Abstract
Abstract We have studied crustal deformation in the Nankai subduction zone, southwest Japan, based on three-dimensional GPS velocity fields. Oblique subduction of the Philippine Sea plate has caused two different modes of deformation of the overriding plate: interseismic crustal shortening in the direction of plate convergence, and permanent lateral movement of the forearc. The block boundary dividing the forearc is the Median Tectonic Line (MTL); however, we assumed that its shallower portion is fully or partially locked to a certain depth. The plate boundary and the MTL are represented by many rectangular faults. We carried out inversion analyses with a priori information to estimate simultaneously slip deficit rates at those rectangular faults, together with the rate of lateral movement of the forearc. The results show that the seismogenic subduction faults at a depth of 5–25 km are strongly locked. As for the transition zone at 25–35 km, the inversion analysis results in stronger locking than shown in previous studies, especially when the vertical velocity data are weighted. The rates of lateral forearc movement and slip deficit at the MTL are nearly comparable but in a reverse sense to each other. This shows that the shallower portion of the MTL is strongly locked but that stationary aseismic slip is occurring in the deeper part.
Highlights
Plate locking on the subduction thrust fault is the main source of deformation of an overriding continental margin during the interseismic period in the earthquake cycle at a convergent plate boundary
Interseismic crustal deformation in southwest Japan after the most recent events, the 1944 Tonankai (M = 8.0) and the 1946 Nankai (M = 8.2) earthquakes, has been dominated by interaction with the Philippine Sea plate that subducts beneath the southwest Japan arc at the Nankai Trough (Sagiya and Thatcher, 1999; Miyazaki and Heki, 2001)
Combining crustal velocities from dense campaign Global Positioning System (GPS) measurements with those from the GEONET, Tabei et al (2002) showed that oblique subduction of the Philippine Sea plate at the Nankai Trough has formed a mobile forearc block that is moving slowly parallel to the southwest Japan arc
Summary
Plate locking on the subduction thrust fault is the main source of deformation of an overriding continental margin during the interseismic period in the earthquake cycle at a convergent plate boundary. Interseismic crustal deformation in southwest Japan after the most recent events, the 1944 Tonankai (M = 8.0) and the 1946 Nankai (M = 8.2) earthquakes, has been dominated by interaction with the Philippine Sea plate that subducts beneath the southwest Japan arc at the Nankai Trough (Sagiya and Thatcher, 1999; Miyazaki and Heki, 2001). Combining crustal velocities from dense campaign GPS measurements with those from the GEONET, Tabei et al (2002) showed that oblique subduction of the Philippine Sea plate at the Nankai Trough has formed a mobile forearc block that is moving slowly parallel to the southwest Japan arc. The mobile forearc block may be a characteristic common to the modern oblique subduction zones worldwide (McCaffrey, 1996) It should be taken into consideration when interseismic crustal deformation in the oblique subduction zone is modeled, along with crustal shortening in the direction of plate convergence (Wang et al, 2003)
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