Nicholas J. van der Elst

Enhanced remote earthquake triggering at fluid-injection sites in the midwestern United States

Movers and Shakers We tend to view earthquakes as unpredictable phenomena caused by naturally shifting stresses in Earths crust. In reality, however, a range of human activity can also induce earthquakes. Ellsworth (p. 10.1126/science.1225942) reviews the current understanding of the causes and mechanics of earthquakes caused by human activity and the means to decrease their associated risk. Notable examples include injection of wastewater into deep formations and emerging technologies related to oil and gas recovery, including hydraulic fracturing. In addition to directly causing increased local seismic activity, activities such as deep fluid injection may have other ramifications related to earthquake occurrence. Van der Elst et al. (p. 164; see the news story by Kerr) demonstrate that in the midwestern United States, some areas with increased human-induced seismicity are also more prone to further earthquakes triggered by the seismic waves from large, remote earthquakes. Improved seismic monitoring and injection data near deep disposal sites will help to identify regions prone to remote triggering and, more broadly, suggest times when activities should, at least temporarily, be put on hold. Wastewater injected deep underground can make some faults more susceptible to triggering by large remote earthquakes. A recent dramatic increase in seismicity in the midwestern United States may be related to increases in deep wastewater injection. Here, we demonstrate that areas with suspected anthropogenic earthquakes are also more susceptible to earthquake-triggering from natural transient stresses generated by the seismic waves of large remote earthquakes. Enhanced triggering susceptibility suggests the presence of critically loaded faults and potentially high fluid pressures. Sensitivity to remote triggering is most clearly seen in sites with a long delay between the start of injection and the onset of seismicity and in regions that went on to host moderate magnitude earthquakes within 6 to 20 months. Triggering in induced seismic zones could therefore be an indicator that fluid injection has brought the fault system to a critical state.

Connecting near-field and far-field earthquake triggering to dynamic strain

triggering at dynamic strain amplitudes down to 3 × 10 −9 , orders of magnitude smaller than previously reported. This threshold appears to be an observational limit and shows that extremely small dynamic strains can trigger faults that are sufficiently near failure. Using a probabilistic model to transform measured interevent times to seismicity rate changes, we find that triggering rates in the far field scale with peak dynamic strain. This scaling, projected into the near field, accounts for 15%–60% of earthquakes within 6 km of magnitude 3–5.5 earthquakes. Statistical seismicity simulations validate the interevent time method and show that the data are consistent with the number of far‐field triggered earthquakes being linearly proportional to peak dynamic strain. We interpret the additional near‐field component as reflecting either static stress triggering, more effective dynamic triggering at higher frequencies, or a concentration of aftershock nucleation sites very near main shocks.

Auto‐acoustic compaction in steady shear flows: Experimental evidence for suppression of shear dilatancy by internal acoustic vibration

