Faults In California Research Articles

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.

The northern San Gregorio fault zone: Evidence for the timing of late Holocene earthquakes near Seal Cove, California

AbstractThe San Gregorio fault is the principal active fault west of the San Andreas fault in central coastal California, yet it remains the largest known fault in the region whose seismogenic potential is not known. In this study, we integrate traditional paleoseismic and archaeologic investigations to define the location, style, and timing of slip events on the northern San Gregorio fault at a site near Seal Cove in Moss Beach, California. The on-land portion of the San Gregorio fault at Seal Cove is a late-Holocene active dextral slip fault. Trench excavations revealed a broad zone of faulting, at least 22 m wide, consisting of five Holocene-active strands. These include a single mid-Holocene east-vergent reverse fault and four late-Holocene near-vertical strike-slip faults. The most recent event occurred after the deposition of a native Californian cooking hearth dated A.D. 1270 to A.D. 1400, but prior to the arrival of Spanish missionaries ca. 1775. The penultimate event at the site is less well constrained but appears to have occurred between A.D. 620 and A.D. 1400. The penultimate event was associated with horizontal displacement on the order of 3 m, based on reconstruction of a thrust wedge within the fault zone. The geometry of midden deposits shows a 5 (−2, +6) m deflection along the projection of faults associated with the most recent event (MRE). All or part of this deflection may be associated with the MRE. These displacements are consistent with Mw 7 to 7¼ earthquakes and show that the San Gregorio fault is an active seismogenic source that should be considered in seismic hazard assessments in the San Francisco Bay area.

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.

Bulk-friction modeling of afterslip and the modified Omori law

Afterslip data from the Superstition Hills fault in southern California, a creep event on the same fault, the modified Omori law, and cumulative moments from aftershocks of the 1957 Aleutian Islands earthquake all indicate that the original formulation by Dieterich (1981) [Constitutive properties of faults with simulated gouge. AGU, Geophys. Monogr. 24, 103–120] for friction evolution is more appropriate for systems far from instability than the commonly used approximation developed by Ruina (1983) [Slip instability and state variable friction laws. J. Geophys. Res. 88, 10359–10370] to study instability. The mathematical framework we use to test the friction models is a one-dimensional, massless spring-slider under the simplifying assumption, proposed by Scholz (1990) [The Mechanics of Earthquakes and Faulting. Cambridge University Press] and used by Marone et al. (1991) [On the mechanics of earthquake afterslip. J. Geophys. Res., 96: 8441–8452], that the state variable takes on its velocity-dependent steady-state value throughout motion in response to a step in stress. This assumption removes explicit state-variable dependence from the model, obviating the need to consider state-variable evolution equations. Anti-derivatives of the modified Omori law fit our data very well and are very good approximate solutions to our model equations. A plausible friction model with Omori-law solutions used by Wesson (1988) [Dynamics of fault creep. J. Geophys. Res. 93, 8929–8951] to model fault creep and generalized by Rice (1983) [Constitutive relations for fault slip and earthquake instabilities. Pure Appl. Geophys. 121, 443–475] to a rate-and-state variable friction model yields exactly Omori's law with exponents greater than 1, but yields unstable solutions for Omori exponents less than 1. We estimate from the Dieterich formulation the dimensionless parameter a∗ which is equal to the product of the nominal coefficient of friction and the more commonly reported friction parameter a. We find that a∗ is typically positive, qualitatively consistent with laboratory observations, although our observations are considerably larger than laboratory values. However, we also find good model fits for a∗ < 0 when data correspond to Omori exponents less than 1. A modification of the stability analysis by Rice and Ruina (1983) [Stability of steady frictional slipping. J. Appl. Mech. 50, 343–349] indicates that a∗ < 0 is not a consequence of our assumption regarding state-variable evolution. A consistent interpretation of a∗ < 0 in terms of laboratory models appears to be that the data are from later portions of processes better characterized by two-state-variable friction models. a∗ < 0 is explained by assuming that our data cannot resolve the co-seismic evolution of a short-length-scale state variable to a velocity-weakening state; our parameterization leads to an apparent negative instantaneous viscosity. We estimate the largest critical slip distance associated with afterslip to be ∼1–10 cm, consistent with other estimates for near-surface materials. We assume that our observed large values for a∗ reflect the fact that our model ignores the geometrical complexities of three-dimensional stresses in fractured crustal materials around a fault zone with frictional stresses that vary on a fault surface. Our one-dimensional model parameters reflect spatially averaged, bulk, stress and frictional properties of a fault zone, where we clearly cannot specify the details of the averaging process. Our analysis of Omori's law suggests that bulk-frictional properties of a fault zone are well described by our simple laboratory-based models, but they would need to change during the seismic cycle for a mainshock instability to recur, unless a mainshock-aftershock sequence were characterized by a process similar to the arrested instabilities possible in two-state-variable systems.

