Seasonal Seismicity Associated with Western United States Volcanoes

We examined volcanic areas in the western United States for evidence of "seasonal" seismicity (annual periodicity). Data sets cover a 20-year period and include six distinct zones of focused seismicity in large calderas (3 zones apiece at Long Valley caldera and Yellowstone National Park) and four Cascade Range stratovolcanoes (Mts. Lassen, Hood, St. Helens, and Rainier). We developed a method to identify and remove large swarm events associated with magmatic intrusion from the time series in order to better detect annual trends. We then applied statistical techniques to test for annual seismic patterns.

Seven of the ten study areas show statistically significant seasonality (>80% probability in Schuster tests). The exceptions are two areas in Long Valley caldera that were dominated by seismic swarms during the majority of the 20-year period and Mt. St. Helens, where the apparent seasonality fails to meet our 80% probability threshold. Elsewhere in the Cascade Range and in the other Long Valley study area, peaks in annual seismicity occur in the late summer and autumn. In Yellowstone National Park, seismicity peaks earlier, during late spring and summer. In order to determine the most plausible cause for the observed trends, we quantify possible forcing mechanisms that could induce seasonal seismicity. Possible triggers include barometric pressure changes, solid earth tides, unloading associated with annual snowmelt, and groundwater recharge. Annual cycles of barometric pressure and solid earth tides cause changes in annual loads of <5 kPa, which are probably not sufficient to trigger seismicity. The stress changes associated with snow unloading and groundwater recharge are larger (>10 kPa) and may serve as the trigger for seasonal seismicity.