Abstract

During the 1906 magnitude 7.8 earthquake, surface rupture along the San Andreas fault propagated through the Town of Portola Valley on the San Francisco Peninsula. Subsequently, a number of studies came to conflicting conclusions regarding the exact location and nature of the 1906 surface rupture through the area. Our study provides new evidence for correctly locating the 1906 rupture within the town using field observations, a detailed analysis of 3D, bare‐earth Light Detection and Ranging (lidar) images, and a review of photographs and other historical documents related to the 1906 rupture. Our composite images combine lidar views with historical photographs to aid in locating the 1906 fault trace and a previously unrecognized splay of the fault. Both the main fault trace and newly proposed splay‐fault locations are significantly different from interpretations of the 1906 surface rupture developed over the last 50 years. Assembled evidence shows that the primary rupture in 1906 occurred only on the western of two main fault traces, rather than stepping to the eastern trace as proposed previously, and that a major section (about 2.6 km) of the western trace has been inaccurately mapped. From lidar‐image analysis, we demonstrate that the previously mapped 1906 trace in the southern part of Portola Valley cuts through a homoclinal sequence of strata. These moderately dipping strata form prominent bedding lineaments have been mapped previously as quaternary fault traces.

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