Cascadia's Fault Read Online Free PDF Page A

two seconds another shockwave thundered in from the coast like a horizontal pile driver hitting the foundations of the ancient city. Concrete and steel joints were cracking, walls were pulling away from floors. Tall structures standing too close together smashed into each other at the top and tore themselves apart from the roof down. Multi-wing complexes that were not perfectly square or rectangular—those with T-shapes or with a number of sections connected to a central hub—yanked themselves apart at the joints because each wing rattled and snapped to its own beat in syncopated self-destruction.
    Anything between six and seventeen stories tall had a special problem—an internal rhythm in near-perfect synch with the earthquake itself. Shockwaves from the quake caused these midsize towers to hum like a tuning fork. This “resonant oscillation” amplified the amount of energy that pulsed through their vertical frames. With the arrival of
each new wave, the foundations would be slammed to the side yet again before the rooftops had flexed back to a vertical position. Like rocks along the fault itself, the urban bricks, mortar, and steel inevitably failed.
    As stress was relieved along the fault, the continental shelf off the west coast came unstuck from the oceanic plate that had snagged it more than ten miles (16 km) below the surface. The overlying crustal plate broke free, rebounded sideways as far as eight feet (2.5 m) and then sagged. Western beaches slumped as much as thirty inches (80 cm), causing the local sea level to rise.
    Offshore, while the continental shelf was rattling loose and rebounding to the west, it also heaved upward. Roughly three thousand square miles (7,500 km 2 ) of the shallow sea floor lifted a huge mound of salt water. When gravity broke this hump of hoisted water into elliptical waves—a train of tsunamis raced across the Pacific.
    Given the magnitude of the earthquake, the size of the tsunami was relatively small. From Manzanillo to Acapulco and Zihuatanejo, the waves measured from three to ten feet (1–3 m) but caused relatively little damage. When the tsunami hit Hilo, Hawaii, hours later the wave was less than nine inches (22 cm) high. One possible explanation is that the ocean floor had been shoved underneath the continental landmass at such a shallow angle that the volume of seawater lifted by the earthquake did not amount to much, compared to other tsunamis generated by large earthquakes.
    But in September 1985 scientists were still struggling to understand what had happened. The intricate details of tectonic motion were still pretty sketchy. One thing, however, was clear: a seemingly logical explanation for the lack of major earthquakes off Mexico’s west coast—if we haven’t seen any in all of recorded history, they must not happen here—was wrong.
    Since a great earthquake had just happened in a supposedly aseismic zone like Mexico’s, where else might the same thing occur? What about the coast of northern California, Oregon, Washington, and
British Columbia, also thought to be aseismic? A major disaster like Mexico’s had not happened in the Pacific Northwest in all of recorded history either. By the same logic, if mega-quakes were likely to happen up there, surely we would have seen one by now.
    On the other hand, maybe not. Overnight it seemed anything was possible. Perhaps a megathrust quake could happen on any offshore “subduction zone.” Teams of scientists scrambled to Mexico as quickly as they could to examine this newest twist in the young science of plate tectonics. Ideas about how the world’s largest earthquakes are created would change in the months and years ahead. For some scientists, the Mexico City disaster would be a tipping point.
    As I fell asleep that night in New York, I too faced a knowledge gap. I did not know the Mexico City earthquake had become the opening chapter of a story I would soon be covering