Fall 2014

Uncovering Evidence of Past Tsunamis in California

DIGGING DEEP IN THE DIRT, two HSU geologists recently discovered a buried treasure invaluable to the scientific community.

A team of HSU geologists examines samples from Orekw Marsh in Redwood National and State Parks.

HSU Geology Research Associate Eileen Hemphill-Haley, Research Associate Harvey Kelsey, and a team of graduate students discovered an extensive sedimentary deposit formed by a 1946 tsunami. The deposit provides the first known evidence of the event and furthers understanding of the hazards of tsunamis on the California coast.

The Humboldt State group was part of the U.S. Geological Survey’s Science Application for Risk Reduction team. Their goal was to search for geological evidence of past tsunamis in marshlands from Crescent City to the Tijuana River.

Their work provides the first evidence that earthquakes along the Aleutian Islands could potentially generate larger tsunamis at other locations on the California coast.

“Evidence of the 1964 tsunami is only found around Crescent City, and not elsewhere in coastal California,” Hemphill-Haley said. “That is because the location of the actual earthquake across the Pacific will control where the largest waves may reach our shores.”

The history of tsunamis in California only goes back to the late 1700s. Understanding the older geologic history of tsunamis reaching California will lead to a better awareness of the tsunami potential in coastal communities and help local authorities devise improved evacuation plans.

“This study is the most comprehensive paleotsunami exploration project to be conducted in the state of California,” USGS geologist Bruce Richmond said. “No one has looked at so many locations over this large of a geographical area.”

The study found that strong evidence of tsunamis was absent from most marshlands examined, except for two locations in the state. In Crescent City, new information helped to better define the extent of flooding from a historic tsunami that occurred in 1964 as well as a pre-historic tsunami from 1700. The extensive 1946 tsunami deposit was found near Half Moon Bay, and a third location, Carpinteria Marsh near Santa Barbara, contains multiple sand layers that are still being evaluated for a possible tsunami origin.