synthesize our results into cumulative along‐strike displacement distributions. Using these data, we calculate displacement gradients and compare our results with historical strike‐slip ruptures in the eastern California shear zone. For the M w "> M w 6.4 rupture, we report 96 displacements measured along 18 km of northeast‐striking rupture. Cumulative displacement curves for the rupture yield a mean left‐lateral displacement of 0.3–0.5 m and maximum of 0.7–1.6 m. Net mean vertical displacement based on the difference of down‐to‐the‐west (DTW) and down‐to‐the‐east (DTE) displacement curves is close to zero (0.02 m DTW). The M w "> M w 6.4 displacement distribution shows that the majority of displacement occurred southwest of the intersection with the M w "> M w 7.1 rupture. The M w "> M w 7.1 rupture is northwest‐striking and 50 km long based on 576 field measurements. Displacement curves indicate a mean right‐lateral displacement of 1.2–1.7 m and a maximum of 4.3–7.0 m. Net vertical displacement in the rupture averages 0.3 m DTW. The M w "> M w 7.1 displacement distributions demonstrate that maximum displacement occurred along a 12‐km‐long portion of the fault near the M w "> M w 7.1 epicenter, releasing 66% of the geologically based seismic moment along 24% of the total rupture length. Using our displacement distributions, we calculate kilometer‐scale displacement gradients for the M w "> M w 7.1 rupture. The steepest gradients ( ⁠ ∼ 1 – 3    m / km "> ∼ 1 – 3 m / km ⁠ ) flank the 12‐km‐long region of maximum displacement. In contrast, gradients for the 1992 M w "> M w 7.3 Landers and 1999 M w "> M w 7.1 Hector Mine earthquakes are < 0.6    m / km "> < 0.6 m / km . Our displacement distributions are important for understanding the influence of cross‐fault rupture on M w "> M w 6.4 and 7.1 rupture length and displacement and will facilitate comparisons with distributions generated remotely and at broader scales.

California↗

USGS2020

Minimal clustering of injection-induced earthquakes observed with a large-n seismic array

The clustering behavior of injection‐induced earthquakes is examined using one month of data recorded by the LArge‐ n Seismic Survey in Oklahoma (LASSO) array. The 1829‐node seismic array was deployed in a 25 km×32 km area of active saltwater disposal in northern Oklahoma between 14 April and 10 May 2016. Injection rates in the study area are nearly constant around the time of the deployment. We develop a local magnitude ( ⁠ M L ⁠ ) equation for the region and estimate magnitudes for 1104 earthquakes recorded by the deployment. The determined earthquake magnitudes range from M L 0.01 to 3.0. The majority of earthquakes occurred between 1.5 and 5.5 km depth, and the shallowest earthquake depths overlap with the base of injection wells at depths between 1.5 and 2.5 km. We compute focal mechanisms of the largest events ( ⁠ M L > 2.0 ⁠ ), and find a mix of normal‐ and strike‐slip‐faulting types. Earthquakes occur regularly in time during the deployment, but are not evenly distributed in space across the study area, that is, they are spatially clustered. Analysis of the nearest‐neighbor distances in the space–time–magnitude domain shows the seismicity is dominated by single‐event clusters (i.e., independent events). This high proportion of single‐event clusters compared with multievent clusters has been previously noted for induced events at geothermal sites. When clustering occurs, the number of events in a cluster is typically small. We observe only four clusters with 10 or more events. For these larger clusters, we find equivalent numbers of foreshocks and aftershocks; however, the foreshock sequences are significantly longer in duration lasting days to tens of days, while aftershock sequences are observed only on the order of one day. The minimal clustering observed for events in the LASSO array suggests that the majority of events are being directly driven by stress changes due to local saltwater disposal.

