“WE’RE crossing a fault scarp . . . now.” The truck bounces slightly at a sudden dip in the asphalt, but away from the road the drop is much more severe. On both sides, the dull-brown bushes and dry grass of this desert landscape lurch down abruptly, leaving a step some three metres high that stretches to both horizons. Only grinding and shifting of the Earth’s crust can create crumpling on such a large scale. There’s no doubt about it: this place is seismic.
The road winds upwards into a mountain range and the scarps become harder to spot. Soon they are disguised by a coating of snow, and by stands of Jeffrey pine trees whose bunches of needles reach upwards like absurdly long fingernails. The very presence of these mountains is another sign of the Earth’s restlessness here. And over the narrow pass, there is more. A great azure lake more than 30 kilometres long comes into view, formed where a series of faults have forced the land to drop by hundreds of metres. This is Lake Tahoe, on the border between California and Nevada, a playground for Silicon Valley entrepreneurs and Reno casino owners.
It’s also a playground for geologists. For the past four years, Richard Schweickert from the University of Nevada at Reno and his colleagues have been painstakingly mapping the region’s many geological faults, and their findings have led them to a startling conclusion. The combination of seismic activity and lake water means only one thing. It doesn’t matter that we’re more than 400 kilometres from the open ocean: Schweickert and his colleagues are convinced this is a prime spot for tsunamis.
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Out in the ocean, tsunamis occur when an earthquake lifts up part of the seafloor, disturbing the water and setting a giant wave in motion. In coastal inlets – fjords, bays and the like – they can be triggered by a giant landslide. Here in Lake Tahoe, and in many other lakes around the world, it now seems that both of these effects could be in play. Several fault lines run across the floor of the lake, and if a slip on any of these caused the lake bed to jolt, a tsunami would follow within a matter of minutes. That same seismic shaking could also cause large blocks to tumble down from the steep mountains around the lake, displacing the water yet again and setting another tsunami in motion. What’s more, because this is an enclosed lake basin, a tsunami would result in a train of “seiches”, waves that slosh back and forth for hours before they eventually die down.
The size of the waves depends critically on the geometry of the lake. In deep water, tsunamis are barely noticeable. But when these waves reach shallow water, they drag on the bottom and slow down, causing a destructive wall of water to pile up. In April this year Gene Ichinose, a graduate student from the University of Nevada, published the results of a modelling study on Lake Tahoe. He and his colleagues found that tsunamis generated by earthquakes in the lake could reach heights of 10 metres or more – comparable to some of the largest oceanic tsunamis – and that the seiches could last for anything up to 12 hours. If such an event happened today it could destroy roads and sweep away houses around Tahoe’s shore.
Down by the lakeside, nothing seems less likely. Through the huge picture windows of Whittell Mansion – a research and conference facility newly acquired by the University of Nevada – the lake is spectacularly serene. Only the slightest of ripples stir the water’s surface, casting veins of sunlight onto the granite boulders below. As we sip our coffee and admire the view, Mary Lahren, also from the University of Nevada, unfurls a brightly coloured map. This shows the bathymetry (depth measurements) of the lake floor and its surroundings, using data obtained two years ago by researchers from the US Geological Survey.
Superimposed on the map are the faults mapped by the University of Nevada team (see Diagram). Lahren points out three major fault lines that run into the lake: the North Tahoe fault, the West Tahoe fault and the Dollar Point fault. She also points to McKinney Bay, a large, half-moon-shaped inlet on the far side of the lake. We can see this bay across the water, and behind it two deep glacial canyons that have been carved out of the mountains rising up at the back of the bay. Both bay and canyons are crucial to the story.FIG-mg22694501.JPG
It was when they looked closely at the bathymetry of McKinney Bay, in 1998, that Schweickert and Lahren were alerted to the possibility of tsunamis. The floor of the bay, and the lake as a whole, are strewn with debris from landslides. Some of the chunks of rock are vast – up to 140 metres high – and they are scattered over more than a hundred square kilometres of lake floor. Landslides like this would surely have caused massive tsunamis. “The bathymetry brought it all into focus,” says Schweickert. “At that point, it’s inescapable. If there’s a collapse of that size there would have to be big waves.”
In retrospect, says Schweickert, it should not really have been much of a surprise. The group had spent several years mapping earthquake faults on land, and it certainly looked as if they continued under the lake. What’s more, research in the 1970s had already revealed landslide debris on the lake bed, although the results had been more or less forgotten. And McKinney Bay looks for all the world like a bite taken out of the side of the lake – a classic sign that it was created by landslides. Still, the notion of a tsunami took the researchers by surprise.
And then Lahren noticed something else about the McKinney Bay area. When she looked closely at the map, she realised something was missing. Wherever glaciers grind their way down valleys, they create steep ridges called moraines along their flanks. The Lake Tahoe basin was heavily glaciated during the last ice age, and the glaciers created moraines. When the ice finally retreated about 12,000 years ago, these piles of rock should have remained in place, which is exactly what they did in most of the glacial valleys around the lake. But in the two valleys behind McKinney Bay, the moraines had vanished.
“It was one of those epiphanies,” says Schweickert. “We were staring at the figure of McKinney Bay and we suddenly said, `Holy cow! There’s huge glacial valleys with no moraines.’ No one’s ever seen that before.” What could have wiped out the moraines? Could it possibly have been caused by landslides and tsunamis? Schweickert and Lahren teamed up with Jim Howle, another geoscientist from the University of Nevada, and went for a closer look.
