Intensities, aftershock sequences, and the location of the 1936 Milton‐Freewater earthquake near the Oregon–Washington border, U.S.A.

Abstract

The epicenter of the 16 July 1936 M "> M M 6 Milton‐Freewater earthquake, also known as the State Line earthquake and the largest historical earthquake in northeastern Oregon or southeastern Washington, is uncertain. Various studies place the epicenter of the earthquake, which was widely felt in eastern Washington, northeastern Oregon, and northern Idaho, within 30 km of the intersection of the Hite and Wallula faults. In the absence of reported coseismic surface rupture for the earthquake, we sought to determine which epicentral location is most consistent with the intensity observations and with its aftershock sequence, which lasted for at least 27 months. An epicenter between Umapine and Milton‐Freewater best matches the observations. This location falls within the region that experienced the highest intensities of VII and reported the largest number of aftershocks, compares favorably to the (2018) International Seismological Centre‐Global Earthquake Model (ISC‐GEM) epicenter, and is proximal to sites that experienced ground failure and groundwater effects. Modeling of aftershock rates is consistent with this suggested epicenter and with the estimated 10 km long subsurface rupture of the earthquake. This suggested epicenter lies at, or just west of, the intersection of the Hite and Wallula faults. The elongation of ground failure along the Wallula fault and the aftershock distribution appears more consistent with rupture of the Wallula fault or of a subparallel fault than with rupture of the Hite fault. Rupture on faults to the north or east of the Wallula‐Hite fault intersection is inconsistent with most observations, including the perceived impulsivity of the mainshock.