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:

73 Publications

Planar impact experiments were employed to induce dynamic tensile failure in Bedford limestone. Rock discs were impacted with aluminum and polymethyl methacralate flyer plates at velocities of 10 to 25 m/s. This resulted in tensile stresses in the range of ~11 to 160 MPa. Tensile stress durations of 0.5 and 1.3 μs induced microcrack growth…

We compare 2-D, quasi-static earthquake nucleation on rate-and-state faults under both "aging" and "slip" versions of the state evolution law. For both versions mature nucleation zones exhibit 2 primary regimes of growth: Well above and slightly above steady state, corresponding respectively to larger and smaller fault weakening rates. Well…

Nonvolcanic tremor is an important component of the slow slip processes which load faults from below, but accurately locating tremor has proven difficult because tremor rarely contains clear P or S wave arrivals. Here we report the observation of coherence in the shear and compressional waves of tremor at widely separated stations which allows…

During rock friction experiments at large displacement, room temperature and humidity, and following a hold test, the fracture energy increases approximately as the square of the logarithm of hold duration. While it s been long known that failure strength increases with log hold time, here the slip weakening distance, dh, also increases. The…

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…

Abstract The variations in the response of different state evolution laws to large velocity increases can dramatically alter the style of earthquake nucleation in numerical simulations. But most velocity step friction experiments do not drive the sliding surface far enough above steady state to probe this relevant portion of the parameter space…

A new state evolution law has recently been proposed by Nagata et al. (2012) that includes a dependence upon stressing rate through a laboratory derived proportionality constant c. It has been claimed that this law, while retaining the time-dependent healing of the Dieterich (or Aging) law, can also match the symmetric response of the Ruina (or…

The duration of each subevent of 48 earthquakes with magnitude larger than 5.5 and depth greater than 100 km was determined from stacked traces of broadband records of Global Seismograph Network stations. We fitted the source time function by one or more triangles convolved with attenuation. We found that global stacks of displacement…

Abstract We have recently suggested that the nearly constant duration of low-frequency earthquakes (LFEs) (and, equivalently, the band limitation of tectonic tremor) manifests a moment-duration scaling that is fundamentally different from regular earthquakes and is most easily explained as rupture on asperities of roughly constant dimension. In…

Abstract We employ 130 low-frequency earthquake (LFE) templates representing tremor sources on the plate boundary below southern Vancouver Island to examine LFE magnitudes. Each template is assembled from hundreds to thousands of individual LFEs, representing over 269,000 independent detections from major episodic-tremor-and-slip (ETS) events…

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…

The empirical constitutive modeling framework of rate- and state-dependent friction (RSF) is commonly used to describe the time-dependent frictional response of fault gouge to perturbations from steady sliding. In a previous study (Ferdowsi & Rubin, 2020), we found that a granular-physics-based model of a fault shear zone, with time-independent…

Rate‐ and state‐dependent friction (RSF) equations are commonly used to describe the time‐dependent frictional response of fault gouge to perturbations in sliding velocity. Among the better‐known versions are the Aging and Slip laws for the evolution of state. Although the Slip law is more successful, neither can predict all the robust features…

We investigate the dynamics of viscous pressure losses associated with lateral magma transport in volcanic rift zones by performing (1) coupled elastic-hydrodynamic simulations of downrift magma flow in dikes and (2) analog experiments mimicking lateral dike propagation in the presence of an along-rift topographic slope. It is found that near…

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…

High confining pressure fracture tests of Indiana limestone [Abou-Sayed, 1977] and Iidate granite [Hashida et al., 1993] were simulated using boundary element techniques and a Dugdale-Barenblatt (tension-softening) model of the fracture process zone. Our results suggest a substantial (more than a factor of 2) increase in the fracture energy of…

FREQUENT shallow earthquakes within the rift zones of the Hawaiian volcano Kilauea have been interpreted as resulting from stress changes associated with a shallow magma conduit system1,2. Here, by using a precise earthquake relocation technique3, we show that what had been imaged as a diffuse cloud of seismicity in the Upper East Rift in 1991…

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…

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…

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…

The November 30, 1974, ML = 5.5 and November 16, 1983, ML = 6.6 earthquakes generated left-stepping, en echelon ground cracks within the Kaoiki seismic zone, on the southeast flank of Mauna Loa volcano, Hawaii. The general trend of the ruptures, N48°-55°E, parallels a nodal plane of the main shocks' focal mechanisms. The ruptures themselves…

We use seismic waveform cross correlation to determine the relative positions of 2747 microearthquakes near Mount Lewis, California, that have waveforms recorded from 1984 to 1999. These earthquakes include the aftershock sequence of the 1986 ML5.7 Mount Lewis earthquake. Approximately 90% of these aftershocks are located beyond the tips of the…

Eruptions on the ice moon Enceladus provide access to materials from Enceladus’ ocean. The mechanism that drives and sustains the eruptions is unclear, and it is also not known what sets the rate of volcanism. We found that a simple model in which the erupting fissures are underlain by slots that connect the surface to the ocean can explain the…

Abstract Whether rate- and state-dependent friction evolution is primarily slip dependent or time dependent is not well resolved. Although slide-hold-slide experiments are traditionally interpreted as supporting the aging law, implying time-dependent evolution, recent studies show that this evidence is equivocal. In contrast, the slip law…

