Triggered Slip Research Articles

Abnormal apparent supershear rupture with discontinuous jumping propagation during the 2023 Türkiye M7.8 earthquake

Earthquake ruptures along a single fault or along a connected system of faults are generally assumed to progress continuously. However, our analysis of the 2023 M7.8 Türkiye earthquake, using finite-fault joint source inversion, uncovered the occurrence of discontinuous rupture jumps. The main fault area adjacent to the splay fault where the earthquake started, and the deeper portion of the northeastern main fault segment exhibited triggered slip before the main rupture front arrived. Through seismic centroid analysis and finite-fault inversion, we estimated apparent rupture speeds within these slip patches reach approximately 6.0 km s-1, exceeding local S-wave velocity. The dynamic triggering mechanism induced the jumping rupture in these areas, resulting in an apparent rupture velocity surpassing the local shear wave velocity. These findings demonstrate the importance of dynamic triggering in adjacent fault systems during large earthquakes, influencing the extent and complexity of rupture propagation.

Application of the count-circle method to forecast the potential for fault, hanging-wall deformation based on water-pipeline failures: An example from California, USA

The Mw 6.7, 1994 Northridge earthquake (NE) produced water-pipeline failures (WPF) in California's highly urbanized San Fernando Valley (SFV) and Sylmar Basin (SB). The causative fault was a south-dipping blind thrust, one of several that underlie the SFV in the Los Angeles Basin with little, if any, obvious geomorphic expression. Prior NE studies indicated WPF are a proxy for tectonic permanent ground deformations (PGD) and secondary shaking hazards including liquefaction, differential settlement, and slope failure (Johnson and Shlemon, 2013, 2015). To determine the specific causes for WPF, we employed the count-circle method (CCM). The CCM is a novel analysis used to calculate WPF densities with known x, y locations and z values that are equal to one. The CCM analysis involves constructing a study area grid of square cells. A circle is then placed over the center of four adjacent cells and WPF within the circle are counted. This is followed by shifting the circle west to east and south to north until all WPF are counted. Adjacent circles exceed the recommended minimum 50% overlap. The resulting CCM data are then machine contoured to construct the WPF isopleths. These isopleths also enclose several faults associated with ground rupture stemming from the nearby 1971 San Fernando, California earthquake and as well as from others that have no evidence of Holocene slip. The isopleth analysis shows that many regional pipeline failures were associated with PGD caused by multi-fault, hanging-wall triggered slip and epicentral area fracture zones unrelated to known geologic structures. From our WPF documentation in southern California, we suggest that CCM analysis of WPF is a useful tool to forecast potential infrastructure damage and lifeline failures stemming from future blind thrusts and associated hanging-wall deformation.

Buried Aseismic Slip and Off-Fault Deformation on the Southernmost San Andreas Fault Triggered by the 2010 El Mayor Cucapah Earthquake Revealed by UAVSAR.

We use UAVSAR interferograms to characterize fault slip, triggered by the Mw 7.2 El Mayor‐Cucapah earthquake on the 1 San Andreas Fault in the Coachella Valley providing comprehensive maps of short‐term geodetic surface deformation that complement in situ measurements. Creepmeters and geological mapping of fault offsets on Durmid Hill recorded 4 and 8 mm of average triggered slip respectively on the fault, in contrast to radar views that reveal significant off‐fault dextral deformation averaging 20 mm. Unlike slip in previous triggered slip events on the southernmost San Andreas fault, dextral shear in 2010 is not confined to transpressional hills in the Coachella valley. Edge detection and gradient estimation applied to the 50‐m‐sampled interferogram data identify the location (to 20 m) and local strike (to <4°) of secondary surface ruptures. Transverse curve fitting applied to these local detections provides local estimates of the radar‐projected dextral slip and a parameter indicating the transverse width of the slip, which we equate with the depth of subsurface shear. These estimates are partially validated by fault‐transverse interferogram profiles generated using the GeoGateway UAVSAR tool, and appear consistent for radar‐projected slip greater than about 5 mm. An unexpected finding is that creep and triggered slip on the San Andreas fault terminate in the shallow subsurface below a surface shear zone that resists the simple expression of aseismic fault slip. We introduce the notion of a surface locking depth above which fault slip is manifest as distributed shear, and evaluate its depth as 6–27 m.