Two‐dimensional seismic image of the San Andreas Fault in the Northern Gabilan Range, central California: Evidence for fluids in the fault zone

A joint inversion for two‐dimensional P‐wave velocity (Vp), P‐to‐S velocity ratio (Vp/Vs), and earthquake locations along the San Andreas fault (SAF) in central California reveals a complex relationship among seismicity, fault zone structure, and the surface fault trace. A zone of low Vp and high Vp/Vs lies beneath the SAF surface trace (SAFST), extending to a depth of about 6 km. Most of the seismic activity along the SAF occurs at depths of 3 to 7 km in a southwest‐dipping zone that roughly intersects the SAFST, and lies near the southwest edge of the low Vp and high Vp/Vs zones. Tests indicate that models in which this seismic zone is significantly closer to vertical can be confidently rejected. A second high Vp/Vs zone extends to the northeast, apparently dipping beneath the Diablo Range. Another zone of seismicity underlies the northeast portion of this Vp/Vs high. The high Vp/Vs zones cut across areas of very different Vp values, indicating that the high Vp/Vs values are due to the presence of fluids, not just lithology. The close association between the zones of high Vp/Vs and seismicity suggests a direct involvement of fluids in the faulting process.

Compression directions in southern California (from Santa Barbara to Los Angeles Basin) obtained from borehole breakouts

Borehole elongation in 71 drill holes was used to infer breakout orientation and directions of maximum horizontal principal stress SH for six areas west of the San Andreas fault in southern California: Santa Barbara, Ojai, Central Ventura Basin, East Ventura Basin, West Los Angeles Basin, and East Los Angeles Basin. Breakouts were determined from analysis of oriented four‐arm caliper data. The breakouts form at the position of the maximum compressive stress on the borehole wall; if the borehole is vertical and parallel to one of the principal stress directions, the breakouts will form parallel to the minimum horizontal principal stress Sh orthogonal to the maximum horizontal principal stress SH. Observations from deviated boreholes permit some constraints on the relative magnitudes of the principal stresses. In most cases the data permit either a thrust faulting (Sv&lt;Sh&lt;SH) or strike‐slip faulting (Sh&lt;Sv&lt;SH) stress regime with NE to NW directions of SH. These results are broadly consistent with results from focal mechanism studies [Hauksson, 1990; Li, 1996] and with breakout and focal mechanism data present in the world stress map database [Zoback, 1992]. However, we find systematic variations in SH directions suggestive of strong heterogeneity in the stress field at shallow depths, similar to that present in the Cajon Pass borehole [Shamir and Zoback, 1992]. Anomalous NW directions of SH in the San Fernando Valley region and near the Whittier fault may be related to structural complexities and/or lateral ramps in nearby fault systems.

Evidence of transpression along the Clipperton Transform: Implications for processes of plate boundary reorganization

High-resolution bathymetry data of the Clipperton Fracture Zone and focal mechanisms of recent earthquakes along the active transform provide evidence for recent and ongoing transpression along the Clipperton Transform. Structural and morphological features observed in the bathymetry data include: normal and strike-slip faults oriented obliquely to the strike of the transform; curved trends of abyssal hills produced by simple shear of the lithosphere bordering the transform fault; abyssal hills offset by obliquely trending strike-slip faults on lithosphere bordering the transform; and lithospheric flexure and associated volcanism near the active transform. All of these features can be explained by a transpression scenario in which a counter-clockwise change in spreading direction of ∼ 5° began about 0.4–0.5 Ma and subjected the right-stepping Clipperton Transform to compression. A comparison of the median ridge volume to calculated estimates of the convergent volume predicted by the transpression scenario indicates that the median ridge is likely to be a product of the brittle deformation associated with transpression. The convergent volume estimates suggest that larger spreading direction changes (∼ 5°) do not occur instantaneously. Instead, spreading direction changes begin slowly and accelerate to the new spreading direction once compression across the transform has ceased. Many of the structures observed along the Clipperton Transform are very similar to features observed along other known convergent transforms, such as the San Andreas Fault in California, and may have important implications for understanding continental transforms. The transpression scenario also indicates that transforms are very important in constraining the rate and character of global plate motion changes.

