Transient Deformation and Stress Patterns Induced by the 2010 Maule Earthquake in the Illapel Segment

Evaluating the transfer of stresses from megathrust earthquakes to adjacent segments is fundamental to assess seismic hazard. Here, we use a 3D forward model as well as GPS and seismic data to investigate the transient deformation and Coulomb Failure Stresses (CFS) changes induced by the 2010 Maule earthquake in its northern segment, where the Mw 8.3 Illapel earthquake occurred in 2015. The 3D model incorporates the coseismically instantaneous, elastic response, and time-dependent afterslip and viscoelastic relaxation processes in the postseismic period. We particularly examine the impact of linear and power-law rheology on the resulting postseismic deformation and CFS changes that may have triggered the Illapel earthquake. At the Illapel hypocenter, our model results in CFS changes of ∼0.06 bar due to the coseismic and postseismic deformation, where the coseismic deformation accounts for ∼85% of the total CFS changes. This is below the assumed triggering threshold of 0.1 bar and, compared to the annual loading rate of the plate interface, represents a clock advance of approximately only 2 months. However, we find that sixteen events with Mw ≥ 5 in the southern region occurred in regions of CFS changes > 0.1 bar, indicating a potential triggering by the Maule event. Interestingly, while the power-law rheology model increases the positive coseismic CFS changes, the linear rheology reduces them. This is due to the opposite polarity of the postseismic displacements resulting from the rheology model choice. The power-law rheology model generates surface displacements that fit better to the GPS-observed landward displacement pattern.

Continual interaction between the Minahassa subduction interface and the Palu-Koro strike-slip fault in Sulawesi, Indonesia.

<p>Two major fault systems host M<sub>w</sub>>7 earthquakes in Central and Northern Sulawesi, Indonesia: the Minahassa subduction interface and the Palu-Koro strike-slip fault. The Celebes Sea oceanic lithosphere subducts beneath the north arm of Sulawesi at the Minahassa subduction zone. At the western termination of the Minahassa subduction zone, it connects to the left-lateral Palu-Koro strike-slip fault zone. This fault strikes onshore at Palu Bay and then crosses Sulawesi. Interseismic GNSS velocities indicate that the Palu-Koro fault zone accommodates about 4 cm/yr of relative motion in the Palu Bay area, with a ~10 km locking depth. This shallowly locked segment of the Palu-Koro fault around the Palu Bay area ruptured during the devastating, tsunami-generating, 2018 M<sub>w</sub>7.5 Palu earthquake. This complex event highlights the high seismic hazard for the island of Sulawesi.</p><p>We have a >20-year record of GNSS velocities on Sulawesi, where the densest cluster of monument sites surrounds the Palu-Koro fault, specifically around Palu Bay, whereas the rest of the island is less densely covered. High quality estimates of interseismic velocities reveal second-order complex patterns of transient deformation in the wake of major earthquakes: the velocities in northern Sulawesi and around the Palu-Koro fault do not follow their interseismic trends after a major subduction earthquake has occurred, for several years after the event. This effect of transient deformation reaches more than 400km away from the epicentre of the major earthquakes. Surprisingly, a deviation from the background slip rate on the Palu-Koro fault is not accompanied by a deviation from the background (micro)seismic activity.</p><p>We construct a 3D numerical model based on the structural and seismological data in the Sulawesi region. We investigate the post-seismic relaxation pattern from a subduction earthquake and determine whether the slip rate on the Palu-Koro fault changes due to this earthquake through forward model calculations. With a modelling focus on the 1996 M<sub>w</sub>7.9 and 2008 M<sub>w</sub>7.4 earthquakes that ruptured the Minahassa subduction interface, this study outlines the triggering of transient deformation and continual interaction between the Minahassa subduction interface and the Palu-Koro strike-slip fault.</p>

When minerals fight back: The relationship between back stress and geometrically necessary dislocation density

<p>The dynamics of several geophysical phenomena, such as post-seismic deformation and post-glacial isostatic readjustment, are inferred to be controlled by the transient rheology of olivine in Earth’s mantle. However, the physical mechanism(s) that underlie(s) this behavior remain(s) relatively unknown, and most experimental studies focus on quantifying steady-state rheology. Recent studies have suggested that back stresses caused by long-range elastic interactions among dislocations could play a role in transient deformation of olivine. Wallis et al. (2017) identified an internal back stress in olivine single crystals deforming at 1573 K, which gave rise to anelastic transient deformation in stress dip experiments. Hansen et al. (2019) quantified the room-temperature strain hardening of olivine deforming by low-temperature plasticity and measured a back stress that gave rise to a Bauschinger effect, a well-known phenomenon in materials science wherein the yield stress is reduced upon reversing the sense of direction of the deformation.</p><p>To explore deformation at very high dislocation density, we have developed a novel nanoindentation load drop method to measure the back stress in a material at sub-micron length scales. Using a self-similar Berkovich tip, we measure back stresses in single crystals of olivine, quartz, and plagioclase feldspar at a range of indentation depths from 100–1700 nm, corresponding to geometrically necessary dislocation (GND) densities of order 10<sup>14</sup>–10<sup>15</sup> m<sup>-2</sup>. Our results reveal a power-law relationship between back stress and GND density with an exponent ranging from 0.44-0.55 for each material, with an average across all materials of 0.48. Normalizing back stress by the shear modulus measured during the indentation test results in a master curve with a power-law exponent of 0.44, in close agreement with the theoretical prediction (0.5) derived from the classical Taylor hardening equation (Taylor, 1934). For olivine, the extrapolation of our fit quantitatively agrees with other published data spanning over 5 orders of magnitude in GND density and temperatures ranging from 298-1573 K. This work provides the first experimental evidence in support of Taylor hardening in a geologic material, supports the assertion that strain hardening is an athermal process that can occur during high-temperature creep, and suggests that back stresses from long-range interactions among dislocations must be considered in rheological models of transient creep.</p>