On a sunny afternoon, you and your sweetie are out for a stroll along the beach, exploring the tidepools.

Suddenly you feel the ground begin to shake. Not hard enough even to knock you off the algae-covered rocks–just a little shaking, and then it's over. You laugh out of the nervousness we all feel when danger passes. You talk with strangers about earthquakes you have experienced and make guesses on how big this one was. "4.5," says one. "No. Less than 4," says another.

Then something truly remarkable happens. The tide goes out after a wave and keeps going out, revealing a whole range of tidepools and kelp you were unable to explore before. It's beautiful.

Take a few seconds to enjoy the scene, if you wish. It probably makes no difference. You are about to die.

A Real Threat

Many Californians remain nonchalant about earthquakes. You can't predict them, after all–you can only be prepared. But even less thought has been given to the prospect of a 100-foot wall of water barreling out of nowhere at 600 miles per hour, dashing homes and businesses to smithereens. People might think you're talking about the next summer disaster movie.

But guess what. "The threat of a tsunami is very, very real," says Richard Eisner, coastal regional administrator in the governor's Office of Emergency Services for Northern California.

Eisner is no alarmist, but he is a true believer. He recites chilling statistics with a calm little smile, his thickly bearded face tilted thoughtfully. "It's a threat we are just beginning to understand," Eisner continues.

He wants every California coastal town to wake up to the fact that tsunamis are inevitabilities, not probabilities. He knows that, for a town like Pismo Beach, a massive tsunami could make a 5.0 earthquake look like a carnival ride.

In the past, complacency about the tsunami threat resulted from a sense of invulnerability. While the seismic ring circling the Pacific Ocean makes it prime tsunami territory, researchers had long believed the U.S. coast to be relatively safe. According to a 1980 study by scientists from several disciplines led by the U.S. Geological Survey (USGS), 12 major tsunamis caused by earthquakes of magnitude 8 or greater have wreaked havoc on the Pacific coast of North America during the last 6,600 years–an average of one every 550 years.

But new evidence suggests that a major tsunami may strike the West Coast as frequently as every 200 to 500 years–and the last one, according to the historical record, was about 300 years ago. Native American legends recount the devastating tsunami of 1700, as do written records in Japan, where giant waves swallowed entire islands.

"It was about 30 meters [98 feet] high," says Eisner, who explains how marine sand deposits have been found far inland in high-up tree hollows. "All along the Oregon and Northern California coasts you see these swamps full of dead trees," he continues. "These trees have been core-dated. They all died in 1700, correlating with the dates of the sand deposits, which were carried by powerful waves up onto the coastal bluffs."

The Cascadia Subduction Zone (CSZ) is an area off the coast of the Pacific Northwest where the Juan de Fuca plate dives under the North American plate. It's the largest active fault in North America outside Alaska. While the zone doesn't produce as many quakes as the San Andreas fault, it is much longer (625 miles), and the quakes it produces are much bigger in magnitude. This kind of thrust fault is the most frequent source of tsunamis.

Motivated by the recent findings about the CSZ and by the devastating 1994 tsunami in Japan–which killed more than 200 people in spite of a high-tech warning system–the U.S. government is taking its first serious look at the tsunami threat. In 1997, Congress set aside $2.3 million to form the National Tsunami Hazard Mitigation Program.

Guided by the National Oceanic and Atmospheric Administration, the Federal Emergency Management Agency (FEMA) and the USGS, the program focuses on the needs of the five Pacific states. (Due to less seismic activity, tsunamis occur much less frequently in the Atlantic.) Each state forms a Tsunami Steering Committee made up of directors of emergency service offices from each coastal county.

Once erroneously referred to as tidal waves, tsunamis have nothing to do with tides. Extending from the surface to the ocean bottom, tsunamis are actually column-shaped waves that can travel at velocities of up to 600 miles per hour–as fast as a commercial jetliner. Ships at sea cannot recognize a tsunami as it passes, since wave heights only reach a few centimeters in deep ocean. The Japanese word tsunami means "harbor wave," because it passes unnoticed until it reaches the shore.

