PURSUING DARWIN'S GEOLOGICAL OBSERVATIONS OF THE 1835 EARTHQUAKE AND TSUNAMI IN CONCEPCION, CHILE
We relied on Darwin’s and FitzRoy’s accounts to guide our search for geological evidence of the 1835 tsunami, and hence to calibrate the relative magnitude and extent of other historic and prehistoric tsunamis that are preserved in the geological record. We focused our field investigations of potential tsunami deposits and earthquake-induced liquefaction on sites described by Darwin and FitzRoy along 200 km of the central Chilean coast, from Coliumo (36.5˚S Lat) to Tirua (38.5˚S Lat). Historically, this region suffered destructive near-source tsunamis in 1570, 1657, 1751, and 1835, indicating a remarkably regular recurrence that has been suspended during the 20th century. Long-term patterns in the magnitude and frequency of tsunamis will contribute to a more realistic tsunami hazard evaluation for metropolitan Concepción, where the flat lowland that Darwin described as being completely inundated by the tsunami in 1835 now supports a dense population and critical infrastructure.
In the low-lying areas near Concepción, our preliminary observations suggest evidence of a tsunami sand layer and liquefaction features. The southernmost study site at Tirua contains at least 3 sand layers that likely represent past tsunamis. According to eyewitnesses and geological evidence, the uppermost of these is from the tsunami generated by the Mw9.5 earthquake that struck south-central Chile in 1960. The middle layer is possibly from the 1575 tsunami, which was produced by an earthquake similar in extent and magnitude to that of 1960. The lower layer was deposited between AD 1300 and 1500, which overlaps with the age of a previously identified tsunami farther to the south. We observed 1-2 underlying, submerged sand layers that could represent even older tsunami deposits.