Depth extent of the long-term slow slip event in the Tokai district, central Japan: A new insight

Temporal changes to interplate coupling and long-term aseismic slip in the Tokai Region of central Japan from July 1996 to June 2009 are statically inverted using continuous Global Positioning System (GPS) data and leveling observations, and examined using a new interpretation of aseismic slip. In this interpretation, the interplate coupling and the aseismic slip should be inferred and compared with reference to a state of no coupling on the plate interface rather than assuming steady state interplate coupling before the occurrence of the aseismic event as previous works have done. For this purpose, we used original time series data without subtracting any linear trends. The inferred distribution shows that the entire period can be divided into three subperiods based on the existence of the aseismic slip area from 2000 to 2005. The strength of the interplate coupling does not change throughout the observed period even though the distribution narrows along the dip direction after the aseismic slip terminates. The aseismic slip area inferred in this study, based on the no-coupling reference, was determined to be deeper than the area where the long-term slow slip event (SSE), defined as the slip corresponding to the residual displacement from the steady state, was hypothesized to have occurred in numerous previous studies. The total seismic moment released by the aseismic slip is equivalent to a M-w approximate to 6.6 earthquake while the SSE in previous works was considered to be equivalent to a M-w approximate to 7.0 to 7.1 earthquake. This difference can be attributed primarily to the effects on the interplate coupling, which previous studies did not take into account when defining the SSE. The aseismic slip area coincides well with the distribution of other types of plate interactions, such as short-term slow slip events and low-frequency earthquakes.