On the cause of enhanced landward motion of the overriding plate after a major subduction earthquake

<div> <p>During and after a large megathrust earthquake, the overriding plate above the rupture zone moves oceanward. Enigmatically, the post-seismic motion of the overriding plate after several recent large earthquakes, further along strike from the rupture zone, was faster in the landward direction than before the event. Previous studies interpreted these changes as the result of increased mechanical coupling along the megathrust interface, transient slab acceleration, or bulk postseismic deformation with elastic bending mentioned as a possible underlying mechanism. Before invoking additional mechanisms, it is important to understand the contribution of postseismic deformation processes that are inherent features of megathrust earthquakes. We thus aim to quantify and analyse the deformation that produces landward motion during afterslip and viscous relaxation. </p> </div><div> <p>We use velocity-driven 3D mechanical finite element models, in which large megathrust earthquakes occur periodically on the finite plate interface. The model geometry is similar to most present-day subduction zones, but does not exactly match any specific subduction zone. </p> </div><div> <p>The results show increased post-seismic landward motion at (trench-parallel) distances greater than 450 km from the middle of the ruptured asperity. Similar patterns of landward motion are generated by viscous relaxation in the mantle wedge and by deep afterslip on the shear zone downdip of the brittle megathrust interface. Landward displacement due to postseismic relaxation largely accumulates at exponentially decaying rates until ~6 Maxwell relaxation times after the earthquake. The spatial distribution and magnitude of the velocity changes is broadly consistent with observations related to both the 2010 Maule and the 2011 Tohoku-oki earthquakes.  </p> </div><div> <p>Further model experiments show that patterns of landward motion due to afterslip and to viscous relaxation are insensitive to the locking pattern of the megathrust. However, the locking distribution does affect the magnitudes of the displacements and velocities. Results show that the increased landward displacement due to postseismic deformation scales directly proportionally to seismic moment. </p> </div><div> <p>We conclude that the landward motion results from in-plane horizontal bending of the overriding plate and mantle. This bending is an elastic response to oceanward tractions near the base of the plate around the ruptured asperity, causing extension locally and compression further away along-trench. This elastic in-plate bending consistently contributes to earthquake-associated changes in surface velocities for the biggest megathrust earthquakes, producing landward motion along strike from the rupture zone.</p> </div>

Viscous relaxation of mineral Inclusions and its implications for reaction overstepping calculations in metamorphic rocks

<p>Over the recent years, Raman elastic barometry has been developed as an additional method <span>to calculate</span> metamorphic conditions in natural systems. A major advantage of Raman elastic barometry is that it does not depend on thermodynamic databases and classic geobarometry methods <span>but</span> relies on mechanical calculations. As a consequence, Raman elastic barometry offers an independent method for estimating the pressure conditions <span>that prevailed at the</span> <span>time of entrapment</span> of mineral<span>s</span> du<span>ring</span> growth of their host<span>s</span>.</p><p>The di<span>fference between</span> the pressure calculated <span>using</span> elastic geobarometry and <span>that calculated by phase</span> equilibria methods has recently <span>been employed to</span> <span>estimate</span> the extent of metamorphic reaction overstepping in natural systems. <span>Quantification of</span> the <span>latter however implicitly assumes that the rheology</span> of the inclusion-host system <span>is perfectly</span> elastic. This assumption may no<span>t</span> hold at high temperatures, where viscous creep of minerals takes place.</p><p>The amount of viscous relaxation of <span>a host-inclusion</span> system is a path<span>-</span>dependent quantity which mostly depends on the temperature-time (T-t) path <span>followed</span>. <span>Here</span>, we present examples of visco-elastic relaxation of mineral inclusions and calculate the apparent reaction overstepping which results by assuming that the mechanical system is purely elastic. <span>Our modelling shows</span> that host-inclusion systems <span>that</span> experienced large peak temperatures for long period<span>s</span> of time will retain inclusion residual pressures that <span>cann</span>ot be simply related to the growth of the<span>ir hosts</span> and should <span>therefore not</span> be used for reaction overstepping calculations.</p>

