Salton Trough Research Articles

Hydrogeologic and hydrochemical inputs to emerging wetlands on the shores of the receding Salton Sea, California

AbstractThe Salton Sea has experienced significant recession over the past two decades due to changes in the diversion of Colorado River water to the Salton Trough for agricultural irrigation. As a result, wetlands have emerged in some exposed playa areas along the Salton Sea, primarily in regions with extensive agricultural return flows and agricultural drainage. One notable wetland system, known as the Bombay Beach Wetlands, has formed on the north shore of the Salton Sea, in an area devoid of agriculture. In many other areas with limited or no agriculture, wetlands have failed to develop, leaving exposed playa surfaces as the Salton Sea recedes. These dry playa surfaces pose a significant threat to the health of local residents due to the presence of toxins contained in windblown dust associated with playa deposits. In this study, stable water isotope data, combined with other hydrological information, led to identification of two potential water sources for the Bombay Beach Wetlands. The first possibility proposes that thermal artesian waters alone contribute to the wetlands' water source, while the second hypothesis involves a combination of drainage from Salton Sea bank storage water mixing with the thermal artesian water. The thermal artesian water discharges into drainage channels that flow towards the Bombay Beach Wetlands, initially devoid of possible groundwater baseflow until reaching the wetlands. Studies were subsequently done along the full reach of the drainage channels receiving thermal artesian water. Dissolved solids content, P and N nutrients, arsenic, and stable water isotopes were tested synoptically along the drainage channels. Channel investigations led to the development of a novel model of salinization, which is linked to channel discharge, channel morphometrics, and channel incision.

High‐Resolution Near‐Surface Imaging at the Basin Scale Using Dark Fiber and Distributed Acoustic Sensing: Toward Site Effect Estimation in Urban Environments

AbstractNear‐surface seismic structure, particularly the shear wave velocity (Vs), can strongly affect local site response, and should be accurately estimated for ground motion prediction during seismic hazard assessment. The Imperial Valley (California), occupying the southern end of the Salton Trough, is a seismically active basin with thick surficial lacustrine sedimentary deposits. In this study, we utilize ambient noise records and local earthquake events for high‐resolution near‐surface characterization and site effect estimation with an unlit fiber‐optic telecommunication infrastructure (dark fiber) in Imperial Valley by using the distributed acoustic sensing (DAS) technique. We apply ambient noise interferometry to retrieve coherent surface waves from DAS records, and evaluate performances of three different surface wave methods on DAS ambient noise dispersion imaging. We develop a quality control workflow to improve the dispersion measurement of noisy portions of the DAS data set by using a data selection strategy. Using the joint inversion of both the fundamental mode and higher overtones of Rayleigh waves, a high resolution two‐dimensional (2D) Vs structure down to 70 m depth is obtained. We successfully achieve an improved Vs30 (the time‐averaged shear‐wave velocity in the top 30 m) model with higher spatial‐resolution and reliability compared to the existing community model for the area. We also explore the potential for utilizing DAS earthquake events for site amplification estimation. The preliminary results reveal a clear anti‐correlation between the approximated site response and the Vs30 profile. Our results indicate the potential utility of DAS deployed on dark fiber for near‐surface characterization in appropriate contexts.

2000 yrs of earthquakes inferred from subsidence events on the Imperial fault, California: Effect of lake-level changes and implication for variable slip rates

The Salton Trough has filled repeatedly with freshwater Lake Cahuilla from avulsions of the Colorado River. The relationship between lake fillings and large earthquakes is a topic of debate. From a new study on the Imperial fault applying cone penetrometer soundings, we show that the long hiatus in lake fillings between about ca 100 BCE and 950 CE resulted in accelerated slip in the few hundred years after re-inundation, an observation that is also seen on the San Andreas and southern San Jacinto faults. A regional, basin-wide signal of transient accelerated slip is interpreted to be the result of the effects of increased pore pressure on fault strength resulting from the 100 m of water load during full lake inundations. These results demonstrate the effects of dramatically increasing pore pressure, termed the reservoir induced seismicity effect, on fault activity and explain the current “earthquake drought” on the southernmost section of the San Andreas fault.

