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

Abstract

[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.Citation: 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,

The fate of the downgoing oceanic plate: Insight from the Northern Cascadia subduction zone

“…For Cascadia, the low-velocity zone seems to be composed of three layers, such as sediments, hydrated pillow basalts and dykes, and gabbros and other ultra-mafic rocks (Hansen et al, 2013). Despite the large number of studies focused on the Cascadia subduction zone, there are a number of important open questions about the shallow structure in the 40-100 km depth range, as testified by the controversy over the depth of the plate boundary beneath northern Washington, a fundamental parameter that influences the assessment of seismic hazard potential in the area (Audet et al, 2010).…”

Distribution of hydrous minerals in the subduction system beneath Mexico

“…Such large amount of fluid near the plate interface due to subduction related dehydration could also be an explanation for bursts of the nonvolcanic tremor within the subducted oceanic crust and lower continental crust (Payero et al, 2008) and slow slip events (Larson et al, 2007;Kostoglodov et al, 2010;Vergnolle et al, 2010;Radiguet et al, 2011). Recent studies suggest strong correlations between episodic tremor and slip (ETS) events and a low-velocity layer at the top of the subducting oceanic crust (Abers et al, 2009;Song et al, 2009;Audet et al, 2010;Kato et al, 2010).…”

Shear wave velocity structure in West Java, Indonesia as inferred from surface wave dispersion

“…In the late 70s, Langston in [19,20] based on receiver function studies showed that low velocity zone present in the forearc structure in Cascadia subduction zone. In the recent studies at Cascadia subduction zone, it has been shown that the low velocity zone occur to depth of about 45 km [12,21]. Other studies also shown the presence of low velocity zone at the subduction zone revealed from the studies of receiver function or seismic tomography, such as Mariana [10], Honsu [18,22], or Chile [23].…”

“…The thickness of the low velocity zone is estimated to be roughly 40 km. However, the thickness of low velocity zone is still debatable due to the tradeoff between the medium thickness and Poisson's ratio used for the inversion [12,13,14]. Bostock in [15] suggested that the low velocity zone thickness obtained from receiver function studies, which use teleseismic earthquakes, should be interpreted as the upper limit of the thickness.…”