Site Response of Levees From a 4 Element Accelerometer Array at Sherman Island, CA

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

We installed 4 accelerometer stations on the southwest side of Sherman Island in the Sacramento/San Joaquin Delta to investigate linear and non-linear site response of the levees. Each station has a 3-component accelerometer with a 4g clipping level and a sample rate of 200sps. One station is on the crest and one is near the base of the levee. The interstation spacing ranges from 40 to 150m. We have since recorded 3 events on all 4 instruments with magnitudes of 3.1, 2.8 and 3.5. The closest was the first event centered at Oakley, CA, which was only about 6 km away (depth = 10.4 km). Accelerograms for this event were characterized by very impulsive arrivals that were highly polarized with large S waves on the horizontal components , but not on the vertical components. Peak acceleration and velocity show larger values on the levee compared to the base station with peak velocities 66% larger on the crest compared to the base. Spectral ratios of the crest divided by the base show amplification of about 4 between 2 and 3 Hz with sharp peaks at frequencies greater than 10 Hz. The Crocket event (M3.5, Nov. 10, 2018) is further away and has more emergent arrivals. It is located at the southern end of the West Napa fault. Spectral ratios of the crest divided by the base station show a broad amplification between 2 and 15 Hz. In conclusion levee crests do amplify ground motion values as shown by comparing peak acceleration or velocity and as shown in spectral ratios.

Presenting Author: Jon Fletcher

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