Driving around the lake towards McKinney Bay, we find ourselves on a great finger of land jutting out into the water. This promontory is a moraine. A pair of glaciers once spilled down the mountainside here, piling their accumulated debris between them. That debris now forms a headland tens of metres high, whose flanks plunge steeply downwards. It also shows signs of seismic activity in the area. Looking back, we see that the moraine is cut through with fault lines. The crest line of the moraine should slope smoothly down to the water, but it’s more like a staircase, each step created by successive quakes. And in some places, the thick trunks of the sugar pine trees have bizarre kinks. When the Earth moved, the trees found themselves pointing at awkward angles, and were forced to adjust until they were growing vertically once more. “Trees with knees,” says Howle.
The headland is an impressive sight, and there ought to be something similar at McKinney Bay. After all, the valleys behind it were also carved by glaciers. All the more shocking, then, to reach the bay and find nothing of the sort. We climb to the top of Eagle Rock, a hunk of volcanic rock that pokes up out of the landscape behind the bay, and which lies right alongside where the moraine should be. But there’s scarcely a sign of one. Just a few glacial boulders on top and a couple of smallish, tapered hills of glacial debris, lying in the wake of Eagle Rock. Which is just what you’d expect if most of the moraine had disappeared in the landslide and the rest was washed away by repeated waves surging upwards from the shore. “It’s almost too perfect,” says Schweickert. Could a tsunami, or even a series of tsunamis, really have wiped out the moraines here? Schweickert thinks so. “Tsunamis can lift houses off their foundations,” he says, with a shrug.
There is yet more evidence, but for that we have to head south of the bay, to Sugar Pine Point. Schweickert leads us between the pine trees and down a snowy slope that suddenly flattens out for 100 metres or so. We emerge onto a beach, and he points to the pebbles at his feet. Not all of these, he says, can have come from the canyon here – their mineralogy is all wrong. They must have come from somewhere else round the lake. And although we can’t see it now because the ground is covered in snow, the flat surface we have just crossed is scattered with a fine layer of these pebbles. The only explanation, Schweickert believes, is that this part of the lake shore has been bevelled flat by the action of giant waves, up to 30 metres high. The same waves picked up the pebbles and dropped them as they moved inland.
Local damage
If this really is evidence for a tsunami, why are the moraines elsewhere around the lake apparently untouched? The researchers have thought of that. In enclosed bodies of water, tsunami damage can often be very localised, depending on the particular geometry of the wave, and on the bathymetry of the lake bed.
But the team has found signs of tsunamis elsewhere. Near the south-west shore of the lake, they discovered three thin layers of sand in the soil up to 1.5 kilometres from the shoreline, and 30 metres above lake level. The sand could have been deposited by a tsunami, but they can’t be sure without mapping it all the way down to the present shoreline. That’s not easy when so much of the shore is privately owned. Beachfront property here is at a premium, and residents don’t take kindly to geologists digging up their land – even if it could tell them about a devastating tsunami. “We’ve asked the utility companies to let us know whenever they’re digging a trench,” says Schweickert.
What the researchers can do, though, is look for evidence of landslides in the mud that lines the lake bed. Working with oceanographer Bob Karlin, also from the University of Nevada, the team has collected high resolution seismic images of faults and landslides in the lake bottom sediments. And in September, collaborating with a host of institutions, they managed to obtain five sediment cores before bad weather beat them back. So far, they have not had time to analyse the cores in detail. But, Karlin says, they have already thrown up some very interesting findings.
Throughout the cores, the researchers found turbidites – thick layers of coarse sand and fine clay that are characteristic of a landslide. Turbidites form when unconsolidated sediment is shaken loose and tumbles down a slope. Initial results from carbon dating of the cores suggest that there have been eight such major landslides in the past 7000 years. Each was probably associated with a seriously big seismic event. On dry land, it takes an earthquake of at least magnitude 6 on the Richter scale to trigger a landslide, says Karlin. Under the lake, where the weight of the water stabilises the sediment, it would require a quake of at least magnitude 7 on the Richter scale.
Do the cores give any hint as to how often a major tsunami is likely to happen here? No one is prepared to say. It’s hard to tell whether the landslides suggested by the seismic data and sediment cores were spaced out regularly through time or bunched in a few sudden lurches. What’s more, there are so many criss-crossing faults here that it’s almost impossible to predict the next earthquake. “It’s maddening,” Schweickert says. “We don’t know if we’re overdue for an event.”
More geological studies, more cores of the lake bed, and more detailed analysis of those already drilled, should at least reveal how regularly landslides occurred in the past. In the meantime, says Schweickert, “I’d guess these are very rare”.
Karlin now wants to go back and obtain more cores, to try to pinpoint where the landslides came from, and also whether the turbidites were deposited at the same time throughout the lake. He has already studied Lake Washington using the same coring technique. There was plenty of evidence of landslides there, he says, though no direct signs of tsunamis.
Lake Washington is also in a seismically active area. In fact, there are many lakes around the world that sit on seismic faults, and could be at risk of tsunamis. Siberia’s Lake Baikal, for instance. Likewise Lake Titicaca, high in the Andes on the border between Peru and Bolivia, Lucerne in Switzerland, and many more. “It applies to just about any lake in the world where you’ve got mountains,” says Lahren. “If there’s faults, you could get tsunamis.”
From Sugar Pine Point, we trudge back to the trucks through deep snow. The Sun has all but disappeared, and it’s growing very cold. On the far side of the lake, the mountains are touched by a pale pink glow, the reflection of the dying sunset. And the lake is now a perfect, peaceful aquamarine. For most of the time, the worst disturbance you’ll encounter here is a bunch of rowdy skiers, out for a lark. But there’s always the chance of something much more alarming. Tsunamis on inland lakes may be rare, but the evidence suggests that they really do happen. So next time you’re standing in a tranquil alpine setting miles from the sea, you might want to bear that in mind. By all means enjoy the scenery, but keep a weather eye out for the Big Wave.