Related spatio-temporal changes in hypocenters and the b value in the 2017 Kagoshima Bay swarm activity indicating a rise of hot fluids

Abstract

Spatio-temporal characteristics of the b value in relation to changes in hypocentral distribution and the occurrence rate of swarm earthquakes in the 2017 Kagoshima Bay activity were investigated. Noting stepwise changes in the area as well as the occurrence rate of swarm events, we divided the activity from December 2016 to October 2017 into six stages. Characteristics of the activity in each stage were clearly grasped when swarm area was divided into several rectangular zones in a NNW-SSE direction and when hypocenters and the b value in each zone were examined in detail. After the largest shock with a magnitude (M) of 5.3 on July 11, 2017, the swarm area expanded conspicuously. However, the dominant factor that played a leading role in the development of swarm activity was not the occurrence of the M 5.3 shock, but activity in the zone (zone B) just to the west of the focal region. The number of events in zone B was more than two times the whole number of events that occurred in the entire swarm area, except for zone B. The hypocentral distribution in zone B was relatively shallow from early stages, but the most notable feature was the abrupt diffusion of events to a shallower layer at the time of the M 4.4 shock on August 24, 2017. The b value in the central part of zone B was noticeably high from earlier stages, but remarkably high for the shallow events of the burst-like activity after the M 4.4 shock. From these spatio-temporal characteristics of hypocenters and the b value, we propose that hot fluids that had existed beneath the central part of zone B ascended at the time of the M 4.4 shock. Moreover, hot fluids were the major cause of the 2017 Kagoshima Bay swarm activity.