There it may rear up to heights of 100 feet and surge up to two miles inland. Successive waves may be spaced dozens of minutes apart and continue arriving for hours. Many tsunami fatalities occur after the first wave (which is rarely the largest) when people think its safe to return to low land. Sometimes it takes days for the sea to return to normal.

Conventional thinking on what causes tsunamis has changed significantly in recent years. "We used to think that you only got one with a thrust-fault earthquake, which is where the seabed either lifts or drops," Eisner says. "But recently we've seen tsunamis caused by minor earthquakes that trigger landslides. We are also seeing tsunamis caused by strike-slip earthquakes, like one you would have on the San Andreas. We're learning that we're at greater risk than we thought."

In 1979, a submarine landslide in the Mediterranean caused a tsunami that struck the French Riviera without warning. The sea retreated for 975 feet before two 32-foot-high waves rushed back in. Six people died and three were reported missing and presumed dead along 37 miles of coastline.

It was what scientists refer to as a "near-source tsunami." Sometimes, the only indication of such a tsunami comes when water recedes quickly from the shore. The first wave hits within minutes, allowing no time for any kind of official warning. Near-source tsunamis are accompanied by ground shaking, which, if severe, can cause evacuation routes to become blocked by debris.

A local government planning book put out by the Tsunami Steering Committee last month states: "Automated warnings can take up to 15 to 20 minutes to issue, and cannot protect coastal inhabitants located in the immediate area of a near-source tsunami. When a large subduction zone earthquake occurs nearby, the first tsunami waves may reach coastal communities within minutes of the event."

While earthquakes are the most common cause of tsunamis, volcanic eruptions and even meteorites have accounted for some–enough for Erik Asphaug, an asteroid researcher at UC Santa Cruz, to devote himself to the study of tsunami-causing asteroids. The problem, he says, isn't necessarily the big rocks. "It's the smaller group that is the threat," Asphaug says. "We'll never see them before they hit."

Size Doesn't Matter

"The biggest obstacle facing effective tsunami planning right now is that tsunamis are these almost dreamlike occurrences that shock people when they happen but then are quickly forgotten," Eisner explains. "In the U.S., it is especially challenging because we have such a limited written record of these events."

California's meager historical records document 81 tsunamis since 1806. Scientists have determined that 14 had heights higher than 3 feet, six of which were destructive. While that's an average day's surf on much of the coast, the nature of a tsunami wave is such that speed and run-up levels, not necessarily height, pose the greatest threat. A tsunami just keeps coming, surging quickly inland, bending parking meters to the ground and erasing structures. Even a 1-foot wave traveling at 60 miles per hour is a force to be reckoned with, especially if it's carrying debris.

Because distant submarine earthquakes can cause tsunamis, Eisner hopes to cure coastal dwellers of the notion that what happens in the earth's crust thousands of miles away doesn't affect them. Of the 14 tsunamis three feet or higher that have struck California since 1806, only five originated nearby. By comparison, there have been 24 earthquakes in California of magnitude 6 or higher in that period.

And according to FEMA, worldwide more people have died from tsunamis than from earthquakes since 1945.

The most memorable California tsunami was a result of the 1964 Alaskan earthquake, which claimed 16 lives in California alone and caused $20 million in property damage. Hardest hit was Crescent City, where oil storage tanks burned for days. Automobiles were carried as much as 100 feet by the waves and stacked in battered piles or hurled into buildings. Ten died in the town of 3,000, primarily as a result of returning to low land after the first wave instead of waiting for following waves to pass.

In 1992, the Cape Mendocino earthquake shocked experts, who had long believed it would take at least two hours for the first tsunami waves to reach California. Though that wave caused no damage, it reached Humboldt Bay in fewer than 20 minutes. The Cape Mendocino region and the coastal areas of Humboldt and Del Norte counties are among the most seismically active areas in the U.S.

