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Abstract

The Malibu Coast Fault Zone (MCFZ) is an east-west-trending fault system that marks the southern boundary of the western Transverse Ranges along the Santa Monica Mountains of southern California. Focal mechanism solutions for 107 earthquakes in the study area are mostly associated with thrusts or thrust-dominated oblique faults with a small left-lateral component of strike slip. The average azimuth of hanging-wall slip is 206°, which is approximately perpendicular to the trace of the San Andreas fault through the Transverse Ranges. Approximately 60% of the inferred slip vectors had azimuths between 180° and 240°. Six ML ≥5 earthquakes have been located along the MCFZ, most or all of which are attributed to the offshore Anacapa fault or to nonemergent structures to the south of the Anacapa fault. Hundreds of smaller earthquakes have been located in the vicinity of the MCFZ.

The MCFZ merges eastward with the active Potrero, Santa Monica, Hollywood, Raymond, and Cucamonga faults of the western and central Transverse Ranges. Offshore west of Sequit Point, the MCFZ merges with the active Santa Cruz Island and Santa Rosa Island faults. As part of the complex boundary zone between the Pacific and North American plates, the western Transverse Ranges is characterized by rates of uplift and crustal convergence that are comparable to the rates observed in the Himalayan Mountains, between the Indian and Eurasian plates. Documented Holo-cene slip, large gradients in topography and isostatic residual gravity, fault-related geomorphology, active uplift, distribution of micro- and macroearthquakes, and its position along a major structural/tectonic boundary are evidence that the MCFZ is an active fault zone.

The Malibu Coast fault of the MCFZ is an anastamosing zone of fault strands within a few kilometers of the Malibu coastline between longitudes 118.5° and 119°W. The Solstice and Winter Mesa strands of the Malibu Coast fault have been officially recognized as active faults under California’s Alquist-Priolo Act. Rupture of the entire zoned length of the Solstice strand could have been generated by a ML ~5.3 to 5.7 earthquake along the Malibu Coast fault: comparable in size to several historical earthquakes that have been attributed to the Anacapa fault. Although Holocene displacements have been officially recognized across only two strands of the Malibu Coast fault to date, we consider the Malibu Coast fault to be active and capable of producing a magnitude 6.5 to 7 earthquake.

Fault-zone studies involve scientific problems as well as problems of law and applied professional/scientific ethics. In addition to the basic ethics of science, a primary ethic in the engineering geosciences involves the legal and moral necessity to protect the public’s safety. The incomplete nature of the geologic record and technical difficulties associated with dating Holocene geologic materials can make it difficult or impossible to establish the age of the most recent movement on a fault. The dilemma in fault-zone studies involves the potential conflict between the need to protect the safety of the public and the need to protect the property and wealth of the public by not mistakenly zoning fossil faults as active. The resolution of this dilemma must come through the evolution of public policy and professional practice concerning the assessment of fault-zone hazards. The formal reintroduction of the category potentially active, to characterize faults whose most recent displacement is ambiguous but that are likely to be active, might provide a useful intermediate category between active and inactive faults.

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