Tidally Triggered Earthquakes at the Geysers in Northern California

Date: 4/26/2019

Time: 06:00 PM

Room: Fifth Avenue

Seismicity at the Geysers geothermal field in northern California is highly clustered. This clustering is caused by fluid injections and withdrawals, aftershocks, dynamic triggering by seismic waves from distant earthquakes, and possibly other mechanisms. We are investigating whether or not earthquakes are triggered by Earth tides because temporal changes in triggering may reveal evolving stress conditions and help us to forecast behavior. We start with an earthquake catalog that runs from April 2003 to Jun 2016 containing ~470000 earthquakes; using only the ~41000 earthquakes above the magnitude of completeness of 1.75. Because focal mechanisms are unknown, we analyze the relationship between tidally induced volumetric strain with seismicity. Because we know of no way to accurately model and remove all other forms of clustering except for hypothesized clustering by Earth tides, we need an alternative analysis that does not require declustering. We use a likelihood analysis which consists of adding up the tidal stress magnitudes (positive = dilation) for all earthquake times in the catalog. We compare this sum against simulated catalogs that are constructed by segmenting the real catalog using the largest interevent times, then randomly reordering the segments. The number of segments does not significantly affect our results within the range we tested (50-500). This analysis preserves clustering behavior such as aftershock sequences and other forms of clustering that have time scales of several months or less, but randomly shifts them in time. For the entire catalog, less then 1 percent of the simulated catalogs have a higher likelihood value than the real catalog, indicating triggering of the Geysers earthquakes by Earth tides. Further analysis shows that the triggering likelihood during the dry season (May – October) is higher than that during the wet season (November – April).

Presenting Author: Andrew A. Delorey

Authors