SZ4D International Webinars: Short-Term Interaction between Silent and Devastating Earthquakes, and the Role of Interplate Velocity Changes in the Mexican Megathrust
Víctor M. Cruz-Atienza - Instituto de Geofísica, UNAM, México
October 9th, 2020 at 11 AM PDT
Interaction between silent (slow) and devastating earthquakes is one of the most outstanding theories behind the plethora of observations made in the last two decades of the plate interface behavior in subduction zones. The will to re-evaluate seismic hazard from a new perspective is reborn from the discovery of slow slip events (SSE). The role of SSEs in the seismic cycle has been identified as preponderant in the initiation of some megathrust earthquakes. Observations also show that transient waves from teleseismic or regional earthquakes may trigger SSEs and tectonic tremor, which are two closely related phenomena in active faults. Highly pressurized fluids where slow earthquakes happen make frictional conditions very sensitive to small stress or strain
perturbations, thus playing an important role in the generation of slow earthquakes and, certainly, in their interaction with devastating events.
Seismicity rate varies over time and depends on changes in both the state of stress and properties of the solid Earth. The diversity of earthquakes discovered in recent years, together
with new observations of very small transient variations in the crustal properties, offer an unprecedented perspective for exploring causality between different seismotectonic processes. Recent laboratory experiments and theoretical fault models strongly suggest that friction is a sensitive function of the interplate slip-rate where SSEs occur. Slow-slip dynamic instabilities therefore depend on the velocity field discontinuity at the interface (v), which is zero only where both plates are completely locked (i.e. in seldom cases). Since plate interface coupling (PIC, kinematically defined as 1 – v / b, where v is the interplate slip-rate, b is the plate convergence rate and v b) varies over time, creep-rate changes may thus have important implications in the dynamic stability of the megathrust not only because of their frictional counterparts, but also due to the associated stress changes. In fact, from a kinematic point of view, whether a fault undergoes a coupling regime or a SSE it is a matter of definition. We often say that a SSE occur when v > b, which is reasonable because of the associated dynamic implications in terms of stress built-up (coupling regime) or stress release (SSE). However, the mechanical (and thus dynamic) conditions driving the system kinematics (e.g. frictional constitutive parameters, fault-zone rigidity and strengthening dilation, fluids and fault geometry), may significantly vary from one subduction zones to another, reason why SSEs (and slow earthquakes in general) behave so differently across the globe. Systematic and continuous observation of v (i.e. PIC and SSE simultaneously) over the plate interface is thus relevant to confirm or reject theoretical predictions of the interface dynamics, which is our only mean (in the broadest epistemological sense) to understand the underlying physics in subduction systems.
The Mexican subduction zone is one of the most intriguing regions on the globe. Multiple studies in the region have introduced new models and key observations to understand these phenomena that are present in other subduction zones. The recent devastating earthquakes in Mexico of 2017 (Mw8.2 and Mw7.1), 2018 (Mw7.2) and 2020 (Mw7.4), along with an unprecedented sequence of SSEs in Guerrero and Oaxaca provide an exceptional opportunity to test different hypothesis and thus contribute to our understanding of the seismic cycle and its dramatic consequences on society. By means of ELADIN (ELastostatic ADjoint INversion method; Tago et al., GJI, 2020), we have reconstructed the history of v (i.e. SSEs and PIC simultaneously) and the Coulomb Failure Stress across the Mexican megathrust as rigorously as possible from 2016 to 2020 (i.e. six large SSEs and two post-slip relaxations in 2.8 years), and found that the devastating earthquakes are likely related to SSEs and/or PIC changes, describing a cascade of events interacting (most of them) with each other on a regional scale via quasi-static and/or dynamic perturbations (Cruz-Atienza et al., NatComm, 2020; Villafuerte et al., EPSL, 2020). Such interaction seems to be conditioned by the transient memory of Earth materials subject to the “traumatic” stressing produced by the seismic waves of the great Mw8.2 Tehuantepec earthquake (2017), which strongly disturbed the aseismic slip beating over a 650 km long segment of the subduction interface, and may have transiently altered the plate-interface fault-zone mechanical properties facilitating such events interaction. Our results imply that seismic hazard in large populated areas is a short-term evolving function of seismotectonic processes that are often observable.
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