Geology topics

Richard W. Briggs

Publications and source records attributed to Richard W. Briggs.

At least 37 records · Page 2Linked to original sources

USGS2023

The USGS 2023 Conterminous U.S. time‐independent earthquake rupture forecast

We present the 2023 U.S. Geological Survey time‐independent earthquake rupture forecast for the conterminous United States, which gives authoritative estimates of the magnitude, location, and time‐averaged frequency of potentially damaging earthquakes throughout the region. In addition to updating virtually all model components, a major focus has been to provide a better representation of epistemic uncertainties. For example, we have improved the representation of multifault ruptures, both in terms of allowing more and less fault connectivity than in the previous models, and in sweeping over a broader range of viable models. An unprecedented level of diagnostic information has been provided for assessing the model, and the development was overseen by a 19‐member participatory review panel. Although we believe the new model embodies significant improvements and represents the best available science, we also discuss potential model limitations, including the applicability of logic tree branch weights with respect different types of hazard and risk metrics. Future improvements are also discussed, with deformation model enhancements being particularly worthy of pursuit, as well as better representation of sampling errors in the gridded seismicity components. We also plan to add time‐dependent components, and assess implications with a wider range of hazard and risk metrics.

Bulletin of the Seismological Society of America↗

USGS2023

Rapid surface rupture mapping from satellite data: The 2023 Kahramanmaraş, Turkey (Türkiye), earthquake sequence

The 6 February 2023 Kahramanmaraş, Turkey (Türkiye), earthquake sequence produced > 500 km of surface rupture primarily on the left‐lateral East Anatolian (~345 km) and Çardak (~175 km) faults. Constraining the length and magnitude of surface displacement on the causative faults is critical for loss estimates, recovery efforts, rapid identification of impacted infrastructure, and fault displacement hazard analysis. To support these efforts, we rapidly mapped the surface rupture from satellite data with support from remote sensing and field teams, and released the results to the public in near‐real time. Detailed surface rupture mapping commenced on 7 February and continued as high‐resolution (< 1.0 m/pixel) optical images from WorldView satellites (2023 Maxar) became available. We interpreted the initial simplified rupture trace from subpixel offset fields derived from Advanced Land Observation Satellite2 and Sentinel‐1A synthetic aperture radar image pairs available on 8 and 10 February, respectively. The mapping was released publicly on 10 February, with frequent updates, and published in final form four months postearthquake ( Reitman, Briggs, et al. , 2023 ). This publicly available, rapid mapping helped guide fieldwork and constrained U.S. Geological Survey finite‐fault and loss estimate models, as well as stress change estimates and dynamic rupture models.

Kahramanmaraş↗

USGS2023

Rapid characterization of the February 2023 Kahramanmaraş, Turkey, earthquake sequence

The 6 February 2023 M w "> M w 7.8 Pazarcık and subsequent M w "> M w 7.5 Elbistan earthquakes generated strong ground shaking that resulted in catastrophic human and economic loss across south‐central Türkiye and northwest Syria. The rapid characterization of the earthquakes, including their location, size, fault geometries, and slip kinematics, is critical to estimate the impact of significant seismic events. The U.S. Geological Survey National Earthquake Information Center (NEIC) provides real‐time monitoring of earthquakes globally, including rapid source characterization and impact estimates. Here, we describe the seismic characterization products generated and made available by the NEIC over the two weeks following the start of the earthquake sequence in southeast Türkiye, their evolution, and how they inform our understanding of regional seismotectonics and hazards. The kinematics of rupture for the two earthquakes was complex, involving multiple fault segments. Therefore, incorporating observations from rupture mapping was critical for characterizing these events. Dense local datasets facilitated robust source characterization and impact assessment once these observations were obtained and converted to NEIC product input formats. We discuss how we may improve the timeliness of NEIC products for rapid assessment of future seismic hazards, particularly in the case of complex ruptures.

