Every now and then the ground trembles, in some places more often and more dramatically than in others. New York is not California. Even so, in the summer of 1884 in Brooklyn, chimneys collapsed and windows shattered when a quake struck near Coney Island: magnitude 5 or so. (Seismometers weren’t widely used back then.) Anything larger would do quite a bit of damage amid today’s infrastructure. But we have little record of how often such a tremor occurs. “Every thousand years, every ten thousand years, every million years?” William Menke, a seismologist at the Lamont Doherty Observatory in Colombia, recently asked himself, with a view to the possible destruction of the metropolitan area. “It makes a difference!” Many major earthquakes have occurred on the east coast, he explained. We just don’t know when.
Menke hiked up a mountain in Harriman State Park next to the Ramapo Fault to try to fill in the gaps. He was looking for rocks whose shape and placement gave him a sense of existential comfort instead of fear. “That’s why I thought about it,” he said, coming to a bobsled-sized boulder perched on the edge of a flat cliff. “That must say something important about the level of shaking that has occurred since it was placed there. If it shook a lot, it would have fallen.” A hiking guide could not resist a futile nudge. The boulder was deposited there naturally by a glacier. “Everything smells like the Ice Age here,” says Menke. The last glaciers in this area melted about fifteen thousand years ago. Auspicious.
The two continued to climb, searching for increasingly dangerous boulders. Some were too small to preclude human intervention. “You can see how someone put these hefty rocks into a bench configuration,” Menke noted of an arrangement near the remains of a campfire. Another boulder, intriguingly top-heavy, was sitting in a crevasse, making it more difficult to remove and therefore not worth investigating. Menke squatted alongside others, tracing their outlines in a notebook and measuring the slopes of the underlying bedrock with a spirit level and an inclinometer he’d paid Lowe eight dollars for. “Most of what I do is pretty low-tech,” he said. “I occasionally lost things in the field and then found them a little rusty six months later.”
Menke’s gray hair was uncropped and, like some of the rocks he was examining, seemed to defy gravity. Such was his fixation on geology that he failed to notice a buck galloping past, although he did point out a small discoloration in the bedrock at one point. “See the surface here? Something protected this from erosion. Was there a boulder that rolled down? Where is it?” Using some physics on the back cover, he estimated the amount of gravitational acceleration required to slide various candidates downhill in his notebook. “The last one we did was on a gentler slope, and it was about point three in gravity,” he said. “So that would be about seven and a half orders of magnitude.” In contrast, a giant rock shaped like a sea turtle seemed likely to ski at a magnitude 7 on a steeper slope. “That’s So actually an interesting number,” Menke said. “If you can rule out that there have been magnitude 7 earthquakes since the end of the Ice Age, that’s actually quite important in terms of New York’s seismic risk.”
Proper science would require him to delve into sophisticated camera technology for photogrammetry and 3-D computer modelling. “I’ll tell you a funny story about a Greek,” Menke said, referring to the astronomer Aristarchus, who was trying to estimate the distance from the earth to the sun. “He did a pretty good job, but there was one crucial piece of information he needed to know, and that was the angular diameter of the sun. It’s half a degree, and he estimated it was two degrees. If he had taken care to measure things, he would have gotten the right number.” For the moment, however, Menke consoled himself with what the naked eye told him. On the other hand, a magnitude 7 earthquake is a thousand times stronger than a magnitude 5 earthquake. Think Haiti in 2010 instead of Coney Island in 1884.
Menke stopped for a water break before beginning the descent, ran his hand over another boulder and broke off a piece of encrusted tripe or lichen. “Very low nutritional value,” he said. “But given the choice of eating stone tripe or dying, you eat stone tripe.” ♦