Slab Morphology in the Cascadia Fore Arc and Its Relation to Episodic Tremor and Slip

Slab Morphology in the Cascadia Fore Arc and Its Relation to Episodic Tremor and Slip Depths contours of the top of the plate interface from receiver functions (this study, solid lines) and a compilation of various studies (dashed lines, McCrory et al., 2006) along the Cascadia margin. Earthquake epicenters from the GSC and USGS catalogues for M > 2 are shown as green dots; tremor epicenters appear as black dots. Broadband stations used in the analysis are displayed as inverted yellow triangles. Numbers indicate depth (in km) to each contour. Solid black line with arrowheads indicates the location of the trench offshore.
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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 occur- rence of nonvolcanic tremor to the principal structural ele- ments 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 cer- tainty, and its interpretation is debated. In this work we have assembled a large teleseismic body wave data set compris- ing 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 struc- ture, and tremor hypocenters, our results support the thesis that the LVZ represents the signature of subducted oceanic crust, consistent with thermal‐petrological modeling of sub- duction zone metamorphism. The location of tremor epicen- ters 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.
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References
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Audet, P., M. G. Bostock, D. C. Boyarko, M. R. Brudzinski, and R. M. Allen (2010), Slab morphology in the Cascadia fore arc and its relation to episodic tremor and slip, J. Geophys. Res., 115, B00A16
</p><p>McCrory, P. A., J. L. Blair, and D. H. Oppenheimer (2006), Depth to the Juan de Fuca slab beneath the Cascadia subduction margin—A 3‐D model for sorting earthquakes, U.S. Geol. Surv. Data Ser., 91, U.S. Geol. Surv., Reston, Va. (Available at http://pubs.usgs.gov/ds/91/)
</p><p>Acknowledgements: This work was partially funded by the Miller Institute for Basic Research in Science (UC Berkeley) through a Fellowship to P.A., the Natural Science and Engineering Research Council (Canada), and the National Science Foundation. Data are made available by the Canadian National Data Center (CNDC) and the IRIS consortium.</p>

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