Faults In California Research Articles

The improvement of strain estimation using universal kriging

In this paper, universal kriging with linear trend is used to interpolate the strain tensor elements over a region along San Andreas Fault in California. The main goal of this paper is to improve the ordinary kriging interpolation results. A 7-year time series (2006–2012) of 12 permanent stations is utilized to obtain the coordinate changes in UTM coordinates system and calculate the strain tensor elements by means of finite difference method. Comparing the results we can find an improvement about 40 % for universal kriging at critical points in which ordinary kriging can’t be appropriate method of interpolation.

Resilience by Design: Bringing Science to Policy Makers

No one questions that Los Angeles has an earthquake problem. The “Big Bend” of the San Andreas fault in southern California complicates the plate boundary between the North American and Pacific plates, creating a convergent component to the primarily transform boundary. The Southern California Earthquake Center Community Fault Model has over 150 fault segments, each capable of generating a damaging earthquake, in an area with more than 23 million residents (Fig. 1). A Federal Emergency Management Agency (FEMA) analysis of the expected losses from all future earthquakes in the National Seismic Hazard Maps (Petersen et al. , 2014) predicts an annual average of more than $3 billion per year in the eight counties of southern California, with half of those losses in Los Angeles County alone (Federal Emergency Management Agency [FEMA], 2008). According to Swiss Re, one of the world’s largest reinsurance companies, Los Angeles faces one of the greatest risks of catastrophic losses from earthquakes of any city in the world, eclipsed only by Tokyo, Jakarta, and Manila (Swiss Re, 2013). Figure 1. Perspective view of fault segments in the Southern California Community Fault Model and seismicity 1984–2004 (fig. 1 from Plesch et al. , 2007). The last large earthquake in Los Angeles, the M w 6.7, 17 January 1994, Northridge earthquake happened the day before abstracts were due for the last meeting of the Seismological Society of America held in Pasadena. It took 45 min to get the first estimate of location and magnitude (“about 6 and a half in the northwest San Fernando Valley”) and 2 hr to locate the first aftershock, a failing of the computer systems that eventually led to the development of TriNet and the first broadband network in the United States (Hauksson et al. , 2001). When FedEx delivered more than 400 abstracts for that meeting, most of the …

A new GPS velocity field for the Pacific Plate – Part 2: implications for fault slip rates in western California

Lower and upper bounds for present deformation rates across faults in central California between the San Andreas Fault and Pacific coast are estimated from a new Global Positioning System (GPS) velocity field for central, western California in light of geodetic evidence presented in a companion paper for slow, but significant deformation within the Pacific Plate between young seafloor in the eastern Pacific and older seafloor elsewhere on the plate. Transects of the GPS velocity field across the San Andreas Fault between Parkfield and San Juan Buatista, where fault slip is dominated by creep and the velocity field thus reveals the off-fault deformation, show that GPS sites in westernmost California move approximately parallel to the fault at an average rate of 3.4 ± 0.4 mm yr−1 relative to the older interior of the Pacific Plate, but only 1.8 ± 0.6 mm yr−1 if the Pacific Plate frame of reference is corrected for deformation within the plate. Modelled interseismic elastic deformation from the weakly coupled creeping segment of the San Andreas Fault is an order-of-magnitude too small to explain the southeastward motions of coastal sites in western California. Similarly, models that maximize residual viscoelastic deformation from the 1857 Fort Tejon and 1906 San Francisco earthquakes mismatch both the rates and directions of GPS site motions in central California relative to the Pacific Plate. Neither thus explains the site motions southwest of the San Andreas fault, indicating that the site motions measure deformation across faults and folds outboard of the San Andreas Fault. The non-zero site velocities thus constitute strong evidence for active folding and faulting outboard from the creeping segment of the San Andreas Fault and suggest limits of 0–2 mm yr−1 for the Rinconada Fault slip rate and 1.8 ± 0.6 to 3.4 ± 0.4 mm yr−1 for the slip rates integrated across near-coastal faults such as the Hosgri, San Gregorio and San Simeon faults.

