Faults In Southern California Research Articles

Slip partitioning and deformation cycles close to major faults in southern California: Evidence from small‐scale faults

Geometry and kinematics of small‐scale faults adjacent to large seismically active faults have been analyzed for a large region of southern California. The sampled population of small‐scale faults is scattered in orientation and slip but possesses distinct trends. Two important characteristics are, first, the paucity of small‐scale faults that mimic orientations and mechanisms of the adjacent master faults, and, second, the large variation in slip mechanisms of fault planes oriented E‐W and N‐S. In order to test the similarity of the sampled fault population to present‐day deformations, data on geometry and slip of the small‐scale faults have been used for stress inversions. These results are compared with those derived from focal mechanisms of low‐magnitude earthquakes in the same region. Stress orientations derived from small‐scale faults are spatially heterogeneous, but horizontal axes of stress maintain N‐S and E‐W trends over the entire region. The similarities between stress inversions from small faults and seismicity support the conclusion that most of the sampled faults are recent and formed during cyclical events of fracturing in a relatively strong crust adjacent to weak master faults.

Escape tectonics in the Los Angeles metropolitan region and implications for seismic risk

Recent damaging earthquakes in California, including the 1971 San Fernando1, 1983 Coalinga2, 1987 Whittier Narrows3 and 1994 Northridge4 events, have drawn attention to thrust faults as both potentially hazardous seismic sources and as a mechanism for accommodating shortening in many regions of southern California. Consequently, many geological studies5,6 have concluded that thrust faults in Southern California pose the greatest seismic hazard, and also account for most of the estimated 5–7 mm yr−1 of contraction across the greater Los Angeles metropolitan area7,8 indicated by Global Positioning System geodetic measurements9. Our study demonstrates, however, that less than 50% of the geodetically observed contraction is accommodated on the principal thrust systems across the Los Angeles region. We integrate the most recent geological, geodetic and seismological data to assess the spatial distribution of strain across the Los Angeles metropolitan region. We then demonstrate that a significant component of seismic moment release and shortening in this region is accommodated by east–west crustal escape ‘extrusion’ along known strike-slip and oblique-slip faults.

Crustal imaging in southern California using earthquake sequences

An inexpensive means to further understand the geometry of active faults in southern California arises from the use of aftershock recordings to image crustal structures. The advent of regional seismic networks that record digital seismograms from hundreds of stations makes this crustal reflectivity profiling possible even in the absence of conventional active-source seismic data. We show that it is feasible to image fault structure using three-dimensional, wide-angle prestack Kirchhoff migration. We achieve this with the use of aftershock traces recorded on the short-period vertical stations of the Southern California Seismic Network. This work complements seismicity and focal mechanism work by imaging reflectivity volumes and cross-sections rather than having to associate events with certain faults. Further, it can image below the seismogenic zone to resolve current geologic controversies on how proposed faults extend below focal depths. We demonstrate the validity of these images as showing reflective structures, and the ability to use clipped high-gain seismograms as sign-bit data to yield valid geometric imaging. Work with data from the 1991 Sierra Madre earthquake sequence images the prominent lower-crustal reflective zone observed beneath most of the San Gabriel Mountains by the Los Angeles Region Seismic Experiment Line 1. Aftershocks of the 1994 Northridge earthquake allow us to image a north-dipping structure that may represent the fault plane of a crustal-penetrating blind thrust. The images serve as a test for the existence and geometry of thrust ramps and detachments proposed from balanced-section reconstructions of shallow-crustal profiles and borehole data. Our results are more consistent with a thick-skinned tectonic regime in the vicinity of the Northridge earthquake, rather than a thin-skinned model.

Do historical rates of seismicity in southern California require the occurrence of earthquake magnitudes greater than would be predicted from fault length?

Abstract The Working Group on California Earthquake Probabilities reported a discrepancy between the historical rates of large earthquakes in southern California and rates predicted from interpretation of geological, geodetic, and historical seismicity data. It was suggested that the discrepancy may be due to the assumption within their analysis that the magnitude of the largest earthquake on a fault is limited by the mapped fault length. Our analysis of the available data does not support the presence of a historical deficit in the rate of seismicity, nor does it require that earthquakes that rupture beyond the lengths of mapped active faults in southern California, or that rupture numerous subparallel faults, are needed to explain the historical distribution of seismicity.

Southern California Deformation

Southern California is one of the most seismically active places in the United States and has therefore been of long-standing interest to geoscientists. In their Perspective, Jackson et al . discuss recent work to collect geodetic data on the motion and earthquake potential of the faults in southern California.

