Kenneth C. Creager

Non-volcanic tremor driven by large transient shear stresses

Non-impulsive seismic radiation or ‘tremor’ has long been observed at volcanoes and more recently around subduction zones. Although the number of observations of non-volcanic tremor is steadily increasing, the causative mechanism remains unclear. Some have attributed non-volcanic tremor to the movement of fluids, while its coincidence with geodetically observed slow-slip events at regular intervals has led others to consider slip on the plate interface as its cause. Low-frequency earthquakes in Japan, which are believed to make up at least part of non-volcanic tremor, have focal mechanisms and locations that are consistent with tremor being generated by shear slip on the subduction interface. In Cascadia, however, tremor locations appear to be more distributed in depth than in Japan, making them harder to reconcile with a plate interface shear-slip model. Here we identify bursts of tremor that radiated from the Cascadia subduction zone near Vancouver Island, Canada, during the strongest shaking from the moment magnitude Mw = 7.8, 2002 Denali, Alaska, earthquake. Tremor occurs when the Love wave displacements are to the southwest (the direction of plate convergence of the overriding plate), implying that the Love waves trigger the tremor. We show that these displacements correspond to shear stresses of approximately 40 kPa on the plate interface, which suggests that the effective stress on the plate interface is very low. These observations indicate that tremor and possibly slow slip can be instantaneously induced by shear stress increases on the subduction interface—effectively a frictional failure response to the driving stress.

Remote triggering of tremor along the San Andreas Fault in central California

Received 30 August 2008; accepted 16 April 2009; published 18 July 2009. [1] We perform a systematic survey of triggered tremor along the San Andreas Fault in central California for the 31 teleseismic earthquakes with Mw � 7.5 since 2001. We identify 10 teleseismic events associated with clear triggered tremor. About 52% of the tremor is concentrated south of Parkfield near Cholame, where ambient tremor has been identified previously, and the rest is widely distributed in the creeping section of the San Andreas Fault north of Parkfield. Tremor is generally initiated and is in phase with the Love wave particle velocity. However, the pattern becomes complicated with the arrival of the Rayleigh waves, and sometimes tremor continues after the passage of the surface waves. We identify two cases in which tremor is triggered during the teleseismic PKP phase. These results suggest that while shear stress from the passage of the Love waves plays the most important role in triggering tremor in central California, other factors, such as dilatational stresses from the Rayleigh and P waves, also contribute. We also examine the ambient tremor occurrence rate before and after the teleseismic events and find a transient increase of stacked tremor rate during the passage of the teleseismic surface waves. This observation implies that the occurrence time of tremor is temporally advanced by the dynamic stresses of the teleseismic waves. The amplitude of the teleseismic waves correlates with the occurrence of triggered tremor, and the inferred tremor-triggering threshold is � 2–3 kPa. The relatively low triggering threshold indicates that the effective stress at the tremor source region is very low, most likely due to near-lithostatic fluid pressure.

Large-scale variations in inner core anisotropy

I analyze nearly 2000 handpicked differential times of core-penetrating compressional waves to image lateral variations in the anisotropic structure of the solid inner core. The inner core is strongly anisotropic (2–4% on average) throughout most of the western hemisphere from near the surface to its center and into the lowermost several hundred kilometers of the eastern hemisphere. In contrast, the outer half of the eastern hemisphere from 40° to 160°E (the quasi-eastern hemisphere) exhibits very weak anisotropy with an average amplitude of only 0.5%. The symmetry direction is the fast direction and lies on or near the spin axis. Voigts isotropic average of compressional wave speeds is the same in the eastern and western hemispheres, suggesting that there are no large-scale lateral variations in the chemistry or temperature in the inner core. Instead, I suggest that the variations seen in anisotropy represent lateral variations in the degree of crystal alignment. The inner core appears to be organized in a very simple way with 60–90% of its volume containing well-aligned crystals and the remaining part (uppermost 400–700 km of the quasi-eastern hemisphere) containing less well aligned crystals. A mechanism consistent with axisymmetry and large-scale variability of anisotropy is discussed. It incorporates the inference that the inner core is rotating faster than the mantle and that gravitational coupling to the mantle forces large-scale inner core deformation, inducing flow with strain rates as high as 10−14 s−1. A positive feedback mechanism, related to anisotropic viscosity, may reinforce lateral variations in the strength of anisotropy.

