The thermal structure of the lithosphere controls many properties and processes of Earth's crust. The total ∼47-TW heat loss of Earth is key to understanding and modeling this thermal structure, as is partitioning the various sources of that heat into heat entering the base of the lithosphere, heat generated within the lithosphere by radioactive decay (primarily within the continental crust), and secular cooling of the mantle lithosphere (primarily in oceanic lithosphere). A set of framework geotherms for the continental lithosphere explains deep crustal melting in high heat flow regions, metamorphic pressure-temperature (P-T) space in the crust, partial melting at the base of the lithosphere to produce an S-wave low-velocity zone in Phanerozoic and younger terranes, and the P-T fields inferred from mantle xenoliths. Important perturbations to a standard thermal state are produced by orogenic overprints, transient thermal regimes, and exhumation.

Recent megathrust events in Tohoku (Japan), Maule (Chile), and Sumatra (Indonesia) were well recorded. Much has been learned about the dominant physical processes in megathrust zones: (partial) locking of the plate interface, detailed coseismic slip, relocking, afterslip, viscoelastic mantle relaxation, and interseismic loading. These and older observations show complex spatial and temporal patterns in crustal deformation and displacement, and significant differences among different margins. A key question is whether these differences reflect variations in the underlying processes, like differences in locking, or the margin geometry, or whether they are a consequence of the stage in the earthquake cycle of the margin. Quantitative models can connect these plate boundary processes to surficial and far‐field observations. We use relatively simple, cyclic geodynamic models to isolate the first‐order geodetic signature of the megathrust cycle. Coseismic and subsequent slip on the subduction interface is dynamically (and consistently) driven. A review of global preseismic, coseismic, and postseismic geodetic observations, and of their fit to the model predictions, indicates that similar physical processes are active at different margins. Most of the observed variability between the individual margins appears to be controlled by their different stages in the earthquake cycle. The modeling results also provide a possible explanation for observations of tensile faulting aftershocks and tensile cracking of the overriding plate, which are puzzling in the context of convergence/compression. From the inversion of our synthetic GNSS velocities we find that geodetic observations may incorrectly suggest weak locking of some margins, for example, the west Aleutian margin.

In the 1970s, the Shumagin Islands region of the Alaska subduction zone was identified as a seismic gap expected to host a future great [moment magnitude (M_w) ≥8.0] earthquake. More recent geodetic data indicate that this region is weakly coupled, and the geologic record shows little evidence of past large events. From July to October 2020, a series of earthquakes occurred in this region, raising the possibility of greater coupling. The initial M_w 7.8 thrust faulting earthquake straddled the eastern edge of the Shumagin Gap and was followed by an M_w 7.6 strike-slip earthquake within the Shumagin Gap. Stress modeling indicates that this strike-slip earthquake is in fact favored if the Shumagin Gap has low coupling, whereas a highly coupled Shumagin Gap inhibits that type and location of earthquake. The initial thrust earthquake and its afterslip enhanced the strike-slip loading within the subducting slab, helping to trigger the October event.

On 1 April 2007 a great, tsunamigenic earthquake (moment magnitude 8.1) ruptured the Solomon Islands subduction zone at the triple junction where the Australia and Solomon Sea–Woodlark Basin plates simultaneously underthrust the Pacific plate with different slip directions. The associated abrupt change in slip direction during the great earthquake drove convergent anelastic deformation of the upper Pacific plate, which generated localized uplift in the forearc above the subducting Simbo fault, potentially amplifying local tsunami amplitude. Elastic deformation during the seismic cycle appears to be primarily accommodated by the overriding Pacific forearc. This earthquake demonstrates the seismogenic potential of extremely young subducting oceanic lithosphere, the ability of ruptures to traverse substantial geologic boundaries, and the consequences of complex coseismic slip for uplift and tsunamigenesis.

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.