Alluvial fan and fan delta facies architecture recording initial marine flooding in the Mio‐Pliocene syn‐rift sequence of the Fish Creek‐Vallecito Basin, southern California

AbstractThe timing and character of the initial marine flooding of extensional basins has implications for their tectonic history. Yet, the recognition of such flooding is difficult along rift basin margins due to the dominance of coarse‐grained systems and the lack of marine fauna. This study conducts detailed facies and stratigraphic analysis of a Mio‐Pliocene alluvial fan and fan‐delta succession in the Fish Creek Vallecito Basin in southern California. Our goal is to characterize the marine flooding surface, determine the paleogeographic position of the shoreline and estimate the magnitude of relative base‐level rise that occurred during the marine incursion associated with the opening of the Gulf of California. Our results show that the flooding of the Elephant Trees alluvial fans is often marked by an abrupt lithologic and facies change from meter‐scale boulder‐rich subaerial debrites (proximal alluvial fan facies association) to centimetre‐scale granule‐rich subaqueous debrites, ripple‐laminated sandstones and mudstones (prodelta facies association). By delineating the zone of transition between the subaerial and subaqueous facies, we place the initial flooding paleo‐shoreline 4 km up the alluvial fan's paleo‐depositional slope. Considering alluvial fan slope gradients between 1° and 5°, this 4 km transgression would require an estimated 70–350 m of water depth during the initial marine incursion. Interfingering of fan‐delta deposits with subaqueous marine and planktonic‐rich evaporites suggests that the basin was below sea‐level after, and perhaps even before, the marine flooding. Subaerial subsea‐level basins exist in Death Valley and the Salton Trough today within similar extensional and transtentional tectonic regimes. This subaerial subsea‐level interpretation might explain the high magnitude and abrupt relative base‐level rise recorded by the facies transitions in the Fish Creek Vallecito Basin. These results suggest that the Fish Creek Vallecito Basin underwent significant extension during its early and nonmarine depositional phase, allowing it to reach subsea‐level elevations. The tectonic history of the Fish Creek Vallecito Basin maybe similar to other extensional basins where rapid subsidence allows the accumulation of nonmarine strata below sea‐level prior to the marine flooding, then restricted and deep marine strata immediately after flooding.

Defining Regional and Local Sediment Sources in the Ancestral Colorado River System: A Heavy Mineral Study of a Mixed Provenance Unit in the Fish Creek-Vallecito Basin, Southern California

