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Description
The tectonic geomorphology for the southern 55 km of the Clark strand of the San Jacinto fault zone was studied to determine slip in the past 1-2 events, as well as to map larger offsets for future slip rate determinations. In all, 348 offsets were estimated on channel margins, channel thalwegs, ridge noses, and bar crests using field techniques, aerial photography, and new B4 LiDAR imagery. Displacement estimates show that the most recent event (MRE) produced an average of 2.7 m of slip, ranging from a maximum close to 4 m near Anza, California, to less then one meter near the southeast termination of the fault. However, to the north of Anza, no slip information was collected and five rupture scenarios were developed to make accurate estimates of the potential range of earthquake magnitudes that can account for the observed displacements. Magnitude estimates for these scenarios range between Mw=7.2 and Mw=7.5. Historically, no earthquake has been recorded along the Clark fault that is large enough to have produced the displacement observed along the southern half of the Clark fault, with the exception of the November 22nd, 1800 event. In that earthquake, the San Diego and San Juan Capistrano missions reported extensive damage (MMI VII): the event was initially placed in the offshore as there were no constraints from inland areas. However, from paleoseismic work at Hog Lake, the most recent event is dated at ca. 1790, which when considering age uncertainties, could be the November 22nd, 1800 earthquake. By relocating this event on Clark fault, as well as assuming slightly lower intensity values from those interpreted for the missions, magnitude calculations range between Mw=7.2 and Mw=7.5. These estimates are similar to the calculated magnitudes estimated using the slip distribution and various rupture scenarios. Although a more detailed study is required to the north of Anza to complete the measurements of offset features related to the most recent event, from the information collected for this thesis, it is inferred that the displacement recorded on the southern 55 km is associated with the earthquake of November 22nd, 1800.
