Illuminating subduction zone rheological properties in the wake of a giant earthquake

Fast relocking and afterslip-seismicity evolution following the 2015 Mw 8.3 Illapel earthquake in Chile

Large subduction earthquakes induce complex postseismic deformation, primarily driven by afterslip and viscoelastic relaxation, in addition to interplate relocking processes. However, these signals are intricately intertwined, posing challenges in determining the timing and nature of relocking. Here, we use six years of continuous GNSS measurements (2015-2021) to study the spatiotemporal evolution of afterslip, seismicity and locking after the 2015 Illapel earthquake ([Formula: see text] 8.3). Afterslip is inverted from postseismic displacements corrected for nonlinear viscoelastic relaxation modeled using a power-law rheology, and the distribution of locking is obtained from the linear trend of GNSS stations. Our results show that afterslip is mainly concentrated in two zones surrounding the region of largest coseismic slip. The accumulated afterslip (corresponding to [Formula: see text] 7.8) exceeds 1.5 m, with aftershocks mainly occurring at the boundaries of the afterslip patches. Our results reveal that the region experiencing the largest coseismic slip undergoes rapid relocking, exhibiting the behavior of a persistent velocity weakening asperity, with no observed aftershocks or afterslip within this region during the observed period. The rapid relocking of this asperity may explain the almost regular recurrence time of earthquakes in this region, as similar events occurred in 1880 and 1943.

Complex tsunamigenic near-trench seafloor deformation during the 2011 Tohoku-Oki earthquake

The near-trench coseismic rupture behaviour of the 2011 Tohoku-Oki earthquake remains poorly understood due to the scarcity of near-field observations. Differential bathymetry offers a unique approach to studying offshore coseismic seafloor deformation but has a limited horizontal resolution. Here we use differential bathymetry estimates with improved horizontal resolutions to investigate near-trench coseismic slip behaviours in the 2011 Tohoku-Oki earthquake. In the main rupture region, a velocity-strengthening behaviour in the shallow fault is observed. By contrast, the seafloor uplift decreases towards the trench, but the trend inverts near the backstop interface outcrop, revealing significant off-fault deformation features. Amongst various competing off-fault effects observed, we suggest that inelastic deformation plays a predominant role in near-trench tsunami excitation. Large trench-bleaching rupture is also observed immediately north of 39°, delimiting the northern extent of the main rupture region. Overall, striking spatial heterogeneity of the shallow rupture behaviour is revealed for the region.

Rapid shallow megathrust afterslip from the 2021 M8.2 Chignik, Alaska earthquake revealed by seafloor geodesy

The shallower portions of subduction zone megathrust faults host Earth's most hazardous tsunamigenic earthquakes, yet understanding how and when they slip remains elusive because of challenges making seafloor observations. We performed Global Navigation Satellite System Acoustic seafloor geodetic surveys before and ~2.5 months after the 29 July 2021 Mw (moment magnitude) 8.2 Chignik, Alaska, earthquake and determine ~1.4 meters cumulative co- and post-seismic horizontal displacement ~60 kilometers from the megathrust front. Only for the 2011 Mw 9 Tohoku event have closer subduction zone earthquake displacements been observed. We estimate ~2 to 3 meters of megathrust afterslip shallower than 20 kilometers, a portion of the megathrust on which both inter- and co-seismic slip likely had occurred previously. Our analysis demonstrates that by 2.5 months, shallower and deeper moment had effectively equilibrated on the megathrust, suggesting that its tsunamigenic potential remains no more elevated than before the earthquake.

Dislocation theory of steady and transient creep of crystalline solids: Predictions for olivine

In applications critical to the geological, materials, and engineering sciences, deformation occurs at strain rates too small to be accessible experimentally. Instead, extrapolations of empirical relationships are used, leading to epistemic uncertainties in predictions. To address these problems, we construct a theory of the fundamental processes affecting dislocations: storage and recovery. We then validate our theory for olivine deformation. This model explains the empirical relationships among strain rate, applied stress, and dislocation density in disparate laboratory regimes. It predicts the previously unexplained dependence of dislocation density on applied stress in olivine. The predictions of our model for Earth conditions differ from extrapolated empirical relationships. For example, it predicts rapid, transient deformation in the upper mantle, consistent with recent measurements of postseismic creep.

