Seismic energy radiation and along-strike heterogeneities of shallow tectonic tremors at the Nankai Trough and Japan Trench

Shallow slow earthquakes have been documented along shallow plate interfaces near trenches. Recent geophysical observation networks located offshore of Japan enable us to analyze shallow tremors in the Nankai Trough and the Japan Trench. Onshore seismic stations are also important for detecting shallow very low frequency earthquakes (VLFEs) and for evaluating their seismicity prior to the deployment of offshore observation networks. This study analyzes data from ocean bottom seismometers to estimate the seismic energy radiation of such tremors, from which we observe along-strike heterogeneity. Tremors with higher seismic energy rates tend to have longer recurrence intervals, which has also been observed for deep tremors. This study also estimates seismic moment releases of shallow VLFEs at the Japan Trench observed by onshore seismic stations using Green's function in a local three-dimensional seismic velocity model. The scaled energy, which is the ratio of the seismic energy (rate) to the seismic moment (rate), is similar to 10(-9). Shallow slow earthquakes located off the Kii Peninsula and Cape Muroto in the Nankai Trough have higher values (10(-9)-10(-8)), while shallow slow earthquakes located off the Kii channel in the Nankai Trough and in the Japan Trench have lower values (10(-10)-10(-9)), which are similar to the values estimated for deep slow earthquakes. By comparing the cumulative seismic energy of the shallow tremors with the cumulative seismic moment of shallow VLFEs, we evaluate the monitoring of shallow tremors and shallow VLFEs in the Japan Trench, suggesting that monitoring shallow VLFEs based on data from onshore seismic stations could miss many (similar to 90% at most) of the seismic moment releases by small magnitude but frequent events.