The Seismic Record↗

USGS2023

Seismic source characterization in central and eastern United States

This report documents earthquake geology studies conducted in the central and eastern United States (CEUS) between 2018 and 2023. The overarching goal of the earthquake geology studies was to identify and characterize fault sources in the New Madrid seismic zone, the eastern Tennessee seismic zone, and the Charleston seismic zone. These studies primarily relied on the interpretation of high-resolution topography, morphotectonic analysis of 10-meter (m) digital elevation models, field reconnaissance, and integration with new and existing subsurface datasets. Results from each study are placed into a seismic hazard framework. The first two chapters focus on the New Madrid seismic zone, which is the most seismically active region in the CEUS, to better characterize the southern extent of the Reelfoot fault in the Obion River valley, and identify and characterize proposed faults along the margins of Crowleys Ridge in the center of the Mississippi River embayment. Crowleys Ridge is a ~320-kilometer (km) long landform that has been proposed to be formed by intermittent faulting or erosion from the Mississippi River. Legacy seismic reflection data documented Eocene, Mesozoic, and older strata offset by faults on the margins of the ridge (Van Arsdale et al., 1999). A morphotectonic analysis of the topography along Crowleys Ridge indicated southward increases in catchment hypsometric integral and slope. Neotectonic mapping on high-resolution lidar data revealed a series of subparallel linear fault and fold scarps along the margins of Crowleys Ridge. These scarps offset previously mapped fluvial and alluvial surfaces that were <56 thousand years ago (ka) (Rittenour et al., 2007). Integration of new seismic reflection and airborne electromagnetic data, legacy seismic data, with the landscape morphotectonic analysis and neotectonic mapping supports the interpretation that Crowleys Ridge is bound by faults that have been active in the late Quaternary but do not experience modern seismicity. By placing these newly characterized faults into the regional framework, the Crowleys Ridge faults can be interpreted as thrust faults associated with stepovers in the overall dextral fault system, similar to the Reelfoot fault. The results of these studies indicate that seismicity has migrated eastward during the late Quaternary, with the Reelfoot fault accommodating much of the modern-day strain. The Reelfoot fault ruptured in the 1811-1812 New Madrid earthquake sequence (Fuller, 1912) and has a record of past large earthquakes (Tuttle et al., 2002; 2019). Whereas the northern extent of the fault intersects the Mississippi River and has well-expressed surface deformation, questions remained regarding the southern limit of seismicity, deformation, and overall fault length. Analysis of <25 ka river terraces along the Obion River valley in western Tennessee revealed subtle fold scarps, with increased magnitudes of deformation on progressively older terraces, indicative of a longer record of fault movement than has been documented by paleoseismic and paleoliquefaction studies alone (e.g., Kelson et al., 1996; Gold et al., 2019; Tuttle et al., 2019). The third chapter integrates surface and subsurface data to assess the landscape record of surface deformation in the eastern Tennessee seismic zone (ETSZ), which is the second most seismically active region in the CEUS. Seismicity in the ETSZ is deep (5-26 km) and focused in the Proterozoic rock below the Paleozoic detachment. Although the region has experienced moderate (≤ Mw 4.8) seismicity in the instrumental record, the lack of a large historical earthquake and limited paleoseismic and paleoliquefaction evidence of prehistoric large ground shaking events have made seismic hazard characterization of the region challenging. Two models have emerged to predict the style and orientation of active surface deformation in the ETSZ. One suggests that surface rupturing earthquakes would exploit pre-existing faults along the regional structural grain (Cox et al., 2022). The second model advocates for primarily strike-slip motion on east-west or north-south faults, following analyses of recent seismicity (Chapman et al., 1997; Dunn and Chapman, 2006; Daniels and Peng, 2022) and the modern stress field (Levandowski et al., 2018). In our analysis, neotectonic mapping of high-resolution lidar data reveals a concentration of lineaments with east-west orientations, with some corresponding to previously mapped east-west faults that crosscut the northeast-southwest regional structural grain from the Paleozoic orogeny. A morphotectonic analysis of catchments and river segments within the same lithology indicates subtle differences that may be indicative of a longer-term tectonic uplift signal. These changes in morphotectonic metrics spatially correspond to newly mapped lineaments and previously mapped east-west trending faults. Within a regional framework, we suggest that diffuse surface deformation associated with deep seismicity is accommodated on a network on east-west faults. However, further work is needed to better understand the potential late Pleistocene fault activity of these lineaments, and we propose that the ETSZ is still best characterized as an area source in seismic hazard models. The final chapter presents preliminary work to create a geographic information system (GIS) database of recent studies analyzing surface and subsurface datasets and interpretations in the Charleston seismic zone, South Carolina. This GIS database will serve as a foundation for future work to analyze new quality level 1 (QL1; <0.5 m resolution) lidar data over the 1886 magnitude (M) 7 Charleston epicentral region to identify possible fault source sources responsible for the 1886 or other surface-rupturing events. This chapter briefly summarizes three new subsurface datasets, including seismic reflection (Pratt et al., 2022; Liberty, 2022) and seismicity data (Chapman et al., 2016), and one new surface dataset (Marple and Hurd, 2020), and their interpretations of potentially active lineaments and faults in the region.

