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The 1987 Superstition Hills earthquake sequence: Strains and tilts at Piñon Flat Observatory

INSTITUTE OF GEOPHYSICS AND PLANETARY PHYSICS, A-025 UNIVERSITY OF CALIFORNIA,, SAN DIEGO, LA JOLLA, CALIFORNIA 92093

Abstract

We describe observations at Piñon Flat Observatory of preseismic, co-seismic, and postseismic strain and tilt changes associated with the Superstition Hills earthquake sequence of 24 November 1987. The data come from two long-base strainmeters, one long-base tiltmeter, and a borehole dilatometer. Co-seismic offsets were seen for the two largest earthquakes; these offsets roughly agree with those from a dislocation in a half-space, though with disagreements at the 30 per cent level, suggesting some error in one or more of the co-seismic observations. For the main shock, the moment deduced from the offsets is 5 to 6 x 10²⁵ dyne-cm, less than that found from the seismic data. No precursive strain changes were seen. For the 8 hr before the main shock, any strain changes were less than the instrument noise level (1 per cent of the co-seismic strain); during the last 1000 sec before this earthquake, any precursory strains must be less than 0.5 per cent of the co-seismic amount. Postseismic strain during the 2 hr after the main shock was about 10 per cent of the co-seismic strain.

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