Oklahoma↗

USGS2020

Factors influencing the probability of hydraulic fracturing induced seismicity in Oklahoma

Injection‐induced seismicity became an important issue over the past decade, and although much of the rise in seismicity is attributed to wastewater disposal, a growing number of cases have identified hydraulic fracturing (HF) as the cause. A recent study identified regions in Oklahoma where ≥75% of seismicity from 2010 to 2016 correlated with nearly 300 HF wells. To identify factors associated with increased probability of induced seismicity, we gathered publicly available information about the HF operations in these regions including: injected volume, number of wells on a pad, injected fluid (gel vs. slickwater), vertical depth of the well, proximity of the well to basement rock, and the formation into which the injection occurred. To determine the statistical strength of the trends, we applied logistic regression, bootstrapping, and odds ratios. We see no trend with total injected volume in our Oklahoma dataset, in contrast to strong trends observed in Alberta and Texas, but we note those regions have many more multiwell pads leading to larger cumulative volumes within a localized area. We found a > ∼50% lower probability of seismicity with the use of gel compared to slickwater. We found that HF wells targeting older formations had a higher probability of seismicity; however, these wells also tend to be deeper, and we found the trend with well depth to be stronger than the trend with age of formation. When isolated to the Woodford formation, well depth produced the strongest relationship, increasing from ∼5%> to ∼50% probability from 1.5 to 5.5 km. However, no trend was seen in the proximity to basement parameter. Based on previously measured pore pressure gradients, we interpret the strong absolute depth relationship to be a result of the increasing formation overpressure measured in deeper portions of the basin that lower the stress change needed to induce seismicity.

Oklahoma↗

USGS2020

More fault connectivity Is needed in seismic hazard analysis

Did the third Uniform California Earthquake Rupture Forecast (UCERF3) go overboard with multifault ruptures? Schwartz (2018) argues that there are too many long ruptures in the model. Here, I address his concern and show that the UCERF3 rupture‐length distribution matches empirical data. I also present evidence that, if anything, the UCERF3 model could be improved by adding more connectivity to the fault system. Adding more connectivity would improve model misfits with data, particularly with paleoseismic data on the southern San Andreas fault; make the model less characteristic on the faults; potentially improve aftershock forecasts; and reduce model sensitivity to inadequacies and unknowns in the modeled fault system.

Bulletin of the Seismological Society of America↗

USGS2020

VS30 and Dominant Site Frequency (⁠fd⁠) as Provisional Station ML Corrections (⁠dML⁠) in California

New seismic stations added to a regional seismic network cannot be used to calculate local magnitude ( ⁠ M L "> M L ⁠ ) until a revised regionwide amplitude decay function is developed. Each station must record a minimum number of local and regional earthquakes that meet specific amplitude requirements prior to recalibration of the amplitude decay function. Station component adjustments ( ⁠ d M L "> d M L ⁠ ; Uhrhammer et al. , 2011 ) are then calculated after inverting for a new regional amplitude decay function, constrained by the sum of d M L "> d M L ⁠ for long‐running stations. Therefore, there can be significant delay between when a new station starts contributing real‐time waveform packets and when data can be included in magnitude determinations. We propose the use of known estimates of seismic site conditions such as the time‐averaged shear‐wave velocity ( ⁠ V S "> V S ⁠ ) of the upper 30 m ( ⁠ V S 30 "> V S 30 ⁠ ) and the site dominant frequency ( ⁠ f d "> f d ⁠ ) to calculate d M L "> d M L ⁠ ⁠ . Previously established d M L "> d M L ⁠ ⁠ , measured V S 30 "> V S 30 ⁠ ⁠ , and f d "> f d data are available for between 126 and 458 horizontal components (east–west and north–south) at 81 seismic stations in the California Integrated Seismic Network; d M L "> d M L ⁠ d M L "> data range from −1.10 to 0.39, V S 30 "> V S 30 ⁠ values range from 202 to 1464    m / s "> 1464 m / s ⁠ , and 440 f d "> f d values are compiled from earthquake and microseismic records that range from 0.13 to 21 Hz. We find V S 30 "> V S 30 ⁠ and d M L "> d M L ⁠ exhibit a positive coefficient of determination ( ⁠ R = 0.59 "> R = 0.59 ⁠ ), indicating that as V S 30 "> V S 30 ⁠ increases, d M L "> d M L ⁠ increases. This implies that greater site amplification (lower V S 30 "> V S 30 ⁠ ⁠ ) results in smaller d M L "> d M L ⁠ ⁠ . f d "> f d and d M L "> d M L ⁠ also generally exhibit a positive correlation ( ⁠ R 2 < 0.56 "> R 2 < 0.56 ⁠ ), which implies lower d M L "> d M L values are related to site resonance at depth‐dependent frequencies. Using the developed relationships, V S 30 "> V S 30 ⁠ or f d "> f d measurements can be used to establish a provisional d M L "> d M L ⁠ for newly established stations. This procedure allows new stations to contribute to regional network M L "> M L determinations immediately without the need to wait until a minimum set of earthquake data has been recorded.

