Transient electromagnetic survey of a landslide and fault, Santa Susanna Mountains, Southern California

Abstract

Abstract An engineering geologic, geophysical, and geotechnical study was conducted for a proposed 3.8 sq km commercial center in the Santa Susanna Mountains, 50 km northwest of Los Angeles, California. Geologic hazards include an ancient landslide complex overlying the potentially active Santa Susanna (thrust) fault. The landslide covers about 70 percent of the site; the fault underlies about 60 percent of the site. The 1994 Northridge earthquake (Mw = 6.7) occurred about 20 km southeast of the site on the Oak Ridge-"Newhall" fault. No evidence was found that the ancient landslide was reactivated or that the Santa Susanna fault moved. The only earthquake-related geologic effects observed at the site consisted of isolated displaced blocks of bedrock. A transient electromagnetic survey was conducted to determine the thickness of the landslide and to locate the Santa Susanna fault under the landslide. Geologic interpretations of the geophysical data indicate the thickness of the landslide ranges from 0 to 40 m; the depth to the fault ranges from 40 to >75 m below ground surface. Subsequent drilling and geologic logging of four boreholes indicate that the thickness of the landslide and depth to the fault agree to within 5 percent of the geologic interpretations of the geophysical data. Engineering geologic and geotechnical analyses indicate the landslide to be marginally stable. The geologic hazards pose significant constraints on the suitability of the site.