Fig. 1

Study area and hypocenter distribution. a The four regions of earthquake swarm activity (regions N, S, E, and W) based on the epicentral distribution of earthquakes in the study area with flags of K, k, or A, hypocentral depths shallower than 20 km, and magnitudes of \(M\) 1.3 or higher occurring between 1 January 2018 and 31 December 2022 extracted from the JMA unified hypocenter catalog. The color scale shows hypocentral depth, and symbol size is proportional to \(M\). The JMA CMT solution for the largest earthquake (\(M\) 5.4, \({M}_{{\text{w}}}\) 5.1), which occurred in region E on 19 June 2022, is also shown. The outsets show the location of the study area (red) on wider area maps. b Hypocenters projected onto a north–south cross section. Region S is divided into shallow (Ss) and deep regions (Sd). Symbol colors correspond to regions N (blue), E (orange), W (green), Ss (black), and Sd (red). c Cumulative frequency–magnitude distribution. The vertical dashed line indicates the threshold magnitude \({M}_{{\text{th}}}=1.3\). d Time series of hypocentral depths. e Time series of the cumulative number of earthquakes normalized by the total number of earthquakes in each region (3528, 1521, 630, 203, and 320 in regions N, E, W, Ss, and Sd, respectively). Symbol colors in c–e are the same as in b