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USGS2017

The HayWired Earthquake Scenario

Foreword The 1906 Great San Francisco earthquake (magnitude 7.8) and the 1989 Loma Prieta earthquake (magnitude 6.9) each motivated residents of the San Francisco Bay region to build countermeasures to earthquakes into the fabric of the region. Since Loma Prieta, bay-region communities, governments, and utilities have invested tens of billions of dollars in seismic upgrades and retrofits and replacements of older buildings and infrastructure. Innovation and state-of-the-art engineering, informed by science, including novel seismic-hazard assessments, have been applied to the challenge of increasing seismic resilience throughout the bay region. However, as long as people live and work in seismically vulnerable buildings or rely on seismically vulnerable transportation and utilities, more work remains to be done. With that in mind, the U.S. Geological Survey (USGS) and its partners developed the HayWired scenario as a tool to enable further actions that can change the outcome when the next major earthquake strikes. By illuminating the likely impacts to the present-day built environment, well-constructed scenarios can and have spurred officials and citizens to take steps that change the outcomes the scenario describes, whether used to guide more realistic response and recovery exercises or to launch mitigation measures that will reduce future risk. The HayWired scenario is the latest in a series of like-minded efforts to bring a special focus onto the impacts that could occur when the Hayward Fault again ruptures through the east side of the San Francisco Bay region as it last did in 1868. Cities in the east bay along the Richmond, Oakland, and Fremont corridor would be hit hardest by earthquake ground shaking, surface fault rupture, aftershocks, and fault afterslip, but the impacts would reach throughout the bay region and far beyond. The HayWired scenario name reflects our increased reliance on the Internet and telecommunications and also alludes to the interconnectedness of infrastructure, society, and our economy. How would this earthquake scenario, striking close to Silicon Valley, impact our interconnected world in ways and at a scale we have not experienced in any previous domestic earthquake? The area of present-day Contra Costa, Alameda, and Santa Clara Counties contended with a magnitude-6.8 earthquake in 1868 on the Hayward Fault. Although sparsely populated then, about 30 people were killed and extensive property damage resulted. The question of what an earthquake like that would do today has been examined before and is now revisited in the HayWired scenario. Scientists have documented a series of prehistoric earthquakes on the Hayward Fault and are confident that the threat of a future earthquake, like that modeled in the HayWired scenario, is real and could happen at any time. The team assembled to build this scenario has brought innovative new approaches to examining the natural hazards, impacts, and consequences of such an event. Such an earthquake would also be accompanied by widespread liquefaction and landslides, which are treated in greater detail than ever before. The team also considers how the now-prototype ShakeAlert earthquake early warning system could provide useful public alerts and automatic actions. Scientific Investigations Report 2017–5013 and accompanying data releases are the products of an effort led by the USGS, but this body of work was created through the combined efforts of a large team including partners who have come together to form the HayWired Coalition (see chapter A). Use of the HayWired scenario has already begun. More than a full year of intensive partner engagement, beginning in April 2017, is being directed toward producing the most in-depth look ever at the impacts and consequences of a large earthquake on the Hayward Fault. With the HayWired scenario, our hope is to encourage and support the active ongoing engagement of the entire community of the San Francisco Bay region by providing the scientific, engineering, and economic and social science inputs for use in exercises and planning well into the future. As HayWired volumes are published, they will be made available at https://doi.org/10.3133/sir20175013 .

California↗

USGS2016

The Elizabeth Lake paleoseismic site: Rupture pattern constraints for the past ~800 years for the Mojave section of the south-central San Andreas Fault

The southern San Andreas Fault in California has hosted two historic surface-rupturing earthquakes, the ~M7 1812 Wrightwood earthquake and the ~M7.9 1857 Fort Tejon earthquake (e.g., Sieh, 1978; Jacoby et al., 1988). Numerous paleoseismic studies have established chronologies of historic and prehistoric earthquakes at sites along the full length of the 1857 rupture (e.g., Sieh, 1978; Scharer et al., 2014). These studies provide an unparalleled opportunity to examine patterns of recent ruptures; however, at least two significant spatial gaps in high-quality paleoseismic sites remain. At ~100 km long each, these gaps contribute up to 100 km of uncertainty to paleo-rupture lengths and could also permit a surface rupture from an earthquake up to ~M7.2 to go undetected [using scaling relationships of Wells and Coppersmith (1994)]. Given the known occurrence of an ~M7 earthquake on this portion of the SAF (1812), it is critical to fill these gaps in order to better constrain paleo-rupture lengths and to increase the probability of capturing the full spatial record of surface rupturing earthquakes. In this study, we target a new site within the 100 km long stretch of the San Andreas Fault between the Frazier Mountain and Pallett Creek paleoseismic sites (Figure 1), near Elizabeth Lake, California. Prior excavations at the site during 1998-1999 encountered promising stratigraphy but these studies were hindered by shallow groundwater throughout the site. We began our current phase of investigations in 2012, targeting the northwestern end of a 40 x 350 m fault-parallel depression that defines the site (Figure 2). Subsequent investigations in 2013 and 2014 focused on the southeastern end of the depression where the fault trace is constrained between topographic highs and is proximal to an active drainage. In total, our paleoseismic investigations consist of 10 fault-perpendicular trenches that cross the depression (Figure 2) and expose a >2000 year depositional record. These trenches reveal that the thickest section of young stratigraphy occurs at the southeastern end of the site where the fault zone projects through an area of relatively continuous sediment accumulation from a northeast-flowing drainage. This portion of the site contains a 3-m-wide pop-up structure within the fault zone that separates alternating alluvial and paludal deposits south of the fault zone from a thick organic-rich loam on the north side of the fault zone. Faults, fissures, and tilted blocks provide evidence for 4 to 5 paleoearthquakes since ca. 1250 A.D. Radiocarbon dating established that the site has a significant component of detrital charcoal producing an age spread of up to 500 years. To supplement our age chronology we incorporated ages from collections of micro-scale organic fractions and post-IR infrared stimulated luminescence dating in order to better estimate true layer ages.

