Tidal and surface wave triggering of earthquakes at injection and geothermal sites across the United States

Abstract:

We searched for evidence of tidal and surface wave triggering of earthquakes at thirteen different well injection sites around the United States. We assembled earthquake catalogs in regions surrounding thirteen injection/geothermal sites, seven of which are in California, one in Oregon, and five in the Midwestern United States. After temporally declustering the catalogs, we tested each site for evidence of tidal triggering by applying the Schuster test at periods between 10.8 and 26.4 hours. We defined “triggering” at a given period as the time when the Schuster probability exceeds 99% confidence. In general, there was little evidence for tidal triggering of earthquakes at the injection/geothermal sites we investigated. The one exception was seismicity at the Geysers, CA, which was triggered at the M₂ semidiurnal lunar period. Many sites also had amplitudes that exceeded 99% confidence at 24 hour periods, however since these sites had no evidence of triggering at the larger M₂ tidal period, we consider this to be an anthropogenic effect.

Next we searched for triggering by M_w 7 or above global earthquakes and M_w 4-7 regional earthquakes. We quantified the influence of the regional and global earthquakes on seismicity at each injection/geothermal site using the beta statistic, which is a measure of the change in seismicity rate before and after the triggering event occurs. In general, we found evidence for triggering by regional/global earthquakes at most sites. We also explored the relationship of activation to other site variables such as wellhead pressure, monthly injection, monthly production, and cumulative injection/production volumes when available. While the beta statistic does correlate with obvious variables such as the distance between the site and the triggering event it doesn’t appear to correlate with the aforementioned site-specific variables.