Abstract

From September 1987 through March 1988, an earthquake sequence which induded shocks of ML 4.8 and 4.7 on September 25 and October 26, respectively, and a total of 8 moderate-sized events of ML ≥ 3.8, occurred in NW Utah beneath a desert basin west of the Great Salt Lake (center of activity: 41° 12.0′ N, 113° 10.5′ W). Wood-Anderson seismograms indicate nearly identical magnitudes for the two largest earthquakes but a factor of two to five larger seismic moment for the first. Significant aspects of the 1987–1988 sequence induded: foreshock activity, proximity (epicentral distance, Δ, of 7 to 12 km) to a major pumping facility completed in early 1987 to lower the level of the Great Salt Lake, an unambiguous strike-slip focal mechanism for the ML 4.8 mainshock, and the lack of a clear association with late Quaternary surface faults.

Despite constraints on accessibility to the epicentral area, the stations of the regional seismic network (Δ ≥ 60 km) were supplemented with local stations—initially four portable seismographs and later up to four telemetered stations (2 ≤ Δ ≤ 27 km) that operated continuously from October 7, 1987, through March 1988. Well-located aftershock foci form a 6-by-6-km zone between 6 and 12 km depth which is steeply dipping and trends SSE, parallel to the right-lateral nodal plane of the mainshock focal mechanism. Despite coincidental timing and proximity of the earthquakes to major pumping activity at the surface, the case for induced seismicity related to the pumping is weak.

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