Reconciling an Early Nineteenth-Century Rupture of the Alpine Fault at a Section End, Toaroha River, Westland, New Zealand

ABSTRACTThe Alpine fault is a high slip-rate plate boundary fault that poses a significant seismic hazard to southern and central New Zealand. To date, the strongest paleoseismic evidence for the onshore southern and central sections indicates that the fault typically ruptures during very large (Mw≥7.7) to great “full-section” earthquakes. Three paleoseismic trenches excavated at the northeastern end of its central section at the Toaroha River (Staples site) provide new insights into its surface-rupture behavior. Paleoseismic ruptures in each trench have been dated using the best-ranked radiocarbon dating fractions, and stratigraphically and temporally correlated between each trench. The preferred timings of the four most recent earthquakes are 1813–1848, 1673–1792, 1250–1580, and ≥1084–1276 C.E. (95% confidence intervals using OxCal 4.4). These surface-rupture dates correlate well with reinterpreted timings of paleoearthquakes from previous trenches excavated nearby and with the timing of shaking-triggered turbidites in lakes along the central section of the Alpine fault. Results from these trenches indicate the most recent rupture event (MRE) in this area postdates the great 1717 C.E. Alpine fault rupture (the most recent full-section rupture of the southern and central sections). This MRE probably occurred within the early nineteenth century and is reconciled as either: (a) a “partial-section” rupture of the central section; (b) a northern section rupture that continued to the southwest; or (c) triggered slip from a Hope-Kelly fault rupture at the southwestern end of the Marlborough fault system (MFS). Although, no single scenario is currently favored, our results indicate that the behavior of the Alpine fault is more complex in the north, as the plate boundary transitions into the MFS. An important outcome is that sites or towns near fault intersections and section ends may experience strong ground motions more frequently due to locally shorter rupture recurrence intervals.

Coseismic Displacements and Surface Fractures from Sentinel-1 InSAR: 2019 Ridgecrest Earthquakes

Abstract Interferometric Synthetic Aperture Radar is an important tool for imaging surface deformation from large continental earthquakes. Here, we present maps of coseismic displacement and strain from the 2019 Ridgecrest earthquakes using multiple Sentinel-1 images. We provide three types of interferometric products. (1) Standard interferograms from two look directions provide an overview of the deformation and can be used for modeling coseismic slip. (2) Phase gradient maps from stacks of coseismic interferograms provide high-resolution (∼30 m) images of strain concentration and surface fracturing that can be used to guide field surveys. (3) High-pass filtered, stacked, unwrapped phase is decomposed into east–west and up–down, south–north components and is used to determine the sense of fault slip. The resulting phase gradient maps reveal over 300 surface fractures, including triggered slip on the Garlock fault. The east–west component of high-pass filtered phase reveals the polarity of the strike-slip offset (right lateral or left lateral) for many of the fractures. We find a small number of fractures that have slip polarity that is retrograde to the background tectonic stress. This is similar to observations of retrograde slip observed near the 1999 Mw 7.1 Hector Mine rupture, but the Ridgecrest observations are more completely imaged by the frequent and high-quality acquisitions from the twin Sentinel-1 spacecrafts. Determining whether the retrograde features are triggered slip on existing faults, or compliant fault deformation in response to stress changes from the Ridgecrest earthquakes, or new Coulomb-style failures, will require additional field work, modeling, and analysis.