A scaling law to characterize fault-damage zones at reservoir depths

We analyze fracture-density variations in subsurface fault-damage zones in two distinct geologic environments, adjacent to faults in the granitic SSC reservoir and adjacent to faults in arkosic sandstones near the San Andreas fault in central California. These damage zones are similar in terms of width, peak fracture or fault (FF) density, and the rate of FF density decay with distance from the main fault. Seismic images from the SSC reservoir exhibit a large basement master fault associated with 27 seismically resolvable second-order faults. A maximum of 5 to 6 FF/m (1.5 to 1.8 FF/ft) are observed in the 50 to 80 m (164 to 262 ft) wide damage zones associated with second-order faults that are identified in image logs from four wells. Damage zones associated with second-order faults immediately southwest of the San Andreas Fault are also interpreted using image logs from the San Andreas Fault Observatory at Depth (SAFOD) borehole. These damage zones are also 50–80 m wide (164 to 262 ft) with peak FF density of 2.5 to 6 FF/m (0.8 to 1.8 FF/ft). The FF density in damage zones observed in both the study areas is found to decay with distance according to a power law . The fault constant is the FF density at unit distance from the fault, which is about 10–30 FF/m (3.1–9.1 FF/ft) in the SSC reservoir and 6–17 FF/m (1.8–5.2 FF/ft) in the arkose. The decay rate ranges from 0.68 to 1.06 in the SSC reservoir, and from 0.4 to 0.75 in the arkosic section. This quantification of damage-zone attributes can facilitate the incorporation of the geometry and properties of damage zones in reservoir flow simulation models.

Rapid mapping of ultrafine fault zone topography with structure from motion

Structure from Motion (SfM) generates high-resolution topography and coregistered texture (color) from an unstructured set of overlapping photographs taken from varying viewpoints, overcoming many of the cost, time, and logistical limitations of Light Detection and Ranging (LiDAR) and other topographic surveying methods. This paper provides the first investigation of SfM as a tool for mapping fault zone topography in areas of sparse or low-lying vegetation. First, we present a simple, affordable SfM workflow, based on an unmanned helium balloon or motorized glider, an inexpensive camera, and semiautomated software. Second, we illustrate the system at two sites on southern California faults covered by existing airborne or terrestrial LiDAR, enabling a comparative assessment of SfM topography resolution and precision. At the first site, an ∼0.1 km2 alluvial fan on the San Andreas fault, a colored point cloud of density mostly >700 points/m2 and a 3 cm digital elevation model (DEM) and orthophoto were produced from 233 photos collected ∼50 m above ground level. When a few global positioning system ground control points are incorporated, closest point vertical distances to the much sparser (∼4 points/m2) airborne LiDAR point cloud are mostly <3 cm. The second site spans an ∼1 km section of the 1992 Landers earthquake scarp. A colored point cloud of density mostly >530 points/m2 and a 2 cm DEM and orthophoto were produced from 450 photos taken from ∼60 m above ground level. Closest point vertical distances to existing terrestrial LiDAR data of comparable density are mostly <6 cm. Each SfM survey took ∼2 h to complete and several hours to generate the scene topography and texture. SfM greatly facilitates the imaging of subtle geomorphic offsets related to past earthquakes as well as rapid response mapping or long-term monitoring of faulted landscapes.

CyberShake-derived ground-motion prediction models for the Los Angeles region with application to earthquake early warning

Real-time applications such as earthquake early warning (EEW) typically use empirical ground-motion prediction equations (GMPEs) along with event magnitude and source-to-site distances to estimate expected shaking levels. In this simplified approach, effects due to finite-fault geometry, directivity and site and basin response are often generalized, which may lead to a significant under- or overestimation of shaking from large earthquakes (M > 6.5) in some locations. For enhanced site-specific ground-motion predictions considering 3-D wave-propagation effects, we develop support vector regression (SVR) models from the SCEC CyberShake low-frequency ( 415 000 finite-fault rupture scenarios (6.5 ≤ M ≤ 8.5) for southern California defined in UCERF 2.0. We use CyberShake to demonstrate the application of synthetic waveform data to EEW as a ‘proof of concept’, being aware that these simulations are not yet fully validated and might not appropriately sample the range of rupture uncertainty. Our regression models predict the maximum and the temporal evolution of instrumental intensity (MMI) at 71 selected test sites using only the hypocentre, magnitude and rupture ratio, which characterizes uni- and bilateral rupture propagation. Our regression approach is completely data-driven (where here the CyberShake simulations are considered data) and does not enforce pre-defined functional forms or dependencies among input parameters. The models were established from a subset (∼20 per cent) of CyberShake simulations, but can explain MMI values of all >400 k rupture scenarios with a standard deviation of about 0.4 intensity units. We apply our models to determine threshold magnitudes (and warning times) for various active faults in southern California that earthquakes need to exceed to cause at least ‘moderate’, ‘strong’ or ‘very strong’ shaking in the Los Angeles (LA) basin. These thresholds are used to construct a simple and robust EEW algorithm: to declare a warning, the algorithm only needs to locate the earthquake and to verify that the corresponding magnitude threshold is exceeded. The models predict that a relatively moderate M6.5–7 earthquake along the Palos Verdes, Newport-Inglewood/Rose Canyon, Elsinore or San Jacinto faults with a rupture propagating towards LA could cause ‘very strong’ to ‘severe’ shaking in the LA basin; however, warning times for these events could exceed 30 s.

