Margaret S. Boettcher

Numerical simulations of granular shear zones using the distinct element method: 1. Shear zone kinematics and the micromechanics of localization

Two-dimensional numerical simulations were conducted using the distinct element method (DEM) to examine the influences of particle size distribution (PSD) and interparticle friction μp on the nature of deformation in granular fault gouge. Particle fracture was not allowed in this implementation but points in PSD space were examined by constructing assemblages of particles with self-similar size distributions defined by the two-dimensional power law exponent D. For these numerical “experiments,” D ranged from 0.81 to 2.60, where D=1.60 represents the two-dimensional equivalent of a characteristic PSD to which cataclastically deforming gouge is thought to evolve. Experiments presented here used μp values of 0.10 and 0.50 and were conducted using normal stress σn on the shear zone walls of 70 MPa. Shear strain within these simulated assemblages was accommodated by intermittent displacement along discrete slip surfaces, alternating between broadly distributed deformation along multiple slip planes and highly localized deformation along a single, sharply defined, subhorizontal zone of slip. Slip planes corresponded in orientation and sense of shear to shear structures observed in natural gouge zones, specifically Riedel and Y shears; the oblique Riedel shears showed more extreme orientations than typical, but their geometries were consistent with those predicted for low-strength Coulomb materials. The character of deformation in the shear zone varied with PSD due to changes in the relative importance of interparticle slip and rolling as deformation mechanisms. A high degree of frictional coupling between large rolling particles in low D (coarse-grained) assemblages resulted in wide zones of slip and broadly distributed deformation. In higher D assemblages (D >= 1.60), small rolling particles self-organized into columns that separated large rolling particles, causing a reduction in frictional resistance within the deforming assemblage. This unusual particle configuration appears to depend on a critical abundance of small particles achieved at D ≈ 1.60 and may enable strain localization in both real and simulated granular assemblages.

Foreshock sequences and short-term earthquake predictability on East Pacific Rise transform faults

East Pacific Rise transform faults are characterized by high slip rates (more than ten centimetres a year), predominately aseismic slip and maximum earthquake magnitudes of about 6.5. Using recordings from a hydroacoustic array deployed by the National Oceanic and Atmospheric Administration, we show here that East Pacific Rise transform faults also have a low number of aftershocks and high foreshock rates compared to continental strike-slip faults. The high ratio of foreshocks to aftershocks implies that such transform-fault seismicity cannot be explained by seismic triggering models in which there is no fundamental distinction between foreshocks, mainshocks and aftershocks. The foreshock sequences on East Pacific Rise transform faults can be used to predict (retrospectively) earthquakes of magnitude 5.4 or greater, in narrow spatial and temporal windows and with a high probability gain. The predictability of such transform earthquakes is consistent with a model in which slow slip transients trigger earthquakes, enrich their low-frequency radiation and accommodate much of the aseismic plate motion.

Thermal structure of oceanic transform faults

We use three-dimensional fi nite element simulations to investigate the temperature structure beneath oceanic transform faults. We show that using a rheology that incorporates brittle weakening of the lithosphere generates a region of enhanced mantle upwelling and elevated temperatures along the transform; the warmest temperatures and thinnest lithosphere are predicted to be near the center of the transform. Previous studies predicted that the mantle beneath oceanic transform faults is anomalously cold relative to adjacent intraplate regions, with the thickest lithosphere located at the center of the transform. These earlier studies used simplifi ed rheologic laws to simulate the behavior of the lithosphere and underlying asthenosphere. We show that the warmer thermal structure predicted by our calculations is directly attributed to the inclusion of a more realistic brittle rheology. This temperature structure is consistent with a wide range of observations from ridge-transform environments, including the depth of seismicity, geochemical anomalies along adjacent ridge segments, and the tendency for long transforms to break into small intratransform spreading centers during changes in plate motion.