central and eastern United States↗

USGS2022

The Pondosa fault zone: A distributed dextral-normal-oblique fault system in northeastern California, USA

The tectonic domains of Basin and Range extension, Cascadia subduction zone contraction, and Walker Lane dextral transtension converge in the Mushroom Rock region of northeastern California, USA. We combined analysis of high-resolution topographic data, bedrock mapping, 40 Ar/ 39 Ar geochronology, low-temperature thermochronology, and existing geologic and fault mapping to characterize an extensive dextral-normal-oblique fault system called the Pondosa fault zone. This fault zone extends north-northwest from the Pit River east of Soldier Mountain, California, into moderately high-relief volcanic topography as far north as the Bartle (California) townsite with normal and dextral offset apparent in geomorphology and fault exposures. New and existing 40 Ar/ 39 Ar and radiocarbon dating of offset lava flows provides ages of 12.4 ka to 9.6 Ma for late Cenozoic stratigraphic units. Scarp morphology and geomorphic expression indicate that the fault system was active in the late Pleistocene. The Pondosa fault zone may represent a dextral-oblique accommodation zone between north-south–oriented Basin and Range extensional fault systems and/or part of the Sierra Nevada–Oregon Coast block microplate boundary.

California↗

USGS2022

Rupture scenarios for the 3 June 1770 Haiti earthquake

The 2010 M 7.0 Haiti earthquake provided the impetus to reconsider historical earthquakes in Hispaniola ( Bakun et al. , 2012 ). That earthquake also shed new light on complex fault systems along Haiti’s southern peninsula ( Douilly et al. , 2013 ; Saint Fleur et al. , 2015 ). Recently, the 2021 M 7.2 Nippes earthquake ( Calais et al. , 2022 ; Douilly et al. , 2022 ), and a recent study reconsidering the 1860 sequence ( Martin et al. , 2022 ) further underscored the complexity of fault systems and large earthquake ruptures along the peninsula. Motivated by these studies and recent geological investigations ( Prentice et al. , 2010 ; Saint Fleur et al. , 2020 ), we reconsider the 3 June 1770 Haiti earthquake to explore the conventional assumption that it was the last major ( M ≥7.5) earthquake along the Enriquillo–Plantain Garden fault (EPGF). Accounts provide compelling evidence for substantial liquefaction in the Cul‐de‐Sac plain, one or more likely landslide‐driven tsunami in Gonaïves Bay, and extensive landsliding that created at least three documented landslide dams. We consider three end‐member rupture scenarios that are consistent with available constraints: two scenarios with M 7.7 and rupture lengths of 150–170 km, and one scenario with a ∼90 km rupture and M 7.5. Absent future work to identify and date paleoevents along the southern peninsula, none of these scenarios can be ruled out. Our preferred rupture model extends from the Miragoâne pull‐apart to near la Selle mountain, with a rupture length of 127 km, M 7.6, and a high stress drop. Rupture could have been on the EPGF or on an oblique thrust fault associated with overthrusting of the Massif de la Selle. The results do support the conclusion that the 1770 earthquake was the last major earthquake in southern Haiti, with a magnitude upward of M 7.5 and significantly more severe shaking in southern Haiti than during the 2010 earthquake.

