Spatiotemporal Aftershock Analysis of the M5.8 Lincoln, Montana Event

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

One of the most seismically active regions in the western United States far from any major plate boundary is the Intermountain Seismic Belt (ISB). On 6 July 2017, a M5.8 earthquake occurred 11 km southeast of Lincoln, Montana within the ISB. This was the largest earthquake to occur in the state of Montana since the 1959 M7.3 Hebgen Lake earthquake. Data from the University of Montana Seismic Network and the Montana Regional Seismic Network was used to investigate the aftershock sequences following the M5.8 Lincoln event. We have manually refined P- and S-wave arrival times, computed hypocenter locations and double-difference relocations, and generated focal mechanisms for hundreds of aftershocks in the year following the main shock. We are working to characterize the evolution of the aftershock sequence, identify unmapped faults, and analyze the local stress field. Based on preliminary results, focal mechanisms for aftershock events from August to October of 2018, when we had a dense deployment of temporary broadband stations, primarily represent left-lateral slip on a NNE-trending fault. The hypocenter locations align along a NNE-trending fault consistent with the fault plane for the main shock. A smaller percentage of the hypocenter locations align along two adjacent structures, one trending southwestward and one trending northeastward. Based on the alignment of the hypocenters and focal mechanisms, the Lincoln area appears to host a complex fault system that includes subparallel and perpendicular faults. The results can help to constrain estimates of seismic hazard and crustal stress conditions in Montana.

Presenting Author: Ellen M. Smith

Authors