The global seismographic network surpasses its design goal

This year, the Global Seismographic Network (GSN) surpassed its 128-station design goal for uniform worldwide coverage of the Earth. A total of 136 GSN stations are now sited from the South Pole to Siberia, and from the Amazon Basin to the sea floor of the northeast Pacific Ocean—in cooperation with over 100 host organizations and seismic networks in 59 countries worldwide (Figure 1). Established in 1986 by the Incorporated Research Institutions for Seismology (IRIS) to replace the obsolete, analog Worldwide Standardized Seismograph Network (WWSSN),the GSN continues a tradition in global seismology that dates back more than a century to the network of Milne seismographs that initially spanned the globe. The GSN is a permanent network of state-of-the-art seismological and geophysical sensors connected by available telecommunications to serve as a multi-use scientific facility and societal resource for scientific research, environmental monitoring, and education for our national and international community.

Cascadia Tremor Located Near Plate Interface Constrained by S Minus P Wave Times

Nonvolcanic tremor is difficult to locate because it does not produce impulsive phases identifiable across a seismic network. An alternative approach to identifying specific phases is to measure the lag between the S and P waves. We cross-correlate vertical and horizontal seismograms to reveal signals common to both, but with the horizontal delayed with respect to the vertical. This lagged correlation represents the time interval between vertical compressional waves and horizontal shear waves. Measurements of this interval, combined with location techniques, resolve the depth of tremor sources within ±2 kilometers. For recent Cascadia tremor, the sources locate near or on the subducting slab interface. Strong correlations and steady S-P time differences imply that tremor consists of radiation from repeating sources.

Imaging the source region of Cascadia tremor and intermediate-depth earthquakes

The subduction of hydrated oceanic crust releases volatiles that weaken the plate boundary interface, trigger earthquakes, and regulate transient phenomena such as episodic tremor and slip (ETS). It is not clear how dehydration can separately induce earthquakes within the subducting plate and ETS, partly because few data exist on their relationship to subduction zone structures. We present results of a seismic experiment in the Washington Cascades, United States, that images a region producing both earthquake types. Migration of scattered teleseis-mic waves provides images of low-velocity subducting crust at depths <40–45 km with sharp boundaries above and below it. The sharp upper boundary indicates a layer of weak sediment or an overpressured fault zone that terminates abruptly downdip at 40–45 km depth. Regular earthquakes are at the top of the mantle within the downgoing plate everywhere the plate is <95 km deep, but ETS only exists where the sharp upper boundary occurs. The ETS location supports models of slow slip that require near-lithostatic fluid pressure, whereas regular earthquakes nucleate closer to the origin of metamorphic dehydration. Very low shear stresses on the plate boundary may limit seismicity to ETS and similar phenomena.

Rapid, continuous streaking of tremor in Cascadia

Nonvolcanic tremor is a recently discovered weak seismic signal associated with slow slip on a fault plane and has potential to answer many questions about how faults move. Its spatiotemporal distribution, however, is complex and varies over different time scales, and the causal physical mechanisms remain unclear. Here we use a beam backprojection method to show rapid, continuous, slip-parallel streaking of tremor over time scales of several minutes to an hour during the May 2008 episodic tremor and slip event in the Cascadia subduction zone. The streaks propagate across distances up to 65 km, primarily parallel to the slip direction of the subduction zone, both updip and downdip at velocities ranging from 30 to 200 km/h. We explore mainly two models that may explain such continuous tremor streaking. The first involves interaction of slowly migrating creep front with slip-parallel linear structures on the fault. The second is pressure-driven fluid flow through structurally controlled conduits on the fault. Both can be consistent with the observed propagation velocities and geometries, although the second one requires unlikely condition. In addition, we put this new observation in the context of the overall variability of tremor behavior observed over different time scales.

Topography of the 660‐km seismic discontinuity beneath Izu‐Bonin: Implications for tectonic history and slab deformation

We analyze the P wave codas of 65 paths from deep northwestern Pacific earthquakes recorded by arrays of stations in Germany, the western United States, India, and Turkmenistan. We identify a phase resulting from a near-source S-to-P conversion at a nearly horizontal discontinuity ranging in depth from 650 to 730 km, which we interpret as a thermally depressed spinel to perovskite and magnesiowustite phase transition. We migrate these data along with 39 more from Wicks and Richards [1993], accounting for three-dimensional ray bending by the sloping discontinuity, to produce a high-resolution topography map of the 660-km discontinuity in the Izu-Bonin region. Assuming an equilibrium phase transition, we interpret the discontinuity depth in terms of local temperatures. The slab, if defined by a thermal anomaly greater than −400°K, is only about 100 km thick near 28°N suggesting the slab is penetrating into the lower mantle with little or no advective thickening. Farther to the north, however, cold material appears spread out over a wide region, consistent with the slab having been laid down flat on the 660-km discontinuity as the trench retreated 2000 km eastward. Both the narrow slab to the south and the flat-lying slab to the north are consistent with recent high-resolution tomographic images. The depression to 745 km along the arc is consistent with a maximum thermal anomaly of about 1100°K. Along the entire arc, the depression occurs directly beneath the deepest earthquakes, even where seismicity is dipping at 45° and stops at 450 km depth, suggesting that the slab steepens to a vertical dip at the deepest seismicity. This change to a vertical orientation suggests that the slab loses strength temporarily through a physical process which causes the seismicity to increase dramatically and then abruptly cease.