Bulletin of the Seismological Society of America↗

USGS2022

Climatic influence on the expression of strike-slip faulting

Earthquakes on strike-slip faults are preserved in the geomorphic record by offset landforms that span a range of displacements, from small offsets created in the most recent earthquake (MRE) to large offsets that record cumulative slip from multiple prior events. An exponential decay in the number of large cumulative offsets has been observed on many faults, and a leading hypothesis is that climate controls the rate of decay. We present offset measurements compiled from 31 studies of strike-slip faults with evidence of multiple paleoearthquakes and corresponding climatic and tectonic information to test this hypothesis. Both the global compilation and numerical landscape evolution modeling reveal that the decay rate in large offsets is negatively correlated with mean annual precipitation. Faults in dry regions with high drainage density more commonly preserve small MRE offsets, and faults in wet regions with lower drainage density more commonly preserve a mix of small MRE and large cumulative offsets. Geomorphology of faults in different climates supports this result and illustrates precipitation’s effect on the development and preservation of offset channels. Our findings imply that current and past climate affect how displacement on strike-slip faults is recorded and interpreted to inform earthquake history.

Geology↗

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↗

USGS2022

Western U.S. geologic deformation model for use in the U.S. National Seismic Hazard Model 2023

Fault geometry and slip rates are key input data for geologic deformation models, which are a fundamental component of probabilistic seismic hazard analyses (PSHAs). However, geologic sources for PSHA have traditionally been limited to faults with field‐based slip rate constraints, which results in underrepresentation of known, but partially characterized, active faults. Here, we evaluate fault geometries and geologic fault slip rates for the western United States to construct a new geologic deformation model for the U.S. National Seismic Hazard Model 2023 update (NSHM23). In previous NSHM iterations, only faults with published geologic slip rates were included. In the NSHM23 fault sections database compilation, this inclusion criterion was expanded to include faults without known slip rates. In this updated geologic deformation model, preferred slip rates and associated uncertainty distributions are incorporated for faults with slip rates derived from field studies. For faults without site‐specific slip rates, we evaluate a suite of uncertainty distributions derived from broad slip rate categories in the U.S. Geological Survey Quaternary Fault and Fold Database. Preferred slip rate distributions are selected via comparison with geodetic strain rates in tectonic subregions. The resultant moment of the geologic deformation model is generally in deficit compared with the geodetic moment within each region. Primary advances in the NSHM23 geologic deformation model include the following: (1) slip rates are presented as preferred values with uncertainties rather than single values; (2) the representation of the western U.S. active fault network is more complete; and (3) the geologic deformation model leverages geodetic information to assess regional constraints on geologic fault slip rates.

Seismological Research Letters↗

USGS2022

Simplifying complex fault data for systems-level analysis: Earthquake geology inputs for U.S. NSHM 2023

As part of the U.S. National Seismic Hazard Model (NSHM) update planned for 2023, two databases were prepared to more completely represent Quaternary-active faulting across the western United States: the NSHM23 fault sections database (FSD) and earthquake geology database (EQGeoDB). In prior iterations of NSHM, fault sections were included only if a field-measurement-derived slip rate was estimated along a given fault. By expanding this inclusion criteria, we were able to assess a larger set of faults for use in NSHM23. The USGS Quaternary Fault and Fold Database served as a guide for assessing possible additions to the NSHM23 FSD. Reevaluating available data from published sources yielded an increase of fault sections from ~650 faults in NSHM18 to ~1,000 faults proposed for use in NSHM23. EQGeoDB, a companion dataset linked to NSHM23 FSD, contains geologic slip rate estimates for fault sections included in FSD. Together, these databases serve as common input data used in deformation modeling, earthquake rupture forecasting, and additional downstream uses in NSHM development.