A neotectonic study of the San Miguel-Vallecitos fault, Baja California, Mexico

Abstract The San Miguel strand of the San Miguel-Vallecitos fault zone displaces an alluvial ridge 23+5/−10 m in the vicinity of Dolores Mountain. The same section of fault produced surface rupture during the M6.8 earthquake of 1956. We estimate a minimum age of the ridge at between 110+150/−60 ka by comparing soil development on the offset ridge to two soil chronosequences in southern California. Dividing the measured offset by the age of the ridge places a maximum range on the fault slip rate of 0.2+0.35/−0.15 mm/yr. Excavations along and across the fault at the same site revealed at least 80 cm and probably 115 cm of right-lateral offset of a now-buried stream channel deposit. We attribute the 115-cm offset to the 1956 M6.8 earthquake, a displacement that is slightly larger than previously reported for this earthquake. Dividing the 80 to 115 cm of displacement by the calculated fault slip rate yields an estimated return time for similar-sized events of about 5.8+17/−4.3 ka. Similarly, dividing the slip expected for rupture of the entire 160-km length of the San Miguel-Vallecitos fault zone by the fault slip rate yields an estimate of return time of Mw7.8 earthquakes of about 80 ka. The slip rate determined from this study is at least an order of magnitude less than that contributed by the Agua Blanca fault and indicates that the San Miguel fault zone transfers less than about 1% of the plate motions. The San Miguel fault shows a complex fault trace and registers a small value of cumulative geologic offset (maximum of 0.6 km). When combining the geological estimates of magnitude and return time of the largest earthquakes with magnitude-frequency data recorded along the San Miguel fault by the RESNOR seismic network during the period 1976 to 1991, we observe that the shape of the magnitude-frequency distribution along the fault may be described by the Gutenberg-Richter relationship Log n = a − bM. In contrast, along-strike-slip faults in southern California that are characterized by orders of magnitude more cumulative offset and less complex fault traces, we observe that extrapolation of earthquake frequency statistics from the instrumental record of seismicity severely underestimates the rate of occurrence of maximum expected events. We speculate that the relatively high productivity of small earthquakes along the San Miguel fault reflects a more heterogeneous stress field associated with the incipient and complex nature of the fault trace.

A synthetic seismicity model for southern California: Cycles, probabilities, and hazard

The absence of a long historical catalog of observed seismicity with which to constrain earthquake recurrence behaviors is a fundamental stumbling block to earthquake prediction in California. Conceding that this limitation is not likely to relax in the foreseeable future, alternative approaches must be sought to extend the catalog artificially. In this article, I evaluate the long‐term behaviors of earthquakes on a map‐like set of southern California faults through computer simulations that incorporate the physics of earthquake stress transfer and are constrained by excellent, but restricted, bodies of geological and seismological data. I find that model seismicity fluctuates on both short (decades) and long (centuries) timescales but that it possesses a well‐defined mean and standard deviation. Seismicity fluctuations correlate across different magnitudes, and the long‐term cycles of smaller events seem to lead cycles of larger events. Short‐period seismicity fluctuations do not exhibit this tendency, and short‐term changes in low‐magnitude (M5 +) seismicity are not likely to be an effective predictor of future large events, at least for the region as a whole. As do real faults, the model faults produce characteristic and power law quakes in variable ratios with diverse periodic and nonperiodic behaviors. Generally, larger events tend to occur quasi‐periodically, and smaller ones tend to cluster; however, only for a few earthquake classes and certain locations is recurrence notably non‐Poissonian. An important use of synthetic seismicity is in the construction of earthquake hazard maps because it firmly grounds previously ad hoc assumptions regarding frequency‐magnitude distributions, multiple‐segment failure statistics, and rupture extents, while satisfying a spectrum of geological constraints such as fault slip rate, segment recurrence interval, and slip per event. With its depth of temporal and spatial coverage, synthetic seismicity also provides a means to investigate the time dependence of seismic hazard. Because hazard likelihood is a concatenation of the recurrence statistics from many seismic sources, in only about 40–50% of the regions near the major faults do sequences of 0.1 g or 0.2 g exceedances differ from Poissonian.

