Michael R. Brudzinski

Segmentation in episodic tremor and slip all along Cascadia

The recent discovery of episodic tremor and slip (ETS) in subduction zones is based on slow slip episodes visible in global positioning system observations correlated with nonvolcanic tremor signals on seismometers. ETS occurs just inboard from a region capable of great megathrust earthquakes; however, whether there is any communication between these two processes remains unknown. In this study we use new single-station methods to compile an ETS catalog for the entire Cascadia subduction zone, offshore western North America, and compare the patterns with a variety of along-strike trends for the subducting and overriding plates. Correlated ground vibrations and strain observations are found all along the subduction zone, demonstrating that ETS is an inherent part of the subduction process. There are three broad (300–500 km), coherent zones with different recurrence intervals (14 ± 2, 19 ± 4, 10 ± 2 months), where the interval duration is inversely proportional to upper plate topography and the spatial extent correlates with geologic terranes. These zones are further divided into segments of ETS that occur at times typically offset from each other. The seven largest (100– 200 km) segments appear to be located immediately landward from forearc basins interpreted as manifestations of megathrust asperities, implying that there is a spatial link between ETS and earthquake behavior. It is not yet clear if any temporal link exists, but the regional time between ETS episodes could be controlled by strength variations due to composition of geologic terranes.

Subducting slab ultra-slow velocity layer coincident with silent earthquakes in southern Mexico.

Seismic mapping suggests that silent earthquakes may be related to an ultralow velocity layer on top of a subducting slab. Hot Silent Quakes Subduction zones tend to produce the largest and potentially most destructive earthquakes. Recent observations show that some deformation in several subduction zones seems to be occurring through small or “silent” quakes. The origin of these silent quakes, and their effect on the seismic hazard, is uncertain. Song et al. (p. 502) use a specific seismic signal to map out thin regions with low seismic velocities on the subduction zone beneath southern Mexico. The regions seem to occur at depths below the seismogenic zone where temperatures are higher. These high temperatures and the silent quakes may reflect the release and episodic trapping of fluids from metamorphic reactions. Great earthquakes have repeatedly occurred on the plate interface in a few shallow-dipping subduction zones where the subducting and overriding plates are strongly locked. Silent earthquakes (or slow slip events) were recently discovered at the down-dip extension of the locked zone and interact with the earthquake cycle. Here, we show that locally observed converted SP arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern Mexico reveal that the ultra-slow velocity layer (USL) varies spatially (3 to 5 kilometers, with an S-wave velocity of ~2.0 to 2.7 kilometers per second). Most slow slip patches coincide with the presence of the USL, and they are bounded by the absence of the USL. The extent of the USL delineates the zone of transitional frictional behavior.

Slab morphology in the Cascadia fore arc and its relation to episodic tremor and slip

[1] Episodic tremor and slip (ETS) events in subduction zones occur in the general vicinity of the plate boundary, downdip of the locked zone. In developing an understanding of the ETS phenomenon it is important to relate the spatial occurrence of nonvolcanic tremor to the principal structural elements within the subduction complex. In Cascadia, active and passive source seismic data image a highly reflective, dipping, low‐velocity zone (LVZ) beneath the fore‐arc crust; however, its continuity along the margin is not established with certainty, and its interpretation is debated. In this work we have assembled a large teleseismic body wave data set comprising stations from northern California to northern Vancouver Island. Using stacked receiver functions we demonstrate that the LVZ is well developed along the entire margin from the coast eastward to the fore‐arc basins (Georgia Strait, Puget Sound, and Willamette Valley). Combined with observations and predictions of intraslab seismicity, seismic velocity structure, and tremor hypocenters, our results support the thesis that the LVZ represents the signature of subducted oceanic crust, consistent with thermal‐petrological modeling of subduction zone metamorphism. The location of tremor epicenters along the revised slab contours indicates their occurrence close to but seaward of the wedge corner. Based on evidence for high pore fluid pressure within the oceanic crust and a downdip transition in permeability of the plate interface, we propose a conceptual model for the generation of ETS where the occurrence and recurrence of propagating slow slip and low‐frequency tremor are explained by episodic pore fluid pressure buildup and fluid release into or across the plate boundary.

Global prevalence of double Benioff zones.

