Stress evolution during the megathrust earthquake cycle and its role in triggering extensional deformation in subduction zones

Abstract

Great (moment magnitude Mw ∼8.0 and larger) subduction megathrust earthquakes are commonly followed by increased rates of normal faulting seismicity. Extensional activity within the subducting slab is amplified when megathrust slip propagates close to the trench, and forearc extension is triggered by the largest magnitude (Mw 8.5 and larger) events. To better understand these observations, we develop an earthquake cycle model with a realistic slab geometry and stresses that are in balance with plate interface slip and bulk viscous relaxation. The modeled stresses represent perturbations to the long-term background tectonic stress field. The steady-state inter-seismic earthquake cycle stresses are compressive and their magnitudes depend on the interface locking configuration, from 25 MPa for a fully locked seismogenic zone to 5 MPa for discrete asperities on an otherwise unlocked plate interface. The co-seismic slip and corresponding co-seismic stress changes are similar in these models, independent of the locking configuration. The co-seismic stress change magnitudes are up to 5 MPa. This implies that the earthquake cycle stresses after the mainshock would still be compressive in the case of a continuous seismogenic zone, but would be widely reset to zero in the case of discrete asperities. Models with co-seismic slip confined to shallow depths produce tensional stress changes in the slab and the forearc near the trench, whereas events rupturing the base of the seismogenic zone produce tensional stress changes limited to region immediately surrounding the rupture. The locations of normal faulting aftershocks generally correlate well with the tensional co-seismic stress changes greater than 1 MPa. Following the mainshock, rapid afterslip occurring down-dip of the megathrust rupture expands the region of relative extension, but reduces its magnitude and does not promote additional normal faulting events. Bulk viscous relaxation does little to the state of stress in the elastic parts of the model. Continued plate convergence across a re-locked interface returns the system to the pre-earthquake state of compression.