Images of crust beneath southern California will aid study of earthquakes and their effects (original) (raw)
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Journal of Geophysical Research, 1999
We apply inversion methods to first arriving P waves from explosive source seismic data collected along line 1 of the Los Angeles Region Seismic Experiment (LARSE), extending northeastward from Seal Beach, California, to the Mojave Desert, in order to determine a seismic model of the upper crust along the profile. We use resolution information to quantify the extent of blurring in the LARSE images and to smooth a damped least squares (DLS) image by postinversion filtering (PIF). Most of the original data fit is preserved while minimizing model artifacts. We compare DLS, PIF, and smoothing constraint inversion images using both real and synthetic data. A preferred PIF image includes larger-scale features in the smoothing constraint inversion image and finer-scale features in the DLS inversion image that are consistent with geologic information. We interpret principal model features in terms of geology, including faulting. The maximum depth of low-velocity sedimentary and volcanic rocks in the Los Angeles basin is 8-9 km and in the San Gabriel Valley is 4.5-5 km. A horst-like uplift of basement rocks occurs between these basins. The northeastern boundary of the San Gabriel Valley is imaged as a tabular, moderately north dipping low-velocity zone that projects to the surface at the southernmost trace of the Sierra Madre fault system. In the central and southern San Gabriel Mountains, velocitydepth profiles are consistent with intermediate-velocity mylonites overlying lower-velocity Pelona Schist along a shallowly southwest dipping Vincent thrust fault. Tomography does not provide a definitive dip for the San Andreas fault but, combined with other LARSE results, is consistent with a vertical to steep northeast dip. 1Department Specific imaging targets included the bottoms of the Los Angeles and San Gabriel Valley sedimentary basins, the geometries of basin-bounding faults, including the Whittier and Sierra Madre fault zones, and the deep structure of the San Gabriel Mountains, beneath which at least two major fault zones converge, the San Andreas and Sierra Madre fault zones.
Crustal imaging in southern California using earthquake sequences
Tectonophysics, 1998
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 ...
A tomographic image of mantle structure beneath Southern California
Geophysical Research Letters, 1984
We determined the variations in seismic structure beneath southern California by using a tomographic method of inversion on teleseismic P delays recorded with the Southern California Array. The algorithm employed was a modified form of an Algebraic Reconstruction Technique (ART) used in medical X-ray imaging. Deconvolution with an empirically estimated point spread function was also used to help in focusing the image.
Crustal deformation across and beyond the Los Angeles basin from geodetic measurements
Journal of Geophysical Research, 1996
We combine 6 years of Global Positioning System (GPS) data with 20 years of trilateration data and a century of triangulation, taped distance, and astronomic azimuth measurements to derive 66 interseismic station velocities in the greater Los Angeles region. We interpolate the velocities to construct a regional strain rate map beyond the Los Angeles basin. Our results generally agree with the model proposed by the Working Group on California Earthquake Probabilities in 1995. Important regional findings of this study are as follows: (1) There is a significant N-S convergence and E-W extension, about 0.22 and 0.17 •: 0.05 #strain/yr, respectively, for the two components, along the southern frontal fault system of the San Gabriel Mountains. (2) The crustal deformation around the Big Bend of the San Andreas fault (SAF) cannot be explained solely by wrench-style motion across the SAF. Remaining motion could be part of a NW-SE extension which is the response to NF•SW compression in the central Transverse Ranges region. Alternatively, it could be caused by left-lateral faulting on an oblique blind thrust beneath the San Gabriel Mountains. (3) Low strain rates axe found along the Elsinore fault and Newport-Inglewood fault. (4) North-south compression decreases from the Raymond Hill fault westward to the Santa Monica fault. There is little east-west extension along this fault system. ary between the Pacific and the North America plates to the San Andreas fault (SAF) east of the basin and created a left step, known as the Big Bend, along a rightlateral strike-slip SAF north and east of the basin [Atwater, 1989]. The region staxted to undergo compression as it moved NW toward the Big Bend. Evolution of the basin involved volcanism, uplift, extension, block rotation, pulling apart, shear faulting, compression, and folding [Campbell and Yerkes, 1976; Wright, 1987]. At present, the sediments in the central basin are more than 10 km thick, forming a northwest-southeast elongated synclinorium, with its flanks folded and cut by a group of Quaternaxy active faults [Ziony and Yerkes,
Bulletin of the Seismological Society of America, 2008
To better understand the structure of the San Andreas fault (SAF) at Burro Flats in southern California, we acquired a three-dimensional combined set of seismic reflection and refraction profiles centered on the main active trace at Burro Flats. In this article, we discuss the variation in shallow-depth velocities along each seismic profile, with special emphasis on the 1500 m=sec P-wave velocity contour, which can be an indicator of shallow-depth water-saturated unconsolidated sediments. Along the four seismic profiles, minimum depths of the groundwater table, as inferred from 1500 m=sec velocity contour, range from 10 to about 20 m. The largest variations in depth to the top of the groundwater table occur in areas near mapped faults, suggesting that the groundwater flow in Burro Flats is strongly affected by the locations of fault traces. We also used the seismic data to develop seismic reflection images that show multiple strands of the SAF in the upper 60 m. Reflectors above the 10 m depth probably correspond to Holocene alluvial deposits; reflectors below the 15 m depth probably arise from velocity or density variations within the Precambrian gneiss complex, likely due to weathering. Apparent vertical offsets of reflectors are observed along profiles (lines 1 and 2) that are normal to the SAF, indicating minor apparent vertical offsets on the SAF at shallow depths. Along line 2, the apparently vertically offset reflectors correlate with zones of relatively low P-wave velocity. Along the central part of lines 1 and 2, the faults form a flower structure, which is typical of strike-slip faults such as the SAF.