The Colorado River has flowed across the dextral strike-slip plate boundary between the North American and Pacific plates since the latest Miocene or earliest Pliocene. The Fish Creek-Vallecito Basin (FCVB) lies on the Pacific Plate in southern California, dextrally offset from the point where the modern Colorado river enters the Salton Trough; it contains a record of ancestral Colorado River sedimentation from 5.3–2.5 Ma. The basin stratigraphy exhibits a changing balance between locally derived (L-Suite) and Colorado River (C-Suite) sediments. This paper focuses on the Palm Springs Group (PSG), a thick fluvial and alluvial sequence deposited on the upper delta plain (between 4.2–2.5 Ma) when the Colorado was active in the area, allowing the detailed examination of the processes of sediment mixing from two distinct provenance areas. The PSG consists of three coeval formations: 1) Canebrake Conglomerate, a basin margin that has coarse alluvial fan deposits derived from surrounding igneous basement; 2) Olla Formation, fan-fringe sandstones containing L-Suite, C-Suite, and mixed units; and 3) Arroyo Diablo Formation, mineralogically mature C-Suite sandstones. Stratigraphic analysis demonstrates that the river flowed through a landscape with relief up to 2000 m. Satellite mapping and detailed logging reveal a variable balance between the two suites in the Olla Formation with an apparent upward increase in L-Suite units before abrupt cessation of Colorado sedimentation in the basin. Stable heavy mineral indices differentiate L-Suite (high rutile:zircon index: RZi 40–95) from C-Suite (RZi: 0–20). Both suites have garnet:zircon index (GZi) and apatite:tourmaline index (ATi) mostly above 50, although many L-suite and mixed Olla samples have much lower ATi (20–50), suggesting that the distal floodplain was wet and the local sediment had a longer residence time there, or went through several cycles of erosion and redeposition. Heavy mineral analysis, garnet geochemical analysis, and detrital zircon U-Pb age spectra allow us to quantify the amount of mixing from different sediment sources. These data show that about 30% of the mixed units are derived from the Colorado River and that up to 20% of the L-Suite is also derived from the Colorado River, suggesting that there was mutual cannibalisation of older deposits by fluvial channels in a transitional area at the basin margin. Although this study is local in scope, it provides an insight into the extent and nature of sediment mixing in a two-source system. We conclude that most ‘mixing’ is actually interbedding from separate sources; true mixing is facilitated by low subsidence rates and the rapid migration of fluvial channels.

Earth model-space exploration in Southern California: Influence of topography, geotechnical layer, and attenuation on wavefield accuracy

Accurately predicting the seismic wavefield is important for physics-based earthquake hazard studies and is dependent on an accurate source model, a good model of the subsurface geology, and the full physics of wave propagation. Here, we conduct numerical experiments to investigate the effect of different representations of the Southern California Earthquake Center and Harvard community velocity models on seismic waveform predictions in the vicinity of the San Andreas fault in Salton Trough. We test general preconceptions about the importance of topography, near-surface geotechnical layering, and anelastic attenuation up to a maximum frequency of 0.5 Hz. For the Southern California Earthquake Center model developed without topography, we implement 1D and linear model extensions that preserve the geologic structure and a pull-up approach that adapts the original model to topographic variations and distorts the subsurface. The Harvard model includes an elevation model, so we test the squashed topography representation, which flattens it. For both community models, we modify the top 350 m by partially applying the Ely geotechnical layer using a minimum shear wave velocity of 600 m/s and incorporate an Olsen attenuation model using a ratio of 0.05. We evaluate the resulting 24 model representations using the classical waveform misfit and five moderate-magnitude earthquakes. Only the inclusion of attenuation consistently improves the wavefield predictions. It becomes more impactful at higher frequencies, where it significantly improves the performance levels of the crude 1D and linear extension models close to that of the original version. The pull-up topography representation also enhances the waveform prediction ability of the original model. Squashing the topography of the elevation-referenced Harvard model produces better seismogram fits, suggesting that seismic imagers construct community tomographic models without topography to avoid issues related to missing model parameters near the free surface or discrepancies with a different elevation model. Although full implementation of the Ely geotechnical layer that would permit shear wave velocities as low as 90 m/s proves computationally expensive, our partial implementation provides slightly better results in some cases. Our results can serve as recommendations for implementing these community models for future validation or optimization studies.

Complex Patterns of Past and Ongoing Crustal Deformations in Southern California Revealed by Seismic Azimuthal Anisotropy

AbstractWe present a high‐resolution P‐wave azimuthally anisotropic velocity model for the upper and middle crust beneath southern California by a novel adjoint‐state traveltime tomography technique. Our model reveals significant anisotropy variations between tectonic blocks that clearly reflect both past and current plate boundary deformation. In the shallow crust, seismic anisotropy is mostly controlled by the preferred alignment of microcracks related to the present N‐S compressive stress; while at deeper depths (>∼6 km), seismic anisotropy mainly records paleofabrics formed during the long‐lived Farallon subduction and later extension that have not been fully reset by the present transform motion. Interestingly, our model demonstrates distinct fast axes beneath the western Transverse Ranges from its neighboring blocks, probably reflecting the large‐scale vertical axis clockwise rotation of the block. In addition, we identify layered structures with distinct anisotropy features beneath the Salton Trough, which could be a result of the current transtension.

