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Observations and Numerical Simulations of Guided Waves From Earthquakes Located Near the Gorda Slab Interface

Session: Amphibious Seismic Studies of Plate Boundary Structure and Processes

Type: Oral

Date: 4/20/2021

Presentation Time: 10:00 AM Pacific

Description: 

The interplate boundary in a shallow subduction zone is potentially a thick zone of materials with diverse properties. We report the observation of guided waves generated by a thin low-velocity layer on top of the oceanic crust of the subducted Gorda plate and use numerical simulations to understand their propagation.

From the Cascadia Initiative dataset, we observed long-duration S waves (up to 4 s at 2-10Hz) on station KCT in northern California from earthquakes clustered near the slab interface at ~15 km depth. In contrast, earthquakes located just a few km deeper only show impulsive direct arrivals. We hypothesize that the extended S-wave reflects velocity anomalies near the slab interface. To study the origin of the S waves, we deployed a 20-station array near KCT. We applied 2D beamforming on P and S waves from one earthquake close to a slab interface cluster occurring during the deployment. 2D beamforming shows that the P wave contains two arrivals with different apparent slowness that we refer to as P1 and P2, and the S wave similarly contains two arrivals S1, S2 and lower frequency late coda. P1’s (S1’s) apparent slowness is larger than P2 (S2). We infer that P1 and S1 share similar raypaths, while P2 and S2 phases share a different raypath.

To investigate possible near-source velocity anomalies causing multiple P- and S-wave arrivals, we constructed simple, layered velocity models for 2D wave propagation simulations. Simulations using velocity models with a 6-km-thick layer representing the subducted oceanic crust indicate that the S1 and S2 are likely the direct S wave and the top side Moho reflection phase. Simulations using velocity models with a thin, truncated low S-wave-velocity layer on top of the subducted oceanic crust best replicate the S coda in terms of arrival time, frequency content and amplitude. The coda is likely generated by leaked guided waves from the thin low-velocity layer due to scattering from a discontinuity in the waveguide. The low velocity layer requires ~40% S-wave velocity reduction to match the observed amplitude of the coda waves.

Presenting Author: Jianhua Gong

Student Presenter: Yes

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