LA JOLLA, Calif. -- According to scientists here on the rim of the Pacific Ocean, Lake Tahoe, nestled hundreds of miles inland in the Sierra Nevada Mountains, could be due for a tsunami.

		Graphic: Under the lake

If you're one of the thousands of summer tourists who think ocean beaches too dangerous and flock instead to the shores of the deep, cobalt-blue lake on the California-Nevada border, consider yourself forewarned.

Riding a wave of interest generated by the December tsunami that devastated parts of the Indonesian shoreline and killed almost 300,000 people, researchers say large earthquakes and landslides have occurred under the lake in prehistoric times and when one happens again the lake will slosh like a bathtub, inundating beaches and nearby towns.

Scientists at Scripps Institution of Oceanography at the University of California, using a novel combination of airborne laser imaging, acoustic profiling and sediment core sampling, have determined that a large earthquake ---- a magnitude 7 on the Richter scale or higher ---- occurs in the region every 3,000 years.

Such an earthquake would produce a tsunami wave 10 feet to 33 feet high, according to an earlier study by the University of Nevada.

Because the velocity of such a wave increases with water depth and the lake is the 10th deepest in the world, it would hit every shoreline within just three to six minutes, said Graham Kent, associate research geophysicist and director of the Scripps Visualization Center.

"It's important to know about the risk and the potential for severe damage, but the problem is we don't know when the last one was," said Kent, a Tahoe native. "But when it happens it could generate a wave of the size that hit Sri Lanka."

The biggest and most destructive tsunami to hit the United States in recorded history occurred on Good Friday, March 27, 1964, when an 8.4-magnitude earthquake in the Gulf of Alaska sent a series of tsunami waves crashing into the Pacific coast from Alaska to Mexico. Earthquakes also spawned tsunamis that hit the Pacific coast in 1812, 1923, 1927, 1930 and 1946.

Kent, a member of the Scripps team whose Lake Tahoe research findings appear in this month's issue of the journal Geology, said underwater landslides near steep, almost vertical cliffs like those in McKinney Bay on the west side of the lake, can also produce massive tsunamis.

"We know from core samples that a catastrophic slide happened 50,000 years ago," Kent said. "It produced a wave estimated by the U.S. Geological Survey to be more than 300 feet tall."

Lake Tahoe is about 30 miles southwest of Reno, Nev., and covers 193 square miles over an active fault basin prone to earthquakes and landslides. It has several areas where the water depth is in excess of 1,600 feet.

Kent said the water depth has hampered normal geologic investigation of the West Tahoe, Stateline and Incline Village faults that run under the lake. He said "trenching" research, ongoing since 1998, has been completed on the Incline Village fault and is continuing on the other two to determine when big quakes shook the lake.

The Scripps team used a digital profiler -- a device Kent calls a "$500,000 fish-finder" -- to generate acoustic signals that penetrate the sediment layers of the lake floor and document seismic activity. They also used airborne laser technology to map the lake floor, and deep and shallow water core sampling to analyze and date the lake's geologic history.

The scientists will catalog the individual fault ruptures over the past 20,000 years to determine where each fault lies within its earthquake cycle, a continuum known as "strain, accumulation and rupture."

Technologic advances have enabled researchers to re-calibrate the measurements of Tahoe's geologic hazards and the risks are going up.

"Until we determine the fingerprint of the last several quake events we don't know where we fall on that curve," Kent said. "But people should know this is a high earthquake hazard area."

About 17,000 people live around the lake year round, a number that swells to 100,000 during the summer and then increases again when day tourists fill the beaches and boats.

Local planning agencies, aware that catching a wave is not what brings people to Tahoe, cleared the way for Scripps' scientists to work along the fault line that runs through an elementary school site in Incline Village on the lake's northern arm.

But many local residents, Kent said, have the mistaken impression that Lake Tahoe was formed by a long dormant volcano, rather than by the upheaval, fall and displacement of rock strata along the seismically active fault lines. And they seem too busy sunbathing, boating and skiing to care about earthquakes, landslides and tsunamis.

"When we go out and talk to them, we're not very popular," Kent said. "They don't want to hear this stuff."

What they really don't want to hear is "Surf's up!"