Double Benioff zones provide opportunities for insight into seismogenesis because the underlying mechanism must explain two layers of deep earthquakes and the separation between them. We characterize layer separation inside subducting plates with a coordinate rotation to calculate the slab-normal distribution of earthquakes. Benchmark tests on well-established examples confirm that layer separation is accurately quantified with global seismicity catalogs alone. Global analysis reveals double Benioff zones in 30 segments, including all 16 subduction zones investigated, with varying subducting plate ages and stress orientations, which implies that they are inherent in subducting plates. Layer separation increases with age and is more consistent with dehydration of antigorite than chlorite.

Seismic evidence of negligible water carried below 400-km depth in subducting lithosphere

Strong evidence exists that water is carried from the surface into the upper mantle by hydrous minerals in the uppermost 10–12 km of subducting lithosphere, and more water may be added as the lithosphere bends and goes downwards. Significant amounts of that water are released as the lithosphere heats up, triggering earthquakes and fluxing arc volcanism. In addition, there is experimental evidence for high solubility of water in olivine, the most abundant mineral in the upper mantle, for even higher solubility in olivine’s high-pressure polymorphs, wadsleyite and ringwoodite, and for the existence of dense hydrous magnesium silicates that potentially could carry water well into the lower mantle (deeper than 1,000 km). Here we compare experimental and seismic evidence to test whether patterns of seismicity and the stabilities of these potentially relevant hydrous phases are consistent with a wet lithosphere. We show that there is nearly a one-to-one correlation between dehydration of minerals and seismicity at depths less than about 250 km, and conclude that the dehydration of minerals is the trigger of instability that leads to seismicity. At greater depths, however, we find no correlation between occurrences of earthquakes and depths where breakdown of hydrous phases is expected. Lastly, we note that there is compelling evidence for the existence of metastable olivine (which, if present, can explain the distribution of deep-focus earthquakes) west of and within the subducting Tonga slab and also in three other subduction zones, despite metastable olivine being incompatible with even extremely small amounts of water (of the order of 100 p.p.m. by weight). We conclude that subducting slabs are essentially dry at depths below 400 km and thus do not provide a pathway for significant amounts of water to enter the mantle transition zone or the lower mantle.

Variations of P wave speeds in the mantle transition zone beneath the northern Philippine Sea

Using waveforms and travel times from deep earthquakes, we constructed 16 seismic profiles, each of which constrains the radial variation in Vp over a small area beneath the northern Philippine Sea. Taken together, the azimuthal coverage of these profiles also places tight bounds on the lateral extent of a region of anomalously high Vp (up to 3% faster than average Earth models) originally suggested by travel time tomography. Unlike travel time tomography, which relies heavily on arrival times of the direct P phase, we utilize the waveforms and move-out of later arrivals that mainly sample the mantle transition zone of interest. Our results identify three important characteristics of the northern Philippine Sea anomaly that are distinct from previous results. First, being approximately a subhorizontal, laterally uniform feature, the anomaly is localized beneath the northwestern corner of the Philippine Sea, within a region of approximately 500×500 km2 immediately east of the Ryukyu arc. Second, the anomaly is well constrained to occur in the lower portion of the transition zone, extending all the way down to the 660-km discontinuity. Third, the presence of such a distinct anomaly reduces the contrast in Vp across the 660-km discontinuity from approximately 6% to 3%. Such a configuration is consistent with the interpretation that the anomaly is caused by a remnant of subducted slab, as negative buoyancy should rest the slab just above the 660-km discontinuity where resistance to subduction is expected from a negative Clapeyron slope during the spinel—Mg-Fe-perovskite transition.

Constraining the boundary between the Sunda and Andaman subduction systems: Evidence from the 2002 Mw 7.3 Northern Sumatra earthquake and aftershock relocations of the 2004 and 2005 great earthquakes

The 2004 M w 9.0 Sumatra-Andaman earthquake initiated along the Andaman subduction zone, north of the last great Sumatra earthquake along the Sunda Trench in 1861. During the 2005 M w 8.7 Banyak Islands earthquake, a portion of the 1861 rupture subsequently failed. The boundary between the 2004 and 2005 ruptures broadly coincides with local trench rotation and the southern edge of the Andaman microplate, which suggests structural control on fault segmentation. Aftershock relocations of the 2004 and 2005 earthquakes show little overlap, and the sharp boundary between the series locates near the 2002 M w 7.3 Northern Sumatra earthquake. We posit that these features represent the southern extent of the stable Andaman microplate, ∼50-100 km northwest of what was previously reported. Broadband analyses of the 2002 earthquake yield a bilateral rupture pattern that is used to model Coulomb stress changes near the 2004 hypocenter to assess stress interactions along adjacent fault segments.

