Strain accumulation in the Rocky Mountain states

Abstract

Strain accumulation in trilateration networks in Montana, Utah, and New Mexico has been measured for over a decade. The network in New Mexico spans the Rio Grande rift near Socorro. The absence of any observed strain accumulation in that network places an upper limit (2 standard deviations) of 3 mm/a on east‐west spreading across the 100‐km‐wide rift. The network in Utah spans the Wasatch fault near Ogden. Although the 1972–1984 average rate of strain accumulation in that network is not significant at a 2‐standard‐deviation detection threshold of 0.03 μstrain/a, short‐term strain fluctuations of marginal significance have been detected in the network. The network in Montana spans the epicentral area of the 1959 Hebgen Lake earthquake (magnitude 7.1). The principal strain rates observed in that network for the 1973–1984 interval were and , extension reckoned positive. The principal extension is perpendicular to the 1959 rupture trace. The deformation of this network suggests rather rapid reloading of the 1959 rupture zone, but whether this is steady strain accumulation or still postseismic relaxation is not known. The strain rate does appear to be constant over the 1973–1984 interval.