Why No Tsunami Warning Signs on Sonoma Coast?
There is a conspicuous absence along our coast of those bright blue and white tsunami warning signs that we saw in Hawaii and Southern California during the coverage of the earthquake in Chile last month.
Drive south from here though and you can see them as close as Ocean Beach in San Francisco. From Half Moon Bay south to San Diego, a variety of tsunami signs are in place along the California shoreline.
Drive north and they can be seen in virtually every low coastal point from Fort Bragg to Crescent City.
As long ago as 2008, Humboldt county alone had as many as 400 signs, according to the California Department of Transportation (Caltrans) in a news release dated October 29
of that year. In that same release, Troy Nicolini of the National Weather Service, said, “Eventually, all coastal communities in California will have tsunami warning signs.”
So what happened here or, more to the point, what has not happened? For what reason or reasons are we still without signs?
Deputy Emergency Service Coordinator, Chris Helgren, of the Sonoma County Department of Emergency Services, says that the type of signs and where to place them have been “under study for the past few years, but we were the last county to receive ‘run-up maps,’ models for planning, and approval of signage from Caltrans.”
However, on the website for the State of California Department of Transportation (Caltrans), its Tsunami Signs Policy reads in part: “Local emergency-management officials may designate routes and locations for placement of (signs) or may designate appropriate locations for placement of signs near the coast in parking lots, at parks and beaches.”
The “local emergency-management,” in this case, is of course the Sonoma County Department of Emergency. Yet, a search on their website does not reveal the word tsunami at all.
Is this just a bureaucratic stand-off between state and county then?
“We have also had to work on special concerns for sign placement in Bodega Bay,” Helgren went on to say.
He explained that the U.C.Bodega Marine Lab sits on Bodega Head, an area considered high enough to be safe if a tsunami hit. Yet, to evacuate people from there, it would be necessary to take them down into the “unsafe” area along Westshore Road and then back up to higher ground.
Based on this assessment, this means that all of Doran Beach, including the Coast Guard Station, and virtually the entire town of Bodega Bay would be in the unsafe or “inundation line” zone.
When quizzed as to where else high ground could be found, Helgren agreed that possibly the Bodega Bay Fire House, the Bodega Bay Elementary School, and most of the streets in the Bodega Harbour subdivision could be safe.
These are not givens of course, because just how high a tsunami wave will be when or if it hits land is still not entirely predictable. Jane Lubchenco, Administer of the National Oceanic and Atmospheric Administration (NOAA) says that it is all in the “modeling” based on gathering new information.
An effective improvement in this has been achieved by NOAA with the placement of 32 DART stations in the Pacific Ocean and 7 in the Atlantic. DARTs are Deep-Ocean Assessment & Reporting of Tsunami surface buoys that use bottom pressure recorders to provide “real-time” wave action.
The more information gathered, the better the “model” or prediction of tsunami action. Each is an unknown because it depends on the angle of the wave when it hits a coast, the steepness of the shore, and whether it is a bay or a straight shoreline.
The “inundation line” referred to above is indicated in red on maps from the California State Department of Conservation. The Sonoma County Tsunami Inundation Map shows three “Affected USGS Quads” in Sonoma County as determined by the United States Geologic Service. The affected areas are in yellow with green-lined boxes of “mapped areas” within them. For a better view of this map, go to: www.conservation.ca.gov/cgs/geologic_hazards/Tsunami/Inundation_Maps/Sonoma/Pages/Sonoma.aspx
One quad is named the “Sears Point-Petaluma Point” and has to do with possible flood impact on the Petaluma River from a tsunami hitting San Francisco Bay.
The other two quads are “Bodega Head (Salmon Creek, Bodega Bay)” and “Duncan Mills (Jenner, Ocean View).” These two overlap and basically cover all of Highway One from inland Valley Ford north to Jenner.
Yet, in further explanation of why signs have not been put up in these areas, Helgren says that, “the geologists say there is not a huge concern for a tsunami from the San Andreas fault, even though it cuts right through Bodega Bay.”
“It is a strike/slip fault and not a subduction fault, and tsunamis are not generally caused by strike/slip faults.”
According to the USGS, NOAA, and Oregon State University’s College of Engineering’s Tsunami Wave Basin research, this is true.
The 1906 San Francisco earthquake (estimated Richter scale reading of 7.8) and the Loma Prieta in 1989 (6.9) were both on the San Andreas. Neither of these generated any discernible or significant tsunamis here in California nor across the ocean.
But this is not the point. It was the subduction type earthquake in Chile last month that led to tsunami warnings for the entire California coast. The strike/slip fault of San Andreas running through Bodega Bay had nothing to do with it.
A strike/slip fault is when one tectonic plate, part of the earth’s crust that is fracturing, is usually moving gradually past another plate. For instance, the San Andreas is caused by the Pacific Plate moving north while the North American Plate stays put.
Bodega Head is on the Pacific plate. The Bodega Bay post office is on the North American plate. Movement is generally very slow over many thousands of years. Currently, the range of movement is calculated at about 2” per year.
Occasionally though there are the earthquakes called the “big ones” (around 7.0). When these occur, the plate movement is accelerated in swift and unpredictable ways, such as in the case of the San Francisco and Loma Prieta quakes.
Subduction earthquakes, on the other hand, occur when one plate moves up under another plate in a very sudden way. This generally happens in the ocean at depths as much as 10 miles below, according to seismic geologists.
It is this action that can form tsunami waves that can travel for thousands of miles and reach heights of 35 feet and more. The force of subduction quakes is most often 9.0 or higher on the Richter scale.
Examples of this kind of quake include the two in Chile; a 9.5 in 1960 that generated the tsunami that devastated the seafront city of Hilo, Hawaii, and the most recent at 8.8 last month. The tsunami waves from this one fortunately did not cause the destruction that the 1960 one did nor was it as bad as the 9.3 Indian Ocean quake in 2004.
Closer to home, back in 1964, a 9.2 subduction quake off Alaska sent a tsunami wave as far south as Crescent City on the California-Oregon Border. The harbor and downtown area of the small town were crushed.
Sonoma county needs tsunami signs as much as anywhere else on the California coast.
But when will that happen?
The good news is that Caltrans appears to be on the brink of purchasing the signs, but may leave it up to the County to do the actual work of placing them.
Sonoma County Department of Emergency Services is trying to get it done. Last year, it put on what Helgren calls a “tabletop exercise” to better see the need for signs, evacuation routes, and other concerns.
The presentation had representatives from all “first responders”: in other words, the first emergency personnel on site. These included the Bodega Bay Fire Protection District, Bodega Bay Coast Guard Station, State and Regional (county) parks, the Sonoma County Sheriff’s Office, and the California Highway Patrol.
Helgren says that all of these groups are constantly in touch about ways to handle all kinds of emergencies in the county. Tsunamis are now on the list.
The bad news?
Recent budgetary cuts at both county and state levels will probably cause even more delay. Helgren predicted that, “We could see signs up next year or maybe not for five years.”