Conference Paper↗

USGS2021

Fluid-earthquake and earthquake-earthquake interactions in southern Kansas, USA

An increase in injection activity associated with energy production in southern Kansas starting in 2013 has been linked to the occurrence of more than 130,000 earthquakes (M −1.5 to 4.9) between 2014 and 2017. Studies suggest that the dramatic increase in seismicity rate is related to wastewater injection into the highly permeable Arbuckle formation. Most of the seismicity is located in the underlying crystalline basement, for which hydrological properties and specific fault geometries are unknown. Additionally, some earthquake clusters occurred relatively far (tens of kilometers) from the main injection wells. Therefore, the effect of pore pressure diffusion may be insufficient to explain the relation between the volume of injected fluids and the spatiotemporal evolution of seismicity. Combining physical models (static stress and poroelasticity) and a statistical cluster analysis applied to a high-resolution relocated catalog, we analyze the evolution of seismicity in southern Kansas. We find that pore pressure changes (Δ p ) and Coulomb stress changes (ΔCFS) due to fluid diffusion smaller than 0.1 MPa are enough to initiate seismic sequences, which then evolve depending on their distance from the major injection wells. However, we find that earthquake sequences have different seismogenic responses to Δ p and ΔCFS in terms of triggering threshold. In regions located close to disposal wells (Harper area) our cluster analysis suggests that both earthquake interactions and fluid diffusion control the evolution of seismicity. On the other hand, at greater distances (Milan area), where clustering behavior suggests greater earthquake interactions, we find that coseismic ΔCFS are larger than Δ p .

Kansas, Oklahoma↗

USGS2024

Examining the connections between earthquake swarms, crustal fluids, and large earthquakes in the context of the 2020-2024 Noto Peninsula, Japan, earthquake sequence

Earthquake swarms are most commonly composed of small-magnitude earthquakes – those that may in some cases be felt but without causing damage. However, a recent study by Yoshida et al. (2023, https://doi.org/10.1029/2023GL106023) analyzed a swarm beneath the Noto Peninsula in Japan that, after more than two years of moderate-magnitude seismicity, triggered the moment magnitude (Mw) 6.2 Suza mainshock. Based on high-precision earthquake locations and a slip inversion of the mainshock, these authors found that the Mw 6.2 Suza earthquake occurred on the updip extension of a fault that was active during the swarm, likely driven by fluid pressure perturbations. After publication of that paper, a much larger and more destructive Mw 7.5 event occurred nearby. These events underscore the potential for swarms to be precursors to large, damaging earthquakes. Forecasting the eventual evolution of swarms is currently very challenging but could be aided in the future by new observations and models.

Noto Peninsula↗

USGS2025

Broadband stochastic simulation of earthquake ground motions with multiple strong phases with an application to the 2023 Kahramanmaraş, Turkey (Türkiye), earthquake

Stochastic ground motion simulation models are often less accurate at lower frequencies than at higher frequencies when fitting recorded data unless supplemented by a deterministic forward directivity velocity pulse model. Moreover, time-modulated stochastic models, which adjust ground motion amplitudes over time, typically use functions that fail to capture multiple strong-motion phases. The February 2023 Turkey (Türkiye) earthquake exhibited diverse recordings, including near-fault and far-field motions with pulse-like and non-pulse-like characteristics, along with single and multiple strong-motion phases. To better represent such a diverse set of recordings, this study enhances a fully non-stationary site-based stochastic model without combining it with a deterministic model. Improvements include a new band-pass filter with upper- and lower-frequency limits, which refines the representation of the low-frequency content. Moreover, a time-modulating function that can represent energy arrival in multiple strong phases is introduced. The reference model’s parameters are identified by fitting to the energy content, zero-level crossings, and cumulative counts of positive-minima and negative-maxima of a target accelerogram. This fitting procedure is modified to address the increased number of parameters. These improvements broaden the reference model’s applicability while preserving its simplicity, a key aspect appealing to engineering practitioners. The improved model’s applicability is demonstrated by simulating a dataset from the February 2023 Türkiye earthquake, and the accuracy is tested using a pulse-like Next Generation Attenuation Relationships for Western United States dataset. Validations are performed based on total energy, zero-level crossings, Fourier amplitude spectrum, elastic response spectra, and peak ground motion parameters. Validations are performed schematically in the time and frequency domains and quantitatively using goodness-of-fit scores, various validation-metrics errors, and inter-period correlations. Overall, the improved stochastic model can effectively simulate a set of diverse ground motion recordings, including near-fault pulse-like records, records with multiple strong phases, and far-field motions across a broad frequency range.

