By Laura Read

THE RECENT QUAKES AROUND LAKE TAHOE have not only jiggled residents’ homes, they’ve also unleashed thoughts of the “Big One.” The fears are not unfounded, scientists say; but a prediction is elusive, and there’s still a lot to learn. One thing we know is that the story begins deep in the past.

Between 12,000 and 21,000 years ago, a strong earthquake shook Lake Tahoe and loosened about 3.5 cubic miles of earth on the West Shore around McKinney Bay. The debris tumbled downward and across the lake bottom with such force that chunks scattered as far as 9 miles out. Slabs a mile across and 500 feet high stayed intact, according to recent measurements. The McKinney Bay Landslide cut a chunk out of Lake Tahoe’s shoreline and gave it the west-bulging shape it has today.

But the drama wasn’t over. The cascading earth sent a giant wave surging eastward in a swell that reached 330 feet high. The action is animated in a 15-minute film produced by scientists at the UC Davis Tahoe Environmental Research Center (TERC), called Tahoe In Depth 2D. Images generated by sonar and radar equipment reveal just how much impact the tsunami probably had.
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The powerful wave offset by the crumbling earth moved so quickly it took less than 10 minutes to cross the lake and roar up canyons on the East, North, and South shores. In 10 more minutes, the water receded from those hillsides, then splashed up on all sides of the lake. Sloshing back and forth, then, like water in a tilted tub, it scoured the slopes and pulled every living and loose thing downhill. According to geologists, Eagle Rock, a volcanic plug at the mouth of Blackwood Canyon, received a sort of neck scrubbing; its base shows the evidence to anyone looking closely today. The new lake bottom was now scattered with monolithic rocks and deposits of sand and gravel from the mountainsides. Today, the subaqueous wonderland is a fascinating place for scientists to explore, which they do using submersible diving vehicles, remote-operated vehicles, and underwater photography.

It’s not crystal clear what caused the McKinney Bay Landslide, but scientists like Richard Schweickert, emeritus professor of geology at the University of Nevada, Reno, say it probably was a profound quake. Sediments on that side of the lake contain weaker layers of an ancestral lake, Schweickert said. They did not hold up to the earth-ripping jolt.

Despite the dramatic past, tsunamis haven’t been top of mind for many geologists; but this spring, on the morning of May 28, one of Lake Tahoe’s three faults, the Stateline-North Tahoe Fault, shifted twice, 30 minutes apart, at magnitudes 4.2 and 3.1. Not only the scientists paid attention. Other observers wondered, “Are we in for The Big One?”

Alert!
“It’s a wakeup call,” said Jim Howle, with the U.S. Geological Survey. “This is a good time to think about earthquake preparedness.”
Tension and Release

In fact, Lake Tahoe is due for a major rupture. If it occurs, it will most likely be along a 25-mile-long crack that lies west of the Stateline-North Tahoe Fault Zone, called the West Tahoe-Dollar Point Fault Zone, scientists said. Whether that happens tomorrow or in a thousand years is anybody’s guess. “Take a yard stick and bend it, and tell me when it’s going to break. If you do it gradually, it won’t break; then, boom, it breaks,” said Graham Kent, director of the Nevada Seismological Laboratory at UNR. “Sometime in the next thousand years — or maybe tomorrow — we’re going to get that West Tahoe Fault going, and it will be a catastrophic event.”

The idea that precursor quakes, or foreshocks, will warn of such a catastrophe is a misnomer, Howle said: Sometimes earthquakes have foreshocks and sometimes they don’t. “There’s no such thing as an early warning system for earthquakes,” he explained. That said, there have been thousands of little quakes in the Lake Tahoe Basin and surrounding areas that have not drawn much scrutiny, but the bigger quakes of May 28 caught his attention. “This is about as close as we get,” he said.

One place the public has turned to for information is TERC. The research center does not have a geologist on staff, said Heather Segale, TERC’s education and outreach director, but its focus on the watershed and lake health intersects with geological research. After the quakes, more than 100,000 people tuned in to the center’s YouTube channel to see Tahoe In Depth 2D, Segale said. “The earthquakes definitely have people interested in learning more.”

Heat, Ice, and Water
It’s easy to admire the lake’s sublime beauty, and to be comforted by the fact that in recent centuries, the features haven’t changed much. But geologists — who think in much broader timespans, in millions and millions of years — know that all of the earth’s surface is constantly rising and falling as the tectonic plates that form it shift under gravity’s pull.

As these tectonic plates push against each other, tension builds, Howle explained. The pressure or energy must be released, and is usually let go in a tremor, or quake, which can cause a rift in the earth called a fault. Ruptures can appear as “normal,” or “step,” faults, which slide vertically against each other and make a cliff drop pattern; or they can take the form of a “strike-slip” fault, which shifts from side-to-side the way two palms rub together, fingers to thumb.

Even though Lake Tahoe is huge and seems like its own entity, it is nevertheless part of the much larger region extending eastward into Nevada and westward all the way to the Pacific Ocean. Two million years ago, the Basin was less dramatic and held a shallow lake that captured rain and snowmelt, according to Frances Wahl Pierce, a retired geologist and TERC docent who hosted a video about the lake’s geology for TERC’s YouTube channel. Over time, as the Sierra Nevada lifted and a “basin and range” landscape began to form to the east, faults evolved around Lake Tahoe. As the faulting grew, the Tahoe Basin got deeper — even more so than today, for a while — holding a great volume of rainfall and snowmelt.

