Characteristics and interactions between non-volcanic tremor and related slow earthquakes in the Nankai subduction zone, southwest Japan

Non-volcanic tremor and related slow earthquakes in subduction zones are one of the most significant and exciting geophysical discoveries of the 21st century. In Japan, some types of slow earthquakes associated with subduction of the Philippine Sea Plate have been detected by dense seismic and geodetic observation networks equipped with continuous data-recording systems. At the deepest part of the transition between the Nankai megathrust seismogenic zone and the deep stable sliding zone, short-term slow slip events (SSE) occur on the plate interface with durations of days, accompanied by tremor and deep very-low-frequency (VLF) earthquakes resulting from interplate shear stick-slip motions. Along-strike source regions of tremor are divided into segments where these three coupling phenomena (tremor, short-term SSEs, and deep VLF earthquakes) occur at regular recurrence intervals, with durations of 2–6 months. On the updip side of the tremor zone, long-term SSEs with durations of years occur at intervals of 5–10 yrs and trigger tremor at the downdip part of the source region of long-term slip. Near the Nankai trough, shallow VLF earthquakes occur in the accretionary prism. At the eastern edge of the subducting Philippine Sea Plate, short-term SSEs recur every 6 yrs, associated with active earthquake swarms. Some of these slow earthquakes have been detected in other subduction zones; however, the properties of each constituent member of slow earthquakes are different in each subduction zone. Slow earthquakes represent transient shear slip around the seismogenic portion of major interplate megathrust faults; therefore, monitoring the relationship between slow earthquakes and interplate megathrust earthquakes is important for intermediate- and long-term predictions of the next major earthquake.