A new crustal deformation map of southern California based on Global Positioning System (GPS) and other geodetic data gives unexpected results.

The Southern California Earthquake Center is collecting and interpreting geodetic survey data for southern California to monitor fault motions and earthquake potential. A major product of this work is a set of deformation velocity estimates (the rate at which the Earth's crust moves) for 287 sites in southern California. These new estimates reveal the horizontal component of deformation resulting from strain build-up and aseismic motions in the crust.

The resulting map of deformation velocities is so accurate that strain rates (i.e., relative movements between points) can be determined directly. Geodetically determined strain rates on individual faults are in reasonable agreement with geologically estimated fault slip rates. This implies that the crustal deformation between earthquakes is relatively steady, not deviating much from its long-term average ("long-term" refers to thousands of years).

Such agreements are comforting because modern seismic hazard estimates usually assume that immediate (years to decades) earthquake potential is proportional to the long-term slip rate on faults. In spite of the overall agreement between long-term and short-term slip rates, the spatial distribution of the present strain rate is still surprising. The regions of highest shear strain rate are not on the major faults as we expected, but rather (with one exception) they are in the regions surrounding previous earthquakes.

These post-seismic effects could complicate efforts to determine long-term slip rates on faults using geodetic data. Relating the strain field (which is being determined from the new southern California permanent GPS network) to future earthquakes will require an understanding of the effects of past ones.

Contacts:

David Jackson, UCLA (310) 825-0421; Zheng-kang Shen, UCLA (310) 825-9084; Ken Hudnut, USGS (818) 583-7232.

References:

David D. Jackson, Zheng-kang Shen, David Potter, Xiao-Bin Ge, Li-yu Sung. "Southern California Deformation," Science, Vol. 277, p. 1621, 12 September 1997.

SCEC Crustal Deformation Study Group Web Site:

http://www.scec.org/research/groupe/index.html