Cascadia's Fault - Jerry Thompson [9]
In the immediate aftermath of the Mexico City disaster, seismologists, marine geologists, and engineers tried to draw conclusions about the underlying cause and what it might mean for other supposedly aseismic zones around the world. Perhaps these monster quakes had happened before but researchers had not looked far enough into the past to find the evidence. Perhaps “all of recorded history” was simply too brief, in geologic terms, to see a repetition of these enormous undersea earthquakes. If it takes several centuries to build up enough stress for a quake this big, perhaps the last one happened so long ago there was nobody around to write it down.
Some scientists thought the Michoacán zone was a “seismic gap” where strong earthquakes (in 1939, in 1973, and again in 1979) had relieved stress on either side of the main segment, but the part in the middle—the Michoacán zone itself—had remained stuck for nearly two centuries. The zone was the only part of this tectonic plate that had not snapped free of the continental crust that had drifted over it. It was a holdout—a ninety-mile (145 km) slab of sub-sea rock that was bound to rip loose sooner or later. And when it did, the amount of strain released was unprecedented.
For half a minute that must have felt like a lifetime, 320,000 square miles (825,000 km2) of Central and North America shuddered and rumbled up and down and from side to side. More than twenty million people, some as far away as Los Angeles, Guatemala City, and Corpus Christi, Texas, felt the earthquake. Even though the rupture zone was thirty miles (50 km) offshore, south and west of Mexico City, the quake might as well have been directly underneath the downtown core.
Like the lowest notes of an upright bass, this fractured slab of sea floor played fatal music, a throbbing rhythm that pulsed with stunning efficiency through 190 miles (300 km) of continental crust to reach the capital city. The first burst of notes lasted roughly thirty seconds—about the duration of most normal earthquakes that occur closer to the surface. But then another segment of the plate broke loose and the vibrations started again. Many towers that had survived the initial attack were too crippled to endure the second.
As if that weren’t enough, a significant part of the center of the city was constructed upon a layer cake thirty feet (9 m) thick of sand and gravel and clay, washed into the Mexico Valley from a ring of volcanic mountains. The modern city occupies the same site as Tenochtitlan, capital of the Aztec empire, built on the shore of an ancient lake called Texcoco. After Hernán Cortés conquered the Aztecs in 1521, Spanish engineers drained the lake to make room for a larger settlement. They knew nothing at all about the risk of massive earthquakes from the sea.
Four centuries later, eighteen million Mexicans had stacked a sprawling metropolis around and on top of this mudflat basin. When deep bass notes from the Michoacán gap hit the loosely packed soil, it throbbed and resonated. Apartment blocks on the dried-up lake moaned and swayed to the rhythm of the quake. Swinging side to side like inverted pendulums, each tower rang at its own frequency, depending on how tall it was. High-rise towers on the sands of Texcoco were in big trouble.