Volcanic flows on the North Shore blocked the Basin’s sole outlet, now called the Truckee River, Wahl Pierce said. The natural dams contained the water and raised the lake level, but then water eroded them and the lake level dropped, leaving evidence of higher shores on the mountainsides, she said.
Mountain to Basin

The shifting and shaking is far from over. Nowadays, geologists are watching three major normal or step faults that trend north-south through Lake Tahoe, Kent explained. These branch into a combination of normal and strike-slip faults. The West Tahoe-Dollar Point Fault Zone starts at the mouth of Emerald Bay and trends north along the West Shore to the east side of Dollar Point and beyond toward Northstar California Resort. Its effects can be seen from the Rubicon Trail at D.L. Bliss State Park, where a cliff plunges into deep blue water that’s 1,400 feet deep, and divers find boulders and fissures to explore. It also appears along Dollar Point, where, within 20 strokes, kayakers can cruise from water that’s 10 feet deep and the color of turquoise to water 30 times the depth and darkened to a haunting indigo hue.

East of the West Tahoe Fault, the Stateline-North Tahoe Fault Zone runs from Stateline into the lake at a southwest angle. It forms a steep drop just to the east of Cal Neva in Crystal Bay, plunging to the deepest part of the lake, 1,644 feet. Paralleling this is the Incline Village Fault Zone, Kent said. It runs southwest into the water out of Incline Village, cutting through land less than 2 miles away from the Tahoe Environmental Research Center.

Relatively recent breaks, the result of fault zone activity, are visible above ground in a couple of places, Kent said — near the old elementary school in Incline Village and at the Angora Lakes trailhead south of Fallen Leaf Lake.

It’s not easy for even scientists to tell exactly where a quake occurs. An earthquake’s epicenter is determined in part by its distance underground, as fault ruptures exist at specific depths, Schweickert said. When they run close to each other, as is the case with sections of the West Tahoe-Dollar Point and Stateline-North Tahoe faults, quake location will be determined by which depth the tremor corresponds to. Best indication so far for the May 28 quakes, according to Kent, is the bigger rupture was “a relay fault zone between West Tahoe Fault and Stateline-North Tahoe Fault — and was strike slip, left lateral,” meaning the epicenter occurred on one of the rifts that branch off the main artery of the fault trending from the north middle section of the lake through Stateline.
Action Everywhere

The Tahoe-area faults do not stand alone. They belong to an extended series of rifts that trends both south and west. Named Walker Lane, this zone is connected to the San Andreas Fault along the Pacific Coast, said Annie Kell, previously an education and outreach seismologist at UNR, in a video lecture for TERC citizen scientists preparing to paddle the Lake Tahoe Water Trail last month. The San Andreas Fault gets about 75% of the overall region’s activity, while Walker Lane exhibits 25%, Kell said. Walker Lane has experienced thousands of jolts across the centuries, from Mina, Nevada, to Mammoth, California, to Independence Lake. From mid-May to mid-June this year, the Stateline-North Tahoe Fault Zone alone ruptured dozens and dozens of times, according to USGS online records.

Kent has a creative interpretation of how the combined movements shaped Lake Tahoe. If you hypothetically combine all of Lake Tahoe’s known previous jolts in one long segment, the time it took to form the lake would comprise just half a day. “Lake Tahoe was formed in six hours of actual hell and terror separated by thousands of years,” he said. “There is mostly quiescence and then there’s a bad 30 seconds.”

Kent has studied the earthquakes recorded along the Pacific Coast and inland: He sees the big picture and finds it alarming: “Lake Tahoe could be the most dangerous geological hazard in the lower 48,” he said.

One reason is that thousands of people are present in the Tahoe Basin on any given day, he said. They’re sunbathing, dining in cafés, playing with dogs, skiing its slopes, or driving to work. Another reason is: At least one primary fault is overdue to rupture in a big way.

When geologists say the West Tahoe-Dollar Point Fault is ready for “the Big One,” they’re assessing risk by timeframe. It’s been 4,500 years since the last large rupture along that line, according to Kent. The known major event before that was 7,800 years ago, he said. The one prior to that was 11,000 years ago. If the time between significant ruptures is a pattern — roughly 3,200-year intervals — then the West Tahoe-Dollar Point Fault could be in for a big quake soon. Normal, or stepped, faults like this tend to rupture in large singular events, Kent said.

Tsunami Watch
That major earthquake could be bad enough, but will it also trigger a tsunami? Kent and other geologists interviewed said that kind of event is hard to predict. “It is a fact, you will generate a tsunami of a certain size by just dropping the Basin floor,” Kent said. A tsunami could also be triggered by a landslide that might come from a rupture in either a normal fault or a slip-strike fault. “The height is dependent on how that rupture happens,” he said. How will it affect people? “It depends on where you are around the lake and how unlucky you are on the shoreline.”

When asked how worried he is that a tsunami will occur soon, Kent said: “I work with fire departments. I’m worrying about Tahoe burning down about 10 to 20 times before we have an earthquake.” On the other hand, he also advises, “If there’s strong shaking at the beach, get off the beach immediately. If you see big changes in water level, get off the beach …. To a degree, we know we are running out of time on the West Tahoe Fault. It does have to relieve its stress.”

Kell — and Howle, too — won’t avoid Lake Tahoe. But they are wary. “It’s a good reminder that bigger earthquakes are not out of the question in this area, and everybody should take heed,” Howle said.