A Double Difference Tomography Study of the Washington Forearc: Does Siletzia Control Crustal Seismicity?

Abstract

We present new seismic tomography of the Washington forearc using a suite of manually picked regional earthquake phase arrivals. The recovery of similarly sampled P and S velocity permits the robust calculation of Poisson's ratio throughout the region. The seismological signature of Siletzia, an accreted oceanic plateau that crops out in Washington as the Crescent Formation, is evident in our models as a continuous high Poisson's ratio body that coincides with subsurface structures estimated from potential field maps. Relocated earthquakes preferentially locate in low Poisson's ratio regions in the forearc crust and, in particular, in a diffuse layer located at 15-25 km depth in the crust beneath relatively aseismic Siletzia. Our imaging of the Puget Sound is consistent with previous interpretations of the architecture of major faults and blocks, as evident by distinct Poisson's ratio signatures that distinguish sedimentary basins from mafic rocks of the Crescent Formation. We speculate that seismicity below the Puget Sound is promoted by slab-derived fluids that are localized beneath Siletzia as a result of intrinsic low vertical permeability.

Complex Structure in the Nootka Fault Zone Revealed by Double‐Difference Tomography and a New Earthquake Catalog

“…Enclosed within the dotted circle on profile AA' (Figure 2), three spatially confined groups of earthquakes appear to locate within the continental crust immediately above the subducting Juan de Fuca plate. They could be described as sub-vertical and cylindrical in geometry, and resemble other concentrations of crustal seismicity in Cascadia inferred to be promoted by slab-derived fluids as their hypocenters locate within reduced Vp/Vs crust (Merrill et al, 2020;Savard et al, 2018).…”

“…Between 175 and 275 km along section, well-constrained earthquakes within the subducting JdF plate define a low Vp/Vs region (<1.65) that is overlain by a thin high Vp/Vs (>1.75) layer. Body-wave tomographic imaging of subduction zones, both in Cascadia and elsewhere, reveal that subducting oceanic plates often appear as a high Vp/Vs layer underlain by a low Vp/Vs layer, usually attributed to oceanic crust and mantle, respectively, and that the low Vp/Vs anomaly is primarily governed by the reduction of Vp (e.g., Merrill et al, 2020;Reyners et al, 2006;Savard et al, 2018;Shelly et al, 2006). This pattern persists farther north along the profile, as the contact between the low and high Vp/Vs layers locates near 25-30 km depth and generally parallels the slab interface model.…”

“…TomoDD jointly inverts for Vp, Vp/Vs, and earthquake hypocenters and achieves regularization through a combination of first and second derivative smoothing on the model perturbations (Zhang, 2003). The tomographic model is parameterized by nodes spaced 12 km laterally and 3 km in depth, similar to previous studies in northern Cascadia (Merrill et al, 2020;Savard et al, 2018) enabling direct comparisons. A 2-D starting velocity model is produced by smoothing the contact between the oceanic and continental 1-D models across the deformation front with a 13.5 km convolutional kernel in the dip direction.…”

Body‐Wave Tomographic Imaging of the Turkana Depression: Implications for Rift Development and Plume‐Lithosphere Interactions

“…Oceanic lithosphere trapped in the uppermost mantle has been proposed in the NW United States: the 55 Ma accretion of the Siletzia microplate during the Cordilleran Orogeny is imaged seismically as a curtain of high wavespeeds to 200 km depth (Schmandt & Humphreys, 2011). Siletzia appears to be a competent crustal block with a diffuse layer of seismicity close to the Moho (Merrill et al., 2020). Zhang et al.…”

Depth and Thickness of Tectonic Tremor in the Northeastern Olympic Peninsula

“…However, this may be due to the uncertainty on the determination of the tremor depth. The tremor and LFEs are slightly closer to the plate 2003) was used as the initial velocity model by Merrill et al (2020) to compute the full P-and S-wave velocity model that we use to get tremor depth from S minus P times. The Preston P-wave velocity model was also used by Creager (2017a, 2017b) to locate the LFE families.…”