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The Los Osos fault zone is a west-northwest-trending reverse fault in the Pacific coastal region of San Luis Obispo County, California. The fault zone extends as a discontinuous en echelon zone from the Hosgri fault zone in Estero Bay southeast to an intersection with the West Huasna fault zone near Twitchell Reservoir, a distance of up to 57 km. The fault zone is divided into four segments based on distinct changes in recency of activity and slip rate along the fault: abrupt changes in elevation of the bordering San Luis Range, en echelon separation of fault traces, intersection with known or inferred branching or crossing structures (e.g., faults, subsiding basins), and changes in geomorphic expression from a range-front fault to an intrarange fault. From northwest to southeast, we propose naming the segments the Estero Bay, Irish Hills, Lopez Reservoir, and Newsom Ridge segments. The Estero Bay segment, 11 to 15 km long, lies primarily offshore in Estero Bay. Its recency of activity is unknown. The segment is poorly imaged on seismic reflection data and is weakly expressed in sea-floor bathymetry, suggesting a low rate of late Quaternary activity. The Irish Hills segment, which is 17 to 21 km long, exhibits the strongest expression of Holocene activity and is a well-defined range-front fault. Detailed mapping of marine terraces and trenching of fluvial deposits show that this segment has had recurrent late Pleistocene and Holocene movement at a long-term slip rate of 0.2 to 0.4 mm/yr. The adjacent Lopez Reservoir segment is a 15- to 19-km-long, poorly defined range-front fault that displaces older Quaternary alluvium. Detailed mapping and trenching indicate no Holocene activity. The Newsom Ridge segment is an 8-km-long, intrarange fault that has poor geomorphic expression and appears not to displace late Pleistocene deposits.

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