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. 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).
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. 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:
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.
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…
Geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the Earth's surface. Elastic modeling of the vertical ground-surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault…
Observations of eroded volcanic rift zones indicate that dikes in Iceland are typically several times thicker than those in Hawaii. Geodetic and seismic observations of active rifts, however, suggest that dike heights in the two regions are similar. Provided the elastic properties of the rift zones are the same, this implies that dikes are…
Shock wave data to provide an equation of state of muscovite (initial density: 2.835 g/cm3) were determined up to a pressure of 141 GPa. The shock velocity (Us) versus particle velocity (Up) data are fit with a single linear relationship: Us=4.62(±0.12) +1.27(±0.04)Up (km/s). Third-order Birch-Murnaghan equation of state parameters (isentropic…
Planar impact experiments were employed to induce dynamic tensile failure in Bedford limestone. Rock discs were impacted with aluminum and polymethyl methacralate (PMMA) flyer plates at velocities of 10 to 25 m/s. Tensile stress magnitudes and duration were chosen so as to induce a range of microcrack growth insufficient to cause complete…
Field observations and geodetic data indicate that dike intrusion in volcanic rift zones typically generates normal faulting and graben subsidence at the Earth's surface. Elastic models indicate that two-dimensional (infinite strike length) dikes do not lower the ground surface above the dike and that normal faults do not lower the surface…
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…
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…
Dikes beginning to propagate away from a magma source are thin and grow slowly, and thus are susceptible to freezing. A self-similar solution is obtained for a dike propagating down a temperature gradient when the wallrock and magma temperatures are equal at the chamber wall. The solution applies only to the special case of a single-component…
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…
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…
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…
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…
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…
New equation of state data for a weathered granite shocked to about 125 GPa are reported and combined with the Westerly granite data of McQueen, Marsh & Fritz (1967). The shock velocity (Us)-particle velocity (Up) relations can be fitted with two linear regressions: Us = 4.40 + 0.6Up for a range of Up up to about 2 km s−1 and Us = 2.66 + 1…
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…
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…
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…
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…
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…
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…
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 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…
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…
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 investigate the ability of magma to propagate along preexisting fractures oblique to the least compressive stress. Relaxation of the preexisting shear stress to zero over the portion of the fracture dilated by magma (the dike) results in slip for some distance along the closed portion of the fracture ahead of the dike tip and a stress…
The main objective of this study is to see if a lower threshold for earthquake triggering exists. Resolving this issue is important for the understanding of earthquake mechanics and for the purpose of hazard analysis. We compute the cumulative static stress changes imposed on 63 M ≥ 4.5 earthquakes in central California between 1969 and 1998,…
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…
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…
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…
We use relocated catalogs of microearthquakes to investigate earthquake interaction along sections of the Sargent, Calaveras, and San Andreas faults in California. We examine the stress dependence of seismicity rate change along the three fault segments and find that the seismicity rate following a mainshock decays approximately as 1/time, the…
Dieterich [1994] modeled the response to a stress step of a population of faults governed by rate- and state-dependent friction. This model assumes that aftershocks nucleate over areas on the fault that at the time of the main shock are already accelerating toward failure and disregards the effect of interactions among aftershocks. The main…
[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…
We obtain quasi-static, two-dimensional solutions for earthquake nucleation on faults obeying Dieterich's ?aging? version of the rate and state friction equations. Two distinct nucleation regimes are found, separated by roughly a/b ? 0.5, where a and b are the constitutive parameters relating changes in slip rate V and state ? to frictional…
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…
Taiwan's 1999 Mw 7.6 earthquake generated over 85 km surface rupture along the Chelungpu thrust fault. Paleoseismic studies at the Shi-Jia site near Nantou city, reveal folding as the predominant form of deformation. Stratigraphic relations across the 1999 fold scarp show the style and degree of deformation caused by the penultimate event is…
[1] 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…
Correction to “Aftershock asymmetry on a bimaterial surface”
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…
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…
For a wide range of conditions, earthquake nucleation zones on rate- and state-dependent faults that obey either of the popular state evolution laws expand as they accelerate. Under the "slip" evolution law, which experiments show to be the more relevant law for nucleation, this expansion takes the form of a unidirectional slip pulse. In…
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…
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…
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…
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…
Earthquakes often occur on faults separating materials with different elastic properties. On theoretical grounds, it is expected that earthquakes on such bimaterial interfaces might have a preferred rupture propagation direction, that being the direction of motion of the more compliant material. The goal of this paper is to determine whether a…