New Tsunami Maps Depict Extreme High-Water Mark

 

California has a yet another seismic threat to prepare for, thanks to a set of new maps that depict a ferocious line of water that may, if earth moves in just the wrong way, someday surge inland along the state's coastline. Experts are saying that these new maps should be used to plan for emergency evacuations, not changes in land use planning. In at least one case, however, the Coastal Commission is already considering policies that would take tsunamis into account when approving developments.

Approximating maximum inundation for a worst-case scenario tsunami, the line washes over all of Newport Bay and Marina del Rey, wipes out parts of downtown Santa Cruz, and makes half of Seal Beach disappear.  Some of the state's most fabled real estate from La Jolla to Malibu to Montecito would wash out with the tide.  It would yet again pummel Crescent City, itself the victim of the nation's most destructive and deadly recorded tsunami, a 20-foot wave that struck on Good Friday, 1964.

An Unpredictable Threat

Hardly an inch of the state’s 1,200-mile coastline is shielded from tsunamis, which can be triggered by local events -- earthquakes and underwater landslides -- or by those that occur almost anywhere in the Pacific basin.

“Earthquakes are pretty localized,” said Rick Wilson, engineering geologist with the California Geological Survey. “But for distant tsunamis that come across the Pacific, we’re dealing with our entire coastline in California.”

The 130 maps, which cover over 90 percent of the state's populated coastline, culminate a multi-year collaboration between the California Geological Survey, the University of Southern California Tsunami Research Center, and the California Governor's Office of Emergency Services, to marry the most current seismic data with precise measurements of offshore bathymetry and coastal topography.

The maps assign to the tsunami threat a visual element that is absent from the prediction of earthquakes.  Contrary to this discomfiting picture, however, researchers and state officials alike caution that the maps' projections in no way mean that the state's coastal communities and nearly 1 million coastal residents should permanently decamp for higher ground.

Memories of the 2004 south Asian tsunami -- and of the statewide warnings that were sounded after the February 25 Chilean earthquake -- make clear the destructive threat of  tsunamis. The maps were prompted by recommendations in a 2005 risk assessment report published by the state Seismic Safety Commission. That report, which was prompted by the south Asian event, noted that roughly 80 tsunamis have struck California over the past 150 years and that two of them were destructive. Overall, the report concluded that tsunamis represent a “significant threat to life and property in California.”

But tsunamis’ infrequency and unpredictability render them, according to the current research, too speculative a threat to warrant changes in coastal land use patterns for the foreseeable future.

"These maps were not produced for land use planning and we don’t suggest they be used for that," said Aggeliki Barberopoulou, engineering professor at USC and co-leader of the mapping project. “There is no time factor in these maps in the sense that this is not a 50- or 100-year occurrence like we have with flood maps.  Maps used for land use planning purposes would have a time factor.”

Worst-Case Scenario

Since their unveiling in December, the maps have been distributed primarily to county emergency management agencies, which, in turn, have distributed them to their coastal communities.  The maps' sole explicit function is to help those agencies craft emergency management plans, including notification systems, signage, and evacuation routes. California's warnings come mainly from the West Coast/Alaska Tsunami Warning Center, run by the National Oceanographic and Atmospheric Administration.

"I consider these maps a prerequisite for evacuation planning and adequate response on the part of cities and counties on the coast," said Jim Goltz, Earthquake and Tsunami Program Manager at the California Emergency Management Agency. "It’s up to [local officials] to develop evacuation maps, evacuation plans, places where people are safe, and to post signs to indicate where there is a tsunami hazard."

"We are focusing on them purely as emergency management tools," said Susan Asturias, senior emergency services coordinator for San Diego County. "We know they’re not legal documents for land use."

The maps apply data from complex models to GIS maps, drafts of which were sent to local officials for their review before the final drafts were published.  The result, Goltz said, is a much more refined picture than the state has ever had before. But even precise maps do not present a clear prescription for emergency management.

“They come at different directions, different levels of magnitude, and each particular location along the coastline is different in its topography and its bathymetry," said Dan Larkin, Director of Emergency Services for Humboldt County.   “The whole subject is so complex that it’s extremely difficult to come out with a one-size-fits all evacuation plan.”

Though the maps' inundation lines can be viewed as an unbroken whole, the chance of a tsunami striking the entire state at once are essentially zero.  Typically, tsunamis will strike in localized areas, so while the maps may be geographically accurate, they make no claims about frequency or the extent of a single event.

"I don’t see these particular maps as being applicable to land use planning because they present a worst-case scenario of the maximum run-up of a tsunami should we get the Big One, or somebody else gets the Big One and we get the run-up," said Sidnie L. Olson, interim director of community development for the City of Eureka. "That’s like Humboldt County trying to plan for a magnitude 10 earthquake.  If we were planning for that, we wouldn’t allow anyone to live there."

