Slip‐Deficit Rate Distribution Along the Nankai Trough, Southwest Japan, With Elastic Lithosphere and Viscoelastic Asthenosphere

Abstract

In southwest Japan, great earthquakes have occurred on the plate interface along the Nankai trough with a recurrence time of approximately 100 years. Most previous studies estimated slip deficits on the seismogenic zone from interseismic Global Navigation Satellite System (GNSS) velocity data assuming slip‐response functions for an elastic medium. The observed surface velocities, however, include effects of viscoelastic relaxation in the asthenosphere caused by slip motion associated with seismic cycles. If the elastic responses cannot adequately approximate the deformation of the viscoelastic medium, the results of an inversion analysis could be biased. If the recurrence interval is greater than the effective relaxation time in the elastic‐viscoelastic layered structure, we were able to formulate an inverse problem for the estimation of slip‐deficit rates from GNSS velocity data with completely relaxed slip‐response functions for a later stage of the seismic cycle. Analyzing surface velocity data from GNSS daily coordinate data between March 2005 and February 2011 together with seafloor geodetic data, we estimated the slip‐deficit rate distribution by the strain data inversion method. There were significant differences between the results using elastic and completely relaxed responses. Although the result with elastic responses shows a high coupling zone in the coastal region, it was located trenchward when completely relaxed responses were used. We found that the peak slip‐deficit rate increases as the thickness of the lithosphere becomes thinner. Moreover, we succeeded in appropriately separating elastic‐viscoelastic deformation due to plate coupling from rigid block motion.

Centroid Moment Tensor Inversion of Shallow Very Low Frequency Earthquakes Off the Kii Peninsula, Japan, Using a Three‐Dimensional Velocity Structure Model

“…By combination of a suitable 3-D model and onshore broadband network, we confirmed that seismic slips due to SVLFEs repeatedly occurred on or near the PHS boundary from 2004 to 2016. The centroid locations of SVLFEs are also correlated with the zone of relatively high slip deficit rate (e.g., Noda et al, 2017;Yokota et al, 2016). Detected low-velocity zones within the accretionary prism toes, which may be characterized by high pore fluid pressure, correlated with SVLFE activity in 2015 determined by DONET stations (Nakano et al, 2016).…”

“…Our CMT inversions also revealed a spatial relationship between the occurrence of SVLFEs and the slip deficit rate. The centroid locations of SVLFEs are also correlated with the zone of relatively high slip deficit rate (e.g., Noda et al, 2017;Yokota et al, 2016). We focused our attention on SVLFE activity in April 2016.…”

“…In the Nankai subduction zone, southwest Japan, megathrust earthquakes occur repeatedly at intervals of approximately 100-150 years (e.g., Ando, 1975) due to subduction of the Philippine Sea plate (PHS) at a rate of 2-6 cm/year (e.g., Heki & Miyazaki, 2001;Seno et al, 1993). Recent observations have illustrated the spatial variation of slip deficits around the Nankai subduction zone (e.g., Noda et al, 2017;Yokota et al, 2016). On the basis of the slip deficit rate and friction law, tsunamigenic megathrust earthquake scenarios have been proposed (e.g., Hok et al, 2011;Hori et al, 2004).…”

Migrations and Clusters of Shallow Very Low Frequency Earthquakes in the Regions Surrounding Shear Stress Accumulation Peaks Along the Nankai Trough

“…TAKEMURA ET AL. 11,836 (Noda et al, 2018), where it can be seen that SVLFEs mostly occur in regions with a shear stress change rate of 0-5 kPa/y. The SVLFEs off Cape Muroto are concentrated in regions with a negative shear stress change rate, but this activity area is adjacent to a stress accumulation peak (>10 kPa/y) off Cape Muroto.…”

Shear Strain Energy Change Caused by the Interplate Coupling Along the Nankai Trough: An Integration Analysis Using Stress Tensor Inversion and Slip‐Deficit Inversion

“…Slip‐deficit rate distribution and shear stress change at the depth of 10 km. (a) Slip‐deficit rate distribution at the plate interface along the Nankai Trough (Noda et al, ). The blue and red color scales show slip‐deficit and slip‐excess rates at intervals of 20 mm/year, respectively.…”