Harold Magistrale

The SCEC Southern California Reference Three-Dimensional Seismic Velocity Model Version 2

We describe Version 2 of the three-dimensional (3D) seismic velocity model of southern California developed by the Southern California Earthquake Center and designed to serve as a reference model for multidisciplinary research activities in the area. The model consists of detailed, rule-based representations of the major southern California basins (Los Angeles basin, Ventura basin, San Gabriel Valley, San Fernando Valley, Chino basin, San Bernardino Valley, and the Salton Trough), embedded in a 3D crust over a variable depth Moho. Outside of the basins, the model crust is based on regional tomographic results. The model Moho is represented by a surface with the depths determined by the receiver function technique. Shallow basin sediment velocities are constrained by geotechnical data. The model is implemented in a computer code that generates any specified 3D mesh of seismic velocity and density values. This parameterization is convenient to store, transfer, and update as new information and verification results become available.

Mantle Heterogeneities and the SCEC Reference Three-Dimensional Seismic Velocity Model Version 3

We determine upper mantle seismic velocity heterogeneities below Southern California from the inversion of teleseismic travel-time residuals. Teleseismic P-wave arrival times are obtained from three temporary passive experiments and Southern California Seismic Network (SCSN) stations, producing good raypath coverage. The inversion is performed using a damped least-squares conjugate gradient method (LSQR). The inversion model element spacing is 20 km. Before the inversion, the effects of crustal velocity heterogeneities represented by the Southern California Earthquake Center (SCEC) seismic velocity model version 2 are removed from the teleseismic travel times. The P-wave inversion produces a variance reduction of 43%. S-wave velocities are determined from laboratory Vp/Vs ratios. The most prominent features imaged in the results are high P-wave velocities (+3%) in the uppermost mantle beneath the northern Los Angeles basin, and the previously reported tabular high-velocity anomaly (+3%) to depths of 200 km beneath the Transverse Ranges, crosscutting the San Andreas fault. We incorporate the upper mantle seismic velocity heterogeneities into the SCEC Southern California reference seismic velocity model. The prior accounting for the crustal velocity heterogeneity demonstrates the utility of the top-down method of the SCEC seismic velocity model development.

Crustal thickness of the Peninsular Ranges and Gulf Extensional Province in the Californias

We estimate crustal thickness along an east-west transect of the Baja California peninsula and Gulf of California, Mexico, and investigate its relationship to surface elevation and crustal extension. We derive Moho depth estimates from P-to-S converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at ;318N latitude. Depth to the Moho is ;33 (63) km near the Pacific coast of Baja California and increases gradually toward the east, reaching a maximum depth of ;40 (64) km beneath the western part of the Peninsular Ranges batholith. The crust then thins rapidly under the topographically high eastern Peninsular Ranges and across the Main Gulf Escarpment. Crustal thickness is ;15-18 (62) km within and on the margins of the Gulf of California. The Moho shallowing beneath the eastern Peninsular Ranges represents an average apparent westward dip of ;258. This range of Moho depths within the Peninsula Ranges, as well as the sharp ;east-west gradient in depth in the eastern part of the range, is in agreement with earlier observations from north of the international border. The Moho depth variations do not correlate with topography of the eastern batholith. These findings suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges and that a local Airy crustal root does not support the highest elevations. We suggest that Moho shallowing under the eastern Peninsular Ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the Gulf Extensional Province that commenced in the late Cenozoic. Support of the eastern Peninsular Ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and/or upper mantle.

3D simulations of multi‐segment thrust fault rupture

Thrust faults, such as those that underlie the Los Angeles basin, are typically segmented by tear faults that offset the thrust fault segments. We perform 3D finite difference simulations of earthquake rupture to evaluate the effectiveness of these offsets in retarding rupture. The simulations include orthogonal, intersecting faults to model the interaction of the thrust fault segments with the tear fault. For reasonable assumptions for fault segment length, strength, and stress drop, rupture can jump offsets of up to 2 km if a tear fault is present, consistent with observations of well studied thrust earthquakes. Absent a tear fault, the maximum offset that can be breached is an order of magnitude smaller.

Regional crustal thickness variations of the Peninsular Ranges, southern California

We used the teleseismic receiver function technique to obtain a profile of the crustal thickness of the northern Peninsular Ranges, California. Depth to the Moho varies from ∼37 km beneath the western Peninsular Ranges batholith to ∼27 km at the western edge of the Salton trough, an average apparent dip of ∼10° to the west over a lateral distance of 60 km. We previously obtained a similar result for a profile ∼100 km to the south (a Moho dip of ∼20° over 30 km lateral distance). In both cases, the Moho depth variations do not correlate with topography of the eastern batholith, but rather appear to parallel the trend of a boundary that separates compositionally distinct eastern and western terranes. These observations suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges, and that compensation is through lateral variations in crustal or upper mantle density rather than through an Airy root.