Improved Stress Drop Estimates for M 1.5 to 4 Earthquakes in Southern California From 1996 to 2019

AbstractWe estimate Brune‐type stress drops for over 70,000 southern California M 1.5–4 earthquakes from 1996 to 2019 using a P‐wave spectral decomposition approach. Based on our recent work documenting hard‐to‐resolve trade‐offs between absolute stress drop, stress drop scaling with moment, high‐frequency falloff rate, and empirical corrections for path and attenuation terms, we adopt a new approach in which the average corner frequency of the smallest earthquakes within a short distance from each target event is fixed to a constant value. This removes any true coherent spatial variations in stress drops among the smallest events but ensures that any spatial variations seen in larger event stress drops are real and not an artifact of inaccurate path corrections. Applying this approach across southern California, we document spatial variations in stress drop that agree with previous work, such as lower‐than‐average stress drops in the Salton Trough, as well as small‐scale stress drop variations along many faults and aftershock sequences. We observe an apparent increase in Brune‐type stress drop with moment for M 3–4 earthquakes, but their spectra can be fit equally well with self‐similar models with a high‐frequency falloff rate shallower than f−2.

Seismostratigraphic analysis of Lake Cahuilla sedimentation cycles and fault displacement history beneath the Salton Sea, California, USA

Abstract The Salton Trough (southeastern California, USA) is the northernmost transtensional stepover of the Gulf of California oblique-divergent plate boundary and is also where the southern terminus of the San Andreas fault occurs. Until recently, the distribution of active faults in and around the Salton Sea and their displacement histories were largely unknown. Subbottom CHIRP (compressed high-intensity radar pulse) surveys in the Salton Sea are used to develop a seismic facies model for ancient Lake Cahuilla deposits, a detailed map of submerged active faults, and reconstructed fault displacement histories during the late Holocene. We observe as many as fourteen Lake Cahuilla sequences in the Salton Sea (last ~3 k.y.) and develop a chronostratigraphic framework for the last six sequences (last ~1200 yr) by integrating CHIRP data and cone penetrometer logs with radiocarbon-dated stratigraphy at an onshore paleoseismic site. The Salton Sea contains northern and southern subbasins that appear to be separated by a tectonic hinge zone, and a subsidence signal across hinge-zone faults of 6–9 mm/yr (since ca. A.D. 940) increases toward the south to >15 mm/yr. The faults mapped to the south of the hinge zone appear to accommodate transtension within the San Andreas–Imperial fault stepover. We identify 8–15 distinct growth events across hinge-zone faults, meaning growth occurred at least once every 100 yr since Lake Cahuilla sedimentation began. Several faults offset the top of the most recent Lake Cahuilla highstand deposits, and at least two faults have offset the Salton Sea flood deposits. Active faults and folds were also mapped to a limited extent within the northern subbasin and display growth, but their kinematics and rupture histories require further study. The broad distribution of active faulting suggests that strain between the San Andreas, San Jacinto, and Imperial faults is highly distributed, thus discrepancies between geologic and geodetic slip-rate estimates from these major fault systems are to be expected.

The Imperial Valley Dark Fiber Project: Toward Seismic Studies Using DAS and Telecom Infrastructure for Geothermal Applications

Abstract The Imperial Valley is a seismically active basin occupying the southern end of the Salton trough, an area of rapid extension, high heat flow, and abundant geothermal resources. This report describes an ongoing large-scale distributed acoustic sensing (DAS) recording study acquiring high-density seismic data on an array between Calipatria and Imperial, California. This 27 km array, operating on dark fiber since 9 November 2020, has recorded a wealth of local seismic events as well as ambient noise. The goal of the broader Imperial Valley Dark Fiber project is to evaluate passive DAS as a tool for geothermal exploration and monitoring. This report is intended to provide installation information, noise characteristics, and metadata for future studies utilizing the data set. Because of the relatively small number of basin-scale DAS studies that have been conducted to date, we also provide a range of lessons learned during the deployment to assist future researchers exploring this acquisition strategy.

