The anomalous seismic response of the ground at the Tarzana hill site during the Northridge 1994 southern California earthquake: A resonant, sliding block?

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Abstract
The Northridge, California, earthquake of 17 January 1994 (Mw = 6.7) caused widespread damage in the San Fernando valley and adjacent areas. In many places, the earthquake motion was amplified or strongly modified by the local ground response, but the most localized and extreme site effect occurred at Tarzana, a small hill of gentle topography at the northern edge of the Santa Monica mountains where strong amplification (horizontal peak acceleration of 1.82g) and other unusual site effects were recorded at a CDMG accelerograph. The anomalous site effect was confined to a small area 50 m in radius around the station, beyond which the ground shaking was down to its theoretically expected level. If strong ground motion can vary so much in such a small scale, microzonation of urban areas may be an impossible goal at worst or, at best, only reliable at those sites where ground response has previously been measured. Hence, to clearly understand the dynamic response of the ground at the Tarzana site has become an important problem in earthquake hazard assessment and microzonation. This article reports results on the characteristics of the seismic response of the Tarzana site based on the investigation of aftershock data recorded on a local array and recorded accelerograms. Preliminary results strongly suggest that the Tarzana hill is formed by an old landslide of the Modelo formation. Such a structure is consistent with the anomalous resonant behavior at C00, the observed resonant frequencies, and the complex body wave recordings observed throughout the local array. The results also suggest that the 1.82g peak acceleration at C00 could have been caused by a small (Mw = 1.3) earthquake, located some 60 m from the station, generated by a downslope sliding of the hill with a maximum displacement of 4 cm over a circular area of nearly 50 m in radius. The slide was triggered by the Northridge mainshock.