Fortnightly modulation of San Andreas tremor and low-frequency earthquakes

Earth tides modulate tremor and low-frequency earthquakes (LFEs) on faults in the vicinity of the brittle-ductile (seismic-aseismic) transition. The response to the tidal stress carries otherwise inaccessible information about fault strength and rheology. Here, we analyze the LFE response to the fortnightly tide, which modulates the amplitude of the daily tidal stress over a 14-d cycle. LFE rate is highest during the waxing fortnightly tide, with LFEs most strongly promoted when the daily stress exceeds the previous peak stress by the widest margin. This pattern implies a threshold failure process, with slip initiated when stress exceeds the local fault strength. Variations in sensitivity to the fortnightly modulation may reflect the degree of stress concentration on LFE-producing brittle asperities embedded within an otherwise aseismic fault.

An apparatus to measure frictional, anelastic, and viscous behavior in ice at temperate and planetary conditions

In this paper, we describe a cryogenic, servo-controlled biaxial friction apparatus designed to measure the deformational behaviors of ice. The apparatus is specifically designed to accurately achieve and measure the low differential stresses applicable to deforming ice on earth and on icy satellites. We can apply loads in the range ∼2-1800 kPa and velocities up to 4 mm/s, with resolution of 39 Pa and 0.7 μm, respectively. Precise temperature control, measurement, and insulation allow testing at constant temperature (from -2 to -30 °C) for prolonged periods of time. The apparatus is tested with various plastics as well as with polycrystalline ice samples and the results are consistent with previously published values. Critical components of the instrument are described along with examples of data collection schemes and preliminary results. The flexibility of the design allows for both glaciological and planetary applications over a range of deformational behaviors including friction, anelastic, and viscous.

Probabilistic model of waiting times between large failures in sheared media

Using a probabilistic approximation of a mean-field mechanistic model of sheared systems, we analytically calculate the statistical properties of large failures under slow shear loading. For general shear F(t), the distribution of waiting times between large system-spanning failures is a generalized exponential distribution, ρ_{T}(t)=λ(F(t))P(F(t))exp[-∫_{0}^{t}dτλ(F(τ))P(F(τ))], where λ(F(t)) is the rate of small event occurrences at stress F(t) and P(F(t)) is the probability that a small event triggers a large failure. We study the behavior of this distribution as a function of fault properties, such as heterogeneity or shear rate. Because the probabilistic model accommodates any stress loading F(t), it is particularly useful for modeling experiments designed to understand how different forms of shear loading or stress perturbations impact the waiting-time statistics of large failures. As examples, we study how periodic perturbations or fluctuations on top of a linear shear stress increase impact the waiting-time distribution.

Stabilizing stick-slip friction

Even the most regular stick-slip frictional sliding is always stochastic, with irregularity in both the intervals between slip events and the sizes of the associated stress drops. Applying small-amplitude oscillations to the shear force, we show, experimentally and theoretically, that the stick-slip periods synchronize. We further show that this phase locking is related to the inhibition of slow rupture modes which forces a transition to fast rupture, providing a possible mechanism for observed remote triggering of earthquakes. Such manipulation of collective modes may be generally relevant to extended nonlinear systems driven near to criticality.