Variations in P wave speeds and outboard earthquakes: Evidence for a petrologic anomaly in the mantle transition zone

We investigate the nature of laterally varying P wave speeds (Vp) and their relationship to a group of unusual deep earthquakes beneath the backarc of Tonga. This subhorizontal swath of “outboard” earthquakes occurs within the transition zone of the mantle but is several hundred kilometers away from the inclined Wadati-Benioff zone. Surprisingly, the seismogenic region is not associated with fast Vp. It follows that the effect of cold temperature, as implied by the presence of large earthquakes, must have been counteracted by other factors such as compositional or mineralogical effects. Our results are based on a sequence of seismic profiles in a fan-shot geometry. For a given azimuthal sector, we seek the simplest model of radially varying Vp that explains observed broadband waveforms and travel times. The lateral extent of anomalous Vp is then constrained by comparing the results from a wide range of azimuths. This approach fully utilizes information from triplicate P arrivals that are sensitive to seismic wave speeds at the depths of interest. Our results identify prominent anomalies of fast Vp (∼3% over average Earth models) both in the transition zone and in the lower mantle beneath the Tonga backarc. However, the region of fast Vp in the transition zone is offset by ∼300 km to the south of that in the lower mantle, and the region of offset coincides with the outboard earthquakes. A straightforward interpretation is that a large flux of subducted Pacific lithosphere lingers in the transition zone before descending into the lower mantle. Within this broad remnant of subducted lithosphere, the outboard earthquakes occur in a region where impounding of subducted material is most significant, a condition favorable for the accumulation of met astable olivine or volatiles. Such materials reduce Vp and are candidates for triggering earthquakes, thus accounting for the absence of fast Vp in the source zone of deep earthquakes.

Distinguishing induced seismicity from natural seismicity in Ohio: Demonstrating the utility of waveform template matching

This study investigated the utility of multistation waveform cross correlation to help discern induced seismicity. Template matching was applied to all Ohio earthquakes cataloged since the arrival of nearby EarthScope TA stations in late 2010. Earthquakes that were within 5 km of fluid injection activities in regions that lacked previously documented seismicity were found to be swarmy. Moreover, the larger number of events produced by template matching for these swarmy sequences made it easier to establish more detailed temporal and spatial relationships between the seismicity and fluid injection activities, which is typically required for an earthquake to be considered induced. Study results detected three previously documented induced sequences (Youngstown, Poland Township, and Harrison County) and provided evidence that suggests two additional cases of induced seismicity (Belmont/Guernsey County and Washington County). Evidence for these cases suggested that unusual swarm-like behaviors in regions that lack previously documented seismicity can be used to help distinguish induced seismicity, complementing the traditional identification of an anthropogenic source spatially and temporally correlated with the seismicity. In support of this finding, we identified 17 additional cataloged earthquakes in regions of previously documented seismicity and away from disposal wells or hydraulic fracturing that returned very few template matches. The lack of swarminess helps to indicate that these events are most likely naturally occurring.

Changes in plate motions and the shape of Pacific fracture zones

Geosat passes, the new 2-min global gravity grid [Smith and Sandwell, 1995], and shipboard bathymetry across central Pacific fracture zones were used to identify features common to fracture zone segments that formed during times of changes in plate motions. These features are not predicted by current “locked fault” fracture zone models. During a change in spreading direction that induces tension across the transform fault, large-offset (greater than ∼500 km) transforms develop multiple parallel faults, spaced 50 to 100 km apart. The gravity signature of small-offset transform faults under tension includes a broader and more symmetric trough than observed on segments that formed during periods of steady spreading. Parts of fracture zones that form subsequent to a spreading reorientation that causes compression across the transform fault generally exhibit a single fault scarp that fits the locked fault model. Seafloor formed during a period of change usually marks a transition between structural styles, for example, between multiple fracture zone strands and a narrower single-fault fracture zone. Widening of the transform fault zone under tension and narrowing under compression are consistent with the assumption that during a change in spreading direction the new spreading ridges propagate to, but not across, the old transform fault.