Episodic Slow Slip and Tremor

Much of my recent work has involved studying slow slip and tectonic tremor in subduction zones.  Discovered about the year 2000 (in leap-frog fashion) in Japan and Cascadia, the two coupled styles of fault slip occur quasi-periodically, down-dip of the locked portion of subduction zone thrust faults and several strike-slip faults around the world.  Although when first discovered the question was “How can fault slip accelerate without leading to an earthquake?”, now that there are several proposed mechanisms that could plausibly generate episodic slow slip, that question has morphed into “How can we distinguish among the proposed physical mechanisms for slow slip?”.  I am tackling this question through a combination of numerical analysis and observations.

Jessica Hawthorne first used Earthscope borehole strainmeter data to show that the moment rate of slow slip in Cascadia is modulated by tidal stresses with amplitudes of only 1 kPa.  At the period of the strongest tide, 12.4 hours, the moment rate varies by about 25% above and below the mean, in phase with the tremor rate. The next step was to use this observation of modulation at the tens-of-percent level (as opposed to a few or nearly 100%) as a constraint on numerical models of slow slip.  Using what is arguably the simplest of the proposed mechanisms (a transition from velocity-weakening to velocity-strengthening behavior at a slip speed of about 1 micron/s), she came up with the first prediction of what controls the slow-slip recurrence interval for any of the proposed mechanisms.  She found that it was possible to match both the observed stress drops (or equivalently the recurrence interval) and tidal modulation, given sufficiently low effective normal stresses, but that to do so required pushing the limits of parameter space more than one might like.  

One lesson I learned from this work is that we need even more observations to judge between the proposed mechanisms for slow slip.  The most promising path, I think, lies in obtaining more accurate and complete tremor catalogs.  Tremor is notoriously difficult to locate because it lacks identifiable impulsive P-wave and S-wave arrivals, and is likely made up of simultaneous sources coming from multiple regions of the fault.  I am working on developing a “cross-station” detection/location algorithm, which compares the same short time window at different stations, as opposed to more traditional “cross-time” methods that compare different time windows at the same station.  Figure 1 shows how coherent the seismic signal can be at stations tens of kilometers apart.

Figure 1

Figure 1. Upper panels show horizontal velocity seismograms at 3 seismic stations on southern Vancouver Island, filtered 1.5-6 Hz and then rotated and time-shifted to maximize the mutual cross-correlation values. Lower panels (cyan curves) show the cross correlation value averaged over the 3 station pairs, using a 0.5-s moving window. Time axis is in seconds. (a) shows a local earthquake “caught” by the detector; (b)-(d) show 18 - 24 seconds of tremor.  The tremor contains both simple coherent arrivals, reminiscent of (a) but with lower frequency content, and extended-duration coherent signals that we interpret as superimposed, nearly co-located sources.

This high degree of coherence has resulted in a tremor catalog that is more accurate than any other from anywhere in the world, with relative location errors in the 0.5­­–1 km range.  This has allowed us to image in unprecedented detail small-scale tremor migrations that piggy-back on top of the main slow slip event.  These tend to (a) start at or within about 1 km of the main tremor front, and propagate back along strike at rates 25-50 times faster, about 10-20 km/hr; (b) less commonly do the reverse, ending at the main front; or (c) propagate up- or down-dip at or within 1-2 kilometers of the main front.  Several examples of these secondary fronts can be seen in Figure 2 below, which shows a 10-km-wide region that was very active in each of the slow slip episodes in 2003, 2004, and 2005.  These images are for the 2005 event; the main front propagates SE to NW at about 10 km/day (this can be seen from the progression of the blue colors from panel to panel).

