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We propose a Bayesian approach for jointly determining the locations of earthquake hypocentres and velocity structures in a region divided by a structural boundary, such as a fault zone. To accommodate the Markov chain Monte Carlo (MCMC) method in a region containing lateral velocity contrast, we incorporated a data-driven station clustering scheme when estimating earthquake hypocentre locations, a 1-D velocity model assigned to each cluster and station corrections for traveltimes. The proposed method realizes the clustering of stations reflecting velocity structures beneath the station network in the framework of the MCMC method. We verified the performances of the proposed method using numerical experiments with different distributions of earthquake hypocentres relative to the structural boundary. The results demonstrated that the proposed method reconstructed earthquake hypocentre locations and velocity structures involving lateral contrast robustly to earthquake distributions. Additionally, the number of 1-D velocity models (the number of station clusters) was optimized by comparing resultant posterior probability proportional to distances between the observed and theoretical traveltimes. We also applied the proposed method to estimate the earthquake hypocentre locations and velocity structures around the source area of the 2004 Mid-Niigata prefecture earthquake, Japan, to examine the applicability of the proposed method to real observation data. The stations were divided into east and west clusters, coinciding with the lateral velocity variation related to the geological setting. Therefore, the proposed method accurately determines earthquake hypocentres in a region divided by a structural boundary without any knowledge regarding the geological structure. [ABSTRACT FROM AUTHOR]

We investigate S-wave attenuation (Qs) structure in and around Hokkaido, Japan, including the source area of the 2018 Hokkaido Eastern Iburi Earthquake (M 6.7) and its aftershocks. From the strong-motion seismograms recorded in the nationwide seismograph network in Japan, we compute spectrum amplitude at 1–10 Hz with every 1 Hz and then estimate Qs structure by adopting tomographic inversion technique. The obtained Qs structure suggests that lateral variations of Qs are remarkably formed in the study areas. The low-Qs anomalies distributing north to south are imaged in both sides of the Hidaka mountain range which is built in the central Hokkaido. These characteristics structures of Qs are extended from the ground surface to depths by about 50 km, indicating that heterogeneous structures resulted from the collision of the Kuril and northeastern Japan arcs develop at the depth ranges. Around the source area of the 2018 Hokkaido Eastern Iburi Earthquake, the abrupt changes of Qs values are identified. The 2018 Hokkaido Eastern Iburi Earthquake and its aftershocks seem to be arraigned on the high-Qs zones, whereas the low-Qs anomalies are also imaged next to the high-Qs zones. The boundary of the low- and high-Qs zones lies with near vertical, corresponding to aftershock distributions of the 2018 Hokkaido Eastern Iburi Earthquake. This agreement would propose that the heterogeneities formed due to the arc–arc collision would characterize faulting processes in the main shock of the Hokkaido Eastern Iburi Earthquake and the activities of its aftershocks. [ABSTRACT FROM AUTHOR]