Juan de Fuca subducting plate geometry and intraslab seismicity
Medema, Guy Frederick
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The geometry of the subducting Juan de Fuca plate beneath the Olympic Peninsula of Western Washington is modeled using wide-angle P mP reflections off the slab Moho. Active-source reflection data collected from the 1998 WET SHIPS project were augmented with earthquake-source reflections to increase the spatial distribution of reflection points over the Juan de Fuca arch structure. Approximately 1100 WET SHIPS and 500 earthquake-source reflections were used in our inversion. PmP travel-times from active sources and PmP--P differential times from earthquakes sources were simultaneously inverted for slab-Moho depth, using a combination of finite-difference and 3-D ray-tracing methods. Results show a tighter arch structure than in previous models with the shallowest dipping portion (∼ 10° dip) concentrated directly beneath the Olympic Mountains. Comparison of our slab model to intraslab earthquake hypocenters reveals a southwest-northeast trending lineament of seismicity situated just beneath the subducted Moho in the slab mantle. We interpret this seismicity as the manifestation of a subducting pseudofault along which high levels of upper mantle hydration occurred prior to subduction. Most of the remaining intraslab seismicity is concentrated on the northern and southern flanks of the arch and is likely due to increased strain rates in these regions from the combined effect of slab arch and subsequent steepening of slab dip. Earthquakes in the northern region appear to occur above the slab Moho in the crust of the oceanic plate, while uncertainties in earthquake locations and 3-D velocities in the southern region prevent an unambiguous interpretation at this time. The southern patch is especially important as it contains 3 large (magnitude 6.5 to 7.1) earthquakes during that past 60 years, including the 2001, Mw 6.8, Nisqually earthquake. Earthquakes occurring between 45 and 65 km depth in these two regions also systematically produce an anomalous low-frequency, high-amplitude secondary P arrival on updip stations with an average horizontal velocity of 6.0 km/s. We identified 13 earthquakes producing this anomalous phase. These arrivals are typically observed for rays that pass under the Olympic Mountains. 2-D ray-tracing was used to interpret travel times of these arrivals as S-to-P conversions at the interface between the top of the subducted oceanic crust and mantle wedge. Efficient conversion from S-to-P required high velocity oceanic crust and an anomalously low velocity mantle wedge. We therefore interpret this secondary phase to indicate eclogitization of the oceanic crust and serpentinization of the mantle wedge in the vicinity of earthquakes producing the phase. Along paths where these phases are observed, 3-D tomography models exhibit a dipping low-velocity zone formed by accretionary sediments underthrusting high-velocity Eocene volcanics. 2-D ray tracing indicates that this acts as a wave guide along which high-amplitude, low-frequency waves can propagate great distances to the seismic stations.
- Geophysics