California↗

USGS2020

Strain-estimated ground motions associated with recent earthquakes in California

Peak ground velocity (PGV) is a commonly used parameter in earthquake ground‐motion models (GMMs) and hazard analyses, because it is closely related to structural damage and felt ground shaking, and is typically measured on broadband seismometers. Here, we demonstrate that strainmeters, which directly measure in situ strain in the bulk rock, can easily be related to ground velocity by a factor of bulk shear‐wave velocity and, thus, can be used to measure strain‐estimated PGV. We demonstrate the parity of velocity to strain utilizing data from borehole strainmeters deployed along the plate boundaries of the west coast of the United States for nine recent M "> M M 4.4–7.1 earthquakes in California, including the largest two events of the July 2019 Ridgecrest earthquake sequence. PGVs derived from maximum horizontal shear strains fall within the range of seismic‐estimated values recorded at the same distances. We compare the strain‐estimated data with GMMs based on seismic PGVs and find consistency in residual polarity (positive vs. negative; the sign of the difference between observed and modeled data) for certain earthquake–station paths, where some paths indicate an overestimation and others indicate an underestimation of strain‐derived PGVs, as compared with the GMMs. We surmise that this may be indicative of over or underestimation of shear‐wave velocity along those paths, as compared with the average velocity used to derive PGV from strain measurements, or indicative of repeatable site and path effects that are not accounted for in our analyses. This direct comparison of strain with velocity can highlight physical path effects, as well as improve the density and capability of ground‐motion recordings.

California↗

USGS2020

San Andreas fault exploration using refraction tomography and S-wave-type and Fϕ-mode guided waves

Surface ruptures from the 18 April 1906 M∼7.9 San Francisco earthquake were distributed over an ∼35 ‐meter‐wide zone at San Andreas Lake on the San Francisco Peninsula in California ( Schussler, 1906 ). Since ∼1906⁠ , the surface ruptures have been largely covered by water, but with water levels at near‐historic low levels in 2008–2011, we observed that the 1906 surface ruptures were no longer visible. As a fault imaging test, we acquired refraction tomography and guided‐wave data across the 1906 surface ruptures in 2011. We found that individual fault traces, as mapped by Schussler (1906) , can be identified on the basis of discrete low‐velocity zones ( ⁠V S and V P⁠ , reduced ∼40% and ∼34%⁠ , respectively) and high‐amplitude guided waves. Guided waves have traditionally been observed as large‐amplitude waveforms over wide (hundreds of meters to kilometers) zones of faulting, but we demonstrate that by evaluating guided waves (including Rayleigh/Love‐ and P / SV ‐types) in terms of peak ground velocity (PGV), individual near‐surface fault traces within a fault zone can be precisely located, even more than 100 yr after the surface ruptures. Such precise exploration can be used to focus paleoseismic trenching efforts and to identify or exclude faulting at specific sites. We evaluated PGV of both S ‐wave‐type and Fϕ ‐mode‐type guided waves and found that both wave types can be used to identify subsurface fault traces. At San Andreas Lake (main fault), S ‐wave‐type guided waves travel up to 18% slower than S body waves, and Fϕ ‐mode guided waves travel ∼60% slower than P body waves but ∼15% faster than S body waves. We found that guided‐wave amplitudes vary with frequency but are up to five times higher than those of body waves, including the S wave. Our data are consistent with the concept that guided waves can be a strong‐shaking hazard during large‐magnitude earthquakes.