Kahramanmaraş↗

USGS2022

Revised earthquake geology inputs for the central and eastern United States and southeast Canada for the 2023 National Seismic Hazard Model

It has been nearly a decade since updates to seismic and fault sources in the central and eastern United States (CEUS) were last assessed for the 2012 Central and Eastern United States Seismic Source Characterization for nuclear facilities (CEUS-SSCn) and 2014 United States Geological Survey National Seismic Hazard Model (NSHM) for the conterminous U.S. In advance of the 2023 NSHM update, we created 3 related geospatial databases to summarize and characterize new fault source information for the CEUS. These include fault section, fault-zone polygon, and earthquake geology (fault slip rate, earthquake recurrence intervals) databases which document updates to fault parameters used in prior seismic hazard models in this region. The 2012 CEUS-SSCn and 2014 NSHM fault models served as a foundation, as we revised and added fault sources where new published studies documented significant changes to our understanding of fault location, geometry, or activity. We added 9 new fault sections that meet the criteria of (1) a length ≥7 km, (2) evidence of recurrent Quaternary tectonic activity, and (3) documentation that is publicly available in a peer-reviewed source. The prior CEUS models only included 6 fault sections (sources) and 10 fault-zone polygons (previously called repeating large magnitude earthquake (RLME) polygons). The revised databases include 15 fault sections and 10 fault zone polygons. Updates to the faults constitute a 150% increase in fault sections, but no change in the number of fault-zone polygons, although some fault-zone polygons differ from RLME polygons used in prior models. No faults were removed from past models. Several seismic zones and suspected faults were evaluated but not included in this update due to a lack of information about fault location, geometry, or recurrent Quaternary activity. These updates to the fault sections, fault-zone polygons, and earthquake geology databases will inform fault geometry and activity rates of CEUS sources during the 2023 NSHM implementation.

Seismological Research Letters↗

USGS2013

Geological and geophysical evaluation of the mechanisms of the great 1899 Yakutat Bay earthquakes

We have used tectonic, geologic, and seismologic observations to reevaluate the mechanisms and seismotectonic significance of the two great ( M w = 8.1 and 8.2) September 1899 Yakutat Bay earthquakes. In their comprehensive study of these earthquakes between 1905 and 1910, Tarr and Martin (1912) showed that these events were accompanied by shoreline changes in Yakutat Bay that ranged from 14.4 m emergence to 2.1 m submergence, uplift of about 1 m at Yakataga 160 km west of Yakutat Bay, and by several zones of surface fissures on land. Although major earthquake faults were not found, Tarr and Martin postulated that the shoreline displacements were caused by vertical movements on a system of concealed steep normal faults and that the fissure zones on ridges were along subsidiary faults. Our geologic studies in the Yakutat Bay region indicate that: (1) the emergent shorelines along Yakutat Bay define a broad upwarp roughly 50 km ́ 30 km that is primarily related to reverse slip on local concealed shallowly dipping thrust faults; (2) the reported subsidence was due largely, or entirely, to nontectonic surficial submergence of unconsolidated deposits; and (3) most, if not all, of the zones of surface fractures related to the 1899 earthquakes are “sackung” that were probably caused by large-scale gravitational slumping of steep slopes, rather than faulting. A small number of early damped seismograms and the vertical uplift data were used to constrain the fault slippage that occurred during the two great earthquakes of 1899. Seismic moments determined from 50-s surface wave amplitudes are ~2 ́x 10 21 N m for these two events, equivalent to M w 8.1. Uplift determined from raised shorelines within Yakutat Bay can be accounted for by the 10 September event alone, and these data can be fit by ~10- to 20-m dip slippage on three dextral oblique thrust faults that dip ~30° northeast or north. Faulting complexity is also shown by the S-wave seismogram of the 10 September shock, which lasted ~3 min and shows at least three distinct long-period pulses. The large seismic moment of the 4 September event and uplift of ~1 m at Yakataga suggest a 150-km westward extension of faulting along the foothills fold-and-thrust zone. Our reassessment suggests that, although some portions of the complex plate boundary zone ruptured in 1899, regional seismic hazard is currently significant. First of all, none of the potentially tsunamigenic offshore thrust faults west of the Pamplona zone slipped in 1899. It is unlikely that all of the onshore thrust faults south of the Chugach–St. Elias thrust fault system did either. Furthermore, more than 100 years of convergence at 48 mm/yr across the region has reloaded faults that slipped in 1899 and added further strains on those that did not. Matters are much less clear for the Yakutat Bay thrusts because although they slipped in 1899, we have no good constraints on the convergence rates across these faults. The most recent pre-1899 uplift event in Yakutat Bay was at least 380 ± 70 years ago.

