Bryan M. Kaproth

Slow Earthquakes, Preseismic Velocity Changes, and the Origin of Slow Frictional Stick-Slip

Slow Stick-Slip While the character of slow earthquakes has been approximated for some time, precise slip histories and the underlying mechanisms have remained elusive. Kaproth and Marone (p. 1229; published online 15 August) have made laboratory observations of repetitive, slow stick-slip in fault-zone materials and developed a mechanical explanation for how earthquake-like dynamic slip nucleation could start and then arrest so as to produce slow slip. As preseismic slip is a precursor to rupture, temporal variations in elastic wave speeds should be monitored in regions of high seismic hazard. Slip-stick experiments reveal the evolution of frictional behavior during slow earthquakes. Earthquakes normally occur as frictional stick-slip instabilities, resulting in catastrophic failure and seismic rupture. Tectonic faults also fail in slow earthquakes with rupture durations of months or more, yet their origin is poorly understood. Here, we present laboratory observations of repetitive, slow stick-slip in serpentinite fault zones and mechanical evidence for their origin. We document a transition from unstable to stable frictional behavior with increasing slip velocity, providing a mechanism to limit the speed of slow earthquakes. We also document reduction of P-wave speed within the active shear zone before stick-slip events. If similar mechanisms operate in nature, our results suggest that higher-resolution studies of elastic properties in tectonic fault zones may aid in the search for reliable earthquake precursors.

Deformation band formation and strength evolution in unlithified sand: The role of grain breakage

[1] We report on laboratory experiments designed to investigate the strength evolution and formation mechanisms of cataclastic deformation bands hosted in unlithified sand, with particular focus on the role of grain breakage. Cataclastic deformation bands are characterized by particle size reduction and increased resistance to weathering compared to parent material. We recovered bands intact from late Quaternary, nearshore marine sand in the footwall of the active McKinleyville thrust fault, Humboldt County, California. Tabular samples 3–5 mm thick and 5 cm × 5 cm in area were sheared at normal stresses representative of in situ conditions, 0.5–1.8 MPa, sliding velocities from 10 mm/s to 10 mm/s, and to shear strain up to 20. Cataclastic deformation bands are stronger than parent material (coefficient of internal friction mi = 0.623 and mi = 0.525, respectively) and exhibit a peak strength followed by weakening. Parent material exhibits significant strain hardening; the frictional yield strength increases up to 9% for a shear strain of 10. Detailed particle size analyses show that strain hardening in parent material is coincident with increased fine particle abundance, resulting from pervasive grain breakage. Our results support the hypothesis that cataclastic deformation bands are stronger than the surrounding parent material due to shear‐driven grain breakage during their formation. We suggest that the combination of strain localization during band formation and strain hardening on individual bands results in dense networks of deformation bands.

Evolution of elastic wave speed during shear‐induced damage and healing within laboratory fault zones

Earthquake faults fail and restrengthen repeatedly during the seismic cycle. Faults restrengthen via a set of processes known collectively as fault healing, which is well documented in the laboratory but less well understood in tectonic fault zones. Recent observations of fault zone wave speed following earthquakes suggest opportunities to connect laboratory and field observations of fault healing. However, existing laboratory data lack detail necessary to identify specific processes linking elastic wave speed to fault damage and healing. Here we document changes in elastic properties during laboratory seismic cycles, simulated via periods of nonshear and quasistatic fault slip. Experiments were conducted on brine-saturated halite under conditions favoring pressure solution, analogous to healing processes within and at the base of the seismogenic zone. We find that elastic wave speed (V) and amplitude (A) correlate with porosity. For each percent of porosity lost during compaction, VP increases by ~3%, VS by ~2%, AP by ~10%, and AS by ~7%. Moreover, V and A decrease with granular dilation during fault slip. With increasing shear strain, fabric formation dominates the ultrasonic signals. We find that fault strength depends on fault porosity, making VP and VS potential proxies for fault strength evolution. Our data show that a 1% change in VP or VS results in a friction increase of 0.01 or 0.02, respectively. Within natural fault zones, advances in monitoring elastic wave speed may provide critical information on the evolution of fault strength and seismic hazard throughout the seismic cycle.

Correction to “Nonlinear dynamical triggering of slow slip on simulated earthquake faults with implications to Earth”

[1] Among the most fascinating, recent discoveries in seismology are the phenomena of dynamically triggered fault slip, including earthquakes, tremor, slow and silent slip—during which little seismic energy is radiated—and low frequency earthquakes. Dynamic triggering refers to the initiation of fault slip by a transient deformation perturbation, most often in the form of passing seismic waves. Determining the frictional constitutive laws and the physical mechanism(s) governing triggered faulting is extremely challenging because slip nucleation depths for tectonic faults cannot be probed directly. Of the spectrum of slip behaviors, triggered slow slip is particularly difficult to characterize due to the absence of significant seismic radiation, implying mechanical conditions different from triggered earthquakes. Slow slip is often accompanied by nonvolcanic tremor in close spatial and temporal proximity. The causal relationship between them has implications for the properties and physics governing the fault slip behavior. We are characterizing the physical controls of triggered slow slip via laboratory experiments using sheared granular media to simulate fault gouge. Granular rock and glass beads are sheared under constant normal stress, while subjected to transient stress perturbation by acoustic waves. Here we describe experiments with glass beads, showing that slow and silent slip can be dynamically triggered on laboratory faults by ultrasonic waves. The laboratory triggering may take place during stable sliding (constant friction and slip velocity) and/or early in the slip cycle, during unstable sliding (stick-slip). Experimental evidence indicates that the nonlinear-dynamical response of the gouge material is responsible for the triggered slow slip.