Comprehensive detection of shallow tremor activities in the Nankai subduction zone, Japan

The Nankai subduction zone is characterized by recurrent megathrust earthquakes with a return period of 90200 years, and the frequent occurrences of slow earthquakes in the deep extension of the frictionally locked seismogenic zone. Recent studies have revealed that slow earthquakes also occur in the shallow part of the Nankai subduction zone. Since the deployment of the Dense Oceanfloor Network system for Earthquake and Tsunami (DONET) offshore the Nankai Trough, several shallow tremor activities coincident with slow slip events (SSEs) have been identified. However, the entire activities have not been fully investigated. Here we present comprehensive shallow tremor activities along the Nankai subduction zone. We analyzed the long-term continuous ocean bottom seismometer (OBS) records of the DONET to detect and locate tremor events. The OBS records often include the noises from offshore marine explorations, such as air gun noises, which we detect as tremor events by mistake. We empirically remove such false detections based on the spectral features of artificial noises. We identified five major tremor burst episodes showing clear epicentral migrations, which infer the propagation of the background SSEs. The spatial distribution of the tremor episodes was quite complementary from each other, which indicates the tremor activities can work for useful stress indicators on the plate interface. We also detected the faster migration of tremor events across the shallow extension of the segment boundary of the historical Nankai and Tonankai earthquakes, which reflect the heterogeneous physical properties on the plate interface associated with the rupture segmentations of megathrust earthquakes.