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…

[1] The recurrence intervals for 194 repeating clusters on the Calaveras fault follow a power‐law decay relation with elapsed time after the 1984 M6.2 Morgan Hill, California, mainshock. The decay rates of repeating aftershocks in the immediate vicinity of a high‐slip patch that failed during the mainshock systematically exceed those that…

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…

We develop a cross-station method to detect and locate tremor and low-frequency earthquakes (LFEs), based on the original work of Armbruster et al. (2014) that compares waveforms from the same time window at stations separated by roughly 10 km. To improve the signal-to-noise ratio, we first rotate the horizontal components into the empirical shear…

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…

Abstract Episodic tremor and slip (ETS) in subduction zones is generally interpreted as the manifestation of shear slip near the base of earthquake-generating portion of the plate interface. Here we devise a new method of cross-correlating stacked Array of Arrays seismic data that provides greatly improved tremor locations, a proxy for the…

Stress inversion methods employed by structural geologists for estimating a regional stress tensor from populations of faults containing slickenlines rely on the basic assumption that slip on each fault plane occurs in the direction of maximum resolved regional shear stress. This premise ignores directional differences in fault compliance…

Recent relocation and focal mechanism analyses of deep earthquakes beneath Kilauea volcano, Hawaii indicate that seismicity is concentrated on a horizontal fault zone at a depth of 30 km, with seaward slip of the upper block on a low-angle plane. We discuss whether the observed localization of the earthquakes can be explained primarily by…

G. Baer & A. Heimann (eds).

This work combines research results with review papers, discussing dykes from different scientific perspectives. Coverage includes: current dyke geometry measurements; field observation of host rock deformation; textural analyses; and geochemical and petrological studies of dyke swarms.

Earthquakes of magnitude 1 and greater seem to be ubiquitous features of dike propagation, but their origin is not well understood. We examine the elastic stress field surrounding propagating fluid‐filled cracks, with an emphasis on assessing the ambient stress required to produce earthquakes with linear dimensions of ∼100 m near dikes with…

Seismic and geodetic data have demonstrated that dikes in the rift zones of Kilauea Volcano in Hawaii and Krafla Volcano in Iceland are typically intruded laterally from a central magma reservoir and acquire a blade-like form. A remarkable feature of many such dikes is that they propagate at shallow depths for 10s of km without erupting. Using…

Using a waveform cross-correlation technique, Rubin and Gillard [2000] obtained precise relative locations for 4300 0.5 < M < 3.5 earthquakes occurring along 50 km of the San Andreas fault. This study adds to that another 5000 earthquakes distributed along 10 km of the San Andreas fault and 20 km of the Calaveras fault. Errors in relative…

In January 1983, a dike intrusion/fissure eruption generated a swarm of 375 magnitude 1 to 3 earthquakes along a 16‐km segment of Kilauea's Middle East Rift Zone. We searched the Hawaiian Volcano Observatory catalog for multiplets of similar events from this region from 1980 through 1985 and obtained precise relative locations by waveform cross…

Geophysical models have traditionally treated diapiric ascent as occurring in purely viscous host rock, and dike intrusion as occurring in purely elastic host rock. Such models are incapable of determining (1) what governs the transition between the two transport mechanisms, (2) the properties of diapirs that ascend via a combination of…

Using a waveform cross-correlation technique, we have obtained precise relative locations for nearly 75% of the Northern California Seismic Network catalog (4300 earthquakes) occurring between 1984 and 1997 along 50 km of the San Andreas fault. Errors in relative location are meters to tens of meters for events separated by tens to hundreds of…

Waveform cross-correlation allows one to measure the relative arrival times of similar microearthquakes with errors of less than 1/10 of 1 sample. Location algorithms based on these measurements have greatly improved images of earthquake distribution. For the Northern California Seismic Network catalog, however, the relative location errors…

To better understand the asymmetric distribution of microearthquake aftershocks along the central San Andreas fault, we study dynamic models of slip-weakening ruptures on an interface separating differing elastic half-spaces. Subshear ruptures grow as slightly asymmetric bilateral cracks, with larger propagation velocities, slip velocities, and…

Crustal faults that produce most of their slip aseismically typically generate large numbers of small earthquakes. These events have generally been interpreted as coming from localized patches of the fault that undergo unstable (stick–slip) sliding, surrounded by larger regions of stable sliding (creep). In published catalogues the…

Field observations indicate that dikes form and grow in magma source regions, but the mechanics of this process are poorly understood. I derive time-dependent and self-similar solutions for the growth of buoyant dikes fed by porous flow in partially molten rock. The host rock is treated as poroelastic; for basaltic (but not rhyolitic) dikes,…

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,…

Zhou et al. (2012) proposed that longitudinal dunes in the Qaidam Basin, China, formed like yardangs: by erosion into sediment that was not deposited by those dunes. Because erosion occurs on the upwind fl anks of most migrating dunes (Rubin and Hunter, 1982, 1985), the key to demonstrating a yardang-like origin is to show that the dunes did…

The mechanism of magma transport at depth influences direction magma moves, the distance it travels before freezing, the degree to which it communicates chemically with the host rock, the form of surficial volcanism, and ultimately the growth of oceanic and continental crust. Commonly envisioned transport processes include porous flow in…

Whether a dike can propagate far from a magma reservoir depends upon the competition between the rate at which propagation widens the dike and the rate at which freezing constricts the aperture available for magma flow. Various formulations are developed for a viscous fluid at temperature Tm intruding a growing crack in an elastic solid. The…