[J06-2] The spectrum of fault-zone deformation processes (from slow slip to earthquake) II

[J06-2-02] Anomalous gravity changes observed during long-term slow slip events and a possible interpretation based on fluid flow

^○Yoshiyuki Tanaka¹, Yuichi Imanishi¹, Shuhei Okubo¹, Kazunari Nawa², Yoshiaki Tamura³, Kenji Yoshida⁴ (1.Earthquake Research Institute, The University of Tokyo, Tokyo, Japan, 2.Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan, 3.Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, Iwate, Japan, 4.Geodetic Department, The Geospatial Information Authority of Japan, Tsukuba, Japan)

Slow-slip events (SSEs) have been observed in many plate boundary zones along the circum-Pacific seismic belt. 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 elucidated. In this presentation, we discuss a possibility of fluid migrations along the plate interface on the basis of spatiotemporal gravity changes observed by absolute and superconducting gravimeters during recent long-term SSEs in Japan, including those in the Tokai district, the Bungo Channel, and the Yaeyama Islands. The limited spatiotemporal resolutions of the gravity data and uncertainties due to hydrological disturbances preclude us from attributing the gravity changes to the SSEs uniquely. However, after removing the apparent effects due to vertical deformation of the observation sites, there still remain anomalous gravity changes with the order of 1 microGal (10^-8 ms^-2) over the periods of the SSEs. A simple fluid flow model through a fault fracture zone employing Darcy's law predicts gravity variation of 1 microGal/year for a change of 0.6 MPa (=0.1% of the lithostatic minus hydrostatic fluid pressure at a depth of 30 km) and a permeability value of 2 x 10^-11 m^2. The latter is a lower limit for which the rupture becomes slow, obtained by a numerical simulation based on a poroelasticity theory (Yamashita, 2013). These results indicate that the observed anomalous gravity changes can be associated with fluid pressure variations in the slow slip areas. We will continue gravity observations, which is needed to exclude possibilities that hydrological disturbances coincidentally cause these anomalies.