PURSUING DARWIN'S GEOLOGICAL OBSERVATIONS OF THE 1835 EARTHQUAKE AND TSUNAMI IN CONCEPCION, CHILE

During their voyage on the Beagle, Charles Darwin and Captain Robert FitzRoy described the impacts of the severe earthquake and tsunami of February 20, 1835 that devastated the cities of Concepción and Talcahuano in central Chile. Their observations form one of the earliest scientific records of co-seismic land-level changes and the geological effects of tsunamis, including descriptions of 3 waves up to 23 ft (7 m) high and, up to 9 ft (2.7 m) of land upheaval on Santa María Island and other sites near Concepción Bay.

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 20^th 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 M_w9.5 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.