Reflectivity Structure Below the San Fernando Valley from Northridge Aftershock Recordings

Project funded by the National Science Foundation,
Geophysics Program

Principal Investigator: John N. Louie
Seismological Laboratory, The University of Nevada, Reno

PROJECT SUMMARY

The broad destruction caused by the Jan. 17, 1994 earthquake on a fault buried below Northridge in the heavily-urbanized San Fernando Valley emphasizes the immediate need for better understanding of hidden fault systems. This project will assess the Northridge earthquake source and related geological structures by applying geophysical techniques borrowed from seismic reflection surveying to seismic network recordings of the earthquake's abundant aftershocks. The earthquake occurred on a fault that does not break the surface, but that may have been evident in oil-industry seismic reflection surveys of the underlying basin sediments. However, neither industry data nor other existing analyses unambiguously show how numerous buried and exposed faults may be related at depth within the basement, or which faults may be more continuous or more recent, thus presenting greater seismic potential.

This project will elucidate regional fault structures by imaging reflectivity structure below the Northridge aftershock zone. As in industry seismic-reflection surveying, imaging of structure through high-frequency reflectivity demands high-multiplicity data, or a large number of overlapping sources and receivers. The large number of aftershocks recorded to date by the many stations of the Southern California Seismic Network (SCSN) provide the required multiplicity within the San Fernando Valley region. Reconnaissance seismic profiles taken worldwide have shown that reflection frequencies between 1 and 35 Hz are particularly sensitive to fault structures in the middle crust. This project will apply both standard and innovative seismic reflection imaging techniques to the aftershock data, resulting in three-dimensional images of the location, depth, and geometry of buried fault systems in the region.

The Data Center at the Southern California Earthquake Center (SCEC) has more 230 Northridge aftershocks of magnitude 3.0 or greater on file, having more than10 stations recorded each. In fact, most of these events have more than 100 seismograms recorded at SCSN stations up to 150 km distant. This provides a data set of between 10,000 and 20,000 seismograms, comparable in multiplicity to a seismic reflection survey. These data are adequate to define fault geometries below theSan Fernando Valley in 3-d at 0.5 to 1 km precision. Initial processing on data from a few events, equivalent to a 2-d stacked seismic section, identifies reflectivestructures at about 15 and 19 km depth below the epicentral area. This project will establish a data pipeline from SCEC to UNR to process data from all 230 andany further aftershocks, and investigate filtering and signal-processing strategies for best reflector imaging from the seismic data. Then this project will employboth standard common-midpoint stacking as well as advanced nonlinear diffractiontomography to produce 3-d reflector geometries below the region. These will be published in the Northridge data report as well as made available via the Internet.

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