Volcanic earthquake swarms at Izu-Oshima in 2023

Abstract

Earthquake swarms are known to reflect underground fluid movement and stress changes. However, their occurrence patterns and the mechanisms underlying hypocenter migration remain poorly understood. In this study, we focused on two earthquake swarms that occurred at Izu-Oshima in September and October 2023. By detecting microearthquakes and determining their hypocenters using seismic waveform correlation, we captured the hypocenter migration likely reflecting the underlying diffusional process. Specifically, in both swarms, the hypocenters exhibited southeastward migration. The diffusion coefficient, which quantifies the speed of diffusive migrations, was larger in the second swarm compared to the first one. These results suggest that fracture structures formed in the first swarm by magma intrusion or associated underground fluid movement may have facilitated hypocenter migration of the second swarm sequence.