Calfornia↗

USGS2020

Human behavioral response in the Ridgecrest earthquakes: Assessing immediate actions based on data from “Did You Feel It?”

Human behavioral response to earthquake ground motion has long been a subject of multidisciplinary interest and research. In most versions of seismic intensity scales, human perceptions and behavior are one component of the assignment of intensity. Public health research has shown that actions taken during earthquakes have a significant impact on the incidence of injury or the maintenance of safety. Based on this research, emergency managers and organizations promoting emergency preparedness have advocated strategies such as drop, cover, and hold on (DCHO) and promoted this safety measure through public education and annual drills. The “Did You Feel It?” (DYFI) mapping system (see Data and Resources) based on an online questionnaire developed and maintained by the U.S. Geological Survey has provided opportunities for those who have experienced an earthquake to report this experience worldwide since 2004. The DYFI questionnaire, although designed to assign intensity, also contains questions regarding the behavior in which one has engaged during the earthquake. The questionnaire includes other important information that may elucidate behavioral response to earthquakes, including assigned intensity, emotional reaction, and whether damage occurred at the location where the earthquake was experienced. The very large number of people who completed DYFI questionnaires following the July 2019 Ridgecrest, California, earthquakes provides a robust dataset for analysis and suggests that as intensity and levels of fear increase, behavior becomes more active in terms of physical movement to locations of presumed safety. Among active responses including DCHO, going to a doorway, and running outside, DCHO was the least likely to be implemented. The study provides possible explanations for low participation in DCHO despite active campaigns to promote this strategy.

California↗

USGS2020

Post-glacial Mw 7.0-7.5 earthquakes on the North Olympic fault zone, Washington

Holocene crustal faulting in the northern Olympic Peninsula of Washington State manifests in a zone of west‐northwest‐striking crustal faults herein named the North Olympic fault zone, which extends for ∼ 80    km "> ∼ 80 km ∼80 km along strike and includes the Lake Creek–Boundary Creek fault to the east and the Sadie Creek fault and newly discovered scarps to the west. This study focuses on the Sadie Creek fault, which extends for > 14    km "> > 14 km >14 km west‐northwest from Lake Crescent. Airborne light detection and ranging (lidar) imagery reveals the trace of the Sadie Creek fault and offset postglacial landforms showing a history of Holocene surface‐rupturing earthquakes dominated by dextral displacement along a steeply dipping fault zone. Paleoseismic trenches at two sites on the Sadie Creek fault reveal till and outwash overlain by progressively buried forest and wetland soils developed on scarp‐derived colluvial wedges. Trench exposures of complex faulting with subhorizontal slickenlines indicate dextral displacement with lesser dip slip. Correlation of broadly constrained time intervals for earthquakes at the Sadie Creek sites and those to the east along the Lake Creek–Boundary Creek fault is consistent with rupture of much of the length of the North Olympic fault zone three to four times: at about 11, 7, 3, and 1 ka, with a shorter rupture at about 8.5 ka. Dated ruptures from trenches only partially coincide with coseismic landslides and megaturbidites in Lake Crescent, indicating that some earthquakes did not trigger megaturbidites, and some turbidites were unrelated to local fault rupture. Landform mapping suggests single‐event dextral displacement of 4 ± 1    m "> 4 ± 1 m 4±1 m on the Sadie Creek fault. Inferred maximum rupture length and single‐event slip imply earthquake magnitudes M w "> M w Mw 7.0–7.5. Dextral slip rates of 1.3 – 2.3    mm / yr "> 1.3 – 2.3 mm / yr 1.3–2.3 mm/yr and the ∼ 11 , 000    yr "> ∼ 11 , 000 yr ∼11,000 yr slip history suggest that the North Olympic fault zone is a prominent contributor to permanent strain in the northern Cascadia fore‐arc.

