New Insights Into the Slip Budget at Nankai: An Iterative Approach to Estimate Coseismic Slip and Afterslip

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 (SSEs), and quasisteady interseismic creep. Long‐term geodetic monitoring at the Nankai subduction zone in southwest Japan has recorded deformation observations spanning nearly a complete earthquake cycle, making this region the best study area for obtaining a full understanding of how the slip budget is partitioned along the interface and how the rheological conditions evolve through space and time. In this work, we conduct an iterative inversion of vertical surface displacement data to jointly estimate coseismic slip during the 1944/46 Tonankai and Nankai earthquakes, spatiotemporal afterslip history following those earthquakes, and postseismic mantle flow. A suite of postseismic models with variable elastic slab and plate geometry and mantle viscosity all show at least 30 years of sustained afterslip within the ETS zone and the gap between the top of the ETS zone and the fully coupled zone. This result demonstrates that stable, long‐duration afterslip is possible in the ETS zone. Summing the estimated cumulative slip from this study with estimates from previous studies of cumulative slip during the interseismic period, including episodic SSEs, we find that eastern Shikoku island has nearly met all of its slip budget for a 150‐year interseismic cycle, whereas western Shikoku island has a slip deficit of nearly half the budget (4 out of 8.25 meters) above 20 km depth on the subduction interface. Key Points The slip budget is nearly met in eastern Shikoku and there is a slip deficit of nearly half the budget, above 20 km depth, in western Shikoku. Postseismic models require at least 30 years of afterslip in the episodic tremor and slip (ETS) zone and the gap between the ETS zone and the fully coupled zone Coseismic slip and afterslip account for 25%–37% of the slip budget within the ETS zone