by R. Darragh, V. Graizer, and A. Shakal
Final Report: Grant No. CMS-9416424 to California Division of Mines and Geology
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Ground motion amplification has been observed at Tarzana in many earthquakes and for both strong and weak motions. Both the Whittier Narrows and Northridge mainshocks produced larger than expected motions at the Tarzana – Cedar Hill Nursery. The peak acceleration, velocity and displacement during the Northridge mainshock was amplified by factors of 5, 3, and 1.5, respectively, compared to nearby sites. The peak horizontal acceleration recorded during the Whittier Narrows mainshock was amplified by a factor of 5 to 10 compared to sites at a similar distance. In addition, several Northridge aftershocks amplified peak acceleration, velocity and displacement by factors near 2. Also, response spectra were amplified by a factor of 4.5 near 0.3 second (3.2 Hz). In contrast, the Landers and Sierra Madre mainshocks, Whittier Narrows aftershock and some Northridge aftershocks did not produce significant site amplifications near this frequency.
This ground motion amplification at Tarzana is also dependent on frequency. For example, Spudich et al. (1996), Hartzell et al. (1996) and this study document a predominance of 2 to 6 Hz motion in weak and strong motion recordings at Tarzana compared to other nearby stations.
In an effort to study the site amplification the Tarzana site was characterized to a depth of 100 m during this study. A low shear-wave velocity near the surface of 100 m/sec increasing to near 750 m/sec at 100 m depth was measured. The 20 m high hill was found to be extremely well drained with a water table near 17 m. Modelo formation (extremely weathered at the surface) to fresh at depth is underneath hill. The subsurface geology and velocities obtained allow classification of this location as a soft-rock site.
The source of the site amplification that produces large motions at Tarzana is still under investigation. The following causes have been proposed: topography (Celebi (1995)); three-dimensional shape including topography (Spudich et al. (1996) and Bouchon and Barker (1996)); variations in site geology and S-wave velocity (Spudich et al. (1996) and Catchings and Lee (1995, 1996)); and resonance during active downslope sliding of the Tarzana hill (Rial, 1996). Specifically, Bouchon and Barker (1996) showed that the topographic effect at this site can amplify ground motion by factors ranging from 30% to 100%. In addition, Wennerberg et al. (1994) studied aftershock response that suggested large linear site amplification at the Nursery.
In summary, the structure (topography, shear-wave velocity profile and three dimensional geometry) of the site all contribute in part to the higher amplification of ground motion at the top of Tarzana hill. Tarzana – Cedar Hill Nursery is classified as a soft-rock site. It is site class C for the top 17 m (above the water table) over site class B using the site classification scheme proposed by Boore et al. (1993).