Propagating Seismic Waves in VTI Attenuating Media Using Fractional Viscoelastic Wave Equation

AbstractSeismic velocity and attenuation anisotropy are ubiquitous in the crust and upper mantle, significantly modulating the characteristics of seismic wave propagation in the Earth's interior. Accurate seismic wave modeling of velocity and attenuation anisotropy is essential for the understanding of wave propagation in the Earth's interior as well as constructing global and region‐scale seismic full waveform tomography. Here, we derive a decoupled fractional Laplacian (DFL) viscoelastic wave equation to characterize the Earth's frequency‐independent Q behavior in the vertical transversely isotropic (VTI) media. We verify the accuracy of the proposed viscoelastic wave equation by 2D synthetic examples; to show its applicability in crustal‐scale seismic modeling, we present an example of 3D seismic wave propagation in the realistic Salton Trough model. Through extensive numerical tests, we conclude that the proposed viscoelastic wave equation is superior in four aspects. First, the viscoelastic wave equation takes VTI anisotropy of both velocity and attenuation into account and can describe the strong direction‐dependent attenuation. Second, our derivation contains spatially independent Laplacians, and thus the proposed wave equation enjoys higher simulation accuracy for heterogeneous Q media. Third, the new viscoelastic wave equation can decouple the amplitude decay and the phase distortion, which is appealing for improving the resolution in seismic imaging and inversion. Lastly, compared to viscoelastic wave equations with time‐fractional operators, our scheme has higher computational efficiency by avoiding substantial wavefield storage.

The late Holocene history of Lake Cahuilla: Two thousand years of repeated fillings within the Salton Trough, Imperial Valley, California

To constrain the timing of the past seven lake highstands in the Salton Trough, we compiled 423 radiocarbon dates, of which 284 are reliable and have good stratigraphic control, from paleoseismic and archeological sites in the basin. We developed two OxCal models that assume most charcoal, wood, seeds, and twigs recovered from organic mats at or near the shoreline are derived from material that grew within the lake footprint, and therefore date a dry period between lakes. Charcoal samples collected from lacustrine clastic strata may have also been derived from fires burned during a dry period. As an initial constraint, we assume that samples older than those in earlier lake deposits have age inheritance. Assuming the dates are accurately described by their respective 2σ uncertainties, we ran all dates that would run in a preliminary OxCal model, and then removed those with a poor agreement index as defined in OxCal. From this, of the 423 total dates in the compilation, 151 dates are used in the base model and 149 dates are used in an alternative model, with the differences in the models resulting from choices of whether to include or exclude specific dates that may or may not be representative of a particular dry period between lakes. Where the two models agree, the results are robust, but where the models differ, any differences are taken as uncertainty in the lake ages. Historical accounts and a high-resolution paleohydrologic reconstruction allow us to refine some lake ages.The age windows for the past seven Lake Cahuilla highstands are 1731–1733 CE (Lake A), 1618–1636 CE (Lake B), 1486–1503 CE (Lake C), 1118–1165 or 1192–1241 CE (Lake D), 1007–1070 CE (Lake E), 930–966 CE (Lake F), and 612–5 BCE (Lake G). These ages represent the maximum allowable ranges during which a lake may have filled the basin up to the +13 m highstand elevation; the basin may have been dry for significant portions of each time window, though the lake filling and desiccation episodes may have extended beyond the stated highstand age range for each lake. If the paleohydrologic constraints are ignored, some of the lakes may have initiated earlier, by up to three decades. Additional dates would be needed to further bracket the ages of the earlier lakes. Notably, 120 of the original 284 reliable dates were rejected because they clearly violate stratigraphic ordering, implying that more than 40% of all radiocarbon dates in the Salton Basin exhibit statistically significant age inheritance.

