Complexity of Deep Low‐Frequency Earthquake Activity in Shikoku (Japan) Imaged From the Analysis of Continuous Seismic Data

Abstract

AbstractWe derive an extensive catalog of low‐frequency earthquakes (LFEs) in western and central Shikoku applying an automatic, coherency‐based detection and location method to 4‐year long continuous data from selected seismic stations. Our catalog is the first LFE catalog in this area that does not rely on a priory templates and results from a homogeneous data processing scheme. It allows investigating in detail main characteristics of LFE activity in space and time. We observe clear heterogeneity of the LFE space distribution in the along‐strike direction of the subducting Philippine Sea plate corresponding to segments with different sizes, some of which are highly productive in LFE generation. More detailed statistical and correlation‐based analysis of LFE occurrence patterns allows quantification of the along‐strike segmentation and examination of the migration and event interaction during the LFE sequences and inter‐sequence periods. The analysis indicates that a strong interaction exists among LFE sources during slow slip events, but it otherwise varies significantly among the along‐strike segments. We suggest that the observed segmentation of LFE activity is related to static heterogeneity, such as structural property variations along the subduction interface, or dynamic heterogeneity, corresponding to memory‐dependent stress variations or possible fluid transients. We also confirm that regions with the highest LFE productivity correspond to spots of tectonic tremor triggered by teleseismic earthquakes' surface waves supporting a localized fluid‐rich environment with possible fluid transients. Our results illustrate how high‐resolution LFE catalog can contribute to the characterization and quantification of slow earthquake processes through detailed statistical analysis of their activity.