Abstract

The 2 September 2017 M 5.3 Sulphur Peak, Idaho, earthquake is one of the largest earthquakes in southern Idaho since the 1983 M 6.9 Borah Peak earthquake. It was followed by a vigorous aftershock sequence for nearly two weeks that included five events above M 4.5. The coseismic and early postseismic deformation was measured with both Interferometric Synthetic Aperture Radar and Global Positioning System (GPS), yielding up to 3 cm subsidence southwest of the mainshock epicenter and horizontal motions of 1  mm at sites 40  km east and west of the epicenter. We derive dislocation models of the net slip during the 14day swarm from Sentinel 1A interferograms and GPS offsets, allowing for both fault‐zone collapse and normal faulting to account for the observed geodetic motions. Slip inversions yield several decimeters of normal slip on one or more normal faults near the mainshock hypocenter. Distributed normal slip on a moderately (55°) east‐dipping fault, normal slip on one or more shallowly west‐dipping faults, or a combination thereof explain the data equally well and are difficult to distinguish from one another on the basis of geodetic data alone. Previously mapped regional Sevier‐age thrust structures and later normal faults dip westward, suggesting that the sequence reactivated one or more ancient thrust structures with normal slip. If a moderately east‐dipping fault accommodated substantial slip, it would imply a nascent fault structure that cuts across the reactivated ancient thrust structures. The inferred geodetic moment of 3.024.39×1017  N·m (Mw 5.62–5.73) greatly exceeds the 1.15×1017  N·m (Mw 5.34) seismic moment of the 2 September mainshock, showing that most of the moment release occurred during the aftershock sequence. Up to 0.2  m of fault‐zone collapse may have occurred on a shallow west‐dipping fault, suggesting possible large‐scale expulsion of fluids from the fault zone at depth.

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