Triggering of 1812 Santa Barbara Earthquake by a Great San Andreas Shock: Implications for future seismic hazards in southern California

We study the evolution of the stress field over the last 200 years in southern California using the stress buildup associated with major faults and stress drops associated with great earthquakes. In this report we calculate the change in the Coulomb Failure Function (ΔCFF) associated with the great Wrightwood earthquake of Dec. 8, 1812 on the San Andreas fault for the region near Santa Barbara that experienced a large shock 13 days later. The rupture length constrained by paleoseismic data is used for the Wrightwood earthquake of 1812; a uniform displacement of 3.5 m from the surface to seismogenic depths is assumed. The geologic setting and focal mechanisms of earthquakes in the Santa Barbara region are dominated by thrust, left‐lateral strike‐slip faulting and combinations of the two. For our preferred rupture model of the earthquake of Dec. 8 the Santa Barbara shock of Dec. 21, 1812 occurred in a region of +0.01 to +0.1 MPa ΔCFF for reasonable fault orientations, indicating that it may have been advanced by years to decades and triggered by the great San Andreas earthquake 13 days earlier. We also calculate ΔCFF on typical kinds of faults in southern California for a future candidate great earthquake along the San Bernardino and Coachella Valley segments of San Andreas fault. This earthquake produces large positive values of ΔCFF for shallow‐dipping thrust faults beneath the greater Los Angeles area and for NW‐trending right‐lateral strike‐slip faults in three regions. This suggests that shocks of magnitude similar to the M 7.1 1812 Santa Barbara earthquake could be triggered by the next great event on the southern San Andreas fault.

Evidence for possible horizontal faulting in southern California from earthquake mechanisms

We find that 36 of the 505 fault-plane solutions (M ≥ 3.0, 1981–1990) in southern California have a nodal plane dipping no more than 30 °. With the assumption of the low-angle nodal planes being the fault planes, four cross sections are constructed to show the possible horizontal faults in the middle and upper crust. More than half of these low-angle faults are located within or adjacent to the Transverse Ranges. The focal depths vary from 1 km in the southern end of the Sierra Nevada and the southwestern Mojave Desert to 20 km in the Transverse Ranges. The slip directions are also diverse. In general, east-west extensional movements are dominant in the boundary between the southern Sierra Nevada extending to the San Emigdio Mountains and the western Mojave Desert, whereas north-south compressional movements are dominant in the Transverse Ranges. In the Peninsular Ranges and the Salton Trough, both the slip directions and focal depths vary. These features suggest that seismically active low-angle faults in southern California may exist at different depths and slip in various directions. Our data do not support the existence of a regional-scale seismically active detachment in southern California. Only in the western Transverse Ranges is there some suggestion of a large detachment surface at a depth of about 13 to 14 km.

Rock‐solid information

The National Science Foundation's Southern California Earthquake Center and the U.S. Geological Survey have collaborated to provide residents of America's most famous earthquake zone with some hard facts about temblors. Putting Down Roots in Earthquake Country, a 32‐page handbook on coping with life near the many earthen faults in Southern California, was distributed in October to all public libraries from San Luis Obispo to Tijuana. The book summarizes for lay people what is known about the San Andreas fault and the many others that cris‐cross California. It also offers guidance on how to prevent earthquake damage, how to retrofit a home, and how to assess earthquake hazards.

Mesoscale and regional kinematics of the northwestern Yalakom fault system: Major Paleogene dextral faulting in British Columbia, Canada

The northwestern Yalakom fault system (YFS) lies on the eastern margin of the Coast Belt in southwestern British Columbia. The Yalakom fault is the most significant structure in the fault system, with at least 115 km of dextral slip, mostly of early Tertiary age. The NW YFS is up to 25 km wide and is bounded on the southwest by the Tchaikazan fault zone, which had 5 km or more of dextral slip. Kinematic and paleostress analysis of mesofaults and map‐scale structures shows that secondary faults within the NW YFS had both dextral and sinistral oblique slip. Three large synclines had increasing amounts of clockwise, vertical axis rotation from southeast to northwest. Block rotation between two synthetic strike‐slip faults with large offset (Yalakom) and moderate offset (Tchaikazan) is similar to current kinematics between the San Andreas and San Jacinto faults in southern California. The NW YFS was active from latest Cretaceous(?) to (mainly) Eocene time and postdates early Late Cretaceous thrust faulting to the southwest. Early stage mesofaults are compatible with the thrust belt and suggest that open folds developed in the foreland of the thrust belt before strike‐slip faulting commenced. The abrupt northwest termination of the YFS may be related to extension within the adjacent Tatla Lake metamorphic complex. Some dextral slip may have been transferred farther northwest through the Coast Belt, but these structures have not been documented in west central British Columbia. Most of the 120 km of dextral slip on the YFS was transferred to the north via a mosaic of normal and strike‐slip faults in central British Columbia. Thus the YFS forms the southwestern margin of a broad zone of transtensional structures that extends to the Tintina‐Rocky Mountain Trench fault zone; these structures accommodated northwestward displacement of the northwestern Cordillera in Paleogene time.

