Over the past two decades, we witnessed a surge of great earthquakes worldwide. The majority of these earthquakes are due to the rupture of subduction interface faults, referred to as megathrusts. The slip behavior of megathrusts and the viscoelastic rheology of Earth’s mantle govern crustal deformation throughout the subduction earthquake cycle. In this talk, I will present my work on two topics: (1) coseismic and postseismic slip of the shallowest segment of megathrusts and (2) postseismic deformation following great subduction earthquakes controlled by mantle viscoelasticity.

On the first topic, I will give two examples showing contrasting mechanical behavior of the shallow megathrusts at two different margins. By modeling high-resolution cross-trench bathymetry surveys before and after the 2011 Mw 9.0 Tohoku-oki earthquake, we determine the magnitude and distribution of coseismic slip for the most near-trench portion of the megathrust. The inferred > 60 m average slip and a gentle trenchward slip increase indicate moderate degree of coseismic weakening of the shallow fault. Using near-trench seafloor and sub-seafloor fluid pressure variations as strain indicators in conjunction with land-based geodetic measurements, we determine coseismic-slip and afterslip distributions of the 2012 Mw 7.6 Costa Rica earthquake. Here, trench-breaching slip similar to the Tohoku-oki rupture did not occur during the earthquake, but substantial afterslip extended to the trench axis after the earthquake, exhibiting a velocity-strengthening behavior. These two contrasting examples bracket a possibly wide range of slip modes of the shallow megathrust. They help us understand why large tsunamis are generated by some but not all subduction earthquakes.

On the second topic, I will start again with the 2011 Tohoku-oki rupture. Due to the asymmetry of megathrust rupture, with the upper plate undergoing greater coseismic tension than the incoming plate, viscoelastic stress relaxation causes the trench and land areas to move in opposite, opposing directions immediately after the earthquake. Seafloor geodetic measurements following the 2011 Tohoku-oki earthquake, modeled in my work, provided the first direct observational evidence for this effect. Systematic modeling studies in my work suggest that such viscoelastic opposing motion should be common to all Mw ≥ 8 subduction earthquakes. As the effect of viscoelastic relaxation decays with time and the effect of fault relocking becomes increasingly dominant, the dividing boundary of the opposing motion continues to migrate away from the rupture area. Comparative studies of ten 8 ≤ Mw ≤ 9.5 subduction earthquakes in my work quantifies the primary role of earthquake size in controlling the “speed” of the evolution of this deformation. Larger earthquakes are followed by longer-lived opposing motion that affects a broader region of the upper plate.