How cementation and fluid flow influence slip behavior at the subduction interface

Much of the complexity of subduction-zone earthquake size and temporal patterns owes to linkages among fluid flow, stress, and fault healing. To investigate these linkages, we introduce a novel numerical model that tracks cementation and fluid flow within the framework of an earthquake simulator. In the model, there are interseismic increases in cohesion across the plate boundary and decreases in porosity and permeability caused by cementation along the interface. Seismogenic slip is sensitive to the effective stress and therefore fluid pressure; in turn, slip events increase porosity by fracturing. The model therefore accounts for positive and negative feedbacks that modify slip behavior through the seismic cycle. The model produces temporal clustering of earthquakes in the seismic record of the Aleutian margin, which has well-documented along-strike variations in locking characteristics. Model results illustrate how physical, geochemical, and hydraulic linkages can affect natural slip behavior. Specifically, coseismic drops in fluid pressure steal energy from large ruptures, suppress slip, moderate the magnitudes of large earthquakes, and lead to aftershocks.

Fault healing plays a key role in creating the spectrum of tectonic faulting styles from seismic to aseismic slip

<p>Tectonic faults fail in a broad spectrum of modes ranging from aseismic creep to fast, ordinary, earthquakes modulated by elastodynamic rupture processes. Laboratory friction experiments with repetitive stick-slip failure have reproduced this complete range of modes with failure durations spanning several orders of magnitude. These works show that the frictional weakening rate with slip (i.e., the rheological critical stiffness <em>k<sub>c</sub> =σ<sub>n</sub>(b-a)/D<sub>c</sub></em>, where <em>σ<sub>n</sub></em> is effective fault normal stress, <em>D<sub>c</sub></em> is the friction critical slip distance and <em>(b-a)</em> represents the friction rate parameter) is the primary control on the mode of slip, but higher-order effects are also important including variation of <em>k<sub>c</sub>  </em>with slip velocity.  Far from the stability boundary, stick-slip occurs when the rate of elastic unloading with slip <em>k</em> is small compared to the frictional weakening rate (i.e., <em>k</em><<<em>k<sub>c</sub></em>). Potential energy, in the form of stored elastic strain, drives rapid fault acceleration. Near the stability boundary, when <em>k ~ k<sub>c</sub></em>, lab experiments document slow and quasi-dynamic failure events, consistent with the observation that earthquake stress drop is negligible for slow earthquakes. Lab data show that stick-slip stress drop decreases systematically as <em>k/k<sub>c</sub></em> approaches 1 from below. There are two possible scenarios for slow slip near the stability boundary, although they are degenerate in most cases. 1) Fault slip relieves elastic stresses prior to failure and thus the potential energy needed to drive fast rupture is absent. 2) Elastic strain accumulates but the fault rheology is velocity strengthening or otherwise inconsistent with rapid slip, for example because the frictional weakening rate <em>k<sub>c</sub></em>  is low.  In Scenario 1, slip can occur early in the seismic cycle, as creep, or later in the cycle when shear stress reaches a critical value for precursory slip.  In either case, slip occurs because the rate of fault healing is low compared to the stressing rate. A low rate of fault healing can also explain Scenario 2 because the friction state evolution parameter <em>b</em> scales directly with the rate of fault healing and <em>k<sub>c</sub></em>. Given that the friction parameter <em>a</em> is positive definite, the frictional healing rate (<em>b</em>) sets the scale of <em>k<sub>c</sub></em> for a given value of <em>D<sub>c</sub></em>. Thus, while these two scenarios for slow slip appear distinct they both derive from the rate of fault healing.  Exceptions would involve faults that are strongly velocity weakening <em>(b-a)</em> >>0 yet have negligible healing rates (<em>b</em> ~ 0), which is indeed rare.  The rate of fault healing is expected to vary with mineralogy, effective stress, temperature and other factors. Thus, while we expect a systematic variation of seismic style with depth, associated with changes in <em>k<sub>c</sub></em>, we should not be surprised to find a spectrum of faulting styles throughout the lithosphere, including a range of styles at a given location.  Discoveries of seismic tremor, low frequency earthquakes, and other modes of fault slip are challenging our views of tectonic faulting and they highlight the need for close connections between field observations, detailed laboratory work and theoretical studies of friction and faulting.</p>

Nanoscale evidence for temperature-induced transient rheology and postseismic fault healing

Friction-generated heat and the subsequent thermal evolution control fault material properties and thus strength during the earthquake cycle. We document evidence for transient, nanoscale fault rheology on a high-gloss, light-reflective hematite fault mirror (FM). The FM cuts specularite with minor quartz from the Pleistocene El Laco Fe-ore deposit, northern Chile. Scanning and transmission electron microscopy data reveal that the FM volume comprises a <50-μm-thick zone of polygonal hematite nanocrystals with spherical silica inclusions, rhombohedral twins, no shape or crystallographic preferred orientation, decreasing grain size away from the FM surface, and FM surface magnetite nanoparticles and Fe2+ suboxides. Sub–5-nm-thick silica films encase hematite grains and connect to amorphous interstitial silica. Observations imply that coseismic shear heating (temperature >1000 °C) generated transiently amorphous, intermixed but immiscible, and rheologically weak Fe-oxide and silica. Hematite regrowth in a fault-perpendicular thermal gradient, sintering, twinning, and a topographic network of nanometer-scale ridges from crystals interlocking across the FM surface collectively restrengthened fault material. Results reveal how temperature-induced weakening preconditions fault healing. Nanoscale transformations may promote subsequent strain delocalization and development of off-fault damage.