Offset of Latest Pleistocene Shoreface Reveals Slip Rate on the Hosgri Strike-Slip Fault, Offshore Central California

Abstract The Hosgri fault is the southern part of the regional Hosgri–San Gregorio dextral strike‐slip fault system, which extends primarily in the offshore for about 400 km in central California. Between Morro Bay and San Simeon, high‐resolution multibeam bathymetry reveals that the eastern strand of the Hosgri fault is crossed by an ∼265 m wide slope interpreted as the shoreface of a latest Pleistocene sand spit. This sand spit crossed an embayment and connected a western fault‐bounded bedrock peninsula and an eastern bedrock highland, a paleogeography resembling modern coastal geomorphology along the San Andreas fault. Detailed analysis of the relict shoreface with slope profiles and slope maps indicates a lateral slip rate of 2.6±0.9 mm/yr, considered a minimum rate for the Hosgri given the presence of an active western strand. This slip rate indicates that the Hosgri system takes up the largest share of the strike‐slip fault budget and is the most active strike‐slip fault west of the San Andreas fault in central California. This result further demonstrates the value and potential of high‐resolution bathymetry in characterization of active offshore faults. Online Material: (1) High‐resolution bathymetric data for the cross‐Hosgri slope and (2) metadata that describes data collection, processing, and formatting.

The UCERF3 Grand Inversion: Solving for the Long-Term Rate of Ruptures in a Fault System

Abstract We present implementation details, testing, and results from a new inversion‐based methodology, known colloquially as the “grand inversion,” developed for the Uniform California Earthquake Rupture Forecast (UCERF3). We employ a parallel simulated annealing algorithm to solve for the long‐term rate of all ruptures that extend through the seismogenic thickness on major mapped faults in California while simultaneously satisfying available slip‐rate, paleoseismic event‐rate, and magnitude‐distribution constraints. The inversion methodology enables the relaxation of fault segmentation and allows for the incorporation of multifault ruptures, which are needed to remove magnitude‐distribution misfits that were present in the previous model, UCERF2. The grand inversion is more objective than past methodologies, as it eliminates the need to prescriptively assign rupture rates. It also provides a means to easily update the model as new data become available. In addition to UCERF3 model results, we present verification of the grand inversion, including sensitivity tests, tuning of equation set weights, convergence metrics, and a synthetic test. These tests demonstrate that while individual rupture rates are poorly resolved by the data, integrated quantities such as magnitude–frequency distributions and, most importantly, hazard metrics, are much more robust.

Lithosphere–asthenosphere interactions near the San Andreas fault

We decipher the strain history of the upper mantle in California through the comparison of the long-term finite strain field in the mantle and the surface strain-rate field, respectively inferred from fast polarization directions of seismic phases (SKS and SKKS), and Global Positioning System (GPS) surface velocity fields. We show that mantle strain and surface strain-rate fields are consistent in the vicinity of San Andreas Fault (SAF) in California. Such an agreement suggests that the lithosphere and strong asthenosphere have been deformed coherently and steadily since >1 Ma. We find that the crustal stress field rotates (up to 40° of rotation across a 50 km distance from 50° relative to the strike of the SAF, in the near-field of SAF) from San Francisco to the Central Valley. Both observations suggest that the SAF extends to depth, likely through the entire lithosphere. From Central Valley towards the Basin and Range, the orientations of GPS strain-rates, shear wave splitting measurements and seismic stress fields diverge indicating reduced coupling or/and shallow crustal extension and/or presence of frozen anisotropy.

