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Research

Geologic Evidence of Past Tsunamis in California



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An extensive sedimentary deposit formed by a tsunami in 1946 was recently discovered at Pillar Point Marsh near Half Moon Bay, California. Although there were photos and eyewitness accounts of the tsunami and resulting damage at the time, finding the tangible evidence in the geologic record is an important part of assessing the long-term hazard that tsunamis pose to California coastal communities. A better awareness of tsunami potential will help local authorities devise improved mitigation and evacuation plans.

Scientists collecting sediment samples at Pillar Point, California, with a vibracorer
Above: (Left to right) Mike Torresan (USGS), Nick Graehl (Humboldt State University [now with Lettis Consultants International]), and Bob Peters (contractor, USGS) collecting sediment samples at Pillar Point, California, with a vibracorer. (This tool vibrates a long core barrel into the sediment; it works best in soft sand and mud.) USGS photograph by Bruce Richmond. [larger version]

For California, the history of tsunamis is limited by a relatively brief written record that goes back only to the late 1700s. To extend that record into the past, scientists have completed a new study looking for pre-historical evidence of tsunamis in California. As part of the U.S. Geological Survey (USGS) Science Application for Risk Reduction (SAFRR) Tsunami Scenario project, experts from the USGS, Humboldt State University, and the California Geological Survey searched for geologic evidence of past tsunamis in marshlands along the entire California coast, from Crescent City in the north to the Tijuana River in the south.

“The evidence of past tsunamis—as well as the lack of evidence—will help establish the baseline for tsunami-hazard analysis and preparedness,” said geologist Rick Wilson of the California Geological Survey. “Continuing this type of work is essential for characterizing the long-term tsunami threat to California.”

Geologist and student assistant collecting gouge-core sediment samples
Above: Humboldt State University geologist Nick Graehl (left; now with Lettis Consultants International) and student assistant collecting gouge-core sediment samples at Morro Estuary Natural Preserve, California. Photograph taken November 23, 2011, by Eileen Hemphill-Haley, Humboldt State University. [larger version]

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 historical tsunami that occurred in 1964, as well as a pre-historical tsunami that occurred in 1700. An extensive deposit from a tsunami that occurred in 1946 was found at Pillar Point Marsh near Half Moon Bay. A third location, Carpinteria Marsh near Santa Barbara, contains multiple sand layers that are still being evaluated for a possible tsunami origin.

This research focused on impacts from tsunamis generated far away at major earthquake-generating fault systems. All Californians should be aware that local, smaller earthquakes can generate submarine landslides capable of producing damaging local tsunamis.

When the ground shakes, move away from the coast.

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

Tsunami deposits are often composed of thin, continuous sand layers with unique grain-size and microfossil characteristics that may help to differentiate them from other kinds of coastal deposits, such as those from large storms. Coastal marshes, ponds, and lagoons are the most likely areas where these tsunami deposits can be preserved in the geologic record and later recognized, because the tsunami-deposited sands contrast with plant-rich material that accumulates in marshes and the dark, muddy deposits of ponds and lagoons.

Map of California, showing sites where scientists looked for geological evidence of tsunamis No tsunami deposits are evident in this cutbank outcrop at Home Bay marsh, Point Reyes National Seashore, California
Above Left: Map of California, showing sites where scientists looked for geological evidence of tsunamis. White squares, sites where scientists did visual reconnaissance; black dots, visual reconnaissance and coring; yellow dots, detailed evaluations and coring. Modified from figure 5 in “The Search for Geologic Evidence of Distant-Source Tsunamis Using New Field Data in California.” [larger version]

Above Right: No tsunami deposits are evident in this cutbank outcrop at Home Bay marsh, Point Reyes National Seashore, California. Photograph shows the subsurface stratigraphy to a depth of 90 centimeters (35 inches) exposed during low tide. Holes about halfway down the exposure are from burrowing crabs populating a narrow zone at the transition from underlying inorganic mud deposits to overlying peaty mud and peat deposits. Photograph taken July 27, 2011, by Eileen Hemphill-Haley, Humboldt State University. [larger version]

“We worked hard to identify locations along the California coast where there would be a potential for finding paleotsunami deposits,” said geologist Eileen Hemphill-Haley of Humboldt State University. “This was a challenge considering the many miles of rocky coastline along California where tsunami deposits would not be preserved, and the fact that many low-lying coastal wetlands—even some that are now parks or preserves—have been impacted in the past 150 years by agricultural practices or other land uses.”

“This report presents results from data collected by a team of 15 scientists working for more than a year,” said USGS geologist Bruce Jaffe. “And there is still much to be done to reconstruct the tsunami history of California.”

The new report, “The Search for Geologic Evidence of Distant-Source Tsunamis Using New Field Data in California,” is now available online. It is a chapter in a large report, “The SAFRR (Science Application for Risk Reduction) Tsunami Scenario.” 

To learn more about the SAFRR Tsunami Scenario, read the Sound Waves article “Experts Team Up on Tsunami Resilience in California” and visit the SAFRR website.

For more information about tsunamis, visit the California Geological Survey tsunami website and the USGS tsunami website.


Related Sound Waves Stories
Search for Evidence of Prehistoric Tsunamis and Great Earthquakes on Chirikof Island, Eastern Aleutians
December 2010
Distinguishing Tsunami from Storm Deposits in the Geologic Record
November 2007
Brief Tsunami Warning Startles U.S. West Coast, Reveals Strengths and Weaknesses in Tsunami Preparedness
July 2005
Could It Happen Here? Tsunamis That Have Struck U.S. Coastlines
Dec. 2004 / Jan. 2005
Japan Lashed by Powerful Earthquake, Devastating Tsunami
March 2011

Related Websites
U.S. Geological Survey (USGS) Science Application for Risk Reduction (SAFRR) Tsunami Scenario
USGS—SAFRR
The Search for Geologic Evidence of Distant-Source Tsunamis Using New Field Data in California
USGS
The SAFRR (Science Application for Risk Reduction) Tsunami Scenario
USGS Open-File Report 2013–1170
California Geological Survey tsunami website
California Geological Survey
Tsunami & Earthquake Research at the USGS
USGS

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in this issue:

Spotlight on Sandy
USGS Coastal Change Hazards Portal

New Tide Gage/Weather Station Near Mashpee, Massachusetts

Oceanographic Gear Retrieved from Offshore of Fire Island, New York

Research
Coral Reefs Along West-Central Guam—Historical Impacts

Geologic Evidence of Past Tsunamis in California

Outreach
USGS Helps Celebrate the Monterey Bay National Marine Sanctuary

Public Lecture on Deep-Sea Corals Takes Audience “Into the Abyss”

Meetings
USGS Gas Hydrates Project Hosts Japanese Colleagues

Use-Case Training for the Woods Hole, Massachusetts, Research Community

Spring 2014 Monterey Bay Marine GIS User Group Meeting

Publications
July / August Publications

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