Earthquake Atlas of Southern California 1978 - 1990

Egill Hauksson, Kate Hutton, Katrin Hafner, & Lucile M. Jones

Published 1992, SCEC Contribution #6

The Southern California Seismographic Network (SCSN) (Fig. 1) has recorded more than 139,000 earthquakes in southern California since 1978. This earthquake atlas presents maps to describe this ongoing earthquake activity and to facilitate a comparison of the activity with mapped tectonic structures such as late Quatemary faults. These maps also show what earthquake data are available from the Southern California Seismographic Network. Furthermore, when new significant activity occurs, these maps will facilitate comparison with past activity.

The relationship between the seismicity and tectonic structures in southern California has been the subject of broad regional studies such as Wood (1947) and Allen et al. (1965). Wood (1947) analyzed a data set of about 1200 earthquakes while Allen et al. (1965) analyzed 10,126 earthquakes. Allen et al. (1965) recognized that events of magnitude 6.0 or larger were usually associated with major late Quaternary faults, but that the smaller events are scattered and in most cases cannot be associated with mapped surficial faults. They also pointed out the lack of small earthquakes or background seismicity along major late Quaternary faults such as the San Andreas fault Since the Allen et al. (1965) study was completed, the number of seismographic stations in southern California has grown from 18 to 220 in 1990. Most of the new stations were deployed in the 1970s and, with the advent of digital recording and data processing in 1977, make the data collected and analyzed since 1978 of significantly higher quality. This atlas thus allows a comparison between the new and old data and provides an opportunity to check if some of the earlier conclusions about seismicity, such as the scattering of the small events, are real or artifacts caused by insufficient data.

The seismicity maps in this atlas cover two time windows, 1978-1990 and 1990. Because the 13-year maps 1978-1990 show all the recorded activity, they are dominated by the smaller earthquakes. These illustrate the relationship between late Quaternary faulting and seismicity. The 1990 maps include lower-hemisphere single-event focal mechanisms of the larger earthquakes, which reflect the local tectonics. Comparison of the two maps illustrates how many years of monitoring can give a much more complete picture of the earthquake activity than a short time interval, such as a year.

Key Words
United States, swarms, Southern California Seismic Network, seismology, seismicity maps, Transverse Ranges, California, Coast Ranges, Southern California, maps, Imperial Valley, Los Angeles Basin, tectonics, San Bernardino Mountains,earthquakes, Elsinore Fault, atlas

Citation
Hauksson, E., Hutton, K., Hafner, K., & Jones, L. M. (1992). Earthquake Atlas of Southern California 1978 - 1990. , : AEG Southern California Sector Special Publication.