The July 2019 Ridgecrest, California, Earthquake Sequence Recorded by Creepmeters: Negligible Epicentral Afterslip and Prolonged Triggered Slip at Teleseismic Distances

AbstractWe report sequential triggered slip at 271–384 km distances on the San Andreas, Superstition Hills, and Imperial faults with an apparent travel-time speed of 2.2 ± 0.1 km/s, following the passage of surface waves from the 4 July 2019 (17:33:49 UTC) Mw 6.4 and 6 July 2019 (03:19:53 UTC) Mw 7.1 Ridgecrest earthquakes. Slip on remote faults was not triggered instantaneously but developed over several minutes, increasing in duration with distance. Maximum slip amplitudes varied from 10 μm to 5 mm within minutes of slip nucleation, but on the southernmost San Andreas fault slip continued for two months and was followed on 16 September 2019 by a swarm of microearthquakes (Mw≤3.8) near Bombay Beach. These observations add to a growing body of evidence that fault creep may result in delayed triggered seismicity. Displacements across surface faults in the southern epicentral region and on the Garlock fault in the months following the Ridgecrest earthquakes were negligible (&amp;lt;1.1 mm), and they are interpreted to characterize surface strain adjustments in the epicentral region, rather than to result from discrete slip on surface faults.

Multicycle Dynamics of the Aksay Bend Along the Altyn Tagh Fault in Northwest China: 2. The Realistically Complex Fault Geometry

AbstractWe study rupture behavior of the Aksay bend along the Altyn Tagh fault in northwest China over multiple earthquake cycles. A finite element method is used to numerically simulate spontaneous rupture during a coseismic process, and a viscoelastic model is used to analytically compute the fault stresses during an interseismic process. We find that the Aksay bend is an effective barrier to halt dynamically propagating ruptures from either side of the bend within a range of model parameters, with statistically only about 10% of ruptures jumping across the bend and propagating through almost the entire local fault system. Secondary complexities in fault geometry within the bend, in particular those portions that align relatively well with the regional strike of the fault system, play a critical role in these occasionally jumping ruptures. Well‐developed fault patches with shear stress close to shear strength allow dynamically propagating ruptures to penetrate into the bend and are more susceptible to the dynamic triggering that enables rupture to jump across the bend onto the other strand. We identify additionally nine large rupture scenarios with different occurrences, and most of them rupture one strand outside the bend with triggered slip on some portions of the same or the other strand within the bend. Slip rate distributions from the models show significantly reduced fault slip within the bend and a permanently locked portion on the south strand near the peak of the Altun Mountains. These findings have important implications for seismic hazard assessments of complex fault systems worldwide.

Slip perturbation during fault reactivation by a fluid injection

Slip orientation inferred from fault striae or focal mechanism datasets is commonly used in stress inversion methods based on the Wallace-Bott hypothesis. The hypothesis postulates that slip on a fault plane is collinear with the orientation of the resolved shear stress. It is valid for a single planar fault subjected to a homogeneous far-field stress. However, the experimental displacement data from an induced fault reactivation experiment, conducted in the Mont Terri rock laboratory, Switzerland, indicated multiple triggered slip orientations, thereby preventing application of the above inversion method. We present numerical and analytical results of slip on a reactivated fracture with a non-uniform fluid pressure distribution. Using these models, we evaluate the reasons for the inconsistency of our observations and the traditional Wallace-Bott hypothesis and test the physical effects of various parameters on fault slip. In the fully coupled hydromechanical numerical model (three-dimensional distinct element method), fluid pressure at a point on the fault surface is increased stepwise (assumed planar and singular) until shear reactivation of the fault is induced. We studied two different models with high and low fault plane stiffness to represent hard and soft rock masses, respectively. The model shows that high fault stiffness preserves the planarity of the fault plane, while low fault stiffness permits dilation and morphological changes of the fracture related to fluid pressure diffusion. The highest slip perturbation was observed in the low stiffness model due to the change of the fracture shape, controlled by the non-uniform pressure distribution. The Eshelby analytical solution confirmed that the more the fracture is dilated, the more the corresponding resolved shear stress is perturbed. Additionally, when compared to dilation and fault aperture, the friction angle has the most influence on the angular difference between geomechanical slip vectors and resolved shear stress.