Figure 2

Figure 2b

Figure 2:  Locations of 4-second windows with detections (colored circles) for 8 tremor bursts during the September 2005 slow slip episode beneath southern Vancouver Island.  Time in each panel progresses from blue to red.  Small black circles in background show locations of all 9200 128-second windows with detections for the slow slip episodes in 2003, 2004, and 2005, indicating where in this box the fault is capable of producing significant tremor.  Small red dots are grid points “hit” during the 2005 episode prior to the time window illustrated, allowing one to locate each tremor “burst” relative to the NW-propagating main front.  Most of these are “back-propagating” fronts; panels (c) and (d) show up- and down-dip propagation.  Green trace in lowermost panel is velocity seismogram at station PGC; red sections show 4-s windows with detections.

Activity as first the main front and then the secondary fronts pass through can best be seen on “space-time” plots such as in Figure 3, which shows both the slow progression of the main front to the NW and the much more rapid tremor “bursts” behind, for two days during each of the 2003 and 2004 slow slip episodes (in each of the 2003-2005 events the region of Figure 2was most active for about 2 days).  Colors in these plots indicate the relative “radiated energy” of the tremor detection.  At each location in each of the 3 episodes the tremor amplitude generally starts out low and progressively increases over a period of about ½ day before leveling off, spanning a range of nearly 3 orders of magnitude.

Figure 3

Figure 3.  Along-strike position as a function of time in the region of Figure 2, for two days of 4-second detections during each of the March 2003 and July 2004 slow slip episodes, color-coded by log10 of the relative radiated energy.  Black curves are computed tidal shear stress on the subduction thrust.  Tidal loads modulate the tremor amplitude to some extent but cannot explain most of the long-term variability seen here (note that the low tremor amplitudes at the start of activity in 2004 coincide with large tidal stresses; the same is true of 2005).


Related publications:

  • Simulating Short-Term Evolution of Slow Slip Influenced by Fault Heterogeneities and Tides

    Y. Peng; A. M. Rubin

    Abstract In this study, we analyze high-resolution tremor catalogs from northern Cascadia, Guerrero, and northern Kii Peninsula. We find that tremor often occurs in short bursts that repeatedly occupy the same source area within a slow slip event. We hypothesize that these bursts are driven by loading from slow slip in areas surrounding the...

    numerical simulation; rate-and-state friction; short-term evolution of slow slip; slow slip; subduction zones; tremor bursts
  • Does fault strengthening in laboratory rock friction experiments really depend primarily upon time and not slip?

    P. Bhattacharya; A. M. Rubin; N. M. Beeler

    The popular constitutive formulations of rate-and-state friction offer two end-member views on whether friction evolves only with slip (Slip law) or with time even without slip (Aging law). While rate stepping experiments show support for the Slip law, laboratory-observed frictional behavior near zero slip rates has traditionally been inferred...

    Bayesian inversion; earthquake cycle; frictional healing; laboratory friction; rate-state friction; state evolution laws
  • Intermittent tremor migrations beneath Guerrero, Mexico, and implications for fault healing within the slow slip zone

    Y. Peng; A. M. Rubin

    Abstract Slow slip events exhibit significant complexity in slip evolution and variations in recurrence intervals. Behavior that varies systematically with recurrence interval is likely to reflect different extents of fault healing between these events. Here we use high-resolution tremor catalogs beneath Guerrero, Mexico, to investigate the...

    cross-station method; fault zone healing; Guerrero, Mexico; permeability evolution; slow slip; tectonic tremor
  • Imaging slow slip fronts in Cascadia with high precision cross-station tremor locations

    A. M. Rubin; J. G. Armbruster

    We apply a new method to obtain accurate locations of tremor sources beneath southern Vancouver Island. Unlike more standard "cross-time" methods, which compare waveforms from different time windows at the same station, this "cross-station" method compares waveforms from the same time window at widely separated stations. It performs well,...

