Spatiotemporal evolution of aseismic interplate slip between 1996 and 1998 and between 2002 and 2004, in Bungo channel, southwest Japan

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Interplate aseismic slip in the Bungo channel area, southwest Japan, occurred during 1996–1998 and 2002–2004. We employed a square root information filter following the time‐dependent inversion technique to time series of daily coordinates of global positioning system (GPS) sites. The results show that aseismic interplate slip occurred beneath the southwestern part of Shikoku Island from late 1996, then shifted southwestward to the Bungo channel area from May 1997 to February 1998, and subsided over time with a moment magnitude (Mw) of 7.1. In contrast, the 2002–2004 event shows slip in the area beneath the Bungo channel in an early stage from March 2003 to August 2003. From August 2003 to January 2004, the slip area expanded by around 30 km northeastward to the southwestern part of Shikoku Island and the northern part of the Bungo channel, and then decayed over time with Mw reaching 7.1. In the 2002–2004 event, the start of the northeastward expansion and acceleration of the aseismic slip coincided with the occurrence of a short‐term slow slip event and low‐frequency earthquakes in a deep area. This is a different spatiotemporal evolution from that of the 1996–1998 event, despite having occurred in almost the same area with similar moment magnitudes. A balance of slip deficit and forward aseismic slip on the plate boundary suggests that part of the Bungo channel area releases strain energy mainly by episodic long‐term slow slip events.