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Session: Imaging Subduction Zones [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Finding Focus in the Pn Shadow Zone of the Sierra Nevada with P Coda Migration

The Pn shadow zone in seismic record sections across the Sierra Nevada has been a perplexing feature to interpret. For earthquakes on the west side the range, Pn often appears with low apparent velocities (~7.4 km/s) on stations within the mountains followed by high (~8.6 km/s) apparent velocities east of the range. Equally peculiar, for most of the seismic stations in the range, the Pn phase has anomalously low amplitude and, in some cases, no discernible Pn arrival at all.

The diminished amplitudes of the Pn phases hint at a diffusion or scattering of seismic energy. To explore this possibility, we utilize a full waveform modeling algorithm to perform a migration of the P wave coda from the stations into the medium. Because the P wave coda is formed as a scattered wavefield within the medium, a time-reversed propagation of the scattered wavefield will constructively interfere at the locations of scatterers in the medium. The procedure is similar to the gradient method of full waveform inversion, in the sense that the observed scattered field is treated as a residual field calculated in a simple background model.

We demonstrate details of this P wave coda migration method using synthetic data, as well as some preliminary results for a seismic section in the Sierra.

Presenting Author: Kyren R. Bogolub

Additional Authors