    Cascadia subduction zone; episodic tremor and slip; slow slip; tectonic tremor; tremor location algorithms
  • Laterally propagating slow slip events in a rate and state friction model with a velocity-weakening to velocity-strengthening transition

    J. C. Hawthorne; A. M. Rubin

    We investigate the behavior of simulated slow slip events using a rate and state friction model that is steady state velocity weakening at low slip speeds but velocity strengthening at high slip speeds. Our simulations are on a one-dimensional (line) fault, but we modify the elastic interactions to mimic the elongate geometry frequently...

    rate and state friction; slow earthquakes; weakening to strengthening transition
  • Short-time scale correlation between slow slip and tremor in Cascadia

    J. C. Hawthorne; A. M. Rubin

    We use borehole strain and seismic data to show that slow slip and tremor in central Cascadia are correlated on a range of time scales shorter than 1 day. The recorded strain rate is our proxy for the slow slip moment rate, and the seismic amplitude is our proxy for the tremor amplitude. We find that, on average, the strain rate is higher when...

    borehole strain data; slow earthquakes; tremor
  • Tidal modulation and back-propagating fronts in slow slip events simulated with a velocity-weakening to velocity-strengthening friction law

    J. C. Hawthorne; A. M. Rubin

    We examine tidal modulation and back-propagating fronts in simulated slow slip events using a rate and state friction law that is steady state velocity weakening at low slip rates and velocity strengthening at high slip rates. Tidal forcing causes a quasi-sinusoidal modulation of the slip rate during the events, with the maximum moment rate...

    rate and state friction; slow earthquakes; slow slip events; velocity-weakening to -strengthening transition
  • Complex characteristics of slow slip events in subduction zones reproduced in multi-cycle simulations

    H. V. Colella; J. H. Dieterich; K. Richards-Dinger; A. M. Rubin

    Since the discovery of slow slip events along subduction zone interfaces worldwide, dense geodetic and seismic networks have illuminated detailed characteristics of these events and associated tremor. High-resolution observations of tremor, where the spatial-temporal evolution is presumed to reflect that of the underlying slow slip events, show...

    analytic solutions; earthquake physics; slip/tremor migration; slow slip events
  • Designer friction laws for bimodal slow slip propagation speeds

    A. M. Rubin

    A striking observation from both Cascadia and Japan is that the tremor associated with slow slip often migrates along strike at speeds close to 10 km/d but updip and downdip at speeds approaching 100 km/h. In this paper I adopt the view that the friction law appropriate for these regions is unknown, and I ask what constraints the observed...

    rate-and-state friction; slow slip
  • Dilatant strengthening as a mechanism for slow slip events

    P. Segall; A. M. Rubin; A. M. Bradley; J. R. Rice

    The mechanics of slow slip events (SSE) in subduction zones remain unresolved. We suggest that SSE nucleate in areas of unstable friction under drained conditions, but as slip accelerates dilatancy reduces pore pressure p quenching instability. Competition between dilatant strengthening and thermal pressurization may control whether slip is...

    slow slip
  • Role of fault gouge dilatancy on aseismic deformation transients

    Y. Liu; A. M. Rubin

    In the vicinity of episodic aseismic transients in several subduction zones, the presence of interstitial fluids and near-lithostatic pore pressure has been proposed to interpret seismic observations of high P to S wave speed ratio and high Poisson's ratio. Under such conditions, fault stabilization by dilatancy-induced suction during increased...

    fault gouge dilatancy; slow slip events; subduction zone
  • Tidal modulation of slow slip in Cascadia

    J. C. Hawthorne; A. M. Rubin

    Several studies have shown that the seismic tremor in episodic tremor and slip is tidally modulated, suggesting a sensitivity to the rather small tidal stresses. We address whether the slip rate in slow slip events is also tidally modulated by examining data from six borehole strainmeters in northwest Washington and southern Vancouver Island....

    borehole strainmeter; slow slip
  • Episodic slow slip events and rate-and-state friction

    A. M. Rubin

    There are several ways of generating episodic slow slip events in models of rate-and-state friction. Here I explore the possibility that they arise on velocity-weakening faults whose length is "tuned" in some sense. Unlike spring-block sliders, which have a unique critical stiffness for instability, elastically deformable faults have multiple...

    episodic slip; rate-and-state friction; slow earthquakes