Broadband Records of Earthquakes in Deep Gold Mines and a Comparison with Results from SAFOD, California

For one week during September 2007, we deployed a temporary network of field recorders and accelerometers at four sites within two deep, seismically active mines. The ground-motion data, recorded at 200 samples/sec, are well suited to de- termining source and ground-motion parameters for the mining-induced earthquakes within and adjacent to our network. Four earthquakes with magnitudes close to 2 were recorded with high signal/noise at all four sites. Analysis of seismic moments and peak velocities, in conjunction with the results of laboratory stick-slip friction experi- ments, were used to estimate source processes that are key to understanding source physics and to assessing underground seismic hazard. The maximum displacements on the rupture surfaces can be estimated from the parameter Rv, where v is the peak ground velocity at a given recording site, and R is the hypocentral distance. For each earthquake, the maximum slip and seismic moment can be combined with results from laboratory friction experiments to estimate the maximum slip rate within the rupture zone. Analysis of the four M 2 earthquakes recorded during our deployment and one of special interest recorded by the in-mine seismic network in 2004 revealed maxi- mum slips ranging from 4 to 27 mm and maximum slip rates from 1.1 to 6:3 m=sec. Applying the same analyses to an M 2.1 earthquakewithin a cluster of repeating earth- quakes near the San Andreas Fault Observatory at Depth site, California, yielded similar results for maximum slip and slip rate, 14 mm and 4:0 m=sec.

Laboratory-based maximum slip rates in earthquake rupture zones and radiated energy

Laboratory stick-slip friction experiments indicate that peak slip rates increase with the stresses loading the fault to cause rupture. If this applies also to earthquake fault zones, then the analysis of rupture processes is simplified inasmuch as the slip rates depend only on the local yield stress and are independent of factors specific to a particular event, including the distribution of slip in space and time. We test this hypothesis by first using it to develop an expression for radiated energy that depends primarily on the seismic moment and the maximum slip rate. From laboratory results, the maximum slip rate for any crustal earthquake, as well as various stress parameters including the yield stress, can be determined based on its seismic moment and the maximum slip within its rupture zone. After finding that our new equation for radiated energy works well for laboratory stick-slip friction experiments, we used it to estimate radiated energies for five earthquakes with magnitudes near 2 that were induced in a deep gold mine, an M 2.1 repeating earthquake near the San Andreas Fault Observatory at Depth (SAFOD) site and seven major earthquakes in California and found good agreement with energies estimated independently from spectra of local and regional ground-motion data. Estimates of yield stress for the earthquakes in our study range from 12 MPa to 122 MPa with a median of 64 MPa. The lowest value was estimated for the 2004 M 6 Parkfield, California, earthquake whereas the nearby M 2.1 repeating earthquake, as recorded in the SAFOD pilot hole, showed a more typical yield stress of 64 MPa.

The relationship between seismicity and fault structure on the Discovery transform fault, East Pacific Rise

There is a global seismic moment deficit on mid-ocean ridge transform faults, and the largest earthquakes on these faults do not rupture the full fault area. We explore the influence of physical fault structure, including step-overs in the fault trace, on the seismic behavior of the Discovery transform fault, 4S on the East Pacific Rise. One year of microseismicity recorded during a 2008 ocean bottom seismograph deployment (24,377 0 ≤ ML ≤ 4.6 earthquakes) and 24 years of Mw ≥ 5.4 earthquakes obtained from the Global Centroid Moment Tensor catalog, are correlated with surface fault structure delineated from high-resolution multibeam bathymetry. Each of the 15 5.4 ≤ Mw ≤ 6.0 earthquakes that occurred on Discovery between 1 January 1990 and 1 April 2014 was relocated into one of five distinct rupture patches using a teleseismic surface wave cross-correlation technique. Microseismicity was relocated using the HypoDD relocation algorithm. The western fault segment of Discovery (DW) is composed of three zones of varying structure and seismic behavior: a zone with no large events and abundant microseismicity, a fully coupled zone with large earthquakes, and a complex zone with multiple fault strands and abundant seismicity. In general, microseismicity is reduced within the patches defined by the large, repeating earthquakes. While the extent of the large rupture patches on DW correlates with physical features in the bathymetry, step-overs in the primary fault trace are not observed at patch boundaries, suggesting along-strike heterogeneity in fault zone properties controls the size and location of the large events.