Depth-dependent crustal azimuthal anisotropy in the Salton Trough, southern California

We present new crustal models of azimuthally anisotropic P-wave velocity, isotropic P-wave velocity (Vp), isotropic S-wave velocity (Vs) and Vp/Vs ratio for the Salton Trough. High Vp/Vs ratio is revealed at depths of 2–8 km along the San Andreas Fault, implying the possible presence of fluids. High Vp and high Vp/Vs ratio (>1.8) structures of the mid-lower crust beneath the Salton Trough possibly reflect the underplated gabbroic rocks as a result of the extension-induced partial melting of the upwelling asthenospheric materials. The fast velocity directions (FVDs) generally correlate with the direction of the maximum horizontal compressive stress except that fault-parallel FVDs are found at main fault traces. However, the FVDs in the Salton Trough show complex features. The northern Slaton Trough basin (the northern Salton Sea basin and Coachella Valley basin) is dominated by NW-SE FVDs, which are possibly due to the active compression happening there. E-W oriented FVDs predominate the upper crust of the southern Salton Trough basin (the southern Salton Sea basin and Imperial Valley basin), possibly due to faulting and/or block rotation. As the depth increases (12–18 km), the FVDs to the south of the Salton Sea have an overall rotation and gradually become NW-SE. The depth-dependent variation of anisotropy orientation may be attributed to a large band of underlying heat source that probably causes a change of stress field by imposing thermal stress on overlying rocks. In all, our velocity models suggest that faults in the Salton Trough may bear a high risk of mechanical failures due to the complexities of stress distribution and geological structures as well as the presence of a large volume of fluids and heat.

Moho Complexity in Southern California Revealed by Local PmP and Teleseismic Ps Waves

AbstractThe Moho discontinuity plays an important role in crustal growth and evolution. In this study, we delineate the Moho geometry in southern California by jointly using local Moho‐reflected waves PmP and teleseismic Moho‐converted waves Ps. To well constrain the Moho geometry, we have developed a two‐stage process to pick PmP waves and have created a reliable PmP travel time data set with a total of 10,192 picks. We have also extracted 38,648 high‐quality P‐wave receiver functions (RFs). The Moho depth is initially estimated via the common conversion point (CCP) stacking of RFs and then refined by inverting the PmP travel time data in a community velocity model (CVM‐H, version 15.1.1). The newly built Moho geometry is generally consistent with the California Moho Model version 1.0 (CMM‐1.0), that is, a shallow Moho beneath the Salton Trough (23 km), a uniformly shallow Moho beneath the Mojave Desert and the Basin and Range (<29 km), and a sliver of deep Moho under the western Peninsular Ranges, the eastern Transverse Ranges, and the western Sierra Nevada (>34 km). However, our Moho model reveals some new features different from the CMM–1.0, such as a deep Moho (∼34 km) beneath the northern end of the central and western Transverse Ranges, consistent with the observation of deep seismicities due to a thick brittle crust there. We also find a gradual transition from the lower crust to the uppermost mantle beneath the western Peninsular Ranges, leading to the rareness of pickable PmP waves as well as weak Moho‐converted signals there.

