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. Plain Language Summary Slow earthquakes are characterized as slips much slower than similar‐size regular earthquakes. Although interactions between the shallow slow earthquakes and large tsunamigenic earthquakes have often been discussed, our knowledge of the source characteristics and spatial variations of the shallow slow earthquakes is still limited. In this study, we quantitatively investigated the activity characteristics of shallow, very low frequency earthquakes (VLFEs) along the Nankai Trough. Activity areas and released cumulative moments of shallow VLFE swarms exhibited a similar scaling law irrespective of regions. However, the duration and cumulative moments of the swarms varied in each region. These characteristics can provide key information on the faulting conditions of slow earthquakes in shallow plate boundaries. Key Points Comprehensive detection and source parameter estimations of shallow very low frequency earthquake swarms along the Nankai Trough The scaling relationship between the activity areas and cumulative moments of the swarms roughly follows Mo ∝ A3/2 Along‐strike variations of the scaling law for swarm durations reflect differences in the faulting conditions of slow earthquakes