Fault coupling and potential for earthquakes on the creeping section of the central San Andreas Fault

AbstractThe 150 km long central section of the San Andreas Fault (CSAF) in central California creeps at the surface and has not produced a large earthquake historically. However, sections of the San Andreas Fault to the north and south are known to have ruptured repeatedly inM~7–8 earthquakes. It is currently unclear whether the creeping CSAF could rupture in large earthquakes, either individually or along with earthquakes on the locked sections to the north and south. We invert Global Positioning System and interferometric synthetic aperture radar data with elastic block models to estimate the degree of locking on the CSAF and place bounds on the moment accumulation rate on the fault. We find that the inferred moment accumulation rate is highly dependent on the long‐term fault slip rate, which is poorly constrained along the CSAF. The inferred moment accumulation rate, normalized by shear modulus, ranges from 3.28 × 104to 5.85 × 107 m3/yr, which is equivalent to aMw = 5.5–7.2 earthquake every 150 years for a long‐term slip rate of 26 mm/yr andMw = 7.3–7.65 for a long‐term slip rate of 34 mm/yr. The comparisons of slip distributions with microseismicity and repeating earthquakes indicate a possible locked patch between 10 and 20 km depth on the CSAF that could potentially rupture withMw = 6.5.

Improving Innovative Mathematical Model for Earthquake Prediction

The Innovative Mathematical Model for Earthquake Prediction (IMMEP) based on Spatial Connection Theory and reverse Poisson’s distribution was developed previously. Using data from National Earthquake Information Center (NEIC), Spatial Connection Models were constructed using KML programming language in Google Earth program for six fault zones around the world: California, Central USA, Northeast USA, Hawaii, Turkey, and Japan. The Poisson Range Identifier (Pri) values were computed, and the Poisson’s Distribution was applied to the Pri values to arrive at a distance factor. Based on the reverse Poisson’s Distribution, earthquake predictions were carried out. To improve the Innovative Mathematical Model for Earthquake Prediction, further analysis was carried out on California fault zone earthquake data, utilizing Poisson’s and Exponential Distributions. The predictions of the Poisson’s and Exponential Distribution were nearby validating the Spatial Connection Theory By using technological advances and improving the probability of future earthquake predictions, this research provides an effective contribution to earth science. Utilizing the results of this research, disaster management agencies around the world can allocate their resources in appropriate locations to assist people during evacuation and save lives.

Is stress accumulating on the creeping section of the San Andreas fault?

The creeping section of the San Andreas fault (CSAF) in central California is a proposed barrier to propagation of large earthquakes. Yet, recent studies show that the creeping section is not entirely uncoupled but is accumulating slip deficit at a rate equivalent to a Mw=7.2–7.4 earthquake every 150years. A critical piece to understanding earthquake potential on the CSAF is determining whether slip deficit is occurring with stress accumulation on stick‒slip regions or without stress accumulation on stable‒sliding regions shadowed by surrounding locked areas. We use a physical model to estimate the spatial distribution of locked, stress‒accumulating areas of the fault constrained by surface creep rate measurements and GPS‒derived velocities. We find that the area of the fault accumulating stress, if ruptured every 150years, would release slip equivalent to at most a Mw=6.75 earthquake, significantly less than the Mw=7.2–7.4, 150year equivalent total slip deficit rate.

Fault network modeling of crustal deformation in California constrained using GPS and geologic observations

Abstract We have developed a kinematic fault network model of crustal deformation in an elastic half-space. Surface deformation is calculated using this model assuming each fault segment slipping beneath a locking depth. Each fault segment connects to its adjacent elements with slip vector continuity imposed at fault nodes or intersections; the degree of the constraints determines whether deformation is block-like or not. We apply this model to invert GPS observations for slip rates on major faults in California with geological rate constraints. Based on the F-test result, we find that lesser block-like models fit the data significantly better than the strictly block-like model. Our final inversion shows a slip rate varying from 20 to 23 mm/yr along the northern San Andreas from the Santa Cruz to the North Coast segment. Slip rates vary from 9 to 13 mm/yr along the Hayward to the Maacama fault segment, and from 15 to 3 mm/yr along the central Calaveras to the West Napa fault segment. For the central California Creeping Zone, the result suggests a depth dependent creep rate with an average of 22 mm/yr over the top 5 km and 32 mm/yr underneath. From the Mojave to San Bernardino Mountain segments, we also find a significant decrease in slip rate along the San Andreas in comparison with the geologic rates, in contrast to a significant increase in slip rate on faults along the eastern California shear zone. Along the southern San Andreas, slip rates vary from 21 to 25 mm/yr from the Coachella Valley to Imperial Valley segments. Slip rates range from 0 to 3 mm/yr across the western Transverse Ranges faults, which is consistent with the regional crustal thickening. Overall slip rates derived from geodetic observations correlate strongly with the geologic slip rates statistically, suggesting high compatibility between geodetic and geologic observations.

