Interseismic deformation of the Nankai subduction zone, southwest Japan, inferred from three-dimensional crustal velocity fields

Abstract

We have studied crustal deformation in the Nankai subduction zone, southwest Japan, based on threedimensional 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.

Sea level change in the Gulf of Thailand from GPS-corrected tide gauge data and multi-satellite altimetry

“…the Nankai in southwest Japan. The Nankai subduction zone is one of the particularly appropriate examples because it has a short recurrence period (100-150 years) and a geodetic/seismological data set spanning almost a complete earthquake cycle (Tabei et al, 2007).…”

“…However, they clearly suggest a gradual decrease of vertical velocity in the far field. In the Nankai subduction zone, Tabei et al (2007) used data from an array of CGPS to generate a vertical velocity field of the crustal deformation. The distribution shows that as distance from the trough increases, the rates of vertical motion become less varied.…”

Characteristics of short‐term slow slip events estimated from deep low‐frequency tremors in Shikoku, Japan

“…Recent geodetic observations have revealed that the strength of the locking of the plate interface depends on the depth of the subducting plate. In the Nankai subduction zone, seismogenic subduction faults at a depth of 5–25 km are strongly locked; however, at the transition zone at a depth of 25–35 km, GPS observations revealed that the locking was weaker than that of the shallow plate interface [ Tabei et al , 2007]. …”

Average slip rate at the transition zone on the plate interface beneath the Kii Peninsula, Japan, estimated from deep low-frequency tremors

“…Geodetic observations have revealed that the strength of the locking of the plate interface depends on the depth of the subducting plate. In the Nankai subduction zone, seismogenic faults at depths of 5-25 km are strongly locked, although locking at the transition zone at depths of 25-35 km, where short-term SSEs and nonvolcanic DLF tremors are located, is weaker than at shallow depths (Tabei et al, 2007). A simulation study revealed that the recurrence interval of short-term SSEs becomes short as the occurrence of a great earthquake becomes imminent (Matsuzawa et al, 2010).…”

“…Hirose et al (2010) applied this method to the DLF tremors of the Shikoku region of southwestern Japan. They found that the average slip rate compensates for the difference between the convergence rate at the trench (Miyazaki and Heki, 2001) and the slip deficit rate at the transition zone (Tabei et al, 2007) of the subducting Philippine Sea plate.…”