Alaska↗

USGS2007

Coseismic source model of the 2003 Mw 6.8 Chengkung earthquake, Taiwan, determined from GPS measurements

A coseismic source model of the 2003 Mw 6.8 Chengkung, Taiwan, earthquake was well determined with 213 GPS stations, providing a unique opportunity to study the characteristics of coseismic displacements of a high-angle buried reverse fault. Horizontal coseismic displacements show fault-normal shortening across the fault trace. Displacements on the hanging wall reveal fault-parallel and fault-normal lengthening. The largest horizontal and vertical GPS displacements reached 153 and 302 mm, respectively, in the middle part of the network. Fault geometry and slip distribution were determined by inverting GPS data using a three-dimensional (3-D) layered-elastic dislocation model. The slip is mainly concentrated within a 44 ?? 14 km slip patch centered at 15 km depth with peak amplitude of 126.6 cm. Results from 3-D forward-elastic model tests indicate that the dome-shaped folding on the hanging wall is reproduced with fault dips greater than 40??. Compared with the rupture area and average slip from slow slip earthquakes and a compilation of finite source models of 18 earthquakes, the Chengkung earthquake generated a larger rupture area and a lower stress drop, suggesting lower than average friction. Hence the Chengkung earthquake seems to be a transitional example between regular and slow slip earthquakes. The coseismic source model of this event indicates that the Chihshang fault is divided into a creeping segment in the north and the locked segment in the south. An average recurrence interval of 50 years for a magnitude 6.8 earthquake was estimated for the southern fault segment. Copyright 2007 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗

USGS2004

Photomosaics and logs of trenches on the San Andreas Fault, Thousand Palms Oasis, California

We present photomosaics and logs of the walls of trenches excavated for a paleoseismic study at Thousand Palms Oasis (Fig. 1). The site is located on the Mission Creek strand of the San Andreas fault zone, one of two major active strands of the fault in the Indio Hills along the northeast margin of the Coachella Valley (Fig. 2). The Coachella Valley section is the most poorly understood major part of the San Andreas fault with regard to slip rate and timing of past large-magnitude earthquakes, and therefore earthquake hazard. No large earthquakes have occurred for more than three centuries, the longest elapsed time for any part of the southern San Andreas fault. In spite of this, the Working Group on California Earthquake Probabilities (1995) assigned the lowest 30-year conditional probability on the southern San Andreas fault to the Coachella Valley. Models of the behavior of this part of the fault, however, have been based on very limited geologic data. The Thousand Palms Oasis is an attractive location for paleoseismic study primarily because of the well-bedded late Holocene sedimentary deposits with abundant layers of organic matter for radiocarbon dating necessary to constrain the timing of large prehistoric earthquakes. Previous attempts to develop a chronology of paleoearthquakes for the region have been hindered by the scarcity of in-situ 14C-dateable material for age control in this desert environment. Also, the fault in the vicinity of Thousand Palms Oasis consists of a single trace that is well expressed, both geomorphically and as a vegetation lineament (Figs. 2, 3). Results of our investigations are discussed in Fumal et al. (2002) and indicate that four and probably five surface-rupturing earthquakes occurred along this part of the fault during the past 1200 years. The average recurrence time for these earthquakes is 215 ± 25 years, although interevent times may have been as short as a few decades or as long as 400 years. Thus, although the elapsed time since the most recent earthquake, about 320 years, is about 50% longer than the average recurrence time, it is not necessarily unprecedented.