Seismic Architecture of the Upper Mantle Underlying California and Nevada

AbstractWe present new P and S teleseismic tomography models of the upper mantle beneath California and Nevada. Improved tomographic methods and the incorporation of new data increase image resolution and seismic structure definition. Technical advancements include the use of 3‐D ray tracing, ray weighting and finite‐frequency sensitivity kernels. Together with the incorporation of modern crustal velocity models to account for crustal delay time, these tomographic advancements reveal slightly higher amplitudes and narrower structures beneath prominent tectonic features in California and Nevada. The addition of new data from the southern California Borderland and northwestern California are particularly useful. Based on our imaging results, we discuss the effects of temperature and partial melt on upper mantle seismic velocity beneath California and Nevada. We interpret the major seismically fast structures in our study area to be Juan de Fuca slab and its downward continuation beneath central Nevada and foundered Farallon lithosphere beneath the southern Sierra Nevada and Transverse Ranges. The seismically slow structures beneath the slab‐free window of northern California Coast Range, the Salton Trough extensional basin, and the Cascadia back arc, are all consistent with the presence of partial melting of decompressing mantle. The imaged seismically fast and slow anomalies imply buoyancy variations and melt productivity that ultimately act on the overlying crust.

Statistical Relationship Between the Decrease of Major Seismicity and Drought in Southern California Since 1900

The seismicity in Southern California significantly decreased over the last decades. The decrease went in parallel with the reduction of meteoric groundwater recharge, which is a well-known factor capable of affecting seismicity. In this work the existence of a systematic statistical relationship was investigated by comparing the time density of Mw ≥ 5.7 earthquakes since 1900 with the time series of the Palmer Drought Severity Index (PDSI), an indicator of soil moisture roughly correlated with groundwater recharge. Given the non-stationarity of the two signals, the formal comparison was performed using both binomial logistic regression and cointegration testing. The analysis showed a significant statistical relationship, with peaks of seismicity 8 years behind those of PDSI. This finding suggests the hypothesis that groundwater recharge might affect earthquakes at a multi-year time scale. Proving this theory requires accurate measures and hydrogeological modeling, which is behind the scope of this work. Nonetheless, according to previous studies, the observed time lag might be explained by the slow propagation of pore pressure from the surface to the seismogenic volume. The ongoing trend towards an arid climate, made more evident by the recent severe droughts, might have contributed to the earthquake reduction of the last decades. The connection is particularly evident in the Salton Trough, with possible implications for the interpretation of its paleoseismicity.

High-resolution seismic imaging of the plate boundary in northern Baja California and southern California using double-pair double-difference tomography

We image seismic properties of the plate boundary through northern Baja California and southernmost California using a double-difference tomography and earthquake relocation algorithm that incorporates both event-pair and station-pair data. The data are derived from seismic networks in southern California (SCSN) and northern Baja California (CICESE). It comprises >700,000 travel times (P and S) associated with ∼200 seismic stations and >21,000 local earthquakes from 2003-2018, along with >2,000,000 event-pair and >1,500,000 station-pair double-differences derived from these travel times. In the upper crust, low VP values in the resultant models imply deep sediments and sedimentary rocks northwest of the Salton Sea and around the northern end of the Imperial fault. Low VS values indicate regions of rock damage at the northern ends of the Sierra Juarez fault zone and Laguna Salada fault, and the southern end of the San Jacinto fault zone (SJFZ). Shallow seismicity (d<956 km) highlights the Salton Buttes and Cerro Prieto volcanoes as high heat-flow thin crust regions. At mid to lower crustal depths, the sharpest contrast in crustal VP and most prominent low VS anomaly (V∼S3.4 km/s, VP/VS ∼ 1.9) occurs along a corridor extending northwestward from the Cerro Prieto fault (CPF), east of and subparallel to the Sierra El Mayor/Cucapah and into the southern SJFZ, indicating a continuous weak fluid-rich zone. At ∼25 km depth, we observe uppermost mantle velocities (V∼P7.8 km/s) in the eastern Salton Trough, crustal velocities (V<P7 km/s) around the Sierra El Mayor/Cucapah and Peninsular Ranges, and near mantle velocities (7.4 < VP < 7.8 km/s) beneath Laguna Salada. These results illuminate a thin crust in the Salton Trough, except for the Sierra El Mayor/Cucapah, and a thicker crust in the Peninsular Ranges. In all, high seismic activity, rapid changes in crustal structure and the prevalence of damage/deformation and fluids in a corridor extending from the CPF to the southern SJFZ emphasize this zone as a key component of the active plate boundary.