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, B09314, doi:10.1029/2011JB008897, 2012 Auto-acoustic compaction in steady shear flows: Experimental evidence for suppression of shear dilatancy by internal acoustic vibration Nicholas J. van der Elst, 1 Emily E. Brodsky, 1 Pierre-Yves Le Bas, 2 and Paul A. Johnson 2 Received 22 September 2011; revised 9 August 2012; accepted 20 August 2012; published 28 September 2012. [ 1 ] Granular shear flows are intrinsic to many geophysical processes, ranging from landslides and debris flows to earthquake rupture on gouge-filled faults. The rheology of a granular flow depends strongly on the boundary conditions and shear rate. Earthquake rupture involves a transition from quasi-static to rapid shear rates. Understanding the processes controlling the transitional rheology is potentially crucial for understanding the rupture process and the coseismic strength of faults. Here we explore the transition experimentally using a commercial torsional rheometer. We measure the thickness of a steady shear flow at velocities between 10 A3 and 10 2 cm/s, at very low normal stress (7 kPa), and observe that thickness is reduced at intermediate velocities (0.1–10 cm/s) for angular particles, but not for smooth glass beads. The maximum reduction in thickness is on the order of 10% of the active shear zone thickness, and scales with the amplitude of shear-generated acoustic vibration. By examining the response to externally applied vibration, we show that the thinning reflects a feedback between internally generated acoustic vibration and granular rheology. We link this phenomenon to acoustic compaction of a dilated granular medium, and formulate an empirical model for the steady state thickness of a shear-zone in which shear-induced dilatation is balanced by a newly identified mechanism we call auto-acoustic compaction. This mechanism is activated when the acoustic pressure is on the order of the confining pressure, and results in a velocity-weakening granular flow regime at shear rates four orders of magnitude below those previously associated with the transition out of quasi-static granular flow. Although the micromechanics of granular deformation may change with greater normal stress, auto-acoustic compaction should influence the rheology of angular fault gouge at higher stresses, as long as the gouge has nonzero porosity during shear. Citation: van der Elst, N. J., E. E. Brodsky, P.-Y. Le Bas, and P. A. Johnson (2012), Auto-acoustic compaction in steady shear flows: Experimental evidence for suppression of shear dilatancy by internal acoustic vibration, J. Geophys. Res., 117, B09314, doi:10.1029/2011JB008897. 1. Introduction [ 2 ] Frictional sliding processes in geophysics often involve granular shear flows at the sliding interface. This is true for landslides and debris flows, as well as for earthquake rup- tures within granulated damage zones or gouge-filled faults. The frictional strength in these contexts is controlled by the rheology of the granular flow, which has a strong dependence Department of Earth and Planetary Science, Univ. of California, Santa Cruz, California, USA. Geophysics Group, Los Alamos National Laboratory, Los Alamos, New Mexico, USA. Corresponding author: Nicholas J. van der Elst, Department of Earth and Planetary Science, 1156 High St., Univ. of California, Santa Cruz, CA 95060, USA. (nvandere@ucsc.edu) ©2012. American Geophysical Union. All Rights Reserved. 0148-0227/12/2011JB008897 on shear rate and boundary conditions [Campbell, 2006; Clement, 1999; Iverson, 1997; Savage, 1984]. [ 3 ] For different shear rates, confining stresses, and pack- ing densities, the description of a granular flow can range from “solid-like” to “gas-like” [Jaeger et al., 1996], albeit with complicated second-order behavior in each regime. The appropriate description for a particular flow is typically determined by the dimensionless inertial number, which compares the magnitude of the grain inertial stresses to the confining stress [Bocquet et al., 2001; Campbell, 2006; Clement, 1999; Iverson, 1997; Lu et al., 2007; Savage, 1984] I ≡ rd g _ 2 p where r is density, d is grain diameter, g _ is the strain rate, and p is the confining (normal) pressure. The shear rate profile in boundary driven flows is commonly observed to decay B09314 1 of 18

Complex Faulting Associated with the 22 December 2003 Mw 6.5 San Simeon, California, Earthquake, Aftershocks, and Postseismic Surface Deformation

We use data from two seismic networks and satellite interferometric synthetic aperture radar (InSAR) imagery to characterize the 22 December 2003 M wxa06.5 San Simeon earthquake sequence. Absolute locations for the mainshock and nearly 10,000 aftershocks were determined using a new three-dimensional (3D) seismic velocity model; relative locations were obtained using double difference. The mainshock location found using the 3D velocity model is 35.704°xa0N, 121.096°xa0W at a depth of 9.7±0.7 km. The aftershocks concentrate at the northwest and southeast parts of the aftershock zone, between the mapped traces of the Oceanic and Nacimiento fault zones. The northwest end of the mainshock rupture, as defined by the aftershocks, projects from the mainshock hypocenter to the surface a few kilometers west of the mapped trace of the Oceanic fault, near the Santa Lucia Range front and the >5 mm postseismic InSAR imagery contour. The Oceanic fault in this area, as mapped by Hall (1991), is therefore probably a second-order synthetic thrust or reverse fault that splays upward from the main seismogenic fault at depth. The southeast end of the rupture projects closer to the mapped Oceanic fault trace, suggesting much of the slip was along this fault, or at a minimum is accommodating much of the postseismic deformation. InSAR imagery shows ∼72 mm of postseismic uplift in the vicinity of maximum coseismic slip in the central section of the rupture, and ∼48 and ∼45 mm at the northwest and southeast end of the aftershock zone, respectively. From these observations, we model a ∼30-km-long northwest-trending northeast-dipping mainshock rupture surface—called the mainthrust—which is likely the Oceanic fault at depth, a ∼10-km-long southwest-dipping backthrust parallel to the mainthrust near the hypocenter, several smaller southwest-dipping structures in the southeast, and perhaps additional northeast-dipping or subvertical structures southeast of the mainshock plane. Discontinuous backthrust features opposite the mainthrust in the southeast part of the aftershock zone may offset the relic Nacimiento fault zone at depth. The InSAR data image surface deformation associated with both aseismic slip and aftershock production on the mainthrust and the backthrusts at the northwest and southeast ends of the aftershock zone. The well-defined mainthrust at the latitude of the epicenter and antithetic backthrust illuminated by the aftershock zone indicate uplift of the Santa Lucia Range as a popup block; aftershocks in the southeast part of the zone also indicate a popup block, but it is less well defined. The absence of backthrust features in the central part of the zone suggests range-front uplift by fault-propagation folding, or backthrusts in the central part were not activated during the mainshock.