California↗

USGS2008

Paleoseismic Investigations of the Walnut Site on the San Jacinto Fault

The Walnut paleoseismic site is located along the northern San Jacinto fault about 3 km southeast of the San Bernardino, California city center (Figures 1, 2). More than 340 meters of trenches were excavated across the fault zone at this site as part of an Alquist-Priolo fault study (Figure 3). We photographed and logged the SE wall and most of the NE wall of trench 1, both walls of trenches 2 and 7, the NW walls of trenches 3 and 4 and the SE wall of trench 6. After carefully cleaning the trench walls we put up a 1m by 0.5m string and nail grid. For trenches 1, 2, 6, and 7, we photographed each 1m by 0.5m panel individually and photologged on these unrectified photos. These large-scale photos were later rectified to remove the distortion due to irregularities in the trench walls and slight distortion introduced by the camera lens. Field linework was then transferred to the rectified photomosaics. We also took a set of overview photographs for each trench taken from the top of the trench towards the opposite wall. We spliced together these overview photos to make photomosaics of all of the trenches. Because the photos were taken at a downward angle, there is significant distortion. Some of this distortion has been corrected: an attempt was made to keep horizontal grid lines horizontal and there has been some horizontal scaling to align vertical lines between benches. Although the string and nail grid spacing is 1 meter by 0.5 meter, because of the distortion in the photos and subsequent adjustments, the scale is variable along the benches, from bench to bench and from trench to trench for these overview mosaics. This report serves principally as a repository for the overview photomosaics. Sheet 1 shows the overview mosaics for both walls of trenches 1 and 2 along with some linework including most of the fault traces, a prominent unconformity within the fluvial deposits and the larger bodies of liquefied sand. Sheet 2 shows the overview mosaics for the SE wall of trench 3 and the NW wall of trench 4 along with photomosaics of both walls of trench 7 and the SE wall of trench 6 that were complied from the rectified, large scale photos. No linework has been portrayed on these photomosaics. Sheet 3 shows the overview mosaics of both walls of trench 1 with the locations of detrital charcoal samples that were collected. A later version of this report will contain photomosaics for trenches 1 and 2 compiled from the individual, fully rectified photos covering each 1m by 0.5m area with detailed linework superimposed. The trenches exposed a main, Holocene-active, fault zone about 5-12 m wide which juxtaposes Late Pleistocene (?) fluvial sand and gravel southwest of the fault against organic-rich, Holocene fine sand, silt and clay apparently deposited in a marsh. Most of the faults in the main zone appear to rupture to the ground surface making it impossible to resolve individual prehistoric earthquakes along this zone. However, the main fault zone is associated with a slight upwarp and growth strata associated with this folding has recorded evidence for at least 6 late-Holocene earthquakes. Deformation due to liquefaction is further evidence of large earthquakes at these horizons. The fine-grained Holocene deposits contain abundant detrital charcoal. We have so far dated 36 samples from 20 stratigraphic layers. We used 27 of these dates in an Oxcal chronological model in order to constrain the ages of the six earthquakes. Too few samples have so far been dated from the uppermost horizons so the ages of the two youngest earthquakes recorded at the site are poorly constrained. However, it appears that the youngest sediment at the site was deposited about 2000 years ago and the thick surface soil indicate that the two youngest earthquake recorded at the site may be about this old. The radiocarbon dates provide good constraints on the ages of the four older earthquakes (Table 1). The ages of these four earthquakes suggest an aver

Open-File Report↗

USGS1999

Timing of paleoearthquakes on the northern Hayward Fault: Preliminary evidence in El Cerrito, California

The Working Group on California Earthquake Probabilities estimated that the northern Hayward fault had the highest probability (0.28) of producing a M7 Bay Area earthquake in 30 years (WGCEP, 1990). This probability was based, in part, on the assumption that the last large earthquake occurred on this segment in 1836. However, a recent study of historical documents concludes that the 1836 earthquake did not occur on the northern Hayward fault, thereby extending the elapsed time to at least 220 yr ago, the beginning of the written record. The average recurrence interval for a M7 on the northern Hayward is unknown. WGCEP (1990) assumed an interval of 167 years. The 1996 Working Group on Northern California Earthquake Potential estimated ~210 yr, based on extrapolations from southern Hayward paleoseismological studies and a revised estimate of 1868 slip on the southern Hayward fault. To help constrain the timing of paleoearthquakes on the northern Hayward fault for the 1999 Bay Area probability update, we excavated two trenches that cross the fault and a sag pond on the Mira Vista golf course. As the site is on the second fairway, we were limited to less than ten days to document these trenches. Analysis was aided by rapid C-14 dating of more than 90 samples which gave near real-time results with the trenches still open. A combination of upward fault terminations, disrupted strata, and discordant angular relations indicates at least four, and possibly seven or more, surface faulting earthquakes occurred during a 1630-2130 yr interval. Hence, average recurrence time could be <270 yr, but is no more than 710 yr. The most recent earthquake (MRE) occurred after AD 1640. Preliminary analysis of calibrated dates supports the assumption that no large historical (post-1776) earthquakes have ruptured the surface here, but the youngest dates need more corroboration. Analyses of pollen for presence of non-native species help to constrain the time of the MRE. The earthquake recurrence estimates described in this report are preliminary and should not be used as a basis for hazard estimates. Additional trenching is planned for this location to answer questions raised during the initial phase of trenching.

