Abstract
Long-term slow slip events occur adjacent to the seismogenic zones of major interplate earthquakes along the Nankai Trough in southwestern Japan. GNSS observations began about 20 years ago, but this is much shorter than the cycle of major interplate earthquakes. Knowing past long-term slow slips gives important information for understanding the preparation process of major earthquakes along the Nankai Trough. We extracted an unsteady component of vertical crustal deformations in the Kii Peninsula before GNSS deployment using leveling and sea-level data. GNSS-derived, steady-state vertical displacements related to ongoing subduction of the Philippine Sea Plate were subtracted from the vertical displacements observed by leveling surveys. Monthly mean sea-level data were corrected for atmospheric pressure and hydrographic effects. As a result, postseismic deformation of the 1944 Tonankai and the 1946 Nankai earthquakes was seen from 1972 to 1980. No unsteady component of vertical crustal deformation exceeding 3 cm was found from 1980 to 2009 in the Kii Peninsula. This means that a long-term slow slip event with at least more than moment magnitude (Mw) 6.5 may have not occurred in this period along the southeastern coast of the Kii Peninsula.
