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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.

Thailand is surrounded by seismically active plate boundary zones and large-offset crustal faults, but there is comparatively little seismic activity within its borders. On 5 May 2014, a moment magnitude (MW) 6.2 earthquake occurred in the Mae Lao district of northern Thailand. To better anticipate future seismic hazards from events like this, we model the state of stress in Thailand and its vicinity using a finite element approach. We identify northern Thailand as a region with significant stress accumulation, consistent with being caused by convergence at the Sumatra-Andaman subduction zone and southward escape of the Himalayan Orogen. The stress field in this region is compatible with the kinematics of the Mae Lao event. Our modeling also shows that the magnitude of the stress field throughout southeast Asia is particularly sensitive to the stage of the earthquake cycle at the Sumatra-Andaman subduction zone, although the Sumatra-Andaman subduction zone produces a smaller effect on the orientations of stresses in the source region of the Mae Lao earthquake. Finally, we find that the stress change effect from the Mae Lao earthquake is geographically limited and the broader region remains in nearly the same state of stress as prior to the event.

The distribution of slip during subduction megathrust earthquakes depends on the slip deficit that accumulates on the plate interface prior to the event. We develop 3-D finite element models of subduction zones to investigate how locked zones restrict surrounding regions on the plate boundary from sliding. What is new is that we quantify the slip around asperities on the megathrust. The models show plate interface slip increasing from zero at the edge of a locked zone to the relative plate motion over a distance of ~200 km along the megathrust. This area of reduced slip accumulates a seismic moment deficit up to 10 times larger than the moment deficit in the asperity alone. Updip of locked areas, slip at the trench can be reduced by more than 50% of the plate motion. Despite large displacements of the upper plate near the trench, this region moves as a semirigid block. Rupture models of the 2011 Tohoku earthquake, its tsunami characteristics, and geophysical observations near the trench can be interpreted to reflect the consequences of slip deficit accumulated on a low friction interface updip of the seismogenic zone. Neighboring asperities affect plate interface slip in a nonlinear way. Multiple asperities have overlapping pseudo-coupled regions that may restrict the magnitude of coseismic slip in single-asperity ruptures. Once an earthquake has a rupture length greater than ~250 km, it may recover the entire accumulated slip deficit. This is consistent with the magnitude of coseismic slip in several recent great megathrust earthquakes.

We constrain epicentroid locations, magnitudes and depths of moderate-magnitude earthquakes in the 2013-2014 Minab sequence using surface-wave cross-correlations, surfacewave spectra and teleseismic body-wave modelling. We estimate precise relative locations of 54 Mw ≥ 3.8 earthquakes using 48 409 teleseismic, intermediate-period Rayleigh and Lovewave cross-correlation measurements. To reduce significant regional biases in our relative locations, we shift the relative locations to align the Mw 6.2 main-shock centroid to a location derived from an independent InSAR fault model. Our relocations suggest that the events lie along a roughly east-west trend that is consistent with the faulting geometry in the GCMT catalogue. The results support previous studies that suggest the sequence consists of leftlateral strain release, but better defines the main-shock fault length and shows that most of the Mw ≥ 5.0 aftershocks occurred on one or two similarly oriented structures. We also show that aftershock activity migrated westwards along strike, away from the main shock, suggesting that Coulomb stress transfer played a role in the fault failure. We estimate the magnitudes of the relocated events using surface-wave cross-correlation amplitudes and find good agreement with the GCMT moment magnitudes for the larger events and underestimation of small-event size by catalogue MS. In addition to clarifying details of the Minab sequence, the results demonstrate that even in tectonically complex regions, relative relocation using teleseismic surface waves greatly improves the precision of relative earthquake epicentroid locations and can facilitate detailed tectonic analyses of remote earthquake sequences.