Results of absolute gravity measurements in the Tokai area from November 1996 to February 2012

Gravity changes observed with FG5 absolute gravimeters during the period 1996-2012 in the Tokai area are reported. There are three observation sites in the area, where gravity changes were measured once or twice a year. In Omaezaki, located at the cusp of the Peninsula, gravity increased 10 microGals over 16 years. In Kikugawa, at the center of the Peninsula, gravity increased 5 microGals from 2008to 2012. In Toyohashi, located to the west of lake Hamana-ko, where the long-term slow slip began in 2000, gravity decreased 6 microGals from 2004 to 2012. The rates of these gravity changes do not agree completely with those inferred from crustal uplift/subsidence observed by GPS, implying the possibility that mass redistributions are occurring underground. Monitoring and modeling those mass redistributions are important to understand inter-seismic crustal activities including slow slips, and to reveal stress accumulation processes in a subduction zone