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The San Andreas and San Jacinto faults are the primary plate-boundary structures in southern California and present a large earthquake hazard for the region. They approach each other in the Cajon Pass area between the San Gabriel and San Bernardino Mountains, where the northern end of the San Jacinto fault forms a 2-km-wide releasing step with the San Andreas fault. In this study, we used paleoseismic data from sites on the San Jacinto and San Andreas faults near their juncture to evaluate spatial and temporal patterns of surface rupture between these major structures of the North American-Pacific transform plate boundary. We present a new 3700 yr paleoseismic record from the northern San Jacinto fault at Mystic Lake, where trench excavations exposed evidence for at least 16 surface ruptures. A sedimentary gap in our trench exposures separates three ruptures in the oldest part of the record from 13 ruptures during the past 2000 yr. For the past 2000 yr, the mean recurrence times varied from 86 to 312 yr, with a mean recurrence interval of 160 yr. This rate of surface rupture is roughly equal to that of the southern San Andreas fault south of the Cajon Pass juncture, but it is half that of the San Andreas fault north of the juncture, indicating that coseismic strain on the San Andreas fault is split between the southern San Andreas and San Jacinto faults south of Cajon Pass. Comparison of the past 2000 yr of the Mystic Lake record to similar paleoseismic records from nearby sections of the San Andreas fault suggests that: (1) the current open interval on these two faults in the study area is longer than their average recurrence intervals, but that similar intervals of quiescence have occurred in the past 2000 yr; (2) the San Andreas and San Jacinto faults have probably ruptured together multiple times in the past 2000 yr; and (3) a joint rupture of the San Jacinto fault with the Mojave section of the San Andreas fault may be a more likely source of a major earthquake in southern California than a rupture on the San Andreas fault from the Mojave segment to the southern end of the fault. [ABSTRACT FROM AUTHOR]

Assessment of seismic hazards in southern California may be improved with more accurate characterization of the active San Andreas fault strands within the San Gorgonio Pass region. Ongoing debate centers on the activity and geometry of the Mill Creek and Mission Creek strands. Here, we investigate crustal deformation models with six geologically plausible geometries of the Mill Creek and Mission Creek strands. Model results suggest that differences in active fault geometry along the San Andreas fault impact slip rates along the San Jacinto fault by up to 3 mm/yr. Each model fits many but none fits all of the available geologic strike-slip rates. The calculated misfits to the geologic strike-slip rates reveal two best-fitting models: the Inactive Mill Creek model and the West Mill Creek model, which incorporate active portions of the Mill Creek, Mission Creek, and Galena Peak strands, consistent with recent studies. The cumulative strike-slip rates across faults of the two best-fitting models differ from each other by ~5 mm/yr, suggesting that fault slip rates do not always sum to the plate rate. Consequently, kinematic slip budgets should consider off-fault deformation. The two best-fitting models produce uplift patterns with significant differences in the hanging walls of dipping faults. New uplift rate data in these regions and additional geologic slip rates along the northern fault strands could further support plausible interpretations of active fault configuration. An assessment of the seismic hazard of the region indicates the potential for a rupture through the San Gorgonio Pass region with Mw ~7.8. [ABSTRACT FROM AUTHOR]

Slip rates represent the average displacement across a fault over time and are essential to estimating earthquake recurrence for probabilistic seismic hazard assessments. We demonstrate that the slip rate on the western segment of the Puente Hills blind thrust fault system, which is beneath downtown Los Angeles, California (USA), has accelerated from ~0.22 mm/yr in the late Pleistocene to ~1.33 mm/yr in the Holocene. Our analysis is based on syntectonic strata derived from the Los Angeles River, which has continuously buried a fold scarp above the blind thrust. Slip on the fault beneath our field site began during the late-middle Pleistocene and progressively increased into the Holocene. This increase in rate implies that the magnitudes and/or the frequency of earthquakes on this fault segment have increased over time. This challenges the characteristic earthquake model and presents an evolving and potentially increasing seismic hazard to metropolitan Los Angeles. [ABSTRACT FROM AUTHOR]

