Historical analysis of the 1884 Bear Lake earthquake, northern Utah and southern Idaho: Slip on Basin and Range faults

We use the results of a detailed historigraphical analysis of accounts of the ~ M 6.3 1884 earthquake in northern Utah to determine its location, nature of ground shaking, and the geologic setting of the hypocenter. Previous reports were based on newspaper reports, and placed the event at ~0130 local time, 10 November 1884, at 42° N 111° W. We found 75 new felt reports from 19 sites to constrain the event. Detailed reports were found by examining journal reports of settlers, year-end reports of local churches, and letters to newspapers. Reports of the time are rich in detail and reflect a relatively uniform lifestyle that allows us to define Mercalli felt zones. Our work reveals that the epicenter of the Mercalli VIII event was approximately 42.3° N, 111.4° W, approximately 30 km NW of the event's original location. 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. The felt zone was a northwest-southeast trending ellipse in the basin, which may reflect settlement patterns at the time. The hypocentral depth was 5.9 +/- 3.2 km. Analysis of the geologic structure of the area indicates that the epicentral area is a graben bounded on the east by the listric Bear Lake fault and on the west by the West Bear Lake fault. The east Bear Lake fault has 4-5 km of slip, and controls the structure of the basin. However, 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, or on a smaller displacement fault in the hanging wall of the Bear Lake fault. Quaternary activity on the West Bear Lake fault is documented by trenching studies (McCalpin, 1993), and by offset Quaternary surfaces. These data suggest that moderate magnitude earthquakes on antithetic or small displacement faults do pose a significant seismic hazard in the northeastern Basin and Range province. We also demonstrate the utility of combining geological and historigraphic analyses to examine pre-instrument earthquakes of the western United States.