Seismic Moment Accumulation Response to Lateral Crustal Variations of the San Andreas Fault System

AbstractRheologic variations in the Earth's crust (like elastic plate thickness [EPT] or crustal rigidity) modulate the rate at which seismic moment accumulates for potentially hazardous faults of the San Andreas Fault System (SAFS). To quantify rates of seismic moment accumulation, Global Navigation Satellite Systems, and Interferometric Synthetic Aperture Radar data were used to constrain surface deformation rates of a four‐dimensional viscoelastic deformation model that incorporates rheological variations spanning a 900 km section of the SAFS. Lateral variations in EPT, estimated from surface heat flow and seismic depth to the lithosphere‐asthenosphere boundary, were converted to lateral variations in rigidity and then used to solve for seismic moment accumulation rates on 32 fault segments. We find a cluster of elevated seismic moment rates (11–20 × 1015 Nm year−1 km−1) along the main SAFS trace spanning the historical Mw 7.9 1857 Fort Tejon earthquake rupture length; present‐day seismic moment magnitude on these segments ranges from Mw 7.2–7.6. We also find that the average plate thickness in the Salton Trough is reduced to only 60% of the regional average, which results in a ∼60% decrease in moment accumulation rate along the Imperial fault. Likewise, a 30% increase of average plate thickness results in at least a ∼30% increase in moment rate and even larger increases are identified in regions of complex plate heterogeneity. These results emphasize the importance of considering rheological variations when estimating seismic hazard, suggesting that meaningful changes in seismic moment accumulation are revealed when considering spatial variations in crustal rheology.

Redefining the age of the lower Colorado River, southwestern United States

Abstract Sanidine dating and magnetostratigraphy constrain the timing of integration of the lower Colorado River (southwestern United States and northern Mexico) with the evolving Gulf of California. The Colorado River arrived at Cottonwood Valley (Nevada and Arizona) after 5.24 Ma (during or after the Thvera subchron). The river reached the proto–Gulf of California once between 4.80 and 4.63 Ma (during the C3n.2r subchron), not at 5.3 Ma and 5.0 Ma as previously proposed. Duplication of section across newly identified strands of the Earthquake Valley fault zone (California) probably explains the discrepancy. The data also imply the start of focused plate motion and basin development in the Salton Trough (California) at 6–6.5 Ma and relative tectonic stability of the southernmost part of the lower Colorado River corridor after its integration. After integration, the Colorado River quickly incised through sediment-filled basins and divides between them as it also likely excavated Grand Canyon (Arizona). The liberated sediment from throughout the system led to deposition of hundreds of meters of Bullhead Alluvium downstream of Grand Canyon after 4.6 Ma as the river adjusted to its lower base level.

An Assessment of Global Positioning System Velocity Uncertainty in California

AbstractWe analyze data from 580 continuous global positioning system (GPS) stations in California to quantify differences in published velocity estimates from five analysis centers. Horizontal and vertical rates for individual stations can differ up to 5 mm/yr, with systematic differences in some areas comparable to deformation rates. Published velocity uncertainties vary between analysis centers and are systematically underreported in the horizontal relative to empirical uncertainties calculated from the scatter of analysis center velocities. In the vertical, published velocity uncertainties are both over and underreported and vary more widely between centers. An interpolated ensemble vertical velocity field shows high‐subsidence regions in the Central Valley and Salton Trough have the largest empirical uncertainties, while station density has a modest impact on uncertainties. Applications that rely on subcentimeter GPS accuracy should consider the possibility that formal errors published with velocity rate estimates understate true velocity uncertainties in both the horizontal and vertical.