Lisa B. Grant

Paleoseismic evidence of clustered earthquakes on the San Andreas Fault in the Carrizo Plain, California

Exposures we have excavated across the San Andreas fault contradict the hypothesis that part of the fault in the Carrizo Plain is unusually strong and experiences relatively infrequent rupture. The exposures record evidence of at least seven surface-rupturing earthquakes which have been approximately dated by accelerated mass spectrometry radiocarbon analysis of detrital charcoal and buried in situ plants. Five large earthquakes have occurred since 1218 A.D. The most recent earthquake, event A, was the 1857 Fort Tejon earthquake, which we have associated with 6.6–10 m of dextral slip along the main fault trace. The penultimate earthquake, event B, most likely occurred within the period A.D. 1405–1510. Slip from either events B and C combined or from event B alone, totals 7–11 m. Three earthquakes, events C, D, and E, occurred in a temporal cluster prior to event B and after approximately A.D. 1218. The average recurrence interval within this cluster is 73–116 years, depending on assumptions. Events F and G occurred after 200 years B.C. A depositional hiatus between events E and F may hide evidence of additional earthquakes. Events B and D within the Carrizo cluster of A.D. 1218–1510 may correlate with events T (A.D. 1329–1363) and V (A.D. 1465–1495) at Pallett Creek on the Mojave “segment” of the fault. This suggests two fault ruptures similar in length to that of 1857. Events C and E apparently did not rupture the Mojave section, which suggests that the Carrizo segment has ruptured independently or in combination with segments to the north. Irregular repeat times of large earthquakes suggest a pattern of clustered events at the end of seismic “supercycles.”

Community Fault Model (CFM) for Southern California

We present a new three-dimensional model of the major fault systems in southern California. The model describes the San Andreas fault and associated strike- slip fault systems in the eastern California shear zone and Peninsular Ranges, as well as active blind-thrust and reverse faults in the Los Angeles basin and Transverse Ranges. The model consists of triangulated surface representations (t-surfs) of more than 140 active faults that are defined based on surfaces traces, seismicity, seismic reflection profiles, wells, and geologic cross sections and models. The majority of earthquakes, and more than 95% of the regional seismic moment release, occur along faults represented in the model. This suggests that the model describes a comprehen- sive set of major earthquake sources in the region. The model serves the Southern California Earthquake Center (SCEC) as a unified resource for physics-based fault systems modeling, strong ground-motion prediction, and probabilistic seismic hazards assessment.

Late Quaternary uplift and earthquake potential of the San Joaquin Hills, southern Los Angeles basin, California

Analysis of emergent marine terraces in the San Joaquin Hills, California, and 230 Th dating of solitary corals from the lowest terraces reveal that the San Joaquin Hills have risen at a rate of 0.21–0.27 m/k.y. during the past 122 k.y. Movement on a blind thrust fault in the southern Los Angeles basin has uplifted the San Joaquin Hills and has the potential to generate an M w 7.3 earthquake within this densely populated area. Our structural modeling suggests that the fault dips to the southwest and slips at ∼0.42–0.79 m/k.y., yielding an estimated minimum average recurrence interval of ∼1650–3100 yr for moderate-sized earthquakes. Recognition of this blind thrust extends the known area of active blind thrusts and fault-related folding southward from Los Angeles into coastal Orange County.

Stream Channel Offset and Late Holocene Slip Rate of the San Andreas Fault at the Van Matre Ranch Site, Carrizo Plain, California

Sets of well-preserved channels offset across the San Andreas fault (SAF) at the Van Matre Ranch (VMR) site in the northwestern Elkhorn Hills area of the Carrizo Plain offer the opportunity to measure slip rate and examine geomorphic development of the channels. The fault zone and offset channels were exposed by excavation in one fault-perpendicular and five fault-parallel trenches. The geomor- phology and stratigraphy in the channels reveal a record of filling by fluvial sedi- mentation, lateral colluviation, and pedogenesis. The buried thalweg of the currently active channel is offset 24.8 1 m, while the geomorphic channel is offset approx- imately 27.6 1 m. Seventeen samples were collected from channel margin deposits for 14 C dating. An OxCal model of the radiocarbon dates with stratigraphic control suggests that the oldest date for channel incision was A.D. 1160. Minimum and maximum slip rates ranging from 29.3 to 35.6 mm/yr are derived from different assumptions about the timing of channel incision and offset. The resulting slip rates at VMR agree well with the late-Holocene slip rate of 33.9 2.9 mm/yr at Wallace Creek, approximately 18 km to the northwest, and imply that within measurement uncertainty the 30-37 mm/yr velocity gradient across the SAF from decadal time- scale geodetic measurements is accommodated across the several-meter-wide SAF zone at VMR over the last millennium.

Activity of the Offshore Newport-Inglewood Rose Canyon Fault Zone, Coastal Southern California, from Relocated Microseismicity

An offshore zone of faulting approximately 10 km from the southern California coast connects the seismically active strike-slip Newport-Inglewood fault zone in the Los Angeles metropolitan region with the active Rose Canyon fault zone in the San Diego area. Relatively little seismicity has been recorded along the offshore Newport-Inglewood Rose Canyon fault zone, although it has long been suspected of being seismogenic. Active low-angle thrust faults and Quaternary folds have been imaged by seismic reflection profiling along the offshore fault zone, raising the question of whether a through-going, active strike-slip fault zone exists. We applied a waveform cross-correlation algorithm to identify clusters of microseismicity consisting of similar events. Analysis of two clusters along the offshore fault zone shows that they are associated with nearly vertical, north-northwest-striking faults, consistent with an offshore extension of the Newport-Inglewood and Rose Canyon strike-slip fault zones. P -wave polarities from a 1981 event cluster are consistent with a right-lateral strike-slip focal mechanism solution. Manuscript received 23 July 2003.

