On earthquake sequences in the Intermountain West

Abstract:

Intraplate shallow (<20 km) seismicity in the Intermountain West (IMW) occurs within a 100-200 km wide north-south-trending zone that extends 1500 km from southern Nevada and northern Arizona to northwestern Montana, known as the Intermountain Seismic Belt. Several M>6.0 earthquakes have occurred in the area, with the largest recorded being the 1959 Mw 7.2 Hebgen Lake, Montana and the 1983 Ms 7.3 Borah Peak, Idaho earthquakes. More recently, strong earthquakes, namely the 2008 Mw 5.9 Wells, Nevada, the 2020 Mw 5.7 Magna, Utah, and the 2020 Mw 6.5 northwest of Stanley, Idaho, have well recorded aftershocks sequences providing a unique opportunity to model the likelihood and duration of potential aftershocks in the IMW. In addition, several small in magnitude earthquakes occur frequently in the IMW producing short-lived burst-like earthquake sequences. Here, we compile catalogs of individual earthquake sequences using a pool of ~70,000 earthquakes recorded in Utah (University of Utah Seismograph Stations) and northwestern Montana (Montana Bureau of Mine and Geology). To accomplish this we apply and compare multiple declustering algorithms and develop quantitative criteria to discriminate mainshock- aftershock-like sequences from earthquake swarms, which are widespread in the IMW region. For the mainshock-aftershock-like sequences, we model the aftershock parameters, such as productivity and decay rate. Last, we use the aftershock sequences from the strongest earthquakes to test the hypothesis that earthquake sequences following large surface-faulting earthquakes in the IMW behave fundamentally differently than those following moderate size earthquakes.