No abstract

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.

[1] We investigate the physics of laboratory earthquake precursors in a biaxial shear configuration. We conduct laboratory experiments at room temperature and humidity in which we shear layers of glass beads under applied normal loads of 2-8 MPa and with shearing rates of 5-10 μm/s. We show that above~3 MPa load, acoustic emission (AE), and shear microfailure (microslip) precursors exhibit an exponential increase in rate of occurrence, culminating in stick-slip failure. Precursors take place where the material is in a critical state-still modestly dilating, yet while the macroscopic frictional strength is no longer increasing.

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.

The Black Canyon of the Gunnison and Unaweep Canyon in western Colorado have long been viewed as classic examples of post-Laramide Plio-Pleistocene uplift, which in the case of Unaweep, is thought to have forced the Gunnison River to abandon the canyon. Ongoing fi eld studies of the incision histories of these canyons and their surrounding regions, however, suggest that post-Laramide rock uplift has been regional, rather than local in nature. River incision rates calculated using ca. 10 Ma basaltic lava fl ows as a late Miocene datum suggest that long-term incision rates range from 61 to 142 m/m.y. with rates decreasing eastward towards the central Rocky Mountains. Incision rates calculated using the ca. 640 ka Lava Creek B ash range from 95 to 162 m/m.y., decrease eastward towards the mountains, and are broadly similar in magnitude to the longer-term incision rates. Locally, incision rates are as high as 500-600 m/m.y. along the lower reaches of the Black Canyon of the Gunnison, and these anomalously high values refl ect transient knickpoint migration upvalley. Knickpoint migration was controlled, in part, by downvalley base-level changes related to stream piracy. For example, abandonment of Unaweep Canyon by the Gunnison River could have led to rapid incision through erodible Mancos Shale as the Gunnison River joined the Colorado River on its course around the northern end of the Uncompahgre Plateau. Geophysical data show that abandonment of Unaweep Canyon was not caused by differential uplift of the crest of Unaweep Canyon relative to the surrounding basins. Instead, the ancestral (Plio-Pleistocene?) Gunnison River fl owed through Cactus Park, a major paleovalley that feeds into Unaweep Canyon, and continued downvalley to its juncture with the Dolores River near present-day Gateway, Colorado. The average gradient of the ancestral Gunnison River through the canyon prior to abandonment was ~7.5-7.6 m/km. Lithological and mineralogical considerations suggest that the Colorado River also fl owed through and helped to carve Unaweep Canyon, although the Colorado River probably exited Unaweep Canyon prior to abandonment by the Gunnison River. The ancestral Gunnison River remained in its course and incised through bedrock for a long enough period of time to produce terrace remnants in the Cactus Park region that range in elevation from 2000 to 1880 m. Abandonment of the canyon by the Gunnison River was followed by formation of a natural dam that probably led to deposition upvalley of ~50 m of lacustrine sediments in Cactus Park. Recent mapping in the lower reaches of Unaweep Canyon indicate that a landslide could have led to damming of Unaweep Canyon, perhaps while it was occupied by underfi t streams.