Tsunami diggers visit the estero

09/15/2011

A group of geologists recently completed fieldwork within Drakes Estero as part of a federally funded project that will comb the coastline from Cape Mendocino to the border of Mexico for sedimentary evidence of prehistoric tsunamis.

While a substantial amount of paleo-tsunami research has been conducted around Humboldt County and the Pacific Northwest because of the Cascadian subduction zone, the more southern and populated sections of the California coast have not been examined as carefully. Humboldt State University geology professor and tsunami expert Lori Dengler, whose department partnered with the California Geological Survey (CGS) to extend the science to the lower reaches of the coast, said this spring’s disaster in Japan reinforced the importance of determining the prehistoric tsunami record.  

“[Japan] had really based their hazard assessment on the past couple of 100 years,” Dengler said. “It turned out that their hazard assessment was woefully inadequate.  In California, we have a really short historic record [of] 150 years. Tsunamis recur on a much longer time scale than that. Anything we can do to push that record [back] and to identify the worst-case kind of event, we really need to do that.”

In order to establish a pattern of recurrence, researchers plunge metal poles, called cores, into flat wetlands and salty marshes, like the estero, to extract sediment. They search the material for coarse-grained debris, which was carried inland by tsunamis hundreds or even thousands of years ago. The scientists then carbon date the materials found above or below the layers with the tsunami deposits to determine when the surge occurred. If multiple deposits are found up and down the coast and are dated to the same period, researchers can establish the probable time frame for a future event.

The suspected source of past and future tsunamis in the West Marin region and in other parts of California is not local. The Aleutian Trench, which stretches horizontally from Russia to Alaska, is capable of producing a 9.2 magnitude earthquake near the Aleutian Islands and sending massive swells toward the California coast.

“Directionality is important,” Dengler said. “A fault that is perpendicular to the coast is going to produce a big tsunami. [It] is kind of like a loaded shotgun. Where it’s pointing, you’re going to get a lot of energy. The biggest shotgun that’s pointed toward us is the Aleutian Islands [subduction zone.]”

Working with the University of Southern California’s Tsunami Research Center and its Method of Splitting Tsunamis (MOST) computer-modeling program, CGS senior engineering geologist and project chief Rick Wilson produced inundation maps for the California coastline in the event of a 9.2 Aleutian Islands earthquake. The maps depicted a 15-foot surge at the mouth of Drakes Estero. 

Given the inundation data, the research team took subsurface core samples from marshes at the head of Barries Bay, the westernmost arm of the estero, and Home Bay, the north-eastern section, during two days in late July. They searched the muddy fine-grained samples for deposits of coarse-grained material, such as sand, shells or gravel, that would indicate the inland presence of a tsunami. The results are still being evaluated, and in coming months scientists plan on working south down the coast to other locales like Half Moon Bay and the Channel Islands to collect the rest of the deposits.

Humboldt research associate and co-principal investigator Eileen Hemphill-Haley cautioned against interpreting negative results as evidence that a prehistoric tsunami had never rushed through the estero or other parts of West Marin. They would be more reflective of the delicate process of selecting a sampling location: It cannot be too close to the beach, where many samples contain sand from storm surges, or too far inland, where tsunamis might not have left their signature debris mark. 

“Tsunami hazards are something to be taken seriously in California,” Hemphill-Haley said. “The hazards are real.”

Wilson agrees with Hemphill-Haley’s assessment. 

“So many things can happen to that deposit,” Wilson said. “We have to caution ourselves that conditions might have been different when the tsunami occurred in prehistoric times.” 

He explained that two distinct tsunami scenarios could strike the West Marin coastline. In the first, the offshore Point Reyes Thrust Fault, which is potentially active, could produce a 7.0 magnitude earthquake and the vertical uplift could generate an almost immediate 10-foot surge headed toward the region. Locals would have about 15 minutes to retreat to higher ground before the wave reached shore. 

“What we tell people is if you feel the earth shake, then you should probably get to high ground to make sure you don’t take very many chances,” Wilson said. 

But the worst-case scenario, the one that really worries him and the other scientists studying the phenomenon, is the potential 9.2 magnitude earthquake in the Aleutian Islands. That would produce a 20 to 25-foot wall of water that would take roughly four hours to reach areas like Tomales Bay, Bolinas and Stinson Beach. During such an event, the West Coast and Alaska Tsunami Warning Center would monitor the wave as it moved south and keep the California Emergency Management Agency (CEMA) updated on the developments. 

As CGS’s head tsunami scientist, Wilson would interpret the data coming out of the warning center, adjust for tidal conditions and advise CEMA and local county officials, who would plan evacuations accordingly.

In West Marin, Stinson Beach’s low-lying and isolated residential section concerns him the most. “The people that are out there on the sand spit have to be cognizant they have to travel outside the sand spit to … get where the higher ground is,” he said.  

Wilson hopes the current project will provide CEMA and other agencies with a better idea of how to approach these potential catastrophes.  

“One of the geological mantras is that the past is the key to the present,” Wilson said. “The bottom line is if we can build a history of tsunamis, we’ll have a better idea of when it will occur in the future and what we need to plan for on our coast.”

The results of the study will be available on the web at http://www.consrv.ca.gov/cgs/geologic_hazards/Tsunami/Pages/About_Tsunam... and will be presented at the annual meeting of the American Geophysical Union (AGU) in San Francisco in December.