Loma Prieta Earthquake: Lessons for Applied Geology

SMITH, ROBERTA K., University of California, Santa Cruz, CA, and Smith-Evernden Associates, Consulting Geologists, Davenport, CA

On October 17, 1989, a magnitude 7.1 earthquake took place on the San Andreas fault. The fault break was 40 km long in the Santa Cruz Mountains, California. The hypocenter was at 18 km depth (deep) and was followed by many aftershocks. The pattern of hypocenter/epicenter distribution and geodetic measurements demonstrated that (1) fault rupture extended upward from approximately 19 km to 4-6 km depth, but not to the ground surface; (2) the fault slip was 1.7 m right lateral and 1.3 reverse (~vertical = thrust); (3) this section of the San Andreas fault is not vertical but inclines southwest at ~75 degrees. Main shock horizontal and vertical ground accelerations and shaking intensities were high; effects including liquefaction and structural damage were pronounced locally and extended onsiderable distances on "bad ground." Distribution of effects reinforced some of our preexisting ideas, for example, that saturated sediments of certain grain sizes will liquefy widely and that unreinforced masonry construction and homes/structures with inadequate horizontal shear support will fail where ground shaking is intensified. Previously unrecognized, however, was that (narrow) ridge tops focus or intensify ground accelerations, leading to ground shattering and structural damage. Directional characteristics of seismic wave propagation, often in combination with topography, can markedly affect intensity. Also previously unrecognized was that San Andreas fault breaks may not extend to the surface (in particular earthquakes). This calls into question the standard geological practic of looking for paleoseismic histories (by ground trenching) at sites in Alquist-Priolo (earthquake hazard) zones proposed for development.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)