Washington↗

USGS2020

Teleseismic P‐qave coda autocorrelation imaging of crustal and basin structure, Bighorn Mountains Region, Wyoming, U.S.A.

We demonstrate successful crustal imaging via teleseismic P ‐wave coda autocorrelation, using data recorded on a 261 station array of vertical‐component high‐frequency geophones in the area of the Bighorn Mountains, Wyoming, U.S.A. We autocorrelate the P ‐wave coda of 30 teleseismic events and use phase‐weighted stacking to yield seismic profiles comparable to low‐passed versions of those produced via controlled‐source vertical seismic reflection. Our process recovers reflections from the bottoms of the Bighorn and Powder River basins that flank the Bighorn Mountains. We also identify a mid‐crustal reflector that aligns with a region of increased reflectivity, previously interpreted as a Precambrian province boundary. Our results demonstrate the utility of crustal imaging with teleseismic P ‐wave coda energy using modern large‐array seismic data, and they corroborate previous interpretations of crustal structures in the study area.

Wyoming↗

USGS2020

Improving paleoseismic earthquake magnitude estimates with rupture length information: Application to the Puget Lowland, Washington State, U.S.A.

Both earthquake displacement and rupture length correlate with magnitude, and therefore observations of each from past earthquakes can be used to estimate the magnitude of those earthquakes in the absence of instrumental records. We extend the Bayesian inversion method of Biasi and Weldon (2006), which estimates paleoearthquake magnitude from displacement observations, to incorporate both rupture length and surface displacement measurements into the magnitude inversion. We then use this method on 27 late Pleistocene to Holocene paleoearthquakes in the Puget Lowland region of Washington. Observations of (typically vertical) fault separation per event range from 0.6 to 7 m, implying net displacement per event of up to 10 ± 4 m for the largest event. Rupture lengths are estimated to vary between the smallest contiguous mapped scarps to the full extent of the faults mapped from geology and geophysical observations. Although a few of the ruptures may be longer than 150 km, the ruptures have a median of 53 km, indicating that earthquakes in the Puget Lowland have relatively high displacement to length ratios. By considering both datasets, we find that all events were between M 6.3 and 7.5, generally consistent with the expected seismicity from the USGS National Seismic Hazard Map for the region. The simultaneous use of both length and displacement data in the magnitude inversion decreases both the estimated earthquake magnitudes and the uncertainty. The magnitude reduction in particular is due to the relatively short rupture lengths possible for Puget Lowland faults. This implies a decrease in the seismic hazard (relative to a displacement-only assessment) to a highly populated and rapidly urbanizing region.

Washington↗

USGS2020

Introduction to the Special Issue on the 2019 Ridgecrest, California, Earthquake Sequence

No abstract available.

California↗

USGS2021

Paleoseismic trenching reveals late quaternary kinematics of the Leech River Fault: Implications for forearc strain accumulation in Northern Cascadia

New paleoseismic trenching indicates late Quaternary oblique right‐lateral slip on the Leech River fault, southern Vancouver Island, Canada, and constrains permanent forearc deformation in northern Cascadia. A south‐to‐north reduction in northward Global Navigation Satellite System velocities and seismicity across the Olympic Mountains, Strait of Juan de Fuca (JDF), and the southern Strait of Georgia, has been used as evidence for permanent north–south crustal shortening via thrust faulting between a northward migrating southern forearc and rigid northern backstop in southwestern Canada. However, previous paleoseismic studies indicating late Quaternary oblique right‐lateral slip on west‐northwest‐striking forearc faults north of the Olympic Mountains and in the southern Strait of Georgia are more consistent with forearc deformation models that invoke oroclinal bending and(or) westward extrusion of the Olympic Mountains. To help evaluate strain further north across the Strait of JDF, we present the results from two new paleoseismic trenches excavated across the Leech River fault. In the easternmost Good Hope trench, we document a vertical fault zone and a broad anticline deforming glacial till. Comparison of till clast orientations in faulted and undeformed glacial till shows evidence for postdeposition faulted till clast rotation, indicating strike‐slip shear. The orientation of opening mode fissuring during surface rupture is consistent with right‐lateral slip and the published regional S H max "> S H max SHmax directions. Vertical separation and the formation of scarp‐derived colluvium along one fault also indicate a dip‐slip component. Radiocarbon charcoal dating within offset glacial till and scarp‐derived colluvium suggest a single surface rupturing earthquake at 9.4 ± 3.4    ka "> 9.4 ± 3.4 ka 9.4±3.4 ka ⁠ . The oblique right‐lateral slip sense inferred in the Good Hope trench is consistent with slip kinematics observed on other regional west‐northwest‐striking faults and indicates that these structures do not accommodate significant north–south shortening via thrust faulting.