Three-Dimensional Excavation and Recent Rupture History along the Cholame Segment of the San Andreas Fault

A paleoseismic study conducted along the Cholame segment of the San Andreas fault provides evidence for three earthquakes and the amount of lateral offset for the most recent event (1857 Fort Tejon earthquake). Excavations at the Las Yeguas (LY4) site include five fault-perpendicular, two parallel, and several hand-dug trenches. Abruptly truncated sand and silt layers that are not correlative across the fault zone constrain the timing of the penultimate event (L2) between cal. A.D. 1030-1300 and 1390-1460. Vertical offset, shearing, and fracturing of silty sand and gravel units that overlie L2 and historical artifacts that bracket the timing of the MRE (L1) provide evidence that the most recent ground-rupturing event, L1, occurred between cal. A.D. 1390-1460 and ∼1865. L1 is likely the 1857 Fort Tejon earthquake. Tectonic silt-filled fractures that dissect historic gray-tan silt and sand suggest a ground shaking or a triggered slip event (L0), which occurred after L1. Three-dimensional excavation of an alluvial fan edge (unit 4) indicates that 3.0 ± 0.70 m of near-fault brittle slip occurred during the 1857 earthquake at this site.

iSERVO: Implementing the International Solid Earth Research Virtual Observatory by Integrating Computational Grid and Geographical Information Web Services

We describe the goals and initial implementation of the International Solid Earth Virtual Observatory (iSERVO). This system is built using a Web Services approach to Grid computing infrastructure and is accessed via a component-based Web portal user interface. We describe our implementations of services used by this system, including Geographical Information System (GIS)-based data grid services for accessing remote data repositories and job management services for controlling multiple execution steps. iSERVO is an example of a larger trend to build globally scalable scientific computing infrastructures using the Service Oriented Architecture approach. Adoption of this approach raises a number of research challenges in millisecond-latency message systems suitable for internet-enabled scientific applications. We review our research in these areas.

Recent Rupture History of the San Andreas Fault Southeast of Cholame in the Northern Carrizo Plain, California

We conducted a paleoseismic study on the San Andreas fault (SAF) southeast of Cholame, California, to investigate the record of earthquakes along an 80-km paleoseismic data gap between Parkfield and the Carrizo Plain. At the LY4 site, located 37.5 km southeast of Highway 46 along the SAF, we excavated a fault-perpendicular trench on the distal end of an alluvial fan that emanates from the Temblor Mountains to the northeast. We found evidence of three and possibly four ruptures recorded within the stratigraphy. The only age constraints are radiocarbon dates on a paleosol three units (50 cm) below the oldest event horizon and the presence of recently introduced exotic pollen species in an upper unit. The radiocarbon dates indicate there have been at least three surface-rupturing events at the LY4 site since cal. A.D. 1058-1291. Exotic (historic) pollen in the top of a unit possibly cut by the youngest event suggests that an earthquake affected at the LY4 site close to A.D. 1873-1874. Manuscript received 11 January 2000.

Introduction to the Special Issue on Paleoseismology of the San Andreas Fault System

The purpose of this special issue of BSSA is to provide a state-of-the-science compendium of data on the rupture history of the San Andreas fault from recent paleoseismic investigations. The San Andreas fault is the primary boundary fault between the Pacific and North American plates and one of the most thoroughly studied faults in the world. Fourteen articles prepared by 74 authors provide dates of surface ruptures and/or measurements of slip or slip rate at 10 paleoseismic investigation sites along the main trace of the San Andreas fault, at one site on the Hayward fault, and along the northern San Jacinto fault. This issue includes documentation of the longest multicycle earthquake sequence in California—and possibly in North America (14 ruptures at the Wrightwood site). Geographic coverage extends almost along the entire San Andreas fault system, from Bodega Bay in northern California to the Mecca Hills in the Coachella Valley. The issue also includes articles linking paleoseismic data with modern instrumental recordings of ground motion and seismicity. Data on the earthquake history of the San Andreas fault form the basis of numerous models of fault behavior and calculations of seismic hazard. Despite the relatively large amount of data available for the San Andreas fault, there remain unresolved questions about segmentation, the rupture patterns of large earthquakes, and characteristics of earthquake recurrence. The data presented in this special issue contribute toward understanding the spatial and temporal rupture history of the San Andreas fault over multiple rupture cycles during the last few hundred to few thousand years. This is the timescale of greatest interest for seismic hazard assessments and fault modeling. This introduction provides an overview of the issue and summarizes results in simplified, tabular form. Table 1 contains dates of surface ruptures and/or average recurrence intervals reported from 10 sites discussed in …

QuakeSim and the Solid Earth Research Virtual Observatory

We are developing simulation and analysis tools in order to develop a solid Earth Science framework for understanding and studying active tectonic and earthquake processes. The goal of QuakeSim and its extension, the Solid Earth Research Virtual Observatory (SERVO), is to study the physics of earthquakes using state-of-the-art modeling, data manipulation, and pattern recognition technologies. We are developing clearly defined accessible data formats and code protocols as inputs to simulations, which are adapted to high-performance computers. The solid Earth system is extremely complex and nonlinear, resulting in computationally intensive problems with millions of unknowns. With these tools it will be possible to construct the more complex models and simulations necessary to develop hazard assessment systems critical for reducing future losses from major earthquakes. We are using Web (Grid) service technology to demonstrate the assimilation of multiple distributed data sources (a typical data grid problem) into a major parallel high-performance computing earthquake forecasting code. Such a linkage of Geoinformatics with Geocomplexity demonstrates the value of the Solid Earth Research Virtual Observatory (SERVO) Grid concept, and advances Grid technology by building the first real-time large-scale data assimilation grid.