Previous to these maps' publication, the north coast counties of Humboldt and Del Norte had relied on locally generated maps and have developed intensive response strategies in light of their historic experience with tsunamis. The Cascadia subduction zone -- a particularly active tectonic plate boundary stretching from Northern California southern British Columbia -- makes the north coast, according to Goltz, a relative “magnet for tsunamis,” which can arrive from such quake-prone areas as Alaska's Aleutian Islands.

However, Southern California too has seen its share of tsunamis, including four that hit Newport Beach in the 1930s and one that hit Santa Barbara in 1812 with such ferocity that it compelled local Native American to move uphill, according to Wilson, of the CGS.

“We knew that in some areas, say, Orange County and L.A. we were going to see very large inundations from worst-case scenarios that would impact a lot of homes,” said Wilson.  The key here is that we don’t really know how often these large events occur.”

Whether, and when, a major threat will arise remains for the next round of maps and studies to assess. These additional maps and studies will attempt to assign probabilities to tsunamis of different sizes and locations by, in part, delving into the geological record to assess the frequency of prehistoric tsunami events.  The state's Seismic Hazards Mapping Act, adopted in 1991, requires that the state catalog all seismic dangers to the extent that technology and funding allow.  Until researchers can assign credible time horizons to potential tsunamis should planners, any efforts to retard development in potential inundation zones would be overly cautious.

"To a certain degree, we're going to get down to an exercise of acceptable risk," said Baskin. “Do we plan for an event that happens every three or four hundred years or do we accept that that’s just beyond our time horizon?"

Nevertheless, some north coast officials are particularly attuned to the threat of tsunamis and their implications for long-term planning.  To them, the maps, however unclear their time frame may be, represent a threat that should not be ignored.

"These maps might be a starting point to have a discussion as far as critical facilities that we consider how we want to deal with them," said Olson, of Eureka.  Olson said that currently, the maximum inundation zone includes Eureka’s sewage treatment plant, a stretch of Highway 101; “our entire industrial waterfront would be wiped out,” she said.

Land Use Policy

Avoiding such a wipeout in the first place is already influencing policy in the region.  The Coastal Commission has recently proposed local policies for Del Norte County that would direct planners to take tsunami risk into account.  Working off of maps devised at Humboldt State University – maps which are, according to Larkin, in close agreement with the new statewide maps – the commission’s North Coast Office has recently suggested modifications to its policy for Del Norte County that would anticipate tsunami risk.

"There are specific policies in there that are addressing the creation of new units by subdivision that would be prone to tsunami risk,” said Jim Baskin, coastal planner with the California Coastal Commissions North Coast Office.  “If they can’t show that their floor elevation couldn’t be shown to be one foot higher than inundation risk, that might be grounds for disapproval."

These policies would not necessarily sway the coastal permitting process singlehandedly, as tsunami inundation zones are likely to be environmentally sensitive in the first place and therefore subject to heavy scrutiny regardless of tsunami danger.

"Just by virtue of their low-lying terrain, more than likely they’re going to be corresponding with water courses or wetland factors or stream course characteristics that would just in the matter of course be environmentally sensitive areas," said Baskin.

Moreover, even if tsunami risk does not warrant reduction in development per se, emergency planners insist that development plans and approvals should still take into account the need for emergency-response infrastructure, such as evacuation routes and staging areas.

"Certainly if there was a development in an area that was inundation-prone, that development would need to include in terms of its permitting some sort of tsunami safety plan," said Baskin.  "For example, the occupants of a hotel they need to be aware of the evacuation routes and high ground."

In order to bring continuity to these efforts and coordinate efforts between land use officials and emergency management officials, CalEMA officials intend to engage planners in their effort both to explain the maps' limitations and to anticipate the next round of research, which may, upon completion bear on coastal development in certain parts of the state.  Wilson said that, pursuant to the Seismic Hazards Mapping Act, the next stage will be to assign probabilities and timeframes to the maps in order to give land-use planners proper context.  The state intends to involve planners in that discussion and, in the meantime, make efforts to educate the planning community about the implications of the current maps.

“Over the next year our plan is to have a workshop with the land use planning community to not necessarily unveil the existing maps but to explain why and what we’ll be doing in the next several years to make a better set of maps for their use,” said Wilson.

Contacts & Resources:

 

  • Aggeliki Barberopoulou, USC Tsunami Research Center (213) 740 5129
  • Jim Baskin, California Coastal Commmission North Coast Office (707) 445-7833
  • Jim Goltz, Earthquake and Tsunami Program (link to maps) Manager, CalEMA (916) 845-8510
  • Rick Wilson, California Geological Survey (916) 445-1923

-- Josh Stephens