WHEN THE BIG ONE HITS
New tsunami mapping adds to Monterey danger zones
Every decade or so, state geologists use sets of scientific and computational tools to answer the question of what would happen to the California coast in the event of a tsunami. The newest maps marking these events have added several blocks worth of Monterey real estate to the areas most likely affected by sudden ocean swells.
Of all the Monterey Peninsula cities, Monterey would suffer the greatest inflow of seawater in the event of a tsunami, although the swells would also inundate areas of Pacific Grove, Carmel, Seaside and Pebble Beach.
The changes from the 2009 maps and the ones just released by the California Geological Survey, part of the state Department of Conservation, aren’t dramatically different, but varied enough to add warnings to new areas of the city.
“They have increased the map area by several blocks in the downtown area, to the west and south,” said Nat Rojanasathira, Monterey’s assistant city manager.
For example, the new maps show warning areas coming all the way up to Pacific Street covering more of the Old Monterey area, passing over the top of Fremont Street at Abrego Street, surrounding the Naval Postgraduate School and now reaching all the way to Highway 1 along Camino Aguajito.
And in Seaside, the area already susceptible to both sea-level rise and tsunami threats along Laguna del Rey has been expanded into neighboring residential areas by a few blocks.
In Pacific Grove, the new maps cover neighborhoods inland of Ocean View Boulevard all the way up to Surf Avenue and then continuing down along Sunset Avenue. In Pebble Beach, the maps show portions of 17-Mile Drive potentially underwater, including Spanish Bay.
Coastal California has not, at least in recent times, seen anything like the 2011 Great East Japan Earthquake and Tsunami off the coast of Tōhoku, Japan, that killed some 18,000 people and set off the worst nuclear accident in the country’s history at the Fukushima Daiichi Nuclear Power Plant. A tsunami from that quake traveled more than 5,000 miles across the northern Pacific Ocean and caused minor damage to the California coast. One man died when he was swept out to sea near the Klamath River in Del Norte County.
But it doesn’t mean a major tsunami couldn’t happen here. California is littered with earthquake faults and could suffer significant tsunami damage from quakes generated from the Cascadia Subduction Zone, a so-called “megathrust” fault more than 600 miles long that stretches from Northern Vancouver Island to Cape Mendocino on the California coast.
These subduction zones are areas where one tectonic plate meets another and is “thrust” underneath it, which can cause major earthquakes and tsunamis. Other causes include ocean landslides, lava entering the sea, seamount collapse or meteorite impact. Davidson Seamount is a seamount located just 80 miles southwest of Monterey off the Big Sur coast and is one of the largest known seamounts in the world.
Rick Wilson, the senior engineering geologist in the seismic hazards program and the tsunami unit manager with the state Geological Survey, said Thursday from his Sacramento office that the new results in the 2021 maps were the result of a number of new technologies that scientists didn’t have at their disposal for prior formulating maps in 2009.
One of the technologies is called LIDAR, which stands for Light Detection and Ranging. It is a remote sensing method that uses light in the form of a pulsed laser from aircraft to measure land contours down to a single meter in high-resolution imaging. The better accuracy provides more precise measurement of vulnerable areas along the coast, including the Monterey Peninsula.
Another tool is called probabilistic analysis, which uses complex computer modeling and geologic sampling to determine what that region’s tsunami history was dating back thousands of years and projects future probabilities, what Wilson called a “statistical representation of the unknown.”
These new technologies, along with existing ones like carbon dating, enables scientists to go back and retrieve 3,500 years of information along the north coast, including seven or eight tsunamis in the Cascadia Subduction Zone, Wilson said.
“Based on this analysis, we upgraded the magnitudes of our sources in the Cascadia and the AlaskaAleutian Islands subduction zones,” he said “These somewhat larger tsunami sources created modest increases to the new California Tsunami Hazard Area maps we have today.
“We work with communities and the public to plan for the worst case,” Wilson added. “We have provided emergency managers and harbor masters secondary decision-support tools, called ‘playbooks,’ which can be used in real-time during distant source tsunamis where there is time for implementing a lowerlevel plan.”
Rojanasathira said that in the event of a tsunami, warnings would come down from the state to Monterey County’s Office of Emergency Services which would then begin notifying cities. Rojanasathira said Monterey would immediately open its Emergency Operations Center that would coordinate with first responders — police, fire and public works — to evacuate people within the new map areas to higher ground.
Alerts would most likely come in the form of emergency cellphone messages. More information on tsunami preparation is available at www.tsunami. ca.gov.