THE SEARCH FOR TSUNAMIS

By Adam Spencer, The Triplicate October 08, 2012 07:45 pm

When living in the most tsunami-prone city in the contiguous United States, having the most accurate and up-to-date flood maps for evacuation zones is paramount. Recently, a team of geologists has been trying to improve these maps by collecting core samples, including from Crescent City-area wetlands, in an attempt to identify the frequency of past “distant source” tsunamis, which are generated from far-flung earthquakes beneath the Pacific Ocean.

“Distant source” tsunamis, like the one in 2011 generated by a Japanese earthquake, are the only kind that Crescent City has experienced in modern history, and knowing more about how far they traveled inland could help with planning for future disasters. 

Last week, while a team of researchers in waterproof wear took core samples from a marsh just south of the Crescent Beach Motel on Highway 101, the Triplicate was invited to observe the process and see what stories the soil might tell.

The team has been taking core samples from the Marhoffer Creek marsh on the McNamara property near Pebble Beach Drive, Elk Creek marsh and marshes near Sand Mine Road, looking for sand deposits.

Scientists believe that sand deposits are carried inland by tsunami surges. The greater the tsunami, the thicker the sand deposit.  

The first step in last week’s process was one of the easiest: identify the thickest sand deposit usually found in the area, which scientists believe is from the Cascadia event that occurred in the year 1700, 
created by an earthquake on the fault line just off Del Norte’s coast: the Cascadia subduction zone. These deposits show up as consistent strips of sand typically 5 to 10 centimeters thick.

The next step is much trickier:  identifying sand deposits from the 1964 “distant source” tsunami that wiped out much of downtown Crescent City, created by an earthquake near Alaska.

The primary goal of this project is to create a more complete record of the “distant source” tsunamis from 1700 to 1964. 

“Much work has been done to assess the hazard of tsunamis produced by an earthquake on the Cascadia subduction zone, but distant-sourced tsunamis are relatively unstudied,” said Dylan Caldwell, a Crescent City local  on the geology team. Caldwell’s local knowledge was beneficial in choosing test areas for the project, especially the McNamara property.

Dr. Harvey Kelsey, of Humboldt State University, who helped Caldwell work on his master’s thesis, was also in the marsh taking cores.

“We’re looking to see if there’s any evidence at all for something between 1700 and 1964 and so far we haven’t seen anything,” said Dr. Eileen Hemphill-Haley, the lead project geologist from HSU. “The stratigraphy (an area’s makeup of mineral layers) is actually very difficult when you get different kinds of vegetation, so preserving a thin sand layer is actually pretty difficult to do.”

Core samples were collected using a gouge auger, a long semi-cylindrical tube (open on one side), which is pushed into the ground, twisted and then removed with earthy material lodged inside. The technique provides a virtually undisturbed profile of the area’s stratigraphy.

The makeup of the coastal marsh near the motel consisted of mostly peat, but there were spots mixed with sand — some more than others.

The 1700 Cascadia sand deposit was obvious even to the unscientifically trained eye compared to the 1964 deposit, which was “really wispy,” Hemphill-Haley said. “If you didn’t know to look for it, you probably wouldn’t think it was a tsunami deposit.”

One particular tsunami that the team hopes to find the effects of is believed to have occurred in 1788, created by an earthquake in the eastern Aleutians subduction zone. 

Evidence for this tsunami is “very speculative,” as it’s based on accounts from Russian fishing villages, Hemphill-Haley said. The team hopes to identify the 1788 event in Del Norte soils, since scientists believe it could’ve been just as dramatic for Del Norte as the 1964 surges.

A team from the U.S. Geological Survey is collecting similar data on the Aleutian islands, which “haven’t been investigated closely because it’s so remote,” Hemphill-Haley said.

A technique for confirming the date of 1964 soil deposits is possible because of extensive nuclear bomb testing conducted globally that year.

Samples of soil are being sent to a lab on the East Coast to look for a spike in cesium-137, a common radioactive isotope that was blown into the atmosphere from bomb testing.

“There is a spike in cesium right at 1964,” Hemphill-Haley said, so if lab testing sees the spike, then the team can be more confident that it is working with 1964 soil.

The project is a collaboration involving the California Geological Survey, the U.S. Geological Survey and the Humboldt State University Department of Geology.

Although the core samples didn’t always produce clear-cut examples of “distant source” tsunamis in this round of testing, any data will help for future tsunamis, as described by Haley-Hemphill:

“Regardless of the data that we get, there will be new information for the people creating computer models with the newest (tsunami) scenarios to come up with the latest version of the hazard maps for tsunamis” 

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