Large subduction earthquakes cause perturbation of the subduction system, whose response depends on its rheological characteristics. The evolution of the subduction system from earthquake to earthquake is defined as subduction earthquake cycle (SEC), and is characterised by three main processes that operate over decades/centuries: postseismic afterslip, viscoelastic mantle relaxation, and fault relocking.
Despite the precise and global coverage of modern geodetic systems, the full SEC of a single subduction zone is still hindered by the short instrumental time compared to the centuries-long recurrence interval of subduction earthquakes. Wang et al. demonstrate in their review article how to overcome this limitation by swapping space for time. By putting together ‘time snapshots’ of several subduction zones at different phases of their SEC, they obtain a full picture of a subduction system, including short-term (1 yr) and long-term (10-100 yr) effects of the mantle behaviour.
This idea followed the discovery of a common pattern of surface velocities of the upper plate among several subduction zones (Fig. 1), following a large subduction earthquake (e.g., Sumatra 2004; Alaska 1964; Chile 1960; Cascadia 1700). Indeed, GNSS velocities show that (numbering refers to Fig. 1): 1) immediately after the earthquake, the surface undergoes seaward motion; 2) some time after the earthquake, a progressive reversal of direction starts at the rupture zone: this results in a temporal opposite motion of coastal (landward) vs. inland (seaward) areas; 3) when the reversal of motion has propagated inland and a wholesale landward motion direction is observed, the fault is locked.
Figure 1: Three primary processes after a subduction earthquake. (1) Aseismic afterslip occurs mostly around the rupture zone, (2) the coseismically stressed mantle undergoes viscoelastic relaxation, and (3) the fault is relocked. Arrows at the top show the sense of horizontal motion of Earth’s surface, relative to distant parts of the upper plate, caused by each of these three processes. From Wang et al., 2012.
Such a gradual reversal of motion suggests a major role of viscoelastic mantle relaxation, which appears to be faster than expected from the traditionally used fault-locking elastic models, reflecting very low mantle wedge viscosity (Wang et al., 2012 and references therein).
In order to reproduce these conditions, the authors use a SEC numerical model where a bi-viscous rheology is used to obtain two timescales of viscoelastic relaxation of the earthquake-induced stress: a shorter one for the transient viscosity and a longer one for the steady-state viscosity. They then observe how steady-state relaxation, transient relaxation, and characteristic afterslip evolve with time after a subduction earthquake, and how the results compare with observations from natural subduction zones worldwide.
Results show that afterslip and transient rheology dominate, as expected, in the immediate post-seismic period (years), causing rapid seaward motion or the surface; further ahead in the seismic cycle (decades after the earthquake), locking prevails in correspondence to the rupture, resulting in landward motion of coastal areas, while sites far from the rupture are still dominated by relaxation and seaward motion. Locking is the main process driving the upper plate landwards, only when earthquake stresses have been completely relaxed, centuries after the event.
Despite not taking into account the along-strike locking heterogeneity affecting the interseismic deformation pattern, and many other parameters such as plate convergence rate, structural/geometrical features, role of fluids, etc, this model successfully reproduces the observed surface velocity patterns. However, the omissions of the complexities of real-world subduction systems indicate that future research must include these factors for deeper insights into SEC processes.
The scaling relationship of steady-state and transient relaxation with the seismic moment reflects the dependence of relaxation time from the initial stress perturbation, represented by the fault rupture, i.e. larger earthquakes will have longer relaxation times, and fault locking will dominate later in the SEC. This study invalidates “a popular belief that interseismic deformation is a subdued mirror image of coseismic deformation” (Wang et al., 2012), and confirms the strongly time-dependent role of SEC processes. This contribution represents the opening venue to a large number of studies exploiting geodesy to understand the interseismic phase and the seismic cycle (e.g., Li & Chen, 2023), including searching for precursors of large subduction zone earthquakes (e.g., Bedford et al., 2020; Bletery & Nocquet, 2023).
Silvia Crosetto, and the TS Must Read team
References
Bedford, J. R., Moreno, M., Deng, Z., Oncken, O., Schurr, B., John, T., … & Bevis, M. (2020). Months-long thousand-kilometre-scale wobbling before great subduction earthquakes. Nature, 580(7805), 628-635.
Bletery, Q., & Nocquet, J. M. (2023). The precursory phase of large earthquakes. Science, 381(6655), 297-301.
Li, S., & Chen, L. (2023). How Long Can the Postseismic and Interseismic Phases of Great Subduction Earthquake Sustain? Toward an Integrated Earthquake‐Cycle Perspective. Geophysical Research Letters, 50(11), e2023GL103976.
Wang, K., Hu, Y., & He, J. (2012). Deformation cycles of subduction earthquakes in a viscoelastic Earth. Nature, 484(7394), 327-332.
