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We present a new three-dimensional model of the major fault systems in southern California. The model describes the San Andreas fault and associated strikeslip 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 comprehensive 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.

[1] Toward understanding the relationship between strain accumulation and strain release in the context of the mechanics of the earthquake and mountain building process and quantifying the seismic hazard associated with the globes largest continental thrust system, we describe the late Quaternary expression and paleoseismic evidence for great surface rupture earthquakes at six sites along the Himalayan Frontal Thrust (HFT) system of India. Our observations span a distance of $250 km along strike of the HFT. Uplifted and truncated fluvial terrace deposits resulting from the Holocene displacements on the HFT are preserved along canyons of the Ghaggar, Markanda, Shajahanpur, and Kosi Rivers. Dividing the elevation of the bedrock straths at each site by their ages yields estimates of the vertical uplift rate of $4-6 mm/yr, which when assumed to be the result of slip on an underlying thrust dipping at $20°-45°are equivalent to fault slip rates of $6-18 mm/yr or shortening rates of $4-16 mm/yr. Trench exposures reveal the HFT to fold and break late Holocene surface sediments near the cities and villages of Chandigarh, Kala Amb, Rampur Ganda, Lal Dhang, and Ramnagar. Radiocarbon ages of samples obtained from the displaced sediments indicate surface rupture at each site took place after $A.D. 1200 and before $A.D. 1700. Uncertainties attendant to the radiocarbon dating currently do not allow an unambiguous definition of the capping bound on the age of the displacement at each site and hence whether or not the displacements at all sites were contemporaneous. Trench exposures and vertical separations measured across scarps at Rampur Ganda, Lal Dhang, and Ramnagar are interpreted to indicate single-event displacements of $11-38 m. Dividing the observed single-event vertical components of displacement by the estimated longer-term uplift rates indicates $1330-3250 or more years should be required to accumulate the slip sufficient to produce similar sized displacements. Surface rupture appears to not have occurred during the historical 1905 Kangra (M w = 7.7), 1934 Bihar-Nepal (M w = 8.1), and 1950 Assam (M w = 8.4) earthquakes, which also occurred along the Himalayan front. Yet we observe clear evidence of fault scarps and displacements in young alluvium and progressive and continued offset of fluvial terrace deposits along the HFT. We suggest on this basis and the size and possible synchroneity of displacements recorded in the trenches that there exists the potential for earthquakes larger than recorded in the historical record and with the potential to rupture lengths of the HFT greater than the $250 km we have studied.

Paleoseismic data from 11 trenches at seven sites excavated across the southern Johnson Valley, Kickapoo, and Homestead Valley faults that ruptured in the 1992 Landers earthquake, as well as the northern Johnson Valley fault which did not fail in 1992, indicate that the return period for large surface rupturing events in this part of the eastern California shear zone is in the range of 5-15 ka. The inferred slip rates, based on their respective recurrence intervals, are in the range of 0.2-0.6 mm/yr for each of the faults studied. A previous large earthquake ruptured the southern Johnson Valley and Kickapoo faults about 5 ka B.P. The northern Johnson Valley fault also failed at about this time at 5.8 ka B.P. and may have been part of the same rupture. In contrast, the penultimate large earthquake that we identify on the Homestead Valley fault occurred about 15 ka B.P., much earlier than other faults involved in the 1992 rupture. From these observations, combined with paleoseismic work by others after the 1992 earthquake, it appears that previous events along the southern Johnson Valley and Kickapoo faults were different than those of 1992 and may have involved other fault segments. It has been over 5 ka since the most recent rupture on the northern Johnson Valley fault. Therefore, it is surprising that it did not fail in the 1992 rupture. From our observations, dextral shear appears to be distributed across the entire eastern California shear zone, with individual faults taking only a small proportion of the overall slip. Release of this regional strain appears to occur in temporal clusters of large (?) earthquakes, with the 1992 event apparently the most recent of a sequence of late Holocene (0-1 ka) earthquakes that have ruptured the nine faults we have trenched in the southwestern Mojave desert. Previous clusters of earthquake activity occurred in the early (8-9 ka) and middle (5-6 ka) Holocene, and possibly the latest Pleistocene (ϳ15 ka).