A steeply dipping discontinuity in the lower mantle beneath Izu-Bonin

We analyze the coda of teleseismic P waves to deterministically map seismic scatterers in a 1000 km on-a-side cube beneath the Izu-Bonin trench by migrating and stacking waveforms. This method was applied to several hundred short-period vertical-component western United States seismometer recordings of 17 deep-focus Izu-Bonin earthquakes. Except for isolated arrivals, the midmantle appears devoid of sharp discontinuities. S-to-P conversions at the 660-km discontinuity generated the largest signals; the 410-km discontinuity occasionally generated signals. Horizontal discontinuities poorly explain other signals; however, S-to-P conversions generated at a nearly north-south trending, steeply dipping discontinuity at 30°N, 145°E, and 1000 km beneath and parallel to the Izu-Bonin trench explain most additional signals. Observed coherent S-to-P conversions of 2-s period waves limit the width of a gradient zone to <7 km. Compressional and shear tomographic models [Grand et al., 1997; Widiyantoro, 1997] image a fast velocity structure at a similar depth and orientation extending southward from the middle of the Izu-Bonin trench to the Mariana trench. We interpret the sharp scattering surface as a subducted crust and the tomographically imaged structure as the cold thermal anomaly associated with an ancient slab. We observe neither the deep scatterer nor high-seismic wavespeeds north of 32°N. This feature is as much as 800 km east of recent deep earthquakes near 30°N and is at a latitude where the slab appears to extend horizontally on the 660-km discontinuity to the west of the seismicity. We suggest that this deep slab fragment was carried north 500–1000 km owing to oblique subduction. In this scenario, it would be associated with the near vertical subduction at the Mariana subduction zone.

DEPTH EXTENT OF INNER-CORE SEISMIC ANISOTROPY AND IMPLICATIONS FOR GEOMAGNETISM

Abstract To constrain the elastic structure of the Earths inner core, we have picked the differential times of 879 core-penetrating body waves from vertical-component, short-period seismograms recorded by global and regional seismic networks. Using a cross-correlation technique, we measure the difference in arrival times of P′BC-P′DF and P′AB-P′DF where the P′DF (PKIKP) phase penetrates the inner core, while both the P′BC (PKP-BC) and P′AB (PKP-AB) phases bottom in the outer core. P′BC-P′DF times for paths that are nearly parallel to the Earths spin axis are consistently 2–4 s larger than predicted using the Preliminary Reference Earth Model (PREM), while rays in other directions have a mean and standard deviation of 0.2 ± 0.4 s. P′AB-P′DF times, which correspond to P′DF rays turning deeper in the inner core, are 3–6 s for rays nearly parallel to the spin axis and 0.3 ± 0.9 s for rays not near the spin axis. These observations lead to the robust conclusion that the inner core is strongly anisotropic. The level of anisotropy in our model is about 3% at a radius of 1000 km (depth of 200 km) and increases to about 4% at a radius of about 700 km. Below this radius, our resolution is poor, but the anisotropy appears to weaken. Resolution is also weak in the outer 200 km of the inner core, but the anisotropy appears to diminish in this region as well. A simple model of hexagonally symmetric anisotropy aligned with the spin axis explains 74% of the variance. The symmetry direction which fits the data the best and explains 79% of the variance is at 80°N, 120°E. The locus of directions which explain 70–79% of the variance includes only 4% of the possible range of directions, and includes the spin axis direction. The observed anisotropy is most likely due to preferred alignment of elastically anisotropic crystals. We propose several new alignment mechanisms and all viable mechanisms seem to be associated with a strong toroidal magnetic field. An outstanding problem that requires further investigation is that first-principles calculations of seismic anisotropy of hexagonal close packed (hcp) iron suggest that anisotropy of order 3% is predicted. Thus, 100% alignment could go a long way towards explaining our observations, but seems highly unlikely.