Résumé
The Santa Barbara coast is at risk for tsunamis generated from tectonic movement in areas of compression and extension associated with the San Andreas Fault, as well as from submarine landslide movement in the Santa Barbara channel. Historical documents and other records indicate Santa Barbara has experienced approximately sixteen historical tsunamis, the largest of which may have occurred on Dec 21, 1812, following a magnitude approximately 7.1 earthquake. We propose that an anomalous sand deposit, Sand Facies One (SF1), which is found within the first meter of sediment throughout Carpinteria Marsh in Carpinteria, CA, may represent deposition related to this event. We have collected 23 vibracores, up to 4.1 m in length, and three Geoprobe cores to approximately 14 m depth in Carpinteria Marsh. SF1 occurs in 20 of the 23 vibracores and exhibits sedimentological characteristics associated with a tsunami genesis such as: fining upward grain size, sharp or erosional basal contact, and thinning of the deposit landward. Mineralogy, deposit geometry, and X-Ray Florescence (XRF) data are used to determine a marine versus terrestrial origin for the layer. It is difficult, however, to differentiate between storm and tsunami deposits based purely on the sedimentary characteristics of a deposit. We show that an improved age chronology which includes exotic pollen stratigraphy and radiocarbon data indicates an age range appropriate for the 1812 event but does not exclude regional flooding events documented in the 1860s. We use the characteristics of SF1 to determine if similar layers occur at other depths in the Geoprobe cores. Preliminary core descriptions indicate that there is at least one layer which exhibits characteristics similar to SF1 and may indicate the occurrence of a similar inundation event. We conclude that tsunami deposition related to the 1812 event is a possible explanation for SF1, but additional analyses are needed to rule out other flooding events. Whether storm or tsunami, the deposit's characteristics imply inundation at a scale and intensity that would be detrimental to the low lying areas of the Santa Barbara coast--the presence of similar layers at greater depths imply this degree of inundation has recurred over time and is an important process to understand for risk assessment for the Santa Barbara coast.