Cumulative offset of the San Andreas fault in central California: A seismic approach

Scattered-wave imaging of upper crustal heterogeneity along nearly 500 km of the San Andreas fault in central California is used to estimate cumulative offset of basement rocks in the fault zone. Optimal cross-fault realignment of scattering patterns is achieved through removal of ∼315 km of right-lateral slip. This value agrees with most previous estimates of early Miocene displacement, placing the initiation of movement on the San Andreas no earlier than ca. 23.1 Ma. Scattering along the fault correlates with segment boundaries established on the basis of historic and paleo seismicity, corroborating evidence from southern California that the upper crustal structures responsible for scattering are important in seismogenesis.

Noise Reduction in Electromagnetic Time Series to Improve Detection of Seismic-Induced Signals

A method is described for removing the ionospheric-induced electromagnetic signals from electromagnetic time series in order to improve the detection of signals caused by oceanic, industrial, environmental, or seismic activity. The method is based on transforming the ionospheric-induced signals at a remote site to the local site using smoothly varying transfer functions. This assumes the remote site is relatively free of the non-ionospheric-induced signals. The transformed ionospheric-induced signal is then subtracted from the local time series. The method is tested on data recorded at Hollister and Parkfield that have been provided by F. Morrison. The sites are both near the San Andreas fault in California. Parkfield is used here as the remote site because it will be less contaminated by man-made sources since it is in a remote part of central California. The electric dipoles are approximately 100 m long with electrodes buried about 3 m deep and the magnetic sensors are three buried coils.

Mantle Fluids in the San Andreas Fault System, California

Fluids associated with the San Andreas and companion faults in central and south-central California have high 3 He/ 4 He ratios. The lack of correlation between helium isotopes and fluid chemistry or local geology requires that fluids enter the fault system from the mantle. Mantle fluids passing through the ductile lower crust must enter the brittle fault zone at or near lithostatic pressures; estimates of fluid flux based on helium isotopes suggest that they may thus contribute directly to fault-weakening high-fluid pressures at seismogenic depths.

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.

Long-period ground motions and dynamic strain field of Los Angeles basin during large earthquakes

AbstractDuring a big earthquake along the San Andreas fault in southern California, high excitation and low attenuation of long-period (3 to 10 sec) strong ground motions will cause wave motions to propagate efficiently far from the epicentral area. These ground motions could potentially be destructive to large-dimension structures in the Los Angeles basin. We performed calculations using the surface-wave Gaussian beam method for a 3D southern California crustal structure. Displacement field as well as the associated dynamic strain field produced by large propagating ruptures along the San Andreas fault are evaluated. Results indicate that in the presence of lateral heterogeneity, focusing and multipathing interference contribute significantly to a complex pattern of the displacement field and the associated dynamic strain field. For a big event on the San Andreas fault with a seismic moment of 1.8 × 1028 dyne-cm, long-period displacement in the Los Angeles basin could reach a maximum amplitude of meters in places. Since this calculation is fast, we have evaluated the displacement field for a dense grid of points; a differentiation gives the corresponding effective horizontal dynamic strain field. At times, the maximum effective dynamic strains may reach mid-10−3 to even 10−3—high enough to be of engineering concern. This computational result probably gives the upper bound values due to the large source assumed. For events of smaller seismic moment release along less extensive ruptures, these results can easily be scaled down proportionally. Different scenarios are considered in this study with different slip distributions. It is found that with a given seismic moment, a more evenly distributed fault slip over the rupture surface will result in lower peak values on both displacements and dynamic strains. Our displacement results give similar values to those obtained by Kanamori using empirical Green's functions but substantially higher than Bouchon and Aki's results.

Spatial radon anomalies on active faults in California

Radon emanation has been observed to be anomalously high along active faults in many parts of the world. We tested this relationship by conducting and repeating soil-air radon surveys with a portable radon meter across several faults in California. The results confirm the existence of fault-associated radon anomalies, which show characteristic features that may be related to fault structures but vary in time due to other environmental changes, such as rainfall. Across two creeping faults in San Juan Bautista and Hollister, the radon anomalies showed prominent double peaks straddling the fault-gouge zone during dry summers, but the peak-to-background ratios diminished after significant rain fall during winter. Across a locked segment of the San Andreas fault near Olema, the anomaly has a single peak located several meters southwest of the slip zone associated with the 1906 San Francisco earthquake. Across two fault segments that ruptured during the magnitude 7.5 Landers earthquake in 1992, anomalously high radon concentration was found in the fractures three weeks after the earthquake. We attribute the fault-related anomalies to a slow vertical gas flow in or near the fault zones. Radon generated locally in subsurface soil has a concentration profile that increases three orders of magnitude from the surface to a depth of several meters; thus an upward flow that brings up deeper and radon-richer soil air to the detection level can cause a significantly higher concentration reading. This explanation is consistent with concentrations of carbon dioxide and oxygen, measured in soil-air samples collected during one of the surveys.

