Quasi‐periodic slow slip events in the afterslip area of the 1996 Hyuga‐nada earthquakes, Japan

The time evolution of afterslip on a plate boundary experiencing interplate earthquakes is expected to show logarithmic decay. The global positioning system network in Japan has been monitoring transient deformation since the occurrence of two large interplate earthquakes with moment magnitudes of 6.8 and 6.7 in the Hyuga‐nada area, southwest Japan, in 1996. The spatial and temporal evolution of aseismic interplate slip based on crustal deformation data indicates that afterslip followed the two earthquakes and gradually declined to background rates by around 2004 with total moment magnitude of 7.3. However, quasi‐periodic slow slip events suddenly began within the afterslip area in 2005 with approximately one year duration and two‐year recurrence interval. The moment magnitudes of the three slow slip events since January 2005 range from 6.7 to 6.8. This differs greatly from the expected behavior of logarithmic decay over time. Both velocity‐strengthening and velocity‐weakening rate‐and‐state modes have been implicated as the cause of afterslip, whose location is complementary to the main shock area of velocity‐weakening, while a slow slip event occurs in the velocity‐weakening area with different frictional properties from those of an afterslip area. In light of the seemingly different frictional properties, the coexistence of afterslip and slow slip events in the same area would provide additional information about precisely how the plate interface is behaving. The monitoring of these slow slip events should give the clues to understanding the coexistence of long‐term afterslip and slow slip events and the increasing risk of earthquakes in neighboring areas.