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The Origin of Unusually High Earthquake Strong Motion Recordings at Three California Stations

Description: 

It is typically assumed that if a free-field strong motion station is properly installed on a concrete pad and the S-wave velocity profile including basin depth are measured using an acceptable method, there is enough information to assess response to the strong ground shaking. This work focuses on recordings from three California stations: Tarzana- Cedar Hill Nursery, Parkfield Fault Zone 16 and Carquinez bridge that demonstrate significantly higher earthquake responses compared to the surrounding stations. Shown examples indicate that assessing site amplification using standard methods may not be always adequate.

The Tarzana hill strong-motion station was installed near the crest of a low (< 20m) natural hill with gentle slopes. The hill is about 500m in length by 130m in width. Significantly amplified ground accelerations at the hill station were recorded during several events including 1.9g during the 1994 Mw 6.7 Northridge earthquake. It was suggested that the hill acts as a magnifying polarizing glass: It polarizes ground motion in the direction perpendicular to the strike of the hill and also amplifies ground motions that were also amplified by a low-velocity layer (Graizer, 2009).

A very high acceleration record was obtained at station Fault Zone 16 during the Mw 6.0 Parkfield earthquake of 2004. The accelerograph at Fault Zone 16 is deployed at a soil site in a fiberglass housing. Post earthquake instrument shake-table tests indicate that the recorded acceleration is relatively accurate even at the high accelerations involved in this record. Maximum estimated PGA exceeds 2.2g (Shakal et al., 2006).

The Carquinez Bridge geotechnical array #1 recorded PGA of approximately 1.0g at the ground surface during the Mw 6.0 2014 South Napa earthquake compared to 0.42g at a nearby station. The high-level shaking was attributed to the non-linear effect with a decrease of apparent shear-wave velocity during strong shaking (Kishida et al., 2018). The results showed that the apparent Vs in the upper 20m clearly decreased when the PGA was greater than 0.07g, confirming soil nonlinear behavior due to the strong shaking.

Session: Station Installations and Site Conditions, a Quest for Improved Strong Motion Database - I

Type: Oral

Date: 4/16/2025

Presentation Time: 02:00 PM (local time)

Presenting Author: Vladimir

Student Presenter: No

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