Nanometric flow and earthquake instability

Fault zones accommodate relative motion between tectonic blocks and control earthquake nucleation. Nanocrystalline fault rocks are ubiquitous in “principal slip zones” indicating that these materials are determining fault stability. However, the rheology of nanocrystalline fault rocks remains poorly constrained. Here, we show that such fault rocks are an order of magnitude weaker than their microcrystalline counterparts when deformed at identical experimental conditions. Weakening of the fault rocks is hence intrinsic, it occurs once nanocrystalline layers form. However, it is difficult to produce “rate weakening” behavior due to the low measured stress exponent, n, of 1.3 ± 0.4 and the low activation energy, Q, of 16,000 ± 14,000 J/mol implying that the material will be strongly “rate strengthening” with a weak temperature sensitivity. Failure of the fault zone nevertheless occurs once these weak layers coalesce in a kinematically favored network. This type of instability is distinct from the frictional instability used to describe crustal earthquakes.

The Cantabrian Fault at Sea. Low Magnitude Seismicity and Its Significance Within a Stable Setting

The Cantabrian fault (CF) is a crustal-scale structure that cuts obliquely the western North Iberian Margin (NIM) for 160 km and continues onshore transecting the Cantabrian Mountains (CM) for another 150 km as the Ventaniella fault (VF). For most of its length inland, the fault system is aseismic, except for a 70 km long segment at its southern end. Within this segment, a gently north-dipping linear arrangement of earthquakes was interpreted as related to the intersection of a slightly oblique fault to VF with the basal thrust of the CM. In addition to earthquake nucleation along parts of its length, the CF–VF also stands out regionally as a major seismotectonic boundary, separating a seismically active area to the West from an essentially aseismic region to the East. Contrasting tectonothermal evolution in the crust on either side during the Mesozoic rifting may underlie the observed differences. On the other hand, the seismicity within the subsea segment is low magnitude, persistent, and understudied. The scarcity of the permanent seismic stations distribution in the area did not allow to establish more than a generalized consensus relating the offshore events to the submarine structure. A recent local seismic network monitored the area providing the highest accuracy information on the offshore events to date. Although the location of foci is partially challenged by the lack of recording stations from northern azimuths at sea, the observed pattern shows indeed a broad linear trend in the submarine domain in relation to the crustal-scale structure. Specifically, this study shows that the distribution of foci offshore display two preferential areas along the CF–VF within its southern crustal block. Considering the basement rock types and the deep architectural disposition of the margin crust, two possible explanations for the origin of the clusters are put forward in this contribution.

State evolution laws and earthquake nucleation

<p>In models of faults as elastic continua with a frictional interface, earthquake nucleation is the initiation of a propagating dynamic fault rupture nucleated by a localized slip instability. A mechanism capturing both the weakening process leading to nucleation as well as fault healing between events, is a slip rate- and state-dependent friction, with so-called direct effect and evolution effects [Dieterich, JGR 1979; Ruina, JGR 1983]. While the constitutive representation of the direct effect is theoretically supported [e.g., Nakatani, JGR 2001; Rice et al., JMPS 2001], that of the evolution effect remains empirical and a number of state-evolution laws have been proposed to fit lab rock friction data [Ruina, JGR 1983; Kato and Tullis, GRL 2001; Bar-Sinai et al., GRL 2012; Nagata et al., JGR 2012]. These laws may share a common linearization about steady-state, such that a linear stability analysis of steady, uniform sliding yields a single critical wavelength for unstable growth of perturbations [Rice and Ruina, JAM 1983]. However, the laws’ differences are apparent at later, non-linear stages of instability development.</p><div>Previously, we showed that instability development under aging-law state evolution could be understood in terms of dynamical systems [Viesca, PR-E 2016, PRS-A 2016]: the non-linear acceleration of slip occurs as the attraction of a fault’s slip rate to a fixed point, corresponding to slip rate diverging with a fixed spatial distribution and rate of acceleration. Here we show that this framework can also be applied to understand slip instability development under all commonly used evolution laws, including the so-called slip and Nagata laws. To do so, we develop an intermediate state evolution law that transitions between the slip and aging laws with the adjustment of a single parameter. We show that, to within a variable transformation, the intermediate law is equivalent to the Nagata law and that fixed-point blow-up solutions exist for any value of the transition parameter. We assess these fixed-points’ stability via a linear stability analysis and provide an explanation for previously observed behavior in numerical solutions for slip rate and state evolution under various evolution laws [Ampuero and Rubin, JGR 2008; Kame et al., 2013; Bar-Sinai et al., PR-E 2013; Bhattacharya and Rubin, JGR 2014].</div>

High-resolution spatio-temporal fault slip using InSAR observations: insights on seismic and aseismic slip during a shallow crust earthquake swarm