Analysis and interpretation of earthquake-related groundwater response and ground deformation: a case study of May 2006 seismic sequence in the Mexicali Valley, Baja California, Mexico

Anomalous groundwater level and temperature changes are compared with ground deformation recorded before and after an earthquake of MW 5.4 and its foreshocks and aftershocks that occurred during 22–28 May 2006 in the Mexicali Valley, Baja California, Mexico. The coseismic groundwater level changes could be attributed to static volumetric strain changes caused by the mainshock, except for one well, where the groundwater level change may have been affected also by a triggered slip event at a nearby fault. Some of the coseismic temperature changes were attributed to increased convection and mixing of groundwater by seismic shaking. Modeling of groundwater level records allowed the estimation of hydraulic diffusivity. The observed ground tilt and groundwater level anomalies in the area close to the source fault before and after the mainshock and before the aftershocks occurrence are explainable by the dilatancy–diffusion theory, or possibly by assuming the occurrence of a slow slip events and/or fault permeability changes.

Tearing the terroir: Details and implications of surface rupture and deformation from the 24 August 2014 M6.0 South Napa earthquake, California

AbstractThe Mw 6.0 South Napa earthquake of 24 August 2014 caused slip on several active fault strands within the West Napa Fault Zone (WNFZ). Field mapping identified 12.5 km of surface rupture. These field observations, near‐field geodesy and space geodesy, together provide evidence for more than ~30 km of surface deformation with a relatively complex distribution across a number of subparallel lineaments. Along a ~7 km section north of the epicenter, the surface rupture is confined to a single trace that cuts alluvial deposits, reoccupying a low‐slope scarp. The rupture continued northward onto at least four other traces through subparallel ridges and valleys. Postseismic slip exceeded coseismic slip along much of the southern part of the main rupture trace with total slip 1 year postevent approaching 0.5 m at locations where only a few centimeters were measured the day of the earthquake. Analysis of airborne interferometric synthetic aperture radar data provides slip distributions along fault traces, indicates connectivity and extent of secondary traces, and confirms that postseismic slip only occurred on the main trace of the fault, perhaps indicating secondary structures ruptured as coseismic triggered slip. Previous mapping identified the WNFZ as a zone of distributed faulting, and this was generally borne out by the complex 2014 rupture pattern. Implications for hazard analysis in similar settings include the need to consider the possibility of complex surface rupture in areas of complex topography, especially where multiple potentially Quaternary‐active fault strands can be mapped.

Multiple fault slip triggered above the 2016 Mw 6.4 MeiNong earthquake in Taiwan

AbstractRapid shortening in convergent mountain belts is often accommodated by slip on faults at multiple levels in upper crust, but no geodetic observation of slip at multiple levels within hours of a moderate earthquake has been shown before. Here we show clear evidence of fault slip within a shallower thrust at 5–10 km depth in SW Taiwan triggered by the 2016 Mw 6.4 MeiNong earthquake at 15–20 km depth. We constrain the primary coseismic fault slip with kinematic modeling of seismic and geodetic measurements and constrain the triggered slip and fault geometry using synthetic aperture radar interferometry. The shallower thrust coincides with a proposed duplex located in a region of high fluid pressure and high interseismic uplift rate, and may be sensitive to stress perturbations. Our results imply that under tectonic conditions such as high‐background stress level and high fluid pressure, a moderate lower crustal earthquake can trigger faults at shallower depth.