Washington↗

USGS2020

Holocene paleoseismology of the Steamboat Mountain Site: Evidence for full‐Llngth rupture of the Teton Fault, Wyoming

The 72‐km‐long Teton fault in northwestern Wyoming is an ideal candidate for reconstructing the lateral extent of surface‐rupturing earthquakes and testing models of normal‐fault segmentation. To explore the history of earthquakes on the northern Teton fault, we hand‐excavated two trenches at the Steamboat Mountain site, where the east‐dipping Teton fault has vertically displaced west‐sloping alluvial‐fan surfaces. The trenches exposed glaciofluvial, alluvial‐fan, and scarp‐derived colluvial sediments and stratigraphic and structural evidence of two surface‐rupturing earthquakes (SM1 and SM2). A Bayesian geochronologic model for the site includes three optically stimulated luminescence ages ( ⁠ ∼ 12 – 17 ka ⁠ ) for the glaciofluvial units and 16 radiocarbon ages ( ⁠ ∼ 1.2 – 8.6 ka ⁠ ) for the alluvial‐fan and colluvial units and constrains SM1 and SM2 to 5.5 ± 0.2 ka , 1 σ (5.2–5.9 ka, 95%) and 9.7 ± 0.9 ka , 1 σ (8.5–11.5 ka, 95%), respectively. Structural, stratigraphic, and geomorphic relations yield vertical displacements for SM1 ( ⁠ 2.0 ± 0.6 m , 1 σ ⁠ ) and SM2 ( ⁠ 2.0 ± 1.0 m , 1 σ ⁠ ). The Steamboat Mountain paleoseismic chronology overlaps temporally with earthquakes interpreted from previous terrestrial and lacustrine paleoseismic data along the fault. Integrating these data, we infer that the youngest Teton fault rupture occurred at ∼ 5.3 ka ⁠ , generated 1.7 ± 1.0 m , 1 σ of vertical displacement along 51–70 km of the fault, and had a moment magnitude ( ⁠ M w ⁠ ) of ∼ 7.0 – 7.2 ⁠ . This rupture was apparently unimpeded by structural complexities along the Teton fault. The integrated chronology permits a previous full‐length rupture at ∼ 10 ka and possible partial ruptures of the fault at ∼ 8 – 9 ka ⁠ . To reconcile conflicting terrestrial and lacustrine paleoseismic data, we propose a hypothesis of alternating full‐ and partial‐length ruptures of the Teton fault, including M w ∼ 6.5 – 7.2 earthquakes every ∼ 1.2 ky ⁠ . Additional paleoseismic data for the northern and central sections of the fault would serve to test this bimodal rupture hypothesis.

