Chastity Aiken

Dynamic triggering of microearthquakes in three geothermal/volcanic regions of California

Geothermal/volcanic regions are most susceptible to local earthquake triggering by regional and remote earthquakes. Transient stresses caused by surface waves of these earthquakes can activate critically stressed faults. Though earthquakes can be triggered in geothermal/volcanic regions, it is less understood how these regions differ in their triggering responses to distant earthquakes. We conduct a systematic survey of local earthquakes triggered by distant earthquakes in three geothermal/volcanic regions of California: Long Valley Caldera, Coso Geothermal Field, and Geysers Geothermal Field. We examine waveforms of distant earthquakes with magnitudes ≥ 5.5 occurring between 2000 and 2012 and compute β statistics to confirm the significance of our findings. We find that Long Valley, Coso, and Geysers vary in triggering frequency—2.0%, 3.8%, and 6.8% in the 12 year period, respectively—and when compared to the triggering of deep tectonic tremors along the Parkfield-Cholame section of San Andreas Fault (9.2% in the 12 year period). Stress triggering thresholds vary among the regions with Long Valley having the highest of ~5 kPa and ~1 kPa for the other regions. Because dynamic stresses from distant earthquakes are similar in these three regions, the varying triggering behavior likely reflects faults having a tendency to be at or near failure. This is compatible with Geysers having a higher a value in the Gutenberg-Richter relationship and higher geothermal productivity than the other two regions. The observation of more frequent triggering of tremor than microearthquakes is consistent with recent laboratory studies on increasing triggerability with lower effective stress.

Delayed triggering of microearthquakes by multiple surface waves circling the Earth

It is well known that direct surface waves of large earthquakes are capable of triggering shallow earthquakes and deep tremor at long-range distances. However, it is not clear whether multiple surface waves circling the Earth could also trigger/modulate seismic activities. Here we conduct a systematic search of remotely triggered microearthquakes near the Coso Geothermal Field in central California following the 2010 Mw 8.8 Chile earthquake. We find a statistically significant increase of microearthquakes in the first few hours after the Chile mainshock. These observations of apparently delayed earthquake triggering do not follow the Omori-law decay with time since the largest ML 3.5 event occurred during the large-amplitude Love waves. Instead, they are better correlated with the first three groups of multiple surface waves (G1 − R1, G2 − R2, and G3). Our observation provides an alternative explanation of delayed triggering of microearthquakes at long-range distances, at least in the first few hours after large earthquakes.