Snowmelt-Triggered Earthquake Swarms at the Margin of Long Valley Caldera, California

Fluids are well known to influence earthquakes, yet rarely are earthquakes convincingly linked to precipitation. Weak modulation or limited data often leads to ambiguous interpretations. In contrast, here we find that shallow seismicity in the Sierra Nevada range near Long Valley Caldera is strongly modulated by snowmelt. Over 33years, shallow seismicity rates were similar to 37 times higher during very wet periods versus very dry periods. Relative earthquake relocations from a swarm in 2017 reveal downward migration from similar to 1- to 3-km depth along a steeply inclined plane. Steeply dipping strata may provide high-permeability pathways and faulting plane. Here we combine the correlated seismicity and hydrologic time series with the propagation observed in the relatively relocated earthquakes. From this combined evidence, we infer that pressure diffusion from groundwater recharge dramatically accelerated shallow seismicity rates, causing seismic swarms unrelated to volcanic processes. Plain Language Summary Water from melting snow in the mountains near Long Valley Caldera filters down along upturned metasedimentary layers and triggers earthquake swarms. Earthquake locations show that the swarms propagate downward; the same direction as the infiltrating water and opposite from other nearby volcanic earthquake swarms. Over a 33-year record of stream flow and earthquake data, the earthquakes are similar to 37 times more active during wet periods than dry periods. This amount of increase is substantially larger than other regions where similar triggering has been documented. Combining the long-term correlation of the stream flow and earthquakes, with the downward propagating swarm we infer that pressure diffusion from the spring groundwater recharge dramatically increases the seismicity rates.