The 1960 Chilean earthquake and tsunami killed more than 2,000 people in that country before spreading to Hawaii, where it killed 61, and Japan, where it took another 122 lives. The tsunami also killed two in the Los Angeles area. Most of the deaths in Chile were caused not by the record-breaking 9.5-magnitude quake, but by the tsunami that followed. Waves rushed inland for more than two miles, causing landslides and wiping out entire villages. Debris left in trees shows that in some places the water was more than 13 feet deep. Coastal land was lowered by eight feet. Tales are told of pregnant women huddling in tree branches all night while the water swirled beneath them. The dead were found in piles the morning after.

And this tsunami was actually mild compared to older, almost forgotten tsunamis that occurred before any kind of warning systems were in place. The Indonesian tsunami of 1883 killed more than 33,000. The Lisbon tsunami of 1755 left 60,000 dead.

Crying Wolf

One of the main goals of the National Tsunami Mitigation Program is to improve the seismic network. Currently the only way tsunamis can be detected is by noting the epicenter of an earthquake and then waiting for characteristic waves to strike a sparse network of tide gauges throughout the Pacific.

The West Coast/Alaska Tsunami Warning Center issues watches and warnings to all five Pacific states. NOAA has described these watches as being based on "indirect and insufficient information." Because data can only be provided after a wave has passed one of the tide gauges, there is no way to predict what height waves will be at other locations. The result is an unacceptably high false-alarm rate of 75 percent. Crying wolf undermines the credibility of warnings and puts people at risk during evacuation.

Part of the answer will come in the form of a system of deep-ocean tsunami detection buoys. Three prototypes are being tested in the North Pacific and off Monterey Bay. These buoys have detectors three miles long embedded in the ocean floor that can measure a tsunami as it passes. They will revolutionize the current primitive practice of inferring from an earthquake whether a tsunami has been generated and then just waiting for it to arrive. Scientists and disaster officials will be able to determine information in real time about the wave as it passes in mid-ocean.

"There is a great concern that if a tsunami does occur there would be virtually no warning," says Eisner quietly, as the machines in the Office of Emergency Services quietly whir all around him. "This is a huge advance in technology that will enable us to better understand tsunamis and issue watches and warnings."

However, the buoys can't detect near-source tsunamis which can arrive within minutes–not enough time to issue a proper warning even if detectable. This is why coastal inhabitants should familiarize themselves with the warning signs (see sidebar) and be familiar with evacuation routes.

Another integral part of the mitigation program is the production of inundation maps for the coasts of the five Pacific states, which will be used to develop more effective evacuation planning. Only five have been produced in California in the last three years.

Surprisingly, these maps are the first of their kind. Similar inundation maps were developed in the 1970s by the USGS and the Department of Housing and Urban Development as part of a national flood insurance program. There was a regulatory purpose to those maps; if you lived in a tsunami inundation zone you had to have flood insurance. But the maps didn't calculate the effects of a tsunami coming from the Cascadia Subduction Zone.

The new maps, based on the new evidence, provide more accurate projections of how a tsunami may affect specific areas, taking into consideration the characteristics of various types of tsunamis. So far only the most populous coastal counties, such as Los Angeles and San Francisco, have received the new inundation maps. Funding problems make it uncertain when, or if, maps for counties like San Luis Obispo will follow.

Before 1997 few counties were aware of the tsunami threat and most had done little or no tsunami planning. The steering committee has succeeding in educating local governments, but ultimately it can only guide and suggest. Local governments have to determine their own acceptable level of risk. Many question the efficacy of spending a lot of money preparing for such a rare event, even if that event may be devastating.

And warning systems alone are not enough. Neither hell nor high water, it seems, can keep people from developing prime California coastal land.

"Cities that have focused a lot of development along the coast are obviously going to suffer the most from a tsunami," says Eisner. "There is tremendous pressure for development on coastal property, which is of course the area most at risk. Obviously the best solution is not to build along coastlines." Æ

Andrea Perkins is a Bay Area journalist.