Forward and inverse three‐dimensional P wave velocity models of the southern California crust

We construct a three-dimensional P wave velocity model of the southern California crust by combining existing one-dimensional models, each describing a region defined by surface geology, and calibrate the model with travel times from three explosions. The model is expressed as blocks, each of a given slowness. The variance of the P wave travel time residuals of ≈1000 earthquakes relocated in and near the Los Angeles basin, where the model is most detailed, is half that of the catalog locations in the standard one-dimensional model for southern California. Starting from the forward model, we invert ≈21,000 P wave arrivals from earthquakes for hypocenters and block slownesses using the technique of Roecker (1981). The variance of these P wave travel time residuals decreases 47% during the inversion. Many of the blocks representing the upper crust and midcrust are well sampled and well resolved. The resulting model is useful both for locating earthquakes and for comparing the geologies of the different regions. For example, the velocity structure of the Los Angeles basin represents seismically slow sediments on top of basement rocks having velocities similar to the granitic rocks under the Peninsular Ranges. Moho is between 26 and 32 km depth. In contrast, the Ventura basin has mostly slower sediments above a deeper, higher-velocity basement. Compared to catalog locations, relocations in the final three-dimensional model of 98 ML ≥4 earthquakes throughout southern California tend to deepen below sediment filled valleys and basins, shallow in regions without sedimentary cover, and have a 44% lower P wave travel time residual variance.

Lithologic Control of the Depth of Earthquakes in Southern California

The depth distribution of southern California earthquakes indicates that areas underlain by schist basement rocks have a shallower (4 to 10 kilometers) maximum depth of earthquakes than do areas with other types of basement rocks. The predominant minerals in the schists become plastic at lower temperatures, and thus at shallower depths, than the minerals in the other basement rocks. The lateral variations in lithology will control the depth extent (and thus the magnitudes) of potential future earthquakes; these depths can be determined from the depth of the current background seismicity.

Evidence from precise earthquake hypocenters for segmentation of the San Andreas Fault in San Gorgonio Pass

We use precise hypocenter patterns and focal mechanisms to investigate the presence or absence of a continuous strike-slip fault at depth connecting the San Bernardino strand of the San Andreas fault with the Coachella Valley segment of the Banning fault. We inverted 560,000 arrival times from 23,000 earthquakes (1981–1993) for high-quality hypocenters and three-dimensional P wave velocity structure in a 1° by 2° area centered on the San Gorgonio Pass. Cross-sectional plots of relocated earthquakes reveal an abrupt 5 to 7 km high step in the maximum depth of hypocenters. The step riser defines a near-vertical, locally curved surface that extends westerly more than 60 km from the Coachella Valley segment of the San Andreas fault to the San Jacinto fault. A hypothetical continuous vertical San Andreas fault through San Gorgonio Pass would cross the step at an oblique angle. We suggest that the step is the expression of the contact between different basement rock types juxtaposed by large-scale right-slip motion on the ancestral San Andreas fault. South of the step in Peninsular Ranges type basement (intrusives), brittle failure occurs down to about 20-km depth, while north of the step in San Bernardino type basement (Pelona schist), brittle failure occurs to only about 13-km depth. The step provides a piercing plane that should be offset about 3 km right laterally by an active, continuous, vertical San Andreas fault. Within the resolution of our mapping the step is not offset in this manner, implying either that there has not been a throughgoing vertical fault at depth, that a throughgoing fault has not experienced enough slip to offset the step, or that a throughgoing fault is not vertical and dips north over the top of the step. Hypocentral patterns and focal mechanisms indicate distributed deformation (thrust, normal, and strike-slip faulting) over a large volume in the San Gorgonio Pass region; there is no evidence of hypocenter or slip vector alignments that would indicate a throughgoing, continuous, near-vertical San Andreas fault. In summary, we find no evidence indicating a continuous fault at seismogenic depth connecting the San Bernardino strand and Coachella Valley segment of the San Andreas fault zone. We speculate that this is because the 3 km of right slip on the San Bernardino stand of the San Andreas fault and Coachella Valley segment of the Banning fault has not been sufficient to form a single new structure through the 15- to 20-km gap between the two previously unconnected segments. This implies that large earthquake rupture on the San Andreas fault may be inhibited from propagating through San Gorgonio Pass, thus limiting the maximum magnitudes on the southern San Andreas fault.

Crustal thickness variations beneath the peninsular ranges, southern California

We investigate the crustal thickness under the Peninsular Ranges using P-to-S converted phases of teleseismic body waves recorded on a temporary broadband seismometer array and isolated by the receiver function method. Ps minus P times at sites west of a compositional boundary that separates the Peninsular Ranges batholith into east and west zones indicate a relatively flat, deep Moho. Ps minus P times at sites east of the compositional boundary decrease eastward, Moho depth estimates (made from the Ps delays and crustal velocities from seismic tomography) indicate a relatively constant 36 to 41 km thick crust in the western zone. In the eastern zone the crust thins rapidly from 35 km thick at the compositional boundary to 25 km at the edge of the Salton trough, a lateral distance of 30 km. The lack of correlation between topography and Moho depths suggests compensation via lateral density variations in the lower crust or upper mantle. We propose that the compositional boundary decouples the eastern and western portions of the batholith, and that the eastern portion has thinned in response to regional Miocene extension, or Salton trough rifting, or both.

P-wave image of the Peninsular Ranges batholith, southern California

We invert earthquake P-wave arrival times to image the 3D distribution of P wave velocities in the Mesozoic Peninsular Ranges batholith and nearby areas in southern California. There is a 3% velocity contrast between the eastern and western Peninsular Ranges at 4 and 20 km depth (west side faster) and a 1 to 1.5% velocity contrast across the San Andreas fault zone (south side faster) in the San Bernardino region at 4 to 14 km depth. The San Andreas velocity contrast is due to the juxtaposition of different rock types by slip along the fault zone. The Peninsular Ranges batholith velocity contrast is due to a difference in rock composition across the batholith. The maximum gradient in the crustal velocities is coincident with a compositional boundary within the batholith that reflects emplacement of the batholith across juxtaposed oceanic and continental crust. Quaternary fault development has been in part concentrated at this boundary.