Inferring fault rheology from low‐frequency earthquakes on the San Andreas

AbstractFamilies of recurring low‐frequency earthquakes (LFEs) within nonvolcanic tremor (NVT) on the San Andreas fault in central California show strong sensitivity to shear stress induced by the daily tidal cycle. LFEs occur at all levels of the tidal shear stress and are in phase with the very small, ~400 Pa, stress amplitude. To quantitatively explain the correlation, we use a model from the existing literature that assumes the LFE sources are small, persistent regions that repeatedly fail during shear of a much larger scale, otherwise aseismically creeping fault zone. The LFE source patches see tectonic loading, creep of the surrounding fault which may be modulated by the tidal stress, and direct tidal loading. If the patches are small relative to the surrounding creeping fault then the stressing is dominated by fault creep, and if patch failure occurs at a threshold stress, then the resulting seismicity rate is proportional to the fault creep rate or fault zone strain rate. Using the seismicity rate as a proxy for strain rate and the tidal shear stress, we fit the data with possible fault rheologies that produce creep in laboratory experiments at temperatures of 400 to 600°C appropriate for the LFE source depth. The rheological properties of rock‐forming minerals for dislocation creep and dislocation glide are not consistent with the observed fault creep because strong correlation between small stress perturbations and strain rate requires perturbation on the order of the ambient stress. The observed tidal modulation restricts ambient stress to be at most a few kilopascal, much lower than rock strength. A purely rate dependent friction is consistent with the observations only if the product of the friction rate dependence and effective normal stress is ~ 0.5 kPa. Extrapolating the friction rate strengthening dependence of phyllosilicates (talc) to depth would require the effective normal stress to be ~50 kPa, implying pore pressure is lithostatic. If the LFE source is on the order of tens of meters, as required by the model, rate‐weakening friction rate dependence (e.g., olivine) at 400 to 600°C requires that the minimum effective pressure at the LFE source is ~ 2.5 MPa.

AUTOKITE

An experimental kite-plane capable of autonomous aerial imaging is introduced as a viable low-cost small-scale civilian UAV imaging platform ideal for field use. The AUTOKITE fulfills a need currently unmet by other fully automated Unmanned Aerial Vehicles (UAVs), resulting from ease of operation, extended flight time, and overall reliability. The AUTOKITE is outfitted with an off-the-shelf autopilot system, and has demonstrated fully autonomous flight in field deployments while collecting high-resolution (~12 cm/pixel) images. The AUTOKITE has been used to map regions historically prone to earthquakes along the Southern San Andreas Fault in California. Comparative image methods enabled by photogrammetric software, like Agisoft's PhotoScan, are then used to discern Structure-from-Motion (SfM) from a multitude of aerial images taken by AUTOKITE [8]. Processing SfM data from overlapping images results in the creation of Digital Elevation Models (DEMs) and Orthophotos for geographic areas of interest. In addition to sample data sets illustrating the SfM process, The AUTOKITE is compared with three alternative UAV systems, and payload integration/automation details are discussed.

Slip rates and off‐fault deformation in Southern California inferred from GPS data and models

In Southern California, fault slip rate estimates along the San Andreas fault (SAF) and Garlock fault from geodetically constrained kinematic models are systematically at the low end or lower than geologic slip rate estimates. The sum of geodetic model slip rates across the eastern California shear zone is higher than the geologic sum. However, the ranges of reported model and geologic slip rate estimates in the literature are sufficiently large that it remains unclear whether these apparent discrepancies are real or attributable to epistemic uncertainties in the two types of estimates. We further examine uncertainties in geodetically derived slip rate estimates on major faults in Southern California by conducting a suite of inversions with four kinematic models. Long‐term rigid elastic block models constrained by the geologic slip rates cannot fit the present‐day GPS‐derived velocity field. Deforming (permanent off‐fault strain) elastic block models and viscoelastic earthquake cycle block models constrained by geologic slip rates can fit the present‐day GPS‐derived velocity field with 28–33% of the total geodetic moment rate occurring as distributed deformation off of the major faults. Models incorporating viscoelastic mantle flow predict systematically higher slip rates than purely elastic models on many of the major Southern California faults with ranges of (elastic/viscoelastic) 29–34/30–37 mm/yr for the Carrizo SAF segment, 20–24/20–32 mm/yr for the Mojave SAF segment, 14–17/18–22 mm/yr for the Coachella SAF segment, 13–19/14–22 mm/yr for the San Jacinto fault, and 5–11/5–11 mm/yr for the western Garlock fault.

