Seismicity in Idaho and Montana Triggered by the Denali Fault Earthquake: A Window into the Geologic Context for Seismic Triggering

We present a case study of dynamically triggered seismicity in Idaho and western Montana from the 2002 MW 7.9 Denali fault earthquake to investigate the relationship between measured geological discriminants and propensity for triggering. We first establish triggering. We find events that are not reported in the Advanced National Seismic System catalog in Idaho and Yellowstone following the Denali fault earthquake by filtering broadband waveforms. An ML 4.6 earthquake is discovered near New Meadows, Idaho, during the passage of the Rayleigh waves and another earthquake probably located near Yellowstone. We find that central western Idaho and Yellowstone have statistically significant seismicity increases by applying a b test to the cataloged events in the 48 hr after the Denali fault earthquake in Idaho and surrounding regions. We also find that Pine, Idaho, may have triggered events, but the measurement is not robust because of uncertainty in the background seismicity rate. Both central western Idaho and Yellowstone triggered previously during the 1992 Landers earthquake. We then try to determine the local geological conditions necessary for triggered seismicity to occur. Geothermal regions with high total dissolved solids and background seismicity appear to be favorable conditions to trigger earthquakes in the study.