Wyoming↗

USGS2021

Toward physics-based nonergodic PSHA: A prototype fully-deterministic seismic hazard model for southern California

We present a nonergodic framework for probabilistic seismic‐hazard analysis (PSHA) that is constructed entirely of deterministic, physical models. The use of deterministic ground‐motion simulations in PSHA calculations is not new (e.g., CyberShake), but prior studies relied on kinematic rupture generators to extend empirical earthquake rupture forecasts. Fully dynamic models, which simulate rupture nucleation and propagation of static and dynamic stresses, are still computationally intractable for the large simulation domains and many seismic cycles required to perform PSHA. Instead, we employ the Rate‐State earthquake simulator (RSQSim) to efficiently simulate hundreds of thousands of years of M ≥ 6.5 earthquake sequences on the California fault system. RSQSim produces full slip‐time histories for each rupture, which, unlike kinematic models, emerge from frictional properties, fault geometry, and stress transfer; all intrinsic variability is deterministic. We use these slip‐time histories directly as input to a 3D wave‐propagation code within the CyberShake platform to obtain simulated F max = 0.5 Hz ground motions. The resulting 3 s spectral acceleration ground motions closely match empirical ground‐motion model (GMM) estimates of median and variability of shaking. When computed over a range of sources and sites, the variability is similar to that of ergodic GMMs. Variability is reduced for individual pairs of sources and sites that repeatedly sample a single path, which is expected for a nonergodic model. This results in increased exceedance probabilities for certain characteristic ground motions for a source–site pair, while decreasing probabilities at the extreme tails of the ergodic GMM predictions. We present these comparisons and preliminary fully deterministic physics‐based RSQSim–CyberShake hazard curves, as well as a new technique for estimating within‐ and between‐event variability through simulation.

California↗

USGS2021

The San Andreas fault paleoseismic record at Elizabeth Lake: Why are there fewer surface-rupturing earthquakes on the Mojave section?

The structural complexity of active faults and the stress release history along the fault system may exert control on the locus and extent of individual earthquake ruptures. Fault bends, in particular, are often invoked as a possible mechanism for terminating earthquake ruptures. However, there are few records available to examine how these factors may influence the along‐fault recurrence of earthquakes. We present a new paleoearthquake chronology for the southern San Andreas fault at Elizabeth Lake and integrate this record with existing paleoearthquake records to examine how the timing and frequency of earthquakes vary through a major restraining bend. This restraining bend features a mature, throughgoing right‐lateral strike‐slip fault, two major fault intersections, proposed subsurface fault dip changes, and a > 200    km "> > 200 km long section of fault misaligned with the regional plate motion. The Frazier Mountain, Elizabeth Lake, Pallett Creek, Wrightwood, and Pitman Canyon paleoseismic sites are located on this relatively linear surface trace of the San Andreas fault between fault bends. Our paleoseismic investigations at Elizabeth Lake document 4–5 earthquakes, since ∼ 1100    C . E . "> ∼ 1100 C . E . , similar to the number of earthquakes recorded at Pallett Creek. In contrast, the Frazier Mountain and Wrightwood sites each record 8–9 earthquakes during this same time period. Differences in earthquake frequency demonstrate that fewer earthquakes rupture the central portion of the restraining bend than occur near the fault bends and intersections. Furthermore, the similarity of earthquake records from the Bidart Fan paleoseismic site northwest of the restraining bend and the Frazier Mountain paleoseismic site suggests that the broad, 30° curve of the Big Bend section of the San Andreas fault exerts less influence on fault rupture behavior than the 3D geometry of the Mojave sections of the fault.

California↗

USGS2020

Generalizing the inversion‐based PSHA source model for an interconnected fault system

This article represents a step toward generalizing and simplifying the procedure for constructing an inversion‐based seismic hazard source model for an interconnected fault system, including the specification of adjustable segmentation constraints. A very simple example is used to maximize understandability and to counter the notion that an inversion approach is only applicable when an abundance of data is available. Also exemplified is how to construct a range of models to adequately represent epistemic uncertainties (which should be a high priority in any hazard assessment). Opportunity is also taken to address common concerns and misunderstandings associated with the third Uniform California Earthquake Rupture Forecast, including the seemingly disproportionate number of large‐magnitude events, and how well hazard is resolved given the overall problem is very underdetermined. However, the main aim of this article is to provide a general protocol for constructing such models.

Bulletin of the Seismological Society of America↗