Title

Geologic Setting of the 1884 Bear Lake, Idaho, Earthquake: Rupture in the Hanging Wall of a Basin and Range Normal Fault Revealed by Historical and Geological Analyses

Document Type

Article

Journal/Book Title/Conference

Bulletin of the Seismological Society of America

Volume

93

Issue

4

Publisher

Seismological Society of America

Publication Date

2003

First Page

1621

Last Page

1632

DOI

10.1785/0120020159

Abstract

Analysis of historical accounts of the ~M 6.3 1884 earthquake in northern Utah reveals that the earthquake had an epicenter near 42.3° N, 111.4° W, approximately 30 km northwest of the event's original location. We use detailed reports of damage to structures and the consequences of ground shaking to define a felt area of approximately 70,000 km2 and estimate the peak ground accelerations as 100-300 cm/sec2. Analysis of the geologic structure of the area indicates that the epicentral area is a half-graben bounded on the east by the listric Bear Lake fault and on the west by the steeply dipping West Bear Lake fault. The earthquake epicenter was on the west side of the basin, and we interpret the event to have been the result of slip on the West Bear Lake fault zone at a depth of 4-5 km. This zone consists of steeply dipping antithetic faults in the hanging wall of the East Bear Lake fault. These data suggest that moderate-magnitude earthquakes on antithetic or small-displacement faults pose a significant, if local, seismic hazard in the northeastern Basin and Range province. We also demonstrate the utility of combining geological and historigraphic analyses to examine pre-instrument-era earthquakes.