Rheologic testing of wet kaolin reveals frictional and bi‐viscous behavior typical of crustal materials

[1]xa0New rheological data for wet kaolin support its use in analog table-top experiments that simulate deformation of the Earths crust. Creep tests at small strain reveal that wet kaolin (62–66% water by mass) exhibits both elastic and viscous deformation characteristic of a Burgers material. When sheared to failure, the shear strength is relatively insensitive to the strain rate. The shear strength appears sensitive to the amount of initial compaction within the rheometer, which may indicate a normal-stress dependency typical of frictional materials. Stepped velocity tests at large strain demonstrate velocity weakening rate and state frictional behavior after yielding. Because the wet kaolin exhibits deformation characteristic of both frictional materials and bi-viscous materials, this material is well suited to simulate a variety of crustal deformational processes in scaled analog experiments.

Remote Triggering Not Evident Near Epicenters of Impending Great Earthquakes

Recently, there have been numerous great ( M w≥8), devastating earthquakes, with a rate in the last seven years that is 260% of the average rate during the 111‐year seismological history. Each great earthquake presents an opportunity to study a major fault at the very beginning and end of the inferred seismic cycle. In this work, we use these events as both targets and sources to probe susceptibility to dynamic triggering in the epicentral region before and after a large earthquake. This study also carefully addresses the possibility that large earthquakes interact in a cascade of remotely triggered sequences that culminates in further large earthquakes. We seek evidence of triggering associated with the 16 great M w≥8 events that occurred between 1998 and 2011, using regional and global earthquake catalogs to measure changes in interevent time statistics. Statistical significance is calculated with respect to a nonstationary reference model that includes mainshock–aftershock clustering. We find limited evidence that a few great earthquakes triggered an increase in seismicity at the site of the next great earthquake in the sequence. However, this evidence is not corroborated by all statistical tests nor all earthquake catalogs. Systematic triggered rate changes in the years to decades before each great earthquake are less than 19% at the 95% confidence level, too small to explain the observed rate increase. The catalogs are insufficient for the purpose of resolving more moderate triggering expected from previous studies. We calculate that an improvement in completeness magnitude from 3.7 to 3.5 could resolve the expected triggering signal in the International Seismic Center (ISC) catalog taken as a whole, but an improvement to M xa02.0 would be needed to consistently resolve triggering on a regional basis.

Frequency dependence of delayed and instantaneous triggering on laboratory and simulated faults governed by rate‐state friction

Earthquake triggering by transient stresses is commonly observed; however, some aspects remain unexplained. The first is the often-observed delay between the triggered earthquakes and the triggering waves, and the second is the unexpected effectiveness of transient stressing in the seismic frequency band. Previous theoretical and laboratory studies have suggested that seismic transients should have little impact on faults if the duration of the transient is smaller than the timescale for nucleation of slip. We reexamine the dynamics of stress triggering during stick-slip sliding on a laboratory fault and make three important observations that pertain to earthquake triggering. (1) Delayed triggering (clock advance) occurs for both bare granite surfaces and granular gouge prior to the onset of instantaneous triggering. (2) Triggering occurs much earlier in the stick-slip cycle than expected for a simple Coulomb stress threshold. (3) Shorter-period (higher stressing rate) pulses are more effective at triggering than longer-period pulses of the same stress amplitude. We use numerical simulations to show that rate-state friction can explain each of the observed features but not all three simultaneously. Only the Ruina slip law for state evolution, in which faults must slip to heal, can reproduce early-onset and stressing rate-dependent triggering. The laboratory and numerical experiments show that faults can remain relatively weak over much of the seismic cycle and that the triggered response depends on a competition between healing and weakening during triggered slip. Transient stressing at seismic frequencies may be more effective at triggering earthquakes than previously recognized.

The magnitude distribution of dynamically triggered earthquakes

Large dynamic strains carried by seismic waves are known to trigger seismicity far from their source region. It is unknown, however, whether surface waves trigger only small earthquakes, or whether they can also trigger large earthquakes. To partially address this question, we evaluate whether current data can distinguish between the magnitude distribution of triggered and untriggered small earthquakes. We use a mixing model approach in which total seismicity is decomposed into two classes: “triggered” events initiated or advanced by far-field dynamic strains and “untriggered” spontaneous events consisting of everything else. The b-value of a mixed data set, bMIX, is decomposed into a weighted sum of b-values of its constituent components, bT and bU. We utilize the previously observed relationship between triggering rate and dynamic strain amplitude to identify the fraction of triggered events in populations of earthquakes and then invert for bT. For Californian seismicity, data are consistent with a single-parameter Gutenberg-Richter hypothesis governing the magnitudes of both triggered and untriggered earthquakes.