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Earthquake Site Amplification in Oceano, California, That May Be Related to Refraction from a Nearby Fault-Bounded Syncline

F. E. Denison1 and J. M. Saenz2
1867 Hartglen Avenue, Westlake, CA 91361, [email protected]
2Oxnard College, Oxnard, CA 93033, [email protected]

According to Holzer and others (2004) in USGS Open File Report 2994-1269, the December 22, 2003, San Simeon, California, (M6.5) earthquake caused damage to several houses, road surfaces, and to underground utilities in the town of Oceano, California. The community of Oceano is located approximately 80 km (50 miles) from the earthquake epicenter. Damage at this distance from a M6.5 earthquake is considered to be unusual.

The USGS investigation identified two earthquake hazards in Oceano that they thought might explain the San Simeon earthquake damage–site amplification and liquefaction. Site amplification in Oceano was confirmed by monitoring aftershocks.

Areas with site amplification conditions similar to those in Oceano are particularly vulnerable to earthquake damage because site amplification may cause shaking from distant earthquakes, which normally would not cause damage, to increase locally to damaging levels. The vulnerability in Oceano is compounded by the widespread distribution of highly liquefiable soils near the surface that could re-liquefy when ground shaking is amplified as it was during the San Simeon earthquake.

Study of subsurface geology in the Oceano area reveals a syncline of Miocene strata over Jurassic basement rock that is bounded by the Wilmar Avenue fault. We suggest that this fault-bounded syncline may be responsible for earthquake amplification in this area.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009