Refining the shallow slip deficit

Geodetic slip inversions for three major (M_w > 7) strike-slip earthquakes (1992 Landers, 1999 Hector Mine and 2010 El Mayor–Cucapah) show a 15–60 per cent reduction in slip near the surface (depth < 2 km) relative to the slip at deeper depths (4–6 km). This significant difference between surface coseismic slip and slip at depth has been termed the shallow slip deficit (SSD). The large magnitude of this deficit has been an enigma since it cannot be explained by shallow creep during the interseismic period or by triggered slip from nearby earthquakes. One potential explanation for the SSD is that the previous geodetic inversions lack data coverage close to surface rupture such that the shallow portions of the slip models are poorly resolved and generally underestimated. In this study, we improve the static coseismic slip inversion for these three earthquakes, especially at shallow depths, by: (1) including data capturing the near-fault deformation from optical imagery and SAR azimuth offsets; (2) refining the interferometric synthetic aperture radar processing with non-boxcar phase filtering, model-dependent range corrections, more complete phase unwrapping by SNAPHU (Statistical Non-linear Approach for Phase Unwrapping) assuming a maximum discontinuity and an on-fault correlation mask; (3) using more detailed, geologically constrained fault geometries and (4) incorporating additional campaign global positioning system (GPS) data. The refined slip models result in much smaller SSDs of 3–19 per cent. We suspect that the remaining minor SSD for these earthquakes likely reflects a combination of our elastic model's inability to fully account for near-surface deformation, which will render our estimates of shallow slip minima, and potentially small amounts of interseismic fault creep or triggered slip, which could ‘make up’ a small percentages of the coseismic SSD during the interseismic period. Our results indicate that it is imperative that slip inversions include accurate measurements of near-fault surface deformation to reliably constrain spatial patterns of slip during major strike-slip earthquakes.

Dynamic triggering of creep events in the Salton Trough, Southern California by regional [formula omitted] earthquakes constrained by geodetic observations and numerical simulations

Since a regional earthquake in 1951, shallow creep events on strike-slip faults within the Salton Trough, Southern California have been triggered at least 10 times by M≥5.4 earthquakes within 200 km. The high earthquake and creep activity and the long history of digital recording within the Salton Trough region provide a unique opportunity to study the mechanism of creep event triggering by nearby earthquakes. Here, we document the history of fault creep events on the Superstition Hills Fault based on data from creepmeters, InSAR, and field surveys since 1988. We focus on a subset of these creep events that were triggered by significant nearby earthquakes. We model these events by adding realistic static and dynamic perturbations to a theoretical fault model based on rate- and state-dependent friction. We find that the static stress changes from the causal earthquakes are less than 0.1 MPa and too small to instantaneously trigger creep events. In contrast, we can reproduce the characteristics of triggered slip with dynamic perturbations alone. The instantaneous triggering of creep events depends on the peak and the time-integrated amplitudes of the dynamic Coulomb stress change. Based on observations and simulations, the stress change amplitude required to trigger a creep event of a 0.01-mm surface slip is about 0.6 MPa. This threshold is at least an order of magnitude larger than the reported triggering threshold of non-volcanic tremors (2–60 kPa) and earthquakes in geothermal fields (5 kPa) and near shale gas production sites (0.2–0.4 kPa), which may result from differences in effective normal stress, fault friction, the density of nucleation sites in these systems, or triggering mechanisms. We conclude that shallow frictional heterogeneity can explain both the spontaneous and dynamically triggered creep events on the Superstition Hills Fault.

Frequency dependence of delayed and instantaneous triggering on laboratory and simulated faults governed by rate‐state friction

AbstractEarthquake triggering by transient stresses is commonly observed; however, some aspects remain unexplained. The first is the often‐observed delay between the triggered earthquakes and the triggering waves, and the second is the unexpected effectiveness of transient stressing in the seismic frequency band. Previous theoretical and laboratory studies have suggested that seismic transients should have little impact on faults if the duration of the transient is smaller than the timescale for nucleation of slip. We reexamine the dynamics of stress triggering during stick‐slip sliding on a laboratory fault and make three important observations that pertain to earthquake triggering. (1) Delayed triggering (clock advance) occurs for both bare granite surfaces and granular gouge prior to the onset of instantaneous triggering. (2) Triggering occurs much earlier in the stick‐slip cycle than expected for a simple Coulomb stress threshold. (3) Shorter‐period (higher stressing rate) pulses are more effective at triggering than longer‐period pulses of the same stress amplitude. We use numerical simulations to show that rate‐state friction can explain each of the observed features but not all three simultaneously. Only the Ruina slip law for state evolution, in which faults must slip to heal, can reproduce early‐onset and stressing rate‐dependent triggering. The laboratory and numerical experiments show that faults can remain relatively weak over much of the seismic cycle and that the triggered response depends on a competition between healing and weakening during triggered slip. Transient stressing at seismic frequencies may be more effective at triggering earthquakes than previously recognized.

