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

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:

5 Publications
Applied Filters: First Letter Of Title: T Reset

Field observations indicate that zones of inelastic deformation produced at the tips of propagating dikes can be much larger than those produced at the tips of tensile cracks in laboratory experiments. This is in direct conflict with the concept that fracture toughness and fracture energy are rock properties, independent of crack size and…

We consider the thermal history and dynamics of magma emplacement in giant feeder dikes associated with continental flood basalts. For driving pressure gradients inferred for giant dike swarms, thicknesses of <10 m would enable dikes to transport magma laterally over the distances observed in the field (up to thousands of kilometers) without…

We consider solidification of hot fluid flowing through a rigid-wall channel of infinite extent. The calculated ?thermal arrest? lengths are used to investigate the role of magma freezing in limiting the propagation distance of lateral dike intrusions. Our results demonstrate that for reasonable parameters the propagation distances of meter…

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…

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…