As I drove across the I-40 bridge into Memphis, I was reassured: chances were slim that a massive earthquake would wrest the road from its supports, and plunge me more than a hundred feet into the murky Mississippi. Thanks to a recently completed $260 million seismic retrofit, the bridge—a chokepoint for traffic in the central U.S.—is now fortified. It’s also decked out with strong-motion accelerometers and bookended by borehole seismometers to record convulsions in the earth.