On a summer day when the deep-blue water is filled with water-skiers, motorboats and kayaks, it’s hard to think of Lake Tahoe as anything but peaceful.
But every four thousand years or so, that changes. Lake Tahoe sits on top of three fault lines. And when an earthquake happens beneath such a deep lake, it causes big problems. The largest fault, the West Tahoe Fault, shifts vertically during quakes. When it shoots up, it pushes up the lake water. Geologist Gordon Seitz explained that during earthquakes the displaced water, “has to flow downhill and generates a wave. And this is a tsunami wave. And that wave, when it hits the shore, is probably on the order of 20, 30 feet high.”
Seitz, who works for the California Geological Survey, has been studying Tahoe’s faults since 1999. If he has a bit more urgency than you’d expect for someone studying a once-every-four millennia event, that’s because the last big quake happened, well, about 4,000 years ago.
“Do I expect it to happen tomorrow? Perhaps not,” Seitz said. “But if it did happen I wouldn’t be completely surprised.”
On Thursday afternoon, Seitz stood on a barge about 300 yards off the lake’s west shore. The barge is the operating base for a robotic submarine that is spending the week about a thousand feet below the lake’s surface, using high-definition cameras and ultrasound-like technology to examine the West Tahoe Fault.
Seitz and other researchers have been studying sediment deposits and submerged trees on Tahoe’s shore for years, but these images will yield – he hopes, at least – the best evidence yet of what Tahoe’s last earthquake looked like. “We have a pretty good idea of when that happened right now,” he explained. “This will confirm that, hopefully, but also it will give us a new dimension and tell us more about the magnitude. Did it shift ten feet? Did it shift 20 feet?”
At this point, evidence suggests the last quake had a magnitude of 7.1 to 7.4. “Usually, the last earthquake gives you the best picture of what the next earthquake will be doing,” Seitz said.
The Tahoe barge is packed with cables, a crane, a generator and even a propane grill. In a metal shed, scientists monitor video feeds from the sub, along with data showing the machine’s depth, along with the temperature, oxygen and saline levels of the water.
The Tahoe dives are a tune-up for the submarine and its Northern Illinois University crew. Their ultimate destination is Antarctica, where the sub will dive under ice shelves to conduct climate change-related research. So in addition to providing data for California researchers, the crew is working out kinks. Among them: a small leak that left the sub temporarily grounded when reporters visited the barge.
Exactly the type of problem that’s easier to fix on a sunny Lake Tahoe day than deep below an Antarctic ice shelf.