Scientific Data↗

USGS2022

Paleoseismic investigation of the Taylorsville fault at the Airport East site, West Valley fault zone, Salt Lake County, Utah

The West Valley fault zone (WVFZ) and Salt Lake City segment (SLCS) of the Wasatch fault zone comprise Holoceneactive normal faults that bound an intrabasin graben in northern Salt Lake Valley, Utah. Both fault zones have evidence of recurrent Holocene surface-faulting earthquakes. A topic of recent research is the seismogenic relation of the antithetic (subsidiary) WVFZ to the Wasatch fault zone—specifically, to what degree are WVFZ earthquakes independent of slip on the SLCS, or other adjacent segments, of the Wasatch fault zone. To improve paleoseismic data for the WVFZ and better understand the seismogenic relation between the WVFZ and Wasatch fault zone, we conducted a fault-trench investigation at the Airport East site, developed new earthquake recurrence and fault sliprate estimates for the WVFZ, and compared WVFZ earthquake timing data with data from the Wasatch fault zone. The Airport East site is near the northern end of the easternmost traces of the WVFZ, collectively referred to as the Taylorsville fault. At this site, we excavated two parallel trenches across a small (~0.5-m high) east-facing fault scarp. Shallow groundwater severely limited trench depth, and we were able to expose deposits only as old as mid-Holocene. However, the late Holocene section contained evidence for three surfacefaulting earthquakes on the Taylorsville fault, as well as earthquake-related deformation (liquefaction and folding) from a fourth earthquake that was likely sourced elsewhere. Based on OxCal modeling of radiocarbon and optically stimulated luminescence ages, the most recent earthquake, AE1, occurred at 0.4 ± 0.2 ka (mean modeled time ± 2σ), earthquake AE2 occurred at 0.6 ± 0.2 ka, and earthquake AE3 occurred at 2.0 ± 0.3 ka. Timing data from the Airport East site indicate the liquefaction and folding event (LE1) occurred at 5.1 ± 0.3 ka. Net vertical displacement across the fault is 0.6–1.1 m, and calculations of mean per-event displacement range from 0.20 to 0.37 m. Inter-event recurrence intervals for the Taylorsville fault at the Airport East site vary from 200 to 1400 yr, and the mean late Holocene (post-2 ka) recurrence interval is 800 yr. Paleoseismic (closed-interval) slip rates range from 0.23 to 1.4 mm/yr, and geologic (open-interval) slip rates range from 0.1–0.2 mm/yr over the past ~5000 yr to 0.2–0.4 mm/yr over the past ~2500 yr. Combining our new Airport East data with previous paleoseismic data for the WVFZ shows that during individual late Holocene earthquakes, surface faulting has occurred on either the Taylorsville or Granger fault, but not both faults at the same time (at least not on the parts of the faults that have been trenched). However, five mid- to late Holocene WVFZ earthquakes have mean modeled times that are either very similar or identical to mean modeled times of SLCS and Weber-segment earthquakes, and temporal correlations of two additional late Pleistocene WVFZ earthquakes with SLCS earthquakes cannot be ruled out. When comparing the earthquake chronologies of the WVFZ and Wasatch fault zone, our new data lend support to the idea that, more often than not, some part of the WVFZ moves in response to, and possibly synchronously with, slip on the Wasatch fault zone.

Utah↗

USGS2022

Mid- to late Quaternary geomorphic and paleoseismic event history, Cheraw fault, southeastern Colorado

Despite its subdued expression and isolated location within the Great Plains of southeastern Colorado, the 80 km long Cheraw fault may be one of the most active faults in North America east of the Rocky Mountains. We present geomorphic analyses, geochronology, and paleoseismic trenching data to (1) document the rupture history of the ∼45 km long southwestern section of the Cheraw fault over the past ∼19 ka, and (2) evaluate slip rate changes for the entire fault over the past ∼200 ka. Results from new trenches excavated at the Old Ranch site show evidence of four surface rupture events since ∼19 ka, each with an average vertical displacement of 0.75 ± 0.15 m. An additional event is likely only slightly older than ∼19 ka. Evidence for relatively small displacements at and near the Old Ranch site indicates that most of these earthquakes were magnitude ( M ) 7 or less and likely did not rupture the full length of the Cheraw fault. Since ∼19 ka, the average slip rate is ∼0.16 ± 0.3 mm/yr near the Old Ranch site with an average interevent time of 3–5 ka. New geochronologic data for mid‐ to late‐Quaternary geomorphic surfaces cut by the Cheraw fault imply rapid incision by local Arkansas River tributaries from ∼145 to ∼100 ka. Maximum vertical offsets of 7–9 m for these surfaces indicate that from ∼19 to >200 ka the average slip rate was no greater than ∼0.03 mm/yr. The accelerated slip rate since ∼19 ka indicates a possible response to rapid erosional unloading and/or a limited late Cenozoic, <40 ka, paleoseismic history for the Cheraw fault.