Post-entrapment modification of residual inclusion pressure and its implications for Raman elastic thermobarometry

Residual pressure can be preserved in mineral inclusions, e.g. quartz-in-garnet, after exhumation due to differential expansion between inclusion and host crystals. Raman spectroscopy has been applied to infer the residual pressure and provides information on the entrapment temperature and pressure conditions. However, the amount of residual pressure relaxation cannot be directly measured. An underestimation or overestimation of residual pressure may lead to significant errors between calculated and actual entrapment pressure. This study focuses on three mechanisms responsible for the residual pressure modification: (1) viscous creep; (2) plastic yield; (3) proximity of inclusion to the thin-section surface. Criteria are provided to quantify how much of the expected residual pressure is modified due to these three mechanisms. An analytical solution is introduced to demonstrate the effect of inclusion depth on the residual pressure field when the inclusion is close to the thin-section surface. It is shown that for a quartz-in-garnet system, the distance between the thin-section surface and inclusion centre needs to be at least 3 times the inclusion radius to avoid pressure release. In terms of viscous creep, representative case studies on a quartz-in-garnet system show that viscous relaxation may occur from temperatures as low as 600–700 ∘C depending on the particular pressure–temperature (P–T) path and various garnet compositions. For quartz entrapped along the prograde P–T path and subject to viscous relaxation at peak T above 600–700 ∘C, its residual pressure after exhumation may be higher than predicted from its true entrapment conditions. Moreover, such a viscous resetting effect may introduce apparent overstepping of garnet nucleation that is not related to reaction affinity.

Stoneley wave attenuation and dispersion and the dynamic permeability correction

Stoneley waves induce fluid pressure gradients in a permeable formation surrounding the borehole. These gradients are equilibrated through pressure diffusion, that is to say, slow P-waves in the context of Biot’s poroelasticity theory. Because slow P-waves are strongly sensitive to the formation permeability, the Stoneley-slow P-wave interaction can be used to retrieve the formation permeability from the attenuation and dispersion of Stoneley waves. The accuracy of this established technique in high-permeability formations deteriorates when slow P-waves are not pure diffusion waves; hence, the permeability dependence is more complicated. This effect on Stoneley waves is captured by applying the Johnson-Koplik-Dashen dynamic permeability model. Their model depends on a viscous relaxation length. However, in the estimation of formation permeability from Stoneley waves, this parameter is typically not measured but is estimated from an empirical equation, wherein material properties and microstructural descriptors are lumped together. When the so-calculated relaxation length is erroneous, the inverted formation permeability from the Stoneley wave is not correct either. To overcome this limitation and to provide a versatile alternative, the dynamic permeability problem is reformulated within the viscosity-extended Biot framework. Its physical basis is the conversion scattering in random media from slow P- to slow S-waves. The correlation length of this so-called stochastic dynamic permeability model can be derived from pore-scale images, and it also captures the effect of pore interface roughness. This model is then combined with the simplified Biot-Rosenbaum model to predict Stoneley wave attenuation and dispersion. We have applied this hybrid model to interpret laboratory measurements for which the previously suggested choice of the viscous relaxation length does not provide an accurate prediction. The results indicate that the hybrid model can provide another approach to model Stoneley wave attenuation and dispersion across the entire frequency range.

Observing the Viscous Relaxation Process of Silica Optical Fiber at ~1000 °C Using Regenerated Fiber Bragg Grating

A regenerated fiber Bragg grating (RFBG) in silica fiber was used to observe the viscous relaxation process of the host silica fiber at high temperatures of around 1000 °C. Two factors, preannealing time and loaded tension, which affect viscous relaxation, were observed. When an RFBG is stretched after a longer preannealing, the measured viscosity of the optical fiber was observed to reach equilibrium faster, which means that preannealing accelerates viscous relaxation. A similar acceleration phenomenon was also observed when a larger load was applied to stretch the optical fiber, although the acceleration effect of loaded tension was not as strong as in the preannealing case. The results play an active role in establishing effective optical-fiber devices for application in high-temperature environments.