Regional P wave velocity structure of the Northern Cascadia Subduction Zone

Abstract

[1] This paper presents the first regional three-dimensional P wave velocity model for the Northern Cascadia Subduction Zone (SW British Columbia and NW Washington State) constructed through tomographic inversion of first-arrival traveltime data from active source experiments together with earthquake traveltime data recorded at permanent stations. The velocity model images the structure of the subducting Juan de Fuca plate, megathrust, and the fore-arc crust and upper mantle. Beneath southern Vancouver Island the megathrust above the Juan de Fuca plate is characterized by a broad zone (25-35 km depth) having relatively low velocities of 6.4-6.6 km/s. This relative low velocity zone coincides with the location of most of the episodic tremors recently mapped beneath Vancouver Island, and its low velocity may also partially reflect the presence of trapped fluids and sheared lower crustal rocks. The rocks of the Olympic Subduction Complex are inferred to deform aseismically as evidenced by the lack of earthquakes within the low-velocity rocks. The fore-arc upper mantle beneath the Strait of Georgia and Puget Sound is characterized by velocities of 7.2-7.6 km/s. Such low velocities represent regional serpentinization of the upper fore-arc mantle and provide evidence for slab dewatering and densification. Tertiary sedimentary basins in the Strait of Georgia and Puget Lowland imaged by the velocity model lie above the inferred region of slab dewatering and densification and may therefore partly result from a higher rate of slab sinking. In contrast, sedimentary basins in the Strait of Juan de Fuca lie in a synclinal depression in the Crescent Terrane. The correlation of in-slab earthquake hypocenters M > 4 with P wave velocities greater than 7.8 km/s at the hypocenters suggests that they originate near the oceanic Moho of the subducting Juan de Fuca plate.

The fate of fluids released from subducting slab in northern Cascadia

“…The inferred positions of the Juan de Fuca slab and the forearc mantle are shown on the cross-sections. The location of the slab mapped in the present study from S-wave velocity is consistent with the position of the slab inferred in previous tomographic P-wave velocity models by Ramachandran et al (2006). The P-and S-wave velocity models from the present study distinctly image the position of the forearc mantle.…”

“…This 1-D velocity model, along with controlled source travel time picks from Seismic Hazards Investigation in Puget Sound experiment (Ramachandran et al, 2006) and P-wave travel-time picks from the selected earthquakes were first employed to relocate hypocenters and estimate a preliminary P-wave 3-D velocity model. The algorithm used in this study is described in Ramachandran et al (2005) and references therein.…”

“…Step 1: An initial 1-D P-wave velocity model was constructed using the average 1-D model obtained from the previously constructed regional velocity model discussed in Ramachandran et al (2006). This 1-D velocity model, along with controlled source travel time picks from Seismic Hazards Investigation in Puget Sound experiment (Ramachandran et al, 2006) and P-wave travel-time picks from the selected earthquakes were first employed to relocate hypocenters and estimate a preliminary P-wave 3-D velocity model.…”

“…Hyndman et al, 1990). The depth to the Cascadia forearc Moho beneath southwestern British Columbia is ∼36 km (Cassidy and Ellis, 1993;Zhao 2001Zhao et al, 2001Ramachandran et al, 2006, Brocher et al, 2003.…”

Constraining fault interpretation through tomographic velocity gradients: application to northern Cascadia

“…Results from an investigation of the applicability of velocity gradient analysis for structural interpretation of the upper crust, conducted using a previously constructed regional 3-D tomographic P-wave velocity model for the northern Cascadia subduction zone (Ramachandran et al, 2006) are presented in this article. Information from horizontal gradients in X (east-west) and Y (north-south) directions define structural contacts much more clearly than the tomographic velocity model.…”