Intern	Institution	Mentor	Project Title
Tracy Pattelena	Pasadena City College	David Okaya and Nikki Godfrey, USC	Velocity structure of the San Fernando Valley from tomographic inversion of active source data

Abstract

In 1994 the Northridge earthquake (MW = 6.7) struck the Los Angeles area causing significant damage to the San Fernando Valley of southern California. Using existing active-source reflection data collected by Chevron prior to the Northridge earthquake, we obtain both compressional wave (VP) and shear wave (VS) velocity information for the upper 500m of crust. We do this by analyzing the data for three different north-south trending seismic lines. To analyze the VP velocities we pick the refraction phases for VP and apply the tomographic velocity inversion method of Hole (1992) to calculate the first-arrival times. We further compare our VP models to well VP sonic log data available for the area. To analyze the VS velocities we pick the refraction phases for VS after applying bandpass filtering to 12 Hz and apply the tomographic velocity inversion method of Hole to calculate the first-arrival times. We conclude with calculating Poisson's Ratio (s) based on the determined VP to VS ratio to compute the VP-to-VS conversion factor applicable for the San Fernando Valley (SFV).

Our VP models show velocities dipping south into the SFV as do the seismic profiles obtained for all three lines. VP velocity models for all three lines show an overall near-surface velocity range beginning at 1.3 km/s along the base of the Santa Susana Mountains (SSM) decreasing to roughly 1.0 km/s southward into the valley with an overall average velocity convergence to 2.6 km/s at depths of approximately 500 m. Our VS model shows near-surface velocities beginning at 0.3 km/s just beneath the foothills of the SSM and remaining constant southward into the SFV. At depths of roughly 200 to 300 m there is a convergence to 1.2 km/s beneath the SGM which decreases to between 0.8 and 0.9 km/s into the valley. Based on these findings, we calculate s to be 0.4 for near-surface areas beneath the SSM and at depths greater than 200 m once into the SFV and 0.2 for near-surface depths of less than 200 m in the SFV.

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