Along-Dip Variations in Triggering Among Low-Frequency Earthquakes in Shikoku

ABSTRACT Deep low-frequency earthquakes (LFEs) exhibit unique occurrence patterns distinct from those of regular earthquakes, but the physical conditions governing their occurrence and the spatial variations in their activity remain poorly understood. Among various types of slow earthquakes, LFEs are detected as discrete seismic events by ordinary seismometers, and detailed analysis of their activity has the potential to provide important insights into the underlying mechanisms of slow earthquakes. Therefore, we analyze tectonic LFEs in Shikoku using an epidemic-type aftershock sequence (ETAS) model to investigate spatial variations in seismicity. To account for spatial interactions inherent in spatially distributed seismicity, we modify the classic ETAS approach by incorporating a reference region surrounding the target region. The analysis reveals that the background seismicity rate correlates with the total seismicity rate. Furthermore, we focus on the background fraction, which is defined as the ratio of the background seismicity rate to the total seismicity rate. We find a relatively high background fraction at both shallower and deeper ends of the source region. This result indicates that the depth dependency of the seismicity is not a simple monotonic transition, from shallow periodic to deep continuous, but instead reflects a more complex pattern. At the down-dip end, triggering is likely suppressed because of the dominance of ductile deformation, which not only reduces the elastic stress level but also inhibits efficient stress propagation for triggering. In contrast, at the up-dip end, the proximity to the region of megathrust events, where no dislocation is either allowed or at least observed at interseismic periods, may also inhibit stress propagation, leading to a similar increase in the background fraction. These results suggest that spatial variations in both temperature–pressure conditions and frictional properties influence LFEs in a nonmonotonic manner along the dip direction.