California↗

USGS1979

Preliminary report on drilling in and near the San Andreas fault zone, California; Dry Lake Valley No. 1

The purpose of drilling into the San Andreas fault zone is threefold. First, recovery of material from depth will permit determination of the composition and constitutive properties of both solid and fluid phases. Second, it will enable measurements of the physical state of the fault zone to be made (that is, the state of in-situ stress, pore pressure, and temperature). Finally, it will allow emplacement of instruments at depth in the fault zone as part of experiments related to earthquake prediction. This report briefly describes the drilling history and preliminary analysis of fault zone materials from site Dry Lake Valley (DLV) No. 1 (Fig. 1). The site is located at Lat. 36028.09'N, Long. 12103.32'W. This site was chosen in an attempt to drill a 1 km deep well that would reach earthquake foci. Although the entire central section of the San Andreas fault is characterized by moderate seismicity and aseismic creep, Dry Lake Valley has particularly shallow earthquakes as well as a fairly high creep rate (18-20 mm/year, S. Schultz, pers. comm.). Fig. 2 is a longitudinal section along the San Andreas fault showing high precision locations of magnitude one or greater earthquakes that occurred in 1973-1975 (from W. Ellsworth, per. comm.). The depth accuracy of the earthquakes shown is about ± 0.5 km, and it is obvious from Figure 2 that Dry Lake Valley earthquakes are extremely shallow. As shown in Figure 1 the general geology of the central section of the San Andreas fault consists of Tertiary to Jurassic sedimentary and volcanic rocks overlying Cretaceous age Gabilan granite of the Salinian Block on the west side of the fault and Cretaceous-Jurassic sedimentary assemblages of the Franciscan Complex to the east of the fault. Dibblee (1979) summarizes the geology along the central San Andreas fault. At the Dry Lake Valley site as much as 2 km of Pliocene-Miocene marine sediments (Etchegoin and Santa Margarita formation) overlie Franciscan rocks on the northeast side of the San Andreas fault. On the southwestern side of the fault, 1-2 km of Pliocene-Miocene valley sediments and granitic conglomerate overlies the Gabilan granite.

California↗

USGS1994

Some comparisons between mining-induced and laboratory earthquakes

Although laboratory stick-slip friction experiments have long been regarded as analogs to natural crustal earthquakes, the potential use of laboratory results for understanding the earthquake source mechanism has not been fully exploited because of essential difficulties in relating seismographic data to measurements made in the controlled laboratory environment. Mining-induced earthquakes, however, provide a means of calibrating the seismic data in terms of laboratory results because, in contrast to natural earthquakes, the causative forces as well as the hypocentral conditions are known. A comparison of stick-slip friction events in a large granite sample with mining-induced earthquakes in South Africa and Canada indicates both similarities and differences between the two phenomena. The physics of unstable fault slip appears to be largely the same for both types of events. For example, both laboratory and mining-induced earthquakes have very low seismic efficiencies {Mathematical expression} where ??a is the apparent stress and {Mathematical expression} is the average stress acting on the fault plane to cause slip; nearly all of the energy released by faulting is consumed in overcoming friction. In more detail, the mining-induced earthquakes differ from the laboratory events in the behavior of ?? as a function of seismic moment M0. Whereas for the laboratory events ?????0.06 independent of M0, ?? depends quite strongly on M0 for each set of induced earthquakes, with 0.06 serving, apparently, as an upper bound. It seems most likely that this observed scaling difference is due to variations in slip distribution over the fault plane. In the laboratory, a stick-slip event entails homogeneous slip over a fault of fixed area. For each set of induced earthquakes, the fault area appears to be approximately fixed but the slip is inhomogeneous due presumably to barriers (zones of no slip) distributed over the fault plane; at constant {Mathematical expression}, larger events correspond to larger??a as a consequence of fewer barriers to slip. If the inequality ??a/ {Mathematical expression} ??? 0.06 has general validity, then measurements of ??a=??Ea/M0, where ?? is the modulus of rigidity and Ea is the seismically-radiated energy, can be used to infer the absolute level of deviatoric stress at the hypocenter. ?? 1994 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗

USGS2015

The 2011 Virginia M5.8 earthquake: Insights from seismic reflection imaging into the influence of older structures on eastern U.S. seismicity

The Mineral, Virginia (USA), earthquake of 23 August 2011 occurred at 6– 8 km depth within the allochthonous terranes of the Appalachian Piedmont Province, rupturing an ~N36°E striking reverse fault dipping ~50° southeast. This study used the Interstate Highway 64 seismic refl ection profi le acquired ~6 km southwest of the hypocenter to examine the structural setting of the earthquake. The profi le shows that the 2011 earthquake and its aftershocks are almost entirely within the early Paleozoic Chopawamsic volcanic arc terrane, which is bounded by listric thrust faults dipping 30°–40° southeast that sole out into an ~2-km-thick, strongly refl ective zone at 7– 12 km depth. Refl ectors above and below the southward projection of the 2011 earthquake focal plane do not show evidence for large displacement, and the updip projection of the fault plane does not match either the location or trend of a previously mapped fault or lithologic boundary. The 2011 earthquake thus does not appear to be a simple reactivation of a known Paleozoic thrust fault or a major Mesozoic rift basin-boundary fault. The fault that ruptured appears to be a new fault, a fault with only minor displacement, or to not extend the ~3 km from the aftershock zone to the seismic profi le. Although the Paleozoic structures appear to infl uence the general distribution of seismicity in the area, Central Virginia seismic zone earthquakes have yet to be tied directly to specifi c fault systems mapped at the surface or imaged on seismic profiles.