Seismicity and stress rotation in a granular model of the brittle crust

THE basic mechanical process responsible for earthquakes and faulting is not known. The intuitive notion of frictional slip on faults between elastic crustal blocks cannot be reconciled with laboratory measurements of the strength of rocks1, field observations of heat flow2,3 and stress orientation4 around the San Andreas fault in California, and seismological estimates of the energy radiated by earthquakes5. The weakening of large faults by elevated pore-fluid pressure6 has been suggested as one solution to this paradox, but places severe constraints on the hydrological conditions of the faults concerned. Here I propose an alternative model for earthquake mechanics, in which the crust is treated as a system of many interlocking blocks divided by many faults7. The model combines this random granular structure with simple, deterministic mechanical interactions. Numerical simulations of the deformation of an aggregate of rough grains under compressive stress show earthquake-like elastodynamic failures without frictional heat production, and substantial rotation of stresses across shear zones, which mimics field observations. There remain problems of scale in comparing these simulations with nature, but a seismological test of the model may be possible.

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.

Intermediate- and long-term earthquake prediction.

Progress in long- and intermediate-term earthquake prediction is reviewed emphasizing results from California. Earthquake prediction as a scientific discipline is still in its infancy. Probabilistic estimates that segments of several faults in California will be the sites of large shocks in the next 30 years are now generally accepted and widely used. Several examples are presented of changes in rates of moderate-size earthquakes and seismic moment release on time scales of a few to 30 years that occurred prior to large shocks. A distinction is made between large earthquakes that rupture the entire downdip width of the outer brittle part of the earth's crust and small shocks that do not. Large events occur quasi-periodically in time along a fault segment and happen much more often than predicted from the rates of small shocks along that segment. I am moderately optimistic about improving predictions of large events for time scales of a few to 30 years although little work of that type is currently underway in the United States. Precursory effects, like the changes in stress they reflect, should be examined from a tensorial rather than a scalar perspective. A broad pattern of increased numbers of moderate-size shocks in southern California since 1986 resembles the pattern in the 25 years before the great 1906 earthquake. Since it may be a long-term precursor to a great event on the southern San Andreas fault, that area deserves detailed intensified study.

Creep events preceding small to moderate earthquakes on the San Andreas fault

THE San Andreas fault in central California is unusual in that much of the fault displacement occurs as aseismic slip, also known as creep. Changes in creep have been suggested as a possible earthquake precursor1–6, but clear associations between creep changes and subsequent earthquakes are uncommon. Here I report a test of the hypothesis that episodes of rapid creep ('creep events'1–3) precede small to moderate earthquakes on the creeping portion of the San Andreas. The test consists of a comparison of archival creep data to the earthquake catalogue for this area, and an earthquake prediction experiment. Between 1970 and 1994, creep events occurred in the five days preceding half or more of the earthquakes of magnitude > 3.3 in three 1-yr periods of above-average seismic activity. Of five predictions based on the occurrence of creep events between October 1995 and mid-January 1996, four were fulfilled; there were also two 'misses' (earthquakes not preceded by a creep event). If the relationship reported here holds for the entire creeping segment of the San Andreas, and applies to larger earthquakes, these results suggest that a prediction based on precursory creep may be possible at Parkfield, California.

A slow earthquake sequence on the San Andreas fault

EARTHQUAKES typically release stored strain energy on timescales of the order of seconds, limited by the velocity of sound in rock. Over the past 20 years, observations1–13 and laboratory experiments14 have indicated that rupture can also occur more slowly, with durations up to hours. Such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones. The detection of events with larger timescales requires near-field deformation measurements. In December 1992, two borehole strainmeters close to the San Andreas fault in California recorded a slow strain event of about a week in duration, and we show here that the strain changes were produced by a slow earthquake sequence (equivalent magnitude 4.8) with complexity similar to that of regular earthquakes. The largest earthquakes associated with these slow events were small (local magnitude 3.7) and contributed negligible strain release. The importance of slow earthquakes in the seismogenic process remains an open question, but these observations extend the observed timescale for slow events by two orders of magnitude.

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.