Crustal reflectivity from the Santa Monica Mts to the western Mojave Desert, from the Los Angeles Region Seismic Experiment (Phase II), southern California

Trond Ryberg, Gary S. Fuis, William J. Lutter, & Klaus Bauer

In Preparation 2003, SCEC Contribution #771

We acquired combined refraction and low-fold reflection data along a north-south-trending profile through the epicentral regions of the 1994 M 6.7 Northridge and 1971 M 6.7 San Fernando earthquakes as part of the Los Angeles Region Seismic Experiment, Phase II (LARSE II). The chief goal of LARSE is to image sedimentary basins and faults in the Los Angeles region to better understand and mitigate earthquake hazards associated with these geological features. The LARSE II profile crossed the San Fernando and Santa Clarita Valleys, epicentral locations of the Northridge and San Fernando earthquakes, respectively. Conventional CDP data processing, extended by techniques developed in this study, was used to obtain an image of crustal reflectivity. Given the low-fold nature of our reflection data, we first attempted to obtain an image by transforming the unmigrated CDP image into line drawings followed by migration using an independent tomographic velocity model for the profile. This migrated line drawing preserves rich detail in the reflections but retains some criss-crossing reflections. For comparison, we performed standard Kirchhoff poststack migration of the CDP data, using the tomographic velocity model, and obtained a grainy but interpretable image of the entire crust. Both the migrated line drawings and the Kirchhoff-migrated CDP data and the show a prominent, north-dipping reflective zone that extends downward from near the hypocenter of the San Fernando earthquake, at ~13-km depth, to the vicinity of the San Andreas fault, in the central Transverse Ranges, at a depth of ~30 km. A fainter south-dipping reflective zone extends downward from the Northridge hypocenter, at ~16-km depth, to 20-km depth beneath the Santa Monica Mountains. Neither of these reflective zones extend above the hypocenters of the two respective earthquakes, but their upward projections fall at or near the bases of relocated aftershocks for the two events, and we interpret them as conjugate decollements.

We have also examined energy from the bases of three sedimentary basins crossed by the profile. In order to observe this energy, it was necessary to stack all data, including P waves, to distances of 25 km (using only a normal-moveout-stretch mute). This technique enabled us to image wide-angle reflections and rays turning in the sharp velocity gradient between sediments and basement, These phases arrive very closely in time with the P waves, and are ordinarily muted. Our image of the bases of the sedimentary basins gives a qualitative sense of basin shape, but basin depth is very sensitive to the stacking velocities used.

Citation
Ryberg, T., Fuis, G. S., Lutter, W. J., & Bauer, K. (2003). Crustal reflectivity from the Santa Monica Mts to the western Mojave Desert, from the Los Angeles Region Seismic Experiment (Phase II), southern California. Journal of Geophysical Research, (in preparation).