Slip triggered on southern California faults by the 1992 Joshua Tree, Landers, and Big Bear earthquakes : P. Bodin, R. Bilham, J. Behr, J. Gomberg & K. W. Hudnut, Bulletin - Seismological Society of America, 84(3), 1994, pp 806–816

Slip triggered on southern California faults by the 1992 Joshua Tree, Landers, and Big Bear earthquakes : P. Bodin, R. Bilham, J. Behr, J. Gomberg & K. W. Hudnut, Bulletin - Seismological Society of America, 84(3), 1994, pp 806–816

Fault slip rates and earthquake histories for active faults in southern California

Abstract Within the context of the historical record of seismicity, we review the geological data bearing on the Quaternary slip rates and paleoearthquake histories of active faults in southern California.

Slip Triggered on Southern California Faults by the 1992 Joshua Tree, Landers, and Big Bear Earthquakes

Five out of six functioning creepmeters on southern California faults recorded slip triggered at the time of some or all of the three largest events of the 1992 Landers earthquake sequence. Digital creep data indicate that dextral slip was triggered within 1 min of each mainshock and that maximum slip velocities occurred 2 to 3 min later. The duration of triggered slip events ranged from a few hours to several weeks. We note that triggered slip occurs commonly on faults that exhibit fault creep. To account for the observation that slip can be triggered repeatedly on a fault, we propose that the amplitude of triggered slip may be proportional to the depth of slip in the creep event and to the available near-surface tectonic strain that would otherwise eventually be released as fault creep. We advance the notion that seismic surface waves, perhaps amplified by sediments, generate transient local conditions that favor the release of tectonic strain to varying depths. Synthetic strain seismograms are presented that suggest increased pore pressure during periods of fault-normal contraction may be responsible for triggered slip, since maximum dextral shear strain transients correspond to times of maximum fault-normal contraction.

Particle size distribution of cataclastic fault materials from Southern California: A 3-D study

The particle size distributions of fault gouge from the San Andreas, the San Gabriel, and the Lopez Canyon faults in Southern California were measured using sieving and Coulter-Counter techniques over a range of particle sizes from 2 μm to 16 mm. The distributions were found to be power law (fractal) for the smaller fragments and log-normal by mass for sizes near and above the peak size. The apparent fractal dimensionD of the smaller particles in gouge samples from the San Andreas fault, the San Gabriel fault and the Lopez Canyon gouge were 2.4–3.6, 2.6–2.9 and 2.4–3.0, respectively. The averageD for the Lopez Canyon gouge was 2.7±0.2, which is in agreement with earlier studies of this gouge using planar 2-D sections. The fractal dimension of the finer fragments from all three faults is observed to be correlated with the peak fragment size, with finer gouges tending to have a largerD. A computer automaton is used to show that this observation may be explained as resulting from a fragmentation process which has a “grinding limit” at which particle reduction stops.

Changes in static stress on southern California faults after the 1992 Landers earthquake

THE magnitude 7.5 Landers earthquake of 28 June 1992 was the largest earthquake to strike California in 40 years. The slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults; these changes in stress should in turn affect the likelihood of future earthquakes. Stress changes that load faults towards failure have been cited as the cause of small1–5, moderate6 and large7 earthquakes; conversely, those that relax neighbouring faults have been related to a decrease in seismicity5. Here we use an elastic half-space model8 to estimate the stress changes produced by the Landers earthquake on selected southern California faults, including the San Andreas. We find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4.5, and that the largest changes (1–10 bar), occurring on part of the San Bernardino segment of the San Andreas fault, may have decreased the time to the next magnitude 8 earthquake by about 14 years.