Industry seismic reflection data, oil test well data, interpretation of gravity and magnetic data, and seismic refraction deepcrustal profiles provide new perspectives on the subsurface geology of San Fernando Valley , home of two of the most recent damaging earthquakes in southern California. Seismic reflection data provide depths to Miocene-Quaternary horizons; beneath the base of the Late Miocene Modelo Formation are largely nonreflective rocks of the Middle Miocene Topanga and older formations. Gravity and seismic reflection data reveal the North Leadwell fault zone, a set of down-tothe- north faults that does not offset the top of the Modelo Formation; the zone strikes northwest across the valley, and may be part of the Oak Ridge fault system to the west. In the southeast part of the valley, the fault zone bounds a concealed basement high that infl uenced deposition of the Late Miocene Tarzana fan and may have localized damage from the 1994 Northridge earthquake. Gravity and seismic refraction data indicate that the basin underlying San Fernando Valley is asymmetric, the north part of the basin (Sylmar subbasin) reaching depths of 5-8 km. Magnetic data suggest a major boundary at or near the Verdugo fault, which likely started as a Miocene transtensional fault, and show a change in the dip sense of the fault along strike. The northwest projection of the Verdugo fault separates the Sylmar subbasin from the main San Fernando Valley and coincides with the abrupt change in structural style from the Santa Susana fault to the Sierra Madre fault. The Simi Hills bound the basin on the west and, as defined by gravity data, the boundary is linear and strikes ∼N45 E. That northeasttrending gravity gradient follows both the part of the 1971 San Fernando aftershock distribution called the Chatsworth trend and the aftershock trends of the 1994 Northridge earthquake. These data suggest that the 1971 San Fernando and 1994 Northridge earthquakes reactivated portions of Miocene normal faults. [ABSTRACT FROM AUTHOR]

It has been 153 yr since the last large earthquake on the southern San Andreas fault (California, United States), but the average interseismic interval is only 100 yr. If the recurrence of large earthquakes is periodic, rather than random or clustered, the length of this period is notable and would generally increase the risk estimated in probabilistic seismic hazard analyses. Unfortunately, robust characterization of a distribution describing earthquake recurrence on a single fault is limited by the brevity of most earthquake records. Here we use statistical tests on a 3000 yr combined record of 29 ground-rupturing earthquakes from Wrightwood, California. We show that earthquake recurrence there is more regular than expected from a Poisson distribution and is not clustered, leading us to conclude that recurrence is quasi-periodic. The observation of unimodal time dependence is persistent across an observationally based sensitivity analysis that critically examines alternative interpretations of the geologic record. The results support formal forecast efforts that use renewal models to estimate probabilities of future earthquakes on the southern San Andreas fault. Only four intervals (15%) from the record are longer than the present open interval, highlighting the current hazard posed by this fault. [ABSTRACT FROM AUTHOR]

A spectacular band of precariously and semiprecariously balanced rocks extends from Riverside to near Borrego Valley, California, about halfway between the Elsinore and San Jacinto fault zones. The rocks are distributed in a band a few kilometers wide midway between the San Jacinto and Elsinore fault zones, an indication that the distribution is caused by attenuation of strong ground motion from numerous large events along these two fault zones. These rocks have apparently been in place for thousands of years, and thus place important constraints on ground motions from earthquakes. On the basis of field tests and photographic analysis, the estimated quasi-static toppling accelerations for the precariously balanced rocks are ∼0.32 ± 0.10 g (g = gravitational acceleration). The dynamic toppling accelerations are within this range for earthquakes with waveforms similar to those recorded during the Izmit and Denali earthquakes. These constraints are roughly consistent with the median predicted value of ground motion for M7 earthquakes, but somewhat lower than +σ ground motion curves, and much lower than the values from the 2% in 50 yr hazard maps. The evidence from these rocks has importance for some of the assumptions that go into calculating probabilistic seismic hazard assessment, including median and standard deviation of ground motion attenuation curves (especially for hard rock, for which few instrumental data are available), the possible existence of random background earthquakes, and the smoothing distance for historical seismicity. [ABSTRACT FROM AUTHOR]

Presents a study which suggests that aseismic deformation mechanisms attribute to regional shortening strain in southern California. Methodology used to conduct study; Results of study; Discussion on findings.