Cascadia's Fault - Jerry Thompson [11]
CHAPTER 2
Lessons from the Rubble: A Front-Page Story
From the parking lot my first impression of the Pacific Geoscience Centre on southern Vancouver Island was that this sprawling complex of government laboratories belonged on the cover of some architectural magazine. The atrium was covered by a steep, angular skylight of tubular white steel and a grid of glass panels. Against a fetching backdrop of West Coast cedars and ferns, the mature landscaping included a small but brilliant reflecting pool. Nice digs