Slip line model for southern California crustal kinematics

We propose a simple slip line model to explain the faulting and seismicity patterns of southern California. We assume the Sierra Nevada–Great Valley (SNGV) block to be rigid, in view of its low seismicity, and other regions to undergo plastic deformation. From this point of view, the complex deformation patterns observed in southern California, including the Borderland region, result from the “plowing” of the eastern margin of the Pacific plate by the nondeforming SNGV block. A simple analogy can then be made between regional tectonics in southern California and two‐dimensional flow solutions obtained in metallurgy for the extrusion of rigid‐plastic metals. We use a simplified geometry based on the geology and seismicity of southern California to calculate a two‐dimensional slip line field which predicts the shape of the San Andreas fault near the “big bend” surprisingly well. The largely aseismic, high‐topography western Mojave can then be interpreted as an analogue to the nondeforming “dead metal” zone which arises for certain metal extrusion geometries. The slip field agrees quite well with the strikes and slip orientation of major dextral faults in southern California, including the San Jacinto, Elsinore, and Newport‐Inglewood faults. It also predicts correctly the strikes and slip orientation of sinistral “cross faults” found in some areas of the region. In the Transverse Ranges, where thrust faulting dominates, the slip lines are no longer potential fault planes, but a simple argument predicts that they should lie at 45° on the strikes of thrust faults, in fair agreement with the observations.

Earthquake depths and the relation to strain accumulation and stress near strike-slip faults in southern California : Sanders, C O J Geophys ResV95, NB4, April 1990, P4751–4762

Earthquake depths and the relation to strain accumulation and stress near strike-slip faults in southern California : Sanders, C O J Geophys ResV95, NB4, April 1990, P4751–4762

Earthquake depths and the relation to strain accumulation and stress near strike‐slip faults in southern California

In this paper I report and attempt to interpret several observations about the depth distribution of earthquakes in the region of southern California containing the southern San Andreas and San Jacinto fault zones. These observations are as follows: (1) Earthquakes in the major fault zones are predominantly deep (maximum, 22 km; characteristic, 11–18 km). (2) Earthquakes in the crustal blocks bounding the fault zones are predominantly shallow (maximum, 17 km; characteristic, 1–6 km). (3) In the San Jacinto fault zone, maximum earthquake depths correlate with surface heat flow. These relations together with focal mechanisms, geodetic strain measurements, and fault zone models are consistent with the following ideas: (1) Interseismic plate motion is accommodated by aseismic slip along an extension of the major fault zone below a brittle zone that is locked between large earthquakes. (2) The aseismic slip in a narrow fault zone in the brittle‐plastic transition region concentrates strain at the base of the brittle fault zone. (3) Deep earthquakes occur in the lower part of the brittle fault zone due to stick‐slip failure of highly stressed patches. (4) Background earthquakes and aftershocks that occur several kilometers deeper than large earthquake hypocenters suggest that a zone of mixed slip behavior may exist between the stable sliding (deep) and stick‐slip (shallow) regions of the fault zone. This zone of mixed slip may fail predominantly by stable sliding, analogous to the central San Andreas and Calaveras faults of California. Seismic precursors to large earthquakes may occur in this zone, and large earthquakes may nucleate near the upper edge of this zone. Furthermore, the difference in seismicity between the San Jacinto and southern San Andreas faults suggests that the nature of this mixed zone may evolve as total displacement in the fault zone increases. (5) Shear stress may be less in the crustal blocks than in the deep brittle fault zones and generally at a level sufficient to cause brittle failure only shallow in the crustal blocks. (6) In the stress field produced by plate motion and slip in the deep fault zone, the upper brittle fault zone is not oriented favorably for shear failure. Lack of shallow earthquakes in the fault zones and the predominance of shallow earthquakes on favorably oriented fractures in the adjacent crustal blocks suggest that either stress in the upper brittle fault zone is relatively low or the upper fault zone is effectively strong due to its orientation.

Past and Possible Future Earthquakes of Significance to the San Diego Region

The potential for earthquakes that may significantly affect the San Diego, California, region is examined from the viewpoint of geology, seismic history, and strong ground motion. We have compiled the best available data on slip rates and recurrence times for all major faults in southern California and northern Baja California, identified possible fault segments that might rupture in single earthquakes and obtained repeat times for these events which are consistent with (or at least not contradicted by) trenching studies. The most important faults for San Diego's seismic hazard are the Rose Canyon fault, the Elsinore fault, and faults immediately offshore (Coronado Banks, San Diego trough). There have not been any major earthquakes on any of these nearby faults in historical time, but the geological evidence is clear that such events will eventually occur. San Diego is located on the western flank of the Peninsular Range batholith, and there is weak evidence that attenuation in this batholith might be lower than average for California. We combine the geological data with an attenuation model to obtain an estimate for the occurrence rate of various levels of peak ground acceleration in downtown San Diego from events with moment magnitude greater than about 6. We find that peak accelerations of 10% g to 20% g are expected about once every 100 years. There are considerable uncertainties in this estimate, but nevertheless, strong ground shaking from a magnitude 6.0 or greater earthquake near the populated area would not be a scientific surprise.