Hillslopes Record the Growth and Decay of Landscapes

Earth's surface archives the combined history of tectonics and erosion, which tend to roughen landscapes, and sediment transport and deposition, which smooth them. We analyzed hillslope morphology in the tectonically active Dragon's Back Pressure Ridge in California, United States, to assess whether tectonic uplift history can be reconstructed using measurable attributes of hillslope features within landscapes. Hilltop curvature and hillslope relief mirror measured rates of vertical displacement caused by tectonic forcing, and their relationships are consistent with those expected when idealizing hillslope transport as a nonlinear diffusion process. Hilltop curvature lags behind relief in its response to changing erosion rates, allowing growing landscapes to be distinguished from decaying landscapes. Numerical modeling demonstrates that hillslope morphology may be used to infer changes in tectonic rates.

Particle-size distributions of low-angle normal fault breccias: Implications for slip mechanisms on weak faults

Slip on low-angle normal faults is not well understood because they slip at high angles to the maximum principal stress directions. These faults are considered weak and their motion cannot be explained using standard Byerlee friction and Andersonian fault mechanics. One proposed mechanism for weak fault slip is reduction of effective normal stress induced by high pore-fluid pressure. This mechanism is likely to allow dilation of the fault zone and, therefore, affect the particle-size distribution of fault breccia, which has been shown to differ for unconstrained versus constrained comminution. High pore-fluid pressure can cause dilation which leads to unconstrained comminution. We analyze samples from the footwalls of two low-angle normal faults in southern California (West Salton and Whipple detachment faults) to determine the fault-rock textures and grain-size distributions (GSDs). The GSDs are fractal with fractal dimensions ranging from ∼2.6 to 3.4. The lower end of this range is thought to reflect constrained comminution and only occurs in samples from the footwall of a small-offset “minidetachment” fault about 100 m below the Whipple detachment. The higher fractal dimensions are common in cataclasites related to the main faults and also reflect constrained comminution but are overprinted by shear localization. Our GSDs are similar to those from natural and laboratory-deformed fault rocks from strong faults. We conclude that if high pore-fluid pressure aided slip on these faults, it did not strongly affect mechanisms by which brecciation occurs, implying that fluid pressure generally was sublithostatic. Independent evidence exists for lithostatic fluid pressure that having dropped or cycled to hydrostatic levelsin the minidetachment, but our GSD results suggest that periods of high fluid pressure were too short or infrequent for unconstrained comminution to have been the dominant cataclastic mechanism. Fractal dimensions of ∼2.6 for these samples suggest that little subsequent abrasion occurred due to shear localization, consistent with minor offset on the minidetachment. Main detachment footwall samples with fractal dimensions ≥3 reflect constrained comminution followed by shear-related abrasion, and suggest that seismic cycling was important in formation of main detachment cataclasites.

Three-dimensional magnetotelluric inversion in practice—the electrical conductivity structure of the San Andreas Fault in Central California

Three-dimensional magnetotelluric inversion in practice—the electrical conductivity structure of the San Andreas Fault in Central California

A reappraisal of the early slip history of the San Andreas fault, central California, USA

The modern San Andreas fault system (California, United States) is widely considered to have formed in response to the initiation of Pacific–North American plate interaction ca. 27 Ma. Although there is general consensus on the magnitude and timing of Neogene displacement along the San Andreas system, its Paleogene history remains unresolved. In particular, ∼100 km of right-lateral offset between mid-Cretaceous plutonic rocks of the northern Salinian block and the western edge of Sierra Nevada basement remains unaccounted for after restoration of Neogene displacement along strike-slip faults of the San Andreas system. Our detrital zircon data invalidate a key Paleogene piercing point by demonstrating that displaced portions of the hypothesized Middle Eocene Butano–Point of Rocks submarine fan were never contiguous across the San Andreas fault. We instead show that the Eocene provenance characteristics exhibited by northern Salinian strata closely match those of the southern Sierra Nevada and northwestern Mojave Desert. This implies that the northern Salinian block was located at least 75–50 km farther south in Eocene time than previously recognized. Our data require (1) pre–23 Ma dextral slip along the San Andreas fault in central California, and/or (2) slip along a predecessor fault that formed prior to Pacific–North American plate interaction. This previously undocumented slip may indicate that significant Pacific–North American plate interaction propagated from the plate margin into the continental interior much earlier than conventionally believed. Alternatively, late Paleogene slip could predate the development of the modern plate boundary and represent inboard dextral strike-slip displacement along the eastern margin of the Salinian block, similar to the deformation that occurs today along the strike-slip Sumatra fault system.