The subduction zone slip budget in southwest Japan, accounting for coseismic slip and afterslip for the 1944 Tonankai and 1946 Nankai earthquakes

Abstract:

The slip budget at a subduction zone megathrust is met by a spectrum of slip behaviors including coseismic slip from great earthquakes, afterslip, episodic tremor and slip (ETS), long-term slow slip events, and steady interseismic creep. Because of widespread and long-term monitoring, Nankai subduction zone is currently one of the best study areas for full understanding of how the slip budget is partitioned along the interface and how the rheological conditions evolve through space and time. We present a slip budget for cross-sections across western Shikoku, eastern Shikoku, Kii Channel, and Kii Peninsula. We estimate coseismic slip during and afterslip following the 1944/46 Tonankai and Nankai earthquakes using a 3D iterative inversion of vertical surface displacement data. Additionally, we estimate interseismic creep rates using a viscoelastic earthquake cycle model. Summing the estimated coseismic slip, afterslip, and interseismic creep from this study with estimated slip during short-term and long-term slow slip events from previous studies, we find that the subduction interface near the eastern side of Shikoku and Kii Channel has nearly met the slip budget for a 150-year interseismic cycle. The interface near the western side of Shikoku has a slip deficit of nearly half the budget (4 meters out of a total 8.25 meters) above 20 km depth on the subduction interface and the interface near Kii Peninsula has a slip deficit of 3 meters out of a total 8.25 meters between 20-60 km depth on the subduction interface. A suite of postseismic models with variable elastic slab and plate geometry and mantle viscosity all show that sustained afterslip, lasting for at least 20 years, occurred within the ETS zone beneath eastern and western Shikoku, as well as beneath Kii Peninsula. Coseismic slip and afterslip make up 30-50 percent of the total slip budget in the ETS zone beneath Shikoku and Kii Peninsula.