An earthquake sequence during March and April 1975, which included a main shock of magnitude (ML) 6.0 on 28 March, was centered in Pocatello Valley on the Idaho-Utah border. A foreshock of magnitude (ML) 4.2 preceded the main shock by 22 hr and was followed by at least 156 microearthquakes that behaved as aftershocks of the first event. Accurate locations of 587 aftershocks that occurred during the first 20 days after the main shock define irregular zones of concentrated activity beneath the south-central, western, and northern parts of Pocatello Valley. The main shock is interpreted to have involved nonuniform dipslip rupture, without surface faulting, on a previously unrecognized normal fault that dips NW, obliquely transecting the north-trending Pocatello Valley graben. The speace-time pattern of aftershocks and composite focal mechanisms indicate subsidiary faulting and the interaction of diverse fracture trends resulting in a mixture of normal, strike-slip, and oblique faulting. Details of the earthquake sequence imply a complex episode of Basin and Range graben subsidence that may typify similar source regions elsewhere in Utah's Wasatch Front area.

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