InSAR imaging of displacement on flexural‐slip faults triggered by the 2013 Mw 6.6 Lake Grassmere earthquake, central New Zealand

AbstractInterferometric Synthetic Aperture Radar (InSAR) data reveal surface slip on multiple faults triggered by the 2013 Mw 6.6 Lake Grassmere earthquake, New Zealand. Surface offsets of 1–2 cm occurred on previously inferred flexural‐slip faults located ∼4 km from the epicenter. We document dip slip on at least three different northeast striking, northwest dipping, flexural‐slip faults located in the western limb of a syncline. The along‐strike extent of the triggered slip is 1–1.5 km for each fault. Dislocation models suggest that triggered slip is confined to shallow depths (∼800 m). Coulomb stress analysis indicates that slip was not triggered by the static stress change of the main shock but was likely caused by dynamic shaking during the passage of seismic waves. Our finding also provides an important clue on how some slip on shallow flexural‐slip faults takes place.

Three-dimensional discrete element modeling of triggered slip in sheared granular media.

This paper reports results of a three-dimensional discrete element method modeling investigation of the role of boundary vibration in perturbing stick-slip dynamics in a sheared granular layer. The focus is on the influence of vibration within a range of amplitudes and on the fact that above a threshold early slip will be induced. We study the effects of triggering beyond the vibration interval and their origins. A series of perturbed simulations are performed for 30 large slip events selected from different reference runs, in the absence of vibration. For each of the perturbed simulations, vibration is applied either about the middle of the stick phase or slightly before the onset of a large expected slip event. For both cases, a suppression of energy release is on average observed in the perturbed simulations, within the short term following the vibration application. For cases where vibration is applied in the middle of the stick phase, a significant clock advance of the large slip event occurs. In the long term after vibration, there is a recovery period with higher-energy release and increased activity in the perturbed simulations, which compensates for the temporary suppression observed within the short term.

Postseismic deformation following theMw7.2, 23 October 2011 Van earthquake (Turkey): Evidence for aseismic fault reactivation

Geodetic measurements following the 23 October 2011, Mw = 7.2 Van (eastern Turkey) earthquake reveal that a fault splay on the footwall block of the coseismic thrust fault was reactivated and slipped aseismically for more than 1.5 years following the earthquake. Although long-lasting aseismic slip on coseismic ruptures has been documented following many large earthquakes, long-lasting, triggered slip on neighboring faults that did not rupture during the earthquake has not been reported previously. Elastic dislocation and Coulomb stress modeling indicate that the postseismic deformation can be adequately explained by shallow slip on both the coseismic and splay fault and is likely driven mostly by coseismic stress changes. Thus, the slip deficit on the shallow section of the coseismic fault indicated by interferometric synthetic aperture radar-based models has been partially filled by aseismic slip, suggesting a lower likelihood for a large earthquake on the shallow section of the Van fault than suggested by previous studies.

UAVSAR observations of triggered slip on the Imperial, Superstition Hills, and East Elmore Ranch Faults associated with the 2010 M 7.2 El Mayor‐Cucapah earthquake

AbstractThe 4 April 2010 M 7.2 El Mayor‐Cucapah earthquake that occurred in Baja California, Mexico and terminated near the U.S. Mexican border caused slip on the Imperial, Superstition Hills, and East Elmore Ranch Faults. The pattern of slip was observed using radar interferometry from NASA's Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) instrument collected on 20–21 October 2009 and 12–13 April 2010. Right‐lateral slip of 36 ± 9 and 14 ± 2 mm occurred on the Imperial and Superstition Hills Faults, respectively. Left‐lateral slip of 9 ± 2 mm occurred on the East Elmore Ranch Fault. The widths of the zones of displacement increase northward suggesting successively more buried fault motion to the north. The observations show a decreasing pattern of slip northward on a series of faults in the Salton Trough stepping between the El Mayor‐Cucapah rupture and San Andreas Fault. Most of the motion occurred at the time of the M 7.2 earthquake and the UAVSAR observations are consistent with field, creepmeter, GPS, and Envisat observations. An additional 28 ± 1 mm of slip at the southern end of the Imperial Fault over a &lt;1 km wide zone was observed over a 1 day span a week after the earthquake suggesting that the fault continued to slip at depth following the mainshock. The total moment release on the three faults is 2.3 × 1023−1.2 × 1024 dyne cm equivalent to a moment magnitude release of 4.9–5.3, assuming shallow slip depths ranging from 1 to 5 km.

