Source Characteristics and Along-Strike Variations of Shallow Very Low Frequency Earthquake Swarms on the Nankai Trough Shallow Plate Boundary

We detected shallow very low frequency earthquakes (VLFEs) off the Cape Muroto and Kii Channel in the Nankai subduction zone and estimated their moment rate functions. Combining the new and previously estimated catalogs, we obtained the comprehensive catalog of shallow VLFE moment rate functions along the Nankai Trough. We defined the shallow VLFE swarms and investigated the scaling relationships of their cumulative moments, activity area, and durations in each region. Detected swarms were considered candidates for shallow slow slip events. A similar scaling relationship was observed between the cumulative moments and activity areas, irrespective of regions. It indicates similar stress drops in each region. However, the relationship between the cumulative moments and durations varied. This difference was explained by the along-strike variations in the faulting conditions of shallow slow earthquakes, such as material or hydrological properties.

Automated summary

Shallow very low frequency earthquakes (VLFEs) were detected off the Cape Muroto and Kii Channel in the Nankai subduction zone, and their moment rate functions were estimated. The swarms of VLFEs were considered as candidates for shallow slow slip events, and they exhibited similar scaling relationships between cumulative moments and activity areas, but varied in the relationship between cumulative moments and durations, which could be explained by along-strike variations in faulting conditions.