Virginia↗

USGS2018

Modeling the Holocene slip history of the Wasatch fault (Utah): Coseismic and postseismic Coulomb stress changes and implications for paleoseismicity and seismic hazard

The Wasatch fault zone defines the eastern boundary of the actively extending Basin and Range Province (Utah, western United States) and poses a significant seismic hazard to the metropolitan areas along the Wasatch Range. A wealth of paleoseismological data documents ∼24 surface-rupturing Mw ≥ 7 earthquakes along the Wasatch fault during the past 6400 yr. Here, we simulated the Holocene earthquake sequence on the Wasatch, Oquirrh−Great Salt Lake, and West Valley faults using three-dimensional finite-element forward modeling with the goal to calculate coseismic and postseismic Coulomb stress changes and to evaluate the slip and magnitude of hypothetical present-day and future earthquakes. Our results show that a good fit between modeled and observed paleoevents and time-integrated slip rates can be achieved within the uncertainties of the paleoseismological record and model parameters like the fault geometry. The Coulomb stress change analysis for selected paleoearthquakes showed that maximum positive stress changes are induced on faults located along strike of the source fault, while faults parallel to the source fault are generally located in stress shadow zones. Postseismic viscoelastic relaxation considerably modifies the coseismic stress changes; the resulting transient stress changes are recognizable for more than 100 yr after an earthquake. The modeled present-day state of Coulomb stress changes shows that the Brigham City, Salt Lake City, and Provo segments of the Wasatch fault are prone to failure in a Mw ≥ 6.8 earthquake. Our study shows that simulation of an entire earthquake sequence based on a paleoseismological record is feasible and facilitates identification of possible gaps and inconsistencies in the paleoseismological record. Therefore, forward modeling of earthquake sequences may ultimately contribute to improved seismic hazard estimates.

Utah↗

USGS1974

Reconnaissance engineering geology of Sitka and vicinity, Alaska, with emphasis on evaluation of earthquake and other geologic hazards

A program to study the engineering geology of most of the larger Alaska coastal communities and to evaluate their earthquake and other geologic hazards was started following the 1964 Alaska earthquake; this report about Sitka and vicinity is a product of that program. Field-study methods were of a reconnaissance nature, and thus the interpretations in the report are subject to revision as further information becomes available. This report can provide broad geologic guidelines for planners and engineers during preparation of land-use plans. The use of this information should lead to minimizing future loss of life and property due to geologic hazards, especially during very large earthquakes. Landscape of Sitka and surrounding area is characterized by numerous islands and a narrow strip of gently rolling ground adjacent to rugged mountains; steep valleys and some fiords cut sharply into the mountains. A few valley floors are wide and flat and grade into moderate-sized deltas. Glaciers throughout southeastern Alaska and elsewhere became vastly enlarged during the Pleistocene Epoch. The Sitka area presumably was covered by ice several times; glaciers deeply eroded some valleys and removed fractured bedrock along some faults. The last major deglaciation occurred sometime before 10,000 years ago. Crustal rebound believed to be related to glacial melting caused land emergence at Sitka of at least 35 feet (10.7 m) relative to present sea level. Bedrock at Sitka and vicinity is composed mostly of bedded, hard, dense graywacke and some argillite. Beds strike predominantly northwest and are vertical or steeply dipping. Locally, bedded rocks are cut by dikes of fine-grained igneous rock. Host bedrock is of Jurassic and Cretaceous age. Eight types of surficial deposits of Quaternary age were recognized. Below altitudes of 3S feet (10.7 m), the dominant deposits are those of modern and elevated shores and deltas; at higher altitudes, widespread muskeg overlies a mantle of volcanic ash which commonly overlies glacial drift. Alluvial deposits are minor. Man-emplaced embankment fill, chiefly sandy gravel, covers many muskeg and former offshore areas; quarried blocks of graywacke are placed to form breakwaters and to edge large areas of embankment fill and modified ground. The geologic structure of the area is known only in general outlines. Most bedded Mesozoic rocks probably are part of broad northwest-trending complexes of anticlines and synclines. Intrusion of large bodies of plutonic igneous rocks occurred in Tertiary and Cretaceous time. Extensive faulting is suggested by the numerous linear to gently curving patterns of some fiords, lakes, and valleys, and by a group of Holocene volcanoes and cinder cones. Two major northwest-striking fault zones are most prominent: (1) the apparently inactive Chichagof-Sitka fault, about 2.5 miles (4.0 km) northeast of Sitka, and {2) part of the active 800-mile- (1,200-km) long Fairweather-Queen Charlotte Islands fault system, lying about 30 miles (48 km) southwest of the city. Many earthquakes have been reported as felt at Sitka since 1832, when good records were first maintained; several shocks were very strong, but none of them caused severe damage. The closest major earthquake (magnitude about 7.3) causing some damage to the city occurred July 30, 1972, and had an epicenter about 30 miles (48 km) to the southwest. Movement along the Fairweather-Queen Charlotte Islands fault system apparently caused most of the earthquakes felt at Sitka. The probability of destructive earthquakes at Sitka is unknown. The tectonics of the region and the seismic record suggest that sometime in the future an earthquake of a magnitude of about 8 and related to the Fairweather-Queen Charlotte Islands fault system probably will occur in or near the area. Effects from some nearby major earthquakes could cause substantial damage at Sitka. Eight possible effects are as follows: 1. Sudden dis

