Non-volcanic seismic swarms triggered by circulating fluids and pressure fluctuations above a solidified diorite intrusion

A non‐volcanic seismic swarm is often assumed to be tied to fluid movements, based on earthquake migrations obeying a diffusion equation. However, seismological observations that relate to the presence and driving force of migrating fluids are not well documented. Here we present high‐resolution tomographic imaging of seismic velocities in the Wakayama district, Japan, characterized by intense seismic swarms and high heat flow. Earthquake clusters at depths of 4–6 km are distributed within a zone with low‐Vp and anomalously low‐Vp/Vs. The existence of this low‐Vp/Vs anomaly is supported by another estimate using waveform cross‐correlation data from similar event clusters. We suggest that these anomalies are caused by fluid‐filled pores with relatively high aspect ratios. Just beneath the clusters of earthquakes, a prominent high‐velocity zone is clearly imaged. It lacks earthquakes, and could represent a body of intruded diorite related to past volcanism. We propose that non‐volcanic seismic swarms are triggered by circulating fluids and fluid pressure fluctuations driven by the thermal anomaly of the solidified diorite magma.