TRANSIENT STRESSES AT PARKFIELD, CALIFORNIA, PRODUCED BY THE M-7.4 LANDERS EARTHQUAKE OF JUNE 28, 1992 - OBSERVATIONS FROM THE UPSAR DENSE SEISMOGRAPH ARRAY

The M 7.4 Landers earthquake triggered widespread seismicity in the western United States. Because the transient dynamic stresses induced at regional distances by the Landers surface waves are much larger than the expected static stresses, the magnitude and the characteristics of the dynamic stresses may bear upon the earthquake triggering mechanism. The Landers earthquake was recorded on the UPSAR (U.S. Geological Survey Parkfield Small Aperture Array) array, a group of 14 triaxial accelerometers located within a 1-square-km region 10 km southwest of the town of Parkfield, California, 412 km northwest of the Landers epicenter. No triggered earthquakes were observed at Parkfield. Multiple filter analysis shows that the displacements, obtained by double integration, are dominated by the fundamental mode Love and Rayleigh modes, with some higher-mode contributions for periods shorter than 10 s. Most of the surface waves propagated along the great circle path from Landers, but a late arriving surface wave appears to have been scattered from the Sierra Nevada Mountains. We used a standard geodetic inversion procedure to determine the surface strain and stress tensors as functions of time from the observed displacements. Peak dynamic strains and stresses at Earth's surface are about 7 mu strain and 0.035 MPa, respectively, and they have a flat amplitude spectrum between 2-s and 15-s period. These stresses agree well with stresses predicted from a simple equation using the ground velocity spectrum observed at a single station. Peak stresses ranged from about 0.035 MPa at the surface to about 0.12 MPa between 2 and 14 km depth, with the sharp increase of stress away from the surface resulting from the rapid increase of rigidity with depth and from the influence of mode shapes. Because of the free-surface boundary conditions, the horizontal components of the stress tenser tend to dominate in the top 5-6 km of the crust, which might cause triggered seismicity to have strike-slip or normal mechanisms. Comparison of dynamic stresses induced by the Landers, Loma Prieta, and Petrolia earthquakes at a variety of sites indicates that the Landers stresses were not spectacularly larger than those induced by the other sources. Landers dynamic stresses were comparable to Coalinga static stresses at Parkfield. The effective strain caused by Landers at Parkfield, where no earthquakes were triggered, are the same amplitude as those at some sites in Nevada where earthquakes were triggered. Comparing various authors' observations of dynamic stresses, there is no obvious characteristic of these stresses that correlates with the triggered seismicity.