The 4 April 2010 moment magnitude (M w) 7.2 El Mayor-Cucapah earthquake revealed the existence of a previously unidentifi ed fault system in Mexico that extends ~120 km from the northern tip of the Gulf of California to the U.S.-Mexico border. The system strikes northwest and is composed of at least seven major faults linked by numerous smaller faults, making this one of the most complex surface ruptures ever documented along the Pacifi c-North America plate boundary. Rupture propagated bilaterally through three distinct kinematic and geomorphic domains. Southeast of the epicenter, a broad region of distributed fracturing, liquefaction, and discontinuous fault rupture was controlled by a buried, southwest-dipping, dextral-normal fault system that extends ~53 km across the southern Colorado River delta. Northwest of the epicenter, the sense of vertical slip reverses as rupture propagated through multiple strands of an imbricate stack of eastdipping dextral-normal faults that extend ~55 km through the Sierra Cucapah. However, some coseismic slip (10-30 cm) was partitioned onto the west-dipping Laguna Salada fault, which extends parallel to the main rupture and defi nes the western margin of the Sierra Cucapah. In the northernmost domain, rupture terminates on a series of several north-northeast-striking cross-faults with minor offset (<8 cm) that cut uplifted and folded sediments of the northern Colorado River delta in the Yuha Desert. In the Sierra Cucapah, primary rupture occurred on four major faults separated by one fault branch and two accommodation zones. The accommodation zones are distributed in a left-stepping en echelon geometry, such that rupture passed systematically to structurally lower faults. The structurally lowest fault that ruptured in this event is inclined as shallowly as ~20°. Net surface offsets in the Sierra Cucapah average ~200 cm, with some reaching 300-400 cm, and rupture kinematics vary greatly along strike. Nonetheless, instantaneous extension directions are consistently oriented ~085° and the dominant slip direction is ~310°, which is slightly (~10°) more westerly than the expected azimuth of relative plate motion, but considerably more oblique to other nearby historical ruptures such as the 1992 Landers earthquake. Complex multifault ruptures are common in the central portion of the Pacifi c North American plate margin, which is affected by restraining bend tectonics, gravitational potential energy gradients, and the inherently three-dimensional strain of the transtensional and transpressional shear regimes that operate in this region.

New, high-precision U-series ages of solitary corals (Balanophyllia elegans) coupled with molluscan faunal data from marine terraces on the Pacific Coast of NorthAmerica yield information about the timing and warmth of the last interglacial sea-level highstand. Balanophyllia elegans takes up U in isotopic equilibrium with seawater during growth and shortly after death. Corals from the second terrace on San Clemente Island (offshore southern California), the third terrace on Punta Banda (on the Pacific Coast of northern Baja California), and the Discovery Point Formation on Isla de Guadalupe (in the Pacific Ocean offshore Baja California) date to the peak of the last interglacial period and have U-series ages ranging from ca. 123 to 114 ka. The first terrace on Punta Banda has corals with ages ranging from ca. 83 to 80 ka, which corresponds to a sea-level highstand formed in the late last interglacial period. U-series analyses of corals from the Cayucos terrace (central California) and the Nestor terrace at Point Loma (southern California) show that these fossils have evidence of open-system history, similar to what has been reported by other workers for the same localities. Nevertheless, a model of continuous, secondary U and Th uptake shows that two ages of corals are likely present at these localities, representing the ca. 105 and ca. 120 ka sea-level highstands reported elsewhere. . ing the peak of the last interglacial period at ca. 120 ka.

The Ventura Avenue anticline is one of the fastest uplifting structures in southern California, rising at ∼5 mm=yr. We use well data and seismic reflection profiles to show that the anticline is underlain by the Ventura fault, which extends to seismogenic depth. Fault offset increases with depth, implying that the Ventura Avenue anticline is a fault-propagation fold. A decrease in the uplift rate since ∼30 10 ka is consistent with the Ventura fault breaking through to the surface at that time and implies that the fault has a recent dip-slip rate of ∼4:4-6:9 mm=yr.

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