logs of excavations 6, 7, 9, and 14: Supplement 2 from "Earthquake geology of the northern San Andreas fault near Point Arena, California" (Thesis)

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The northern segment of the San Andreas fault last ruptured in 1906, producing the great San Francisco earthquake. This study involves the collection and interpretation of geologic data from the segment of the northern San Andreas fault near Point Arena, California, to determine the recurrence interval and slip rate of this segment of the fault. Holocene sediments deposited on an alluvial fan preserve a record of prehistoric earthquakes near Point Arena, California. Excavations into the fan provided exposures of the sediments across the San Andreas fault zone. At least five earthquakes were recognized in the section. All of these occurred since the deposition of a unit that is approximately 2000 years old. Because deposition in this setting was intermittent and deposition of younger units involved the erosion of underlying units, it is likely that events occurred that were not recorded in the section. Radiocarbon dating of units in the section allows constraints to be placed on the dates of the earthquakes recognized. A buried Holocene (2356-2709 years old) channel has been offset a maximum of 64 ± 2 meters. This implies a maximum slip rate of 25.5 ± 2.5 mm/yr. These data suggest that the average recurrence interval for great earthquakes on this segment of the San Andreas fault is long - between about 200 and 400 years. Offset marine terrace risers near Point Arena and an offset landslide near Fort Ross provide estimates of the average slip rate across the San Andreas fault since Late Pleistocene time. Near Fort Ross, a landslide has been offset approximately 1.7 km across the San Andreas fault. Radiocarbon analysis of charcoal from this deposit indicates that the landslide is older than 43,700 years. This implies a slip rate of less than 39 mm/yr. Correlation of two marine terrace risers across the San Andreas fault near Point Arena suggests offsets of approximately 1.5 and 2.5 km. The U-series age of a solitary coral, altitudinal spacing and correlation with known global high sea-level stands suggest ages of 83,000 and 133,000 years for these surfaces, indicating slip rates of about 18-19 mm/yr since Late Pleistocene time. Tentative correlation of the Pliocene Ohlson Ranch Formation in northwestern Sonoma County with deposits 50 km to the northwest near Point Arena, provides piercing points to use in calculation of a Pliocene slip rate for the northern San Andreas fault. A fission-track age of 3.3 ± 0.8 Ma was determined for zircons separated from a tuff collected from the Ohlson Ranch Formation. The geomorphology of the region, especially of the two major river drainages, supports the proposed 50 km Pliocene offset. This implies a Pliocene slip rate of at least 12-20 mm/yr. These rates for different time periods imply that the slip rate of the northern San Audreas fault has not changed by more than a factor of two since Pliocene time. The rates also imply that much of the Pacific-North American plate motion must be accommodated on other structures at this latitude.

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