<p>How earthquakes initiate and run-away into major ruptures is still a challenging research topic, that will benefit from increasing our capability to observe processes from the seismogenic source regions. In recent years, two models for earthquake nucleation have been proposed to explain earthquake sequences, a slow-slipping model and a cascade model, based mostly on the analysing seismic data. Here we use geodetic data to contribute to the study of seismogenic source regions during earthquake sequences. Earthquake swarms are unusual as they do not obey observational physics laws, e.g., Gutemberg-Richter law. This deviation might be to a disproportioned contribution of aseismic processes, and hence provide an opportunity to investigate the role of aseismic behaviour in the nucleation and propagation of earthquakes.</p><p>Here, we study a shallow seismic swarm in Nevada, USA, in 2011. We process satellite radar images to form differential interferograms and to quantify the surface displacements. From the interferograms, we observe a clear surface displacement signal (~4 cm in line-of-sight direction) consistent with slip along a N-S striking normal fault, before the largest magnitude event (M4.6) in the swarm. We also find that interferograms across the M4.6 are dominated by slip on a NE-SW striking fault. Thus, we consider slip along a fault system with a geometry consisting of two fault planes. To interpret the surface displacement, we invert for its optimal geometry directly using the interferometric wrapped phase. Based on the fault geometry together with inferred surface ruptures, we construct a smooth fault plane with triangular dislocations. Then, we extend our previous method to obtain distributed fault slip models from the wrapped phase. We implement a physics-based linear elastic crack model with no stress singularities, coupled with a linear time inversion with optimal regularization method to estimate the temporal evolution of fault slip. We apply this method to the 2011 Hawthorne swarm geodetic data to test the two conceptual earthquake nucleation and propagation models. The inversion reveals (1) two slip maxima; a narrow (1km<sup>2</sup>) slip area on the southern fault with high average slip (0.8m) occurring before the M4.6 event; and a wider (40km<sup>2</sup>) slip area on the northern fault which ruptured during and after the M4.6 event and with lower average slip (0.1m); (2) our results are more consistent with a cascade model of discrete slip patches, rather than a slow-slipping model thought as a growing elliptical crack; (3) the aseismic (geodetic) moment ratio is variable from 100% before the M4.6 event, but remains larger than 60% after it. </p><p>The study of the 2011 Hawthorne swarm allows us to illuminate fault slip in much greater detail than usually possible. We conclude that there were significant aseismic fault processes, most likely slow-slip or localized fluid-enhanced fault slip, along with discrete segments of the fault plane active before and after the largest earthquake in this swarm. This study contributes to highlighting the importance of using geodetic data to understand the role of aseismic processes during swarms. An important step towards improving our understanding of the nucleation and propagation of earthquakes.</p>

Dilation and compaction accompanying changes in slip velocity in clay-bearing fault gouges

<p>Many natural fault cores comprise volumes of extremely fine, low permeability, clay-bearing fault rocks. Should these fault rocks undergo transient volume changes in response to changes in fault slip velocity, the subsequent pore pressure transients would produce significant fault weakening or strengthening, strongly affecting earthquake nucleation and possibly leading to episodic slow slip events. Dilatancy at slow slip velocity has previously been measured in quartz-rich gouges but little is known about gouge containing clay. In this work, the mechanical behaviour of synthetic quartz-kaolinite fault gouges and their volume response to velocity step changes were investigated in a suite of triaxial deformation experiments at effective normal stresses of 60MPa, 25MPa and 10MPa. Kaolinite content was varied from 0 to 100wt% and slip velocity was varied between 0.3 and 3 microns/s.</p><p>Upon a 10-fold velocity increase or decrease, gouges of all kaolinite-quartz contents displayed measurable volume change transients. The results show the volume change transients are independent of effective normal stress but are sensitive to gouge kaolinite content. Peak dilation values did not occur in the pure quartz gouges, but rather in gouges containing 10wt% to 20wt% kaolinite. Above a kaolinite content of 10wt% to 20wt%, both dilation and compaction decreased with increasing gouge kaolinite content. At 25MPa effective normal stress, the normalised volume changes decreased from 0.1% to 0.06% at 10wt% to 100wt% kaolinite.  The gouge mechanical behaviour shows that increasing the gouge kaolinite content decreases the gouge frictional strength and promotes more stable sliding, rather than earthquake slip. Increasing the effective normal stress slightly decreases the frictional strength, enhances the chance of earthquake nucleation, and has no discernible effect on the magnitude of the pore volume changes during slip velocity changes.</p><p>Low permeabilities of clay-rich fault gouges, coupled with the observed volume change transients, could produce pore pressure fluctuations up to 10MPa in response to fault slip. This assumes no fluid escape from an isolated fault core. Where the permeability is finite, any pore pressure changes will be mediated by fluid influx into the gouge. Volume change transients could therefore be a significant factor in determining whether fault slip leads to earthquake nucleation or a dampened response, possibly resulting in episodic slow slip in low permeability fault rock volumes.</p>

Forecasting Induced Earthquake Hazard Using a Hydromechanical Earthquake Nucleation Model

In response to the dramatic increase in earthquake rates in the central United States, the U.S Geological Survey began releasing 1 yr earthquake hazard models for induced earthquakes in 2016. Although these models have been shown to accurately forecast earthquake hazard, they rely purely on earthquake statistics because there was no precedent for forecasting induced earthquakes based upon wastewater injection data. Since the publication of these hazard models, multiple physics-based methods have been proposed to forecast earthquake rates using injection data. Here, we use one of these methods to generate earthquake hazard forecasts. Our earthquake hazard forecasts are more accurate than statistics-based hazard forecasts. These results imply that fluid injection data, where and when available, and the physical implications of fluid injection should be included in future induced earthquake hazard forecasts.

Migratory earthquake precursors are dominant on an ice stream fault

Simple fault models predict earthquake nucleation near the eventual hypocenter (self-nucleation). However, some earthquakes have migratory foreshocks and possibly slow slip that travel large distances toward the eventual mainshock hypocenter (migratory nucleation). Scarce observations of migratory nucleation may result from real differences between faults or merely observational limitations. We use Global Positioning System and passive seismic records of the easily observed daily ice stream earthquake cycle of the Whillans Ice Plain, West Antarctica, to quantify the prevalence of migratory versus self-nucleation in a large-scale, natural stick-slip system. We find abundant and predominantly migratory precursory slip, whereas self-nucleation is nearly absent. This demonstration that migratory nucleation exists on a natural fault implies that more-observable migratory precursors may also occur before some earthquakes.