Nanoseismicity and picoseismicity rate changes from static stress triggering caused by a Mw 2.2 earthquake in Mponeng gold mine, South Africa

Static stress changes following large earthquakes are known to affect the rate and distribution of aftershocks, yet this process has not been thoroughly investigated for nanoseismicity and picoseismicity at centimeter length scales. Here we utilize a unique data set of M ≥ −3.4 earthquakes following a Mw 2.2 earthquake in Mponeng gold mine, South Africa, that was recorded during a quiet interval in the mine to investigate if rate- and state-based modeling is valid for shallow, mining-induced seismicity. We use Dieterichs (1994) rate- and state-dependent formulation for earthquake productivity, which requires estimation of four parameters: (1) Coulomb stress changes due to the main shock, (2) the reference seismicity rate, (3) frictional resistance parameter, and (4) the duration of aftershock relaxation time. Comparisons of the modeled spatiotemporal patterns of seismicity based on two different source models with the observed distribution show that while the spatial patterns match well, the rate of modeled aftershocks is lower than the observed rate. To test our model, we used three metrics of the goodness-of-fit evaluation. The null hypothesis, of no significant difference between modeled and observed seismicity rates, was only rejected in the depth interval containing the main shock. Results show that mining-induced earthquakes may be followed by a stress relaxation expressed through aftershocks located on the rupture plane and in regions of positive Coulomb stress change. Furthermore, we demonstrate that the main features of the temporal and spatial distributions of very small, mining-induced earthquakes can be successfully determined using rate- and state-based stress modeling.

Moment Tensors and Other Source Parameters of Mining‐Induced Earthquakes in TauTona Mine, South Africa

Abstract Induced seismicity exhibits diverse source mechanisms that are often difficult to constrain for small events. Here, we use data from the in‐mine seismic network, the Natural Earthquake Laboratory in South African Mines network, and a temporary Program for the Array Seismic Studies of the Continental Lithosphere deployment in TauTona Mine, South Africa, to determine full moment tensors of 100 mining‐induced earthquakes in the magnitude range −2.7 M w F ‐test to determine the need for including an isotropic component with an extra degree of freedom in the solution. The results indicate 82% of the events have well‐constrained solutions, and 45% of the well‐constrained events require an isotropic source term. Throughout the magnitude range, both deviatoric and implosive mechanisms are observed, with implosive ratios of volume change to shear deformation (ΔV/Σ τ ) of −1.03 to −0.15. Two explosive events are observed at M w −0.5 and −0.2, withΔV/Σ τ =0.15 and 0.51, respectively. For the largest events, we determine maximum slip and apparent stress ( τ a ) and find values consistent with those of natural tectonic earthquakes, with 0.1≤ τ a ≤9.2  MPa. Our results support previous speculation on the nature of isotropic components of mining‐induced earthquakes, in which events of all sizes begin as shear failure that may intersect a void (tunnel or stope) and cause collapse, whereas only small events result in explosive sources.

Mechanics of fault reactivation before, during, and after the 2015 eruption of Axial Seamount

