Quasi-Real-Time Hypocenter Relocation and Monitoring in the Northeastern Noto Peninsula

The seismicity rate markedly increased in the northeastern Noto Peninsula of Ishikawa Prefecture around the end of 2020, with an M _w 6.2 event on 5 May, 2023, followed by many aftershocks. Previous earthquake relocation studies have detected upward migration of microearthquakes via multiple faults and clusters, suggesting the involvement of crustal fluids in this sequence. Since some active faults exist near the source region, there was concern that the sequence could lead to a larger earthquake; this became a reality with the M _w 7.5 earthquake on 1 January 2024. The objective of this study is to develop an algorithm to precisely relocate the microearthquake hypocenters in quasi-real time for better monitoring. A fine view of seismicity requires relative relocation methods such as the Double-Difference (DD) method with numerous and accurate arrival time difference data derived from the waveform correlation analysis. However, the standard DD method has the disadvantage of huge computational costs when data increases, making it unsuitable for real-time monitoring in such situations. We developed a quasi-real-time algorithm that divides earthquake data into multiple time windows and performs the DD relocation each time new time window data is added. The major improvement is that our method incorporates a traditional simple relative relocation method and preserves constraints between different time windows; the relative locations of new events are constrained from reference events that were already relocated in the previous time windows. We tested a daily relocation algorithm on 11,546 events from 19 June, 2022, to 31 May, 2023, in the Noto Peninsula earthquake sequence. We found that our modification substantially reduced artificial hypocenter offsets between different time windows and succeeded in resolving the fine fault structures from the cloud-like distribution of initial hypocenters. If we do not impose constraints between different windows, the relocated hypocenters are scattered and do not show fine planar structures. Moreover, our algorithm greatly reduces the computational cost, allowing for quasi-real-time earthquake relocation and monitoring. We hope this algorithm will help monitor the spatio-temporal distribution of future earthquake sequences.