Colorado↗

USGS2022

How similar was the 1983 Mw 6.9 Borah Peak earthquake rupture to its surface-faulting predecessors along the northern Lost River fault zone (Idaho, USA)?

We excavated trenches at two paleoseismic sites bounding a trans-basin bedrock ridge (the Willow Creek Hills) along the northern Lost River fault zone to explore the uniqueness of the 1983 M w 6.9 Borah Peak earthquake compared to its prehistoric predecessors. At the Sheep Creek site on the southernmost Warm Springs section, two earthquakes occurred at 9.8−14.0 ka (95% confidence) and 6.5−7.1 ka; each had ∼1.9 m of vertical displacement. About 4 km to the southeast, across the Willow Creek Hills, two ruptures at the Arentson Gulch site on the northernmost Thousand Springs section occurred at 9.0−14.7 ka and 6.1−7.5 ka with ∼1.9 m of vertical displacement each. We synthesize these and previous paleoseismic results into a model of five postglacial (<15 ka) ruptures along a ∼65 km reach of the northern Lost River fault zone. Our results show that the Borah Peak earthquake (34 km; 0.9 m mean displacement) was unique compared to previous ruptures that had both longer and shorter rupture lengths (∼25−38 km), more displacement (mean of ∼1.3−1.4 m), and equal or greater magnitude ( M w 6.9−7.1) than that in the 1983 earthquake. These ruptures support a hypothesis of variable rupture length and displacement on the northern Lost River fault zone and show that predecessors to the 1983 rupture have passed unimpeded through the Willow Creek Hills. Our work demonstrates that normal faults are capable of producing variable spatial-temporal patterns of rupture that, together with comparisons of fault geometry and historical rupture length, improve our understanding of fault segmentation and help inform models of earthquake rupture probability.

Idaho↗

USGS2022

Seismic sources in the aleutian cradle of tsunamis

No abstract available.

Alaska↗

USGS2021

Changing impacts of Alaska-Aleutian subduction zone tsunamis in California under future sea-level rise

The amplification of coastal hazards such as distant-source tsunamis under future relative sea-level rise (RSLR) is poorly constrained. In southern California, the Alaska-Aleutian subduction zone has been identified as an earthquake source region of particular concern for a worst-case scenario distant-source tsunami. Here, we explore how RSLR over the next century will influence future maximum nearshore tsunami heights (MNTH) at the Ports of Los Angeles and Long Beach. Earthquake and tsunami modeling combined with local probabilistic RSLR projections show the increased potential for more frequent, relatively low magnitude earthquakes to produce distant-source tsunamis that exceed historically observed MNTH. By 2100, under RSLR projections for a high-emissions representative concentration pathway (RCP8.5), the earthquake magnitude required to produce >1 m MNTH falls from ~M w 9.1 (required today) to M w 8.0, a magnitude that is ~6.7 times more frequent along the Alaska-Aleutian subduction zone.

