STRAIN ACCUMULATION IN WESTERN UNITED STATES

TLDR

Geodetic strain measurements, primarily from USGS studies, provide the only reliable means of quantifying strain accumulation in western U.S. seismic zones, with long baselines offering accuracy but limiting spatial localization.

Abstract

This review is principally concerned with recent geodetic strain measure­ ments in western United States undertaken by the US Geological Survey as part of the earthquake studies program and, as a consequence, is heavily biased toward the author's own publications. Most of the publications reporting crustal-strain measurements in western United States prior to about 1968 have been compiled in one volume (National Geodetic Survey 1973), and more recent work (with complete bibliographies) is summarized in three successive quadrennial reports to the International Union of Geodesy and Geophysics (Meade 1971 , Savage 1975, Thatcher 1979b). The following conventions are employed in this paper: Strain, a dimensionless quantity, is reported in units of 10-6• Extension is taken as positive. To distinguish between engineering and tensor shear strain, we denote the former by y and quote the units as J,lrad, whereas the latter is denoted by e and given the units of J,lstrain. Uncertainties in all cases are quoted as ± one standard deviation. At the present time, geodetic techniques furnish the only reliable measure of strain accumulation in the seismic regions of the world. The superiority of geodetic techniques derives principally from the long baseline of the measurements. Difficulties in coupling fiducial marks to the Earth have generally made short-baseline measurements of strain accumulation questionable (Wyatt 1982). However, long baselines introduce the dis­ advantage that the measured strain is not closely localized. The current technique of measuring strain accumulation involves the