Abstract
AbstractPreparatory mechanisms accompanying or leading to nucleation of larger earthquakes have been observed at both laboratory and field scales, but conditions favoring the occurrence of observable preparatory processes are still largely unknown. In particular, it remains a matter of debate why some earthquakes occur spontaneously without noticeable precursors as opposed to events that are preceded by an extended failure process. 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 sampling different hypocentral depths and hydraulic conditions within the field. 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 varying differential and mean stresses and damage of the reservoir rocks across the field. About 50% of analyzed sequences exhibit no change in seismicity rate in response to the large main event. However, we find complex waveforms at the onset of the main earthquake, suggesting that small ruptures spontaneously grow into or trigger larger events.
Highlights
Characteristic spatial and temporal patterns of seismicity indicating preparation and nucleation of large‐ magnitude earthquakes would allow to significantly improve short‐term earthquake forecasting and subsequent hazard and risk mitigation
We have generated new high‐resolution seismicity catalogs framing the occurrence of 20 ML > 2.5 earthquakes at The Geysers geothermal field in California
In order to fully understand the presence or lack of these precursory signals, we have studied entire earthquake sequences, namely, foreshocks, main shock, aftershocks, and their relation and interplay from The Geysers geothermal field in California
Summary
Characteristic spatial and temporal patterns of seismicity indicating preparation and nucleation of large‐ magnitude earthquakes would allow to significantly improve short‐term earthquake forecasting and subsequent hazard and risk mitigation. Since the 1980s, the occurrence of foreshocks accompanied by accelerated seismic moment release (e.g., Jones & Molnar, 1979; Papazachos, 1975; Shaw et al, 1992), increase in the seismicity rates (e.g., Ellsworth et al, 1981), or an increase in Benioff strain (e.g., Ben‐Zion & Lyakhovsky, 2002) have been examined as potential signatures to forecast the short‐term occurrence of a larger earthquake. Jones (1985) estimated that only 6% of the earthquakes in Southern California are followed by an earthquake of larger magnitude within five days and a distance of less than 10 km This is in contrast to the more recent work of Bouchon et al (2013) and Brodsky and Lay (2014) who suggest that foreshocks are more common.
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