[1] Slow slip and tectonic tremor in subduction zones take place at depths where there is abundant evidence for distributed shear over broad zones (∼10–103 m) composed of rocks with marked differences in mechanical properties. Here we model quasi-dynamic rupture along faults composed of material mixtures characterized by different rate-and-state-dependent frictional properties to determine the parameter regime capable of producing slow slip in an idealized subduction zone setting. Keeping other parameters fixed, the relative proportions of velocity-weakening (VW) and velocity-strengthening (VS) materials control the sliding character (stable, slow, or dynamic) along the fault. The stability boundary between slow and dynamic is accurately described by linear analysis of a double spring-slider system with VW and VS blocks. Our results place bounds on the volume fractions of VW material present in heterogeneous geological assemblages that host slow slip and tremor in subduction zones.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2012GL053762

Geologic heterogeneity can produce aseismic slip transients

It is widely proposed that the oceanic mantle is hydrated by outer rise normal faults, and carries large amounts of water to the deep mantle. However, the extent of oceanic mantle hydration is poorly constrained by existing observations, and is a major source of uncertainty in determining the total water delivered to the mantle. Full waveform modeling of dispersed P-wave arrivals from events deep within the Wadati-Benioff zone of northern Japan shows that hydrated fault zone structures are present at intermediate depths. Analysis of the P-wave coda associated with events 5-35 km below the top of the slab gives an overall indication of the bulk hydration of the subducting oceanic mantle, and can be explained by a 40-km-thick layer that is 17%-31% serpentinized. This suggests that the top of the oceanic mantle is 2.0- 3.5 wt% hydrated, subducting 170-318 Tg/m.y. of water per meter of arc beneath northern Japan. This order-of-magnitude increase in the estimated H 2 O fl ux in this arc implies that over the age of the Earth, the equivalent of as many as 3.5 present-day oceans of water could be subducted along the Kuril and Izu-Bonin arcs alone. These results offer the fi rst direct measure of the lower lithosphere hydration at intermediate depths, and suggest that regassing of the mantle is more vigorous than has previously been proposed.

/pdf/order-of-magnitude-increase-in-subducted-h2o-due-to-hydrated-10ifl1xaeg.pdf

Order of magnitude increase in subducted H2O due to hydrated normal faults within the Wadati-Benioff zone

Episodic tremor and slip (ETS) along the subduction interface takes place where there is abundant evidence for elevated, near-lithostatic pore pressures, at sufficiently great depths (30–45 km) that chemical dehydration reactions must act as their dominant source. We simulate fluid and heat flow while tracking the location of a vertically oriented, one-dimensional column of material as it subducts through the slow slip and tremor zone. The material in the column is transformed through a pressure-dependent and temperature-dependent dehydration reaction that we describe with a generalized nonlinear kinetic rate law. Column deformation is largely dominated by viscous creep, with a closure rate that depends linearly on porosity. This behavior causes the dehydration reaction to generate traveling porosity waves that transport increased fluid pressures within the slow slip region. To explore the possibility that the observed periodicity of slow slip and tremor in subduction zones can be explained by the migration of such porosity waves, we derive a dispersion relation that accurately describes our numerical results. We also obtain an expression for how the thickness of the dehydrating layer is expected to vary as a function of the parameters in the reaction rate law. Although the amplitudes of pore pressure perturbations rival those that are produced by known external forcings (e.g., tides or passing surface waves), our analysis suggests that given reasonable estimates of rock viscosity, permeabilities in the range 6.5×10−15 to 5×10−10 m2 are required for porosity wave trains to form at periods comparable to those of slow slip and tremor.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015GC006155

Dehydration-induced porosity waves and episodic tremor and slip

For the first time, large quantities of water are identified throughout the length of a subducting plate, and an efficient pathway is defined for injecting water into the deep (>600 km) Earth, by the incorporation of water into the mantle of an oceanic plate as serpentine. Water is thought to facilitate plate tectonics, but the quantities of water involved, and thus its effect, are poorly known. Taking advantage of an anomalous seismic arrival recorded in the Tonga-Fiji subduction zone, the presence, volume, water content, and flux of serpentine and water associated with the subducting oceanic mantle at Tonga is quantified at 2 × 10 8 Tg/Ma H 2 O, many times larger than previous inferences based on surface observations. Water fluxes of this magnitude into the deep Earth will substantially increase the role of water in hypotheses of plate tectonic movements, mantle plumes, and flood basalt activity.

Seismic constraints on the water flux delivered to the deep Earth by subduction

Nature of the Vrancea seismic zone (Eastern Carpathians) – New constraints from dispersion of first-arriving P-waves

SUMMARY

We investigate the geometry of deep subduction zone waveguides (depth >100 km). The wavefield characteristics for up-dip profiles are described and compared with data recorded at the Chile–Peru subduction zone. Observed distorted P onsets at stations in northern Chile near 21°S can be matched by 2-D finite difference simulations of a thin low-velocity layer (LVL) atop the slab in an IASP91 velocity model. The replacement of the LVL by simple random velocity undulations in the slab in the same model cannot explain the observations.



Varying slab geometries are investigated and the distribution of guided wave onsets originating in deep waveguides is predicted relative to the slab surface. Further, double couple source position and orientation is explored and found to be closely limited by the guided wave observations. Sources situated above the layer and at distances more than 2 layer widths below the subducted Moho are not suitable. For the remaining favourable source locations, a strong link between pulse shapes and fault plane dip angle is evident.



We conclude that up-dip guided wave observations at subduction zones follow a simple pattern given by slab geometry and modified by source position. The resulting onsets are shaped by layer thickness and velocity contrast and further influenced by the shape of the slab surface.

S. Martin

Papers

Guided waves at subduction zones: dependencies on slab geometry, receiver locations and earthquake sources