Characteristics of Earthquake Cycles: A Cross-Dimensional Comparison of 0D to 3D Numerical Models

High-resolution computer simulations of earthquake sequences in three or even two dimensions pose great demands on time and energy, making lower-cost simplifications a competitive alternative. We systematically study the advantages and limitations of simplifications that eliminate spatial dimensions in quasi-dynamic earthquake sequence models, from 3D models with a 2D fault plane down to 0D or 1D models with a 0D fault point. We demonstrate that, when 2D or 3D models produce quasi-periodic characteristic earthquakes, their behavior is qualitatively similar to lower-dimension models. Certain coseismic characteristics like stress drop and fracture energy are largely controlled by frictional parameters and are thus largely comparable. However, other observations are quantitatively clearly affected by dimension reduction. We find corresponding increases in recurrence interval, coseismic slip, peak slip velocity, and rupture speed. These changes are to a large extent explained by the elimination of velocity-strengthening patches that transmit tectonic loading onto the velocity-weakening fault patch, thereby reducing the interseismic stress rate and enhancing the slip deficit. This explanation is supported by a concise theoretical framework, which explains some of these findings quantitatively and effectively estimates recurrence interval and slip. Through accounting for an equivalent stressing rate at the nucleation size h* into 2D and 3D models, 0D or 1D models can also effectively simulate these earthquake cycle parameters. Given the computational efficiency of lower-dimensional models that run more than a million times faster, this paper aims to provide qualitative and quantitative guidance on economical model design and interpretation of modeling studies.

The cryptic seismic potential of the Pichilemu blind fault in Chile revealed by off-fault geomorphology

The first step towards assessing hazards in seismically active regions involves mapping capable faults and estimating their recurrence times. While the mapping of active faults is commonly based on distinct geologic and geomorphic features evident at the surface, mapping blind seismogenic faults is complicated by the absence of on-fault diagnostic features. Here we investigated the Pichilemu Fault in coastal Chile, unknown until it generated a Mw 7.0 earthquake in 2010. The lack of evident surface faulting suggests activity along a partly-hidden blind fault. We used off-fault deformed marine terraces to estimate a fault-slip rate of 0.52 ± 0.04 m/ka, which, when integrated with satellite geodesy suggests a 2.12 ± 0.2 ka recurrence time for Mw~7.0 normal-faulting earthquakes. We propose that extension in the Pichilemu region is associated with stress changes during megathrust earthquakes and accommodated by sporadic slip during upper-plate earthquakes, which has implications for assessing the seismic potential of cryptic faults along convergent margins and elsewhere.

Cryptic faults concern earthquake scientists, since they pose a hidden seismic potential which is hard to identify. To address this, the authors here study off-fault deformed geomorphic markers such as marine terraces using high-resolution LiDAR topography, optical dating of sediments and space geodetic observations.

Mantle flow distribution beneath the California margin

Although the surface deformation of tectonic plate boundaries is well determined by geological and geodetic measurements, the pattern of flow below the lithosphere remains poorly constrained. We use the crustal velocity field of the Plate Boundary Observatory to illuminate the distribution of horizontal flow beneath the California margin. At lower-crustal and upper-mantle depths, the boundary between the Pacific and North American plates is off-centered from the San Andreas fault, concentrated in a region that encompasses the trace of nearby active faults. A major step is associated with return flow below the Eastern California Shear Zone, leading to the extrusion of the Mojave block and a re-distribution of fault activity since the Pleistocene. Major earthquakes in California have occurred above the regions of current plastic strain accumulation. Deformation is mechanically coupled from the crust to the asthenosphere, with mantle flow overlaid by a kinematically consistent network of faults in the brittle crust.