Open-File Report↗

USGS1996

Damage and restoration of geodetic infrastructure caused by the 1994 Northridge, California, earthquake

We seek to restore the integrity of the geodetic network in the San Fernando, Simi, Santa Clarita Valleys and in the northern Los Angeles Basin by remeasurement of the network and identification of BMs which experienced non-tectonic displacements associated with the Northridge earthquake. We then use the observed displacement of BMs in the network to portray or predict the permanent vertical and horizontal deformation associated with the 1994 Northridge earthquake throughout the area, including sites where we lack geodetic measurements. To accomplish this, we find the fault geometry and earthquake slip that are most compatible with the geodetic and independent seismic observations of the earthquake. We then use that fault model to predict the deformation everywhere at the earth's surface, both at locations where geodetic observations exist and also where they are absent. We compare displacements predicted for a large number of numerical models of the earthquake faulting to the coseismic displacements, treating the earthquake fault as a cut or discontinuity embedded in a stiff elastic solid. This comparison is made after non-tectonic deformation has been removed from the measured elevation changes. The fault slip produces strain in the medium and deforms the ground surface. The model compatible with seismic observations that best fits the geodetic data within their uncertainties is selected. The acceptable model fault bisects the mainshock focus, and the earthquake size , magnitude, is compatible with the earthquake size measured seismically. Our fault model was used to identify geodetic monuments on engineered structures that were anomalously displaced by the earthquake.

California↗

USGS2002

Paleoseismology at high latitudes: Seismic disturbance of upper Quaternary deposits along the Castle Mountain fault near Houston, Alaska

Most paleoseismic studies are at low to moderate latitudes. Here we present results from a high-latitude (61°30′ N) trenching study of the Castle Mountain fault in south-central Alaska. This fault is the only one known in the greater Anchorage, Alaska, area with historical seismicity and a Holocene fault scarp. It strikes east-northeast and cuts glacial and postglacial sediments in an area of boreal spruce-birch forest, shrub tundra, and sphagnum bog. The fault has a prominent vegetation lineament on the upthrown, north side of the fault. Nine trenches were logged across the fault in glacial and postglacial deposits, seven along the main trace, and two along a splay. In addition to thrust and strike-slip faulting, important controls on observed relationships in the trenches are the season in which faulting occurred, the physical properties of the sediments, liquefaction, a shallow water table, soil-forming processes, the strength of the modern root mat, and freeze-thaw processes. Some of these processes and physical properties are unique to northern-latitude areas and result in seismic disturbance effects not observed at lower latitudes. The two trenches across the Castle Mountain fault splay exposed a thrust fault and few liquefaction features. Radiocarbon ages of soil organic matter and charcoal within and overlying the fault indicate movement on the fault at ca. 2735 cal. (calendar) yr B.P. and no subsequent movement. In the remaining seven trenches, surface faulting was accompanied by extensive liquefaction and a zone of disruption 3 m or more wide. The presence of numerous liquefaction features at depths of <0.5–1.0 m indicates faulting when the ground was not frozen—i.e., from about April to October. Sandy-matrix till, sand, silt, gravel, and pebbly peat were injected up to the base of the modern soil, but did not penetrate the interlocking spruce-birch root mat. The strength of the root mat prohibited development of a nonvegetated scarp face and colluvial wedge. In only one trench did we observe a discrete fault plane with measurable offset. It lay beneath a 2-m-thick carapace of liquefied sand and silt and displayed a total of 0.9–1.85 m of thrust motion since deposition of the oldest deposits in the trenches at ca. 13,500 yr B.P. We found liquefaction ejecta on paleosols at only one other trench, where there were bluejoint ( Calamagrostis canadensis ) tussocks that lacked an extensive root mat. From crosscutting relationships, we interpret three paleoliquefaction events on the main trace of the Castle Mountain fault: 2145–1870, 1375–1070, and 730–610 cal. yr B.P. These four earthquakes on the Castle Mountain fault in the past ∼2700 yr indicate an average recurrence interval of ∼700 yr. As it has been 600–700 yr since the last significant earthquake, a significant (magnitude 6–7) earthquake in the near future may be likely. Paleoseismic data indicate that the timing and recurrence interval of megathrust earthquakes is similar to the timing and recurrence interval of Castle Mountain fault earthquakes, suggesting a possible link between faulting on the megathrust and on “crustal” structures.

Alaska↗