Gravity change observed during 2004-2009 in the Tokai slow slip area and the possibility of detecting high-pressure-fluid flow

Slow-slip events (SSEs) have occurred at plate-subduction zones throughout the world. Previous studies have revealed that high-pressure fluids supplied from the subducted oceanic plate can generate SSEs. However, the behavior of these fluids during an SSE has not been fully described. In this paper, we discuss the possibility of fluid migration along the plate boundary based on spatiotemporal gravity changes observed by absolute and relative gravimeters during a long-term SSE in the Tokai district, Japan. Relative-gravity data is sometimes unreliable due to limited observation accuracy and possible noise produced by groundwater. Nevertheless, the observed gravity changes show a systematic spatial pattern over the slow-slip area. This pattern can be explained by a poroelastic model assuming fluid migration through the plate interface, for which an inversion indicates a permeability of about 10^-15 m^2. This permeability value is within the range of values inferred by other studies for slow-slip areas. Long-term SSEs have occurred repeatedly in the Tokai district. If the permeability is greater than 10^-15 m^2 during a future SSE, we can detect fluid migration by improving observation accuracy to the 1-μ Gal level and evaluating groundwater noise.