Hypocenter migration and temporal variation in background seismic intensity and indicating fluid intrusion and diffusion: earthquake swarm activity in and around Nara Prefecture

Abstract At the end of May 2001, a series of earthquake swarms began in the southwestern part of Nara Prefecture, Japan, near the border with Wakayama Prefecture, followed by a series in central Nara Prefecture at the end of July 2002. The former continued for about 10 years, and the latter is still ongoing, more than 20 years after the first occurrence. A waveform correlation-based hypocenter relocation catalog indicated that these swarms consisted of slow, diffuse, hypocenter migrations. The nonstationary epidemic-type aftershock-sequence model showed an increase in the background seismic intensity of the swarms, and at the same time, a slowdown in the contracting trend of crustal deformation was observed. These results suggest fluid intrusion. The estimated diffusion coefficients and permeabilities were smaller than those of previously reported swarms, and the swarms are expected to last longer. Based on the results of this study and previous studies including seismic surveys, seismic tomography analyses, and compositional analysis of noble gases and major elements, it is believed that fluid released from the slab is rising through the mantle wedge and along the weak planes dipping north to northwest beneath the study area. Because slab-originated fluids are involved in earthquake swarm activity, it is likely that earthquake swarms due to fluid intrusion will occur from time to time in the future. Graphical Abstract