Active accommodation of plate convergence in Southern Iran: Earthquake locations, triggered aseismic slip, and regional strain rates

We present a catalog of interferometric synthetic aperture radar (InSAR) constraints on deformation that occurred during earthquake sequences in southern Iran between 1992 and 2011, and explore the implications on the accommodation of large‐scale continental convergence between Saudi Arabia and Eurasia within the Zagros Mountains. The Zagros Mountains, a salt‐laden fold‐and‐thrust belt involving ~10 km of sedimentary rocks overlying Precambrian basement rocks, have formed as a result of ongoing continental collision since 10–20 Ma that is currently occurring at a rate of ~3 cm/yr. We first demonstrate that there is a biased misfit in earthquake locations in global catalogs that likely results from neglect of 3‐D velocity structure. Previous work involving two M ~ 6 earthquakes with well‐recorded aftershocks has shown that the deformation observed with InSAR may represent triggered slip on faults much shallower than the primary earthquake, which likely occurred within the basement rocks (&gt;10 km depth). We explore the hypothesis that most of the deformation observed with InSAR spanning earthquake sequences is also due to shallow, triggered slip above a deeper earthquake, effectively doubling the moment release for each event. We quantify the effects that this extra moment release would have on the discrepancy between seismically and geodetically constrained moment rates in the region, finding that even with the extra triggered fault slip, significant aseismic deformation during the interseismic period is necessary to fully explain the convergence between Eurasia and Saudi Arabia.

Refining the Southern Extent of the 1872 Owens Valley Earthquake Rupture through Paleoseismic Investigations in the Haiwee Area, Southeastern California

Recent upward revision of the 1872 Owens Valley earthquake from Mw 7.4-7.5 to 7.7-7.9 implies either additional unrecognized rupture length or anoma- louslystronggroundmotionsassociatedwiththisevent.Weinvestigatethefirstpossibility through paleoseismic trenching south of the mapped surface rupture in the Haiwee area, where historical accounts suggest significant surface deformation following the earth- quake. Trenching focused on a prominent north-striking scarp, herein termed the Sage Flat fault, expressed in Pleistocene alluvial fans east of Haiwee Reservoir. Surficial map- ping and ground-based Light Detection and Ranging (lidar) surveying suggest that this faultaccommodateseast-downnormalmotion,andpossiblyacomparableamountofdex- tralslip.Trenchingandluminescencedatingbracketsthetimingofthemostrecentsurface- rupturing earthquake between ∼25:7 and 30.1 ka, and provides evidence for an earlier event predating this time. In combination with scarp profiling, these dates also suggest am aximum rate of normal, dip-slip fault motion up to∼0:1 mm=yr over this period. Although we discovered no evidence for recent surface rupture on the Sage Flat fault, a series of subvertical fractures and fissures cut across young trench stratigraphy, consis- tentwithsecondarydeformationassociatedwithseismicshaking.Assuch,wesuggestthat possible ground disturbance in the Haiwee area during the 1872 event primarily reflected ground shaking or liquefaction-related deformation rather than triggered slip. In addition, we infer a structural and kinematic connection between the Owens Valley fault and oblique-dextral faults north of Lower Cactus Flat in the northwestern Coso Range, rather thanawest-stepintonorthernorwesternRoseValley.Considerationofthesestructuresin the total extent of the Owens Valley fault suggests a length of 140 km, of which at least 113 km ruptured during the 1872 event. Online Material: Procedural details and expanded results from the OSL sample analyses, as well as high-resolution paleoseismic trench logs.