Characterizing Moderate-Magnitude Earthquakes and Their Aftershocks Using Montana Regional Seismic Network Data

Date: 4/25/2019

Time: 06:00 PM

Room: Fifth Avenue

Regional seismograph networks provide critical data for quantifying seismic hazards, however instrumentation and station density affect analytic results. Analysis of two recent moderate-magnitude earthquakes along the northern Intermountain Seismic Belt demonstrates the use and limitations of Montana Regional Seismograph Network (MRSN) data. The 2005 Dillon earthquake, M 5.6, occurred on a normal fault. The 2017 Lincoln earthquake, M 5.8, occurred on a strike-slip fault. The MRSN recorded no foreshocks for the Dillon earthquake and a single magnitude 2.3 foreshock for the Lincoln earthquake. Vigorous aftershock sequences followed both main shocks. A magnitude 4.4 aftershock 36 hours after the Dillon earthquake was the only aftershock above magnitude 4.0. The Lincoln aftershock sequence was significantly more energetic with magnitude 5.0 and 5.1 aftershocks 5 and 67 minutes, respectively, after the main shock, and eight aftershocks in the magnitude 4.0 to 4.7 range. B-values for both sequences are similar at about 0.8. Aftershock decay rates (p-values) are also similar; Dillon 0.81 and Lincoln 1.08 (provisional). However, Lincoln aftershock hypocenter depths are—on average—deeper (12.5 km) than the Dillon hypocenters (8.5 km). Most Dillon aftershocks occurred within 10 km of the main shock. Many Lincoln aftershocks occurred in a north-trending, 8-km-long zone centered on the main shock, but about 5% of Lincoln aftershocks lie up to 30 km northwest and southeast of the main shock, and crudely align with older WNW-trending faults. Coulomb stress changes modeled for an east-dipping normal fault explain Dillon aftershocks well. The Lincoln aftershocks are not as well modeled by coseismic stress changes inferred for sinistral slip on a NNE-trending fault. Focal mechanisms for both main shocks and the larger aftershocks are consistent with NE-SW extension. Despite older instrumentation and low station density, the MRSN provides new insights into Montana seismic hazards.

Presenting Author: Michael C. Stickney

Authors