Preparatory mechanisms accompanying or leading to nucleation of larger earthquakes have been observed at both laboratory and field scale, but the precise conditions favoring extended nucleation processes are still largely unknown. In particular, it remains a matter of debate why earthquakes often occur spontaneously without noticeable precursors as opposed to an extended failure process accompanied by foreshocks. In this study, we have generated new high-resolution seismicity catalogs framing the occurrence of 20 ML > 2.5 earthquakes at The Geysers geothermal field in California. To this end, a seismicity catalog of the 11 days framing each large event was created. We selected 20 sequences in total from different tectonic settings within the field that sample the entire reservoir depth range and temporal periods with high or low injection rates. Seismic activity and magnitude frequency distributions displayed by the different earthquake sequences are correlated with their location within the reservoir. Sequences located in the northwestern part of the reservoir show overall increased seismic activity and low b-values, while the southeastern part is dominated by decreased seismic activity and higher b-values. Periods of high injection coincide with high b-values, and vice versa. These observations potentially reflect differential stresses and damage varying across the field. About 50 % of analyzed sequences exhibit no change in seismicity rate in response to the large main event. Instead, we find complex waveforms at the onset of the main earthquake, suggesting that small ruptures spontaneously grow into or trigger larger events.

Earthquake catalogs belonging to each of the 20 mainshock sequences.

The catalogue is provided in tabular form (ASCII). The file names indate the NCEDC ID of the EQ Sequence mainshock (NCEDC = Northern California earthquake datacenter, Enhanced Geothermal Systems Earthquake Catalog, https://ncedc.org/egs/catalog-search.html). Definition of columns in the data tables (also in the header of the data): - X,Y are in local cartesian coordinates (m) [unlocated events are indicated by NaN]- Z (m) is in depth, negative values point downwards- Coordinate origin in Lat;Lon: 38.8042;-122.7822- dt refers to matlab serial time (i.e. the number of days from January 0, 0000, fractions of days are given as decimal digits)- Ml is local magnitude