Alaska, California↗

USGS2021

Geophysical constraints on the crustal architecture of the transtensional Warm Springs Valley fault zone, northern Walker Lane, western Nevada, USA

The Walker Lane is a zone of distributed transtension where normal faults are overprinted by strike-slip motion. We use two newly-acquired high-resolution seismic reflection profiles and a reprocessed Consortium for Continental Reflection Profiling (COCORP) deep crustal reflection profile to assess the subsurface geometry of the Holocene-active, transtensional Warm Springs Valley fault zone (WSVFZ) near Reno, Nevada, USA. Our multi-scale observations extend to 12 km depth and suggest that the WSVFZ is more complex in the subsurface than implied by late Pleistocene surface fault traces. Two ~4-km-long high-resolution profiles image to a depth of ~2 km and reveal moderately dipping reflections and truncations, some of which project to mapped scarps formed in late Pleistocene surfaces. The shallow lines are co-located with COCORP profile NV 08 along ~40° N latitude. Re-analysis of the COCORP data reveals previously unidentified coherent reflections to a depth of ~12 km and a previously mapped ~30 west-dipping fault at 8-12 km. From these seismic profiles, the WSVFZ is not a simple, sub-vertical fault zone extending through the entire seismogenic crust. Instead, the reflections are consistent with a zone of steep- and moderately-dipping faults that simplify and steepen with depth before intersecting a mid-crustal, low angle (~25-30°) fault. The complex fault geometry of the WSVFZ implies that crustal shear is accommodated by a mix of dipping and subvertical faults in the transtensional northern Walker Lane. If so, transtensional fault zones may present challenges to paleoseismic and geodetic studies and require careful treatment when included in seismic hazard analyses.

Nevada↗

USGS2021

STEPS: Slip time earthquake path simulations applied to the San Andreas and Toe Jam Hill Faults to redefine geologic slip rate uncertainty

Geologic slip rates are a time-averaged measurement of fault displacement calculated over hundreds to million-year time scales and are a primary input for probabilistic seismic hazard analyses, which forecast expected ground shaking in future earthquakes. Despite their utility for seismic hazard calculations, longer-term geologic slip rates represent a time-averaged measure of the tempo of strain release and do not measure variability across earthquake cycles. We have developed a numerical approach called STEPS (Slip Time Earthquake Path Simulations), which is built upon field-based observations and explicitly incorporates realistic variations in displacement per event and variability in the recurrence interval between earthquakes. The STEPS approach, which simulates strain release through time, relies on representing earthquake cycles as stairsteps, rather than straight-line paths, connecting per earthquake time-displacement coordinates. We simulate earthquake histories based on these input constraints using two examples: the Carrizo section of the San Andreas fault and the Toe Jam Hill fault of the Seattle fault zone. We find that modeled slip rate distributions agree with slip rates reported for the sites of interest by the original investigators, while providing a slip rate distribution that reflects the variability of earthquake frequency and displacement. The STEPS approach provides an estimate of fault slip rate uncertainty based on a synthetic suite of plausible time-displacement paths resulting from individual earthquakes, rather than measurement uncertainties associated with offset features. When considering this simulated earthquake behavior between measurements, the uncertainty associated with earthquake paths is greater than that calculated by the long-term rate.

California, Washington↗

USGS2021

Quaternary reelfoot fault deformation in the Obion River Valley, Tennessee, USA

Blind reverse faults are challenging to detect, and earthquake records can be elusive because deep fault slip does not break the surface along readily recognized scarps. The blind Reelfoot fault in the New Madrid seismic zone in the central United States has been the subject of extensive prior investigation; however, the extent of slip at the southern portion of the fault remains unconstrained. In this study, we use lidar to map terraces and lacustrine landforms in the Obion River valley and investigate apparent broad folding resulting from slip on the buried Reelfoot fault. We compare remote surface mapping results with three auger boreholes in the ∼24 ka Finley terrace and interpret apparent warping as due to tectonic folding and not stratigraphic thickening. We combine our results with historical records of coseismic lake formation that indicate surface deformation dammed the Obion River in the 1812 CE earthquake. Older terraces (deposited at least 35–55 ka) record progressive fold scarps ≥1, ≥2, and ≥8 m high indicating a long record of earthquakes predating the existing paleoseismic record. Broad, distributed folding above the Reelfoot fault into the Obion River valley is consistent with a deep active fault tip along the southern reaches of the fault. Our analyses indicate the entire length of the fault (≥70 km) is capable of rupture and is more consistent with longer rupture scenarios.

Tennessee↗