Ocean-bottom seismic and seafloor pressure data from the Ocean Observatories Initiative’s Cabled Array were used to study fault reactivation within Axial Seamount (offshore Oregon, USA). Microearthquakes that occurred during 2015–2016 were located on portions of an outward-dipping ring fault system that was reactivated in response to the inflation and deflation of the underlying magma chamber. Prior to an eruption in April 2015, focal mechanisms showed a pattern of normal slip consistent with the differential vertical uplift of the caldera floor relative to the rim. During the eruption, seismic activity remained localized along these outward-dipping structures; however, the slip direction was reversed as the caldera floor subsided. After the eruption, as the volcano reinflated and the caldera floor uplifted, these faults exhibited sparser seismicity with a more heterogeneous pattern of slip. Monitoring the evolution of ring fault behavior through time may have utility as a metric in future eruption forecasts. INTRODUCTION At active volcanoes, as magma is withdrawn from a chamber during an eruption, faults commonly form in response to the subsidence and collapse of the overlying material—developing as either inward-dipping (normal) or outwarddipping (reverse) structures, depending on the amount of subsidence, geometry of the magma chamber, and tectonic setting (Cole et al., 2005; Holohan et al., 2005; Acocella, 2007; Martí et al., 2008). These collapse structures often exhibit a circular to elliptical pattern in plan view and are commonly referred to as ring faults. Reactivation of ring faults has been documented in regional earthquakes studies (e.g., Nettles and Ekström, 1998; Shuler et al., 2013; Gudmundsson et al., 2016); however, the mechanical role of these structures during different phases of the volcanic cycle remains poorly understood. Axial Seamount is a basaltic shield volcano located at the intersection of the Cobb-Eickelberg hotspot and Juan de Fuca Ridge (offshore Oregon, USA; intermediate spreading rate of 55–60 mm/yr). The summit hosts a caldera at 1500 m below sea level (bsl) that is an elongate depression ~3 km wide and 8.5 km long, with walls up to ~150 m high that are buried by younger lavas to the south (Fig. 1). Multichannel seismic reflection studies have imaged a 3-km-wide by 14-km-long magma chamber offset slightly to the east of the caldera at a depth of 1.1−2.3 km beneath the caldera floor (Arnulf et al., 2014). Volume-predictable eruptive behavior has been proposed based on bottom-pressure recorder (BPR) studies that have tracked the inflation and deflation of Axial Seamount for nearly two decades, capturing diking events in 1998, 2011, and 2015 (Chadwick al., 2012; Nooner and Chadwick, 2016). Seismicity associated with the 1998 and 2011 eruptions was recorded by regional hydrophone arrays (Dziak and Fox, 1999) and by ocean bottom hydrophones (Dziak et al., 2012), respectively. Following the 1998 eruption, temporary arrays of 4–10 ocean bottom seismometers (OBSs) monitored local seismicity for 15 mo (Sohn et al., 2004). During the 1998 and 2011 eruptions, pointsource elastic deformation models indicate that the volume of the magma reservoir decreased by ~0.21 km3 and ~0.15 km3, as dikes propagated 55 km and 33 km, respectively, along the southern rift zone (Chadwick et al., 2012). The most recent eruption at Axial Seamount began on 24 April 2015 and was recorded by a seven-station network of three-component OBSs installed within the caldera as part of the Ocean Observatories Initiative (OOI; http://oceanobservatories.org; Kelley et al., 2014). A dike propagated northward from the eastern margin of the magma chamber, erupting a series of lava flows extending from the northeast caldera floor along the north rift zone up to ~14 km north of the caldera (Chadwick et al., 2016). Seafloor explosions associated with the emplacement of these flows indicate that the 2015 eruption persisted over a period of ~26 d (Wilcock et al., 2016), during which time the volume of the magma reservoir (modeled as a prolate spheroid) decreased by ~0.29 km3 (Nooner and Chadwick, 2016). During the time period immediately surrounding the eruption (January–September 2015), Wilcock et al. (2016) identified two steeply dipping, outward-facing planes of microearthquakes beneath the southern portion of the caldera. These structures were interpreted to represent portions of a ring fault system reactivated in response to the inflation and deflation of the magma chamber. In this study, we created an independent catalog of microearthquakes (median Mw ~1.0) for a longer time period between January 2015 and December 2016. These data confirmed the proposed fault geometry and allowed us to track changes in fault slip direction over a nearly 2 yr period. OBS data were used to generate a time series of composite focal mechanism solutions GEOLOGY, May 2018; v. 46; no. 5; p. 447–450 | GSA Data Repository item 2018138 | https://doi.org/10.1130/G39978.1 | Published online 14 March 2018

Thermal segmentation of mid-ocean ridge-transform faults

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