Induced Aftershock Sequences and Swarms in Geothermal Systems

English Abstract:

Many natural geothermal systems are associated with high seismic activity. This can be related to large scale injection of fluids to enhance geothermal recovery. Other factors, such as changes in the stress field and pore pressure, can also stimulate the occurrence of earthquakes. These systems are also prone to triggering of seismicity by the passage of seismic waves generated by large distant main shocks. In this particular study, we analyze clustering and triggering of seismicity at the Geysers geothermal field, California, due to the occurrence of relatively large local events as well as due to the occurrence of significant long distant main shocks. For the analysis we use the Northern California Earthquake Data Center Earthquake Catalog. The recent Mw 6.0 South Napa main shock triggered a ml 4.5 event and a subsequent swarm of seismic activity at The Geysers. We compare this sequence of events to several other earthquake sequences generated by local large events with magnitudes greater than 4.5 and sequences generated by several other long distant main shocks. We show that the rate of decay of the aftershock sequences generated by local large events in the first day after the main event follows the modified Omori law reasonably well. On the other hand, the swarms of activity triggered by large distant earthquakes cannot be described by the simple modified Omori law. In almost all cases the frequency-magnitude statistics of triggered sequences follow Gutenberg-Richter scaling to a good approximation with relatively large b-values. The analysis indicates that the seismicity triggered by relatively large local events can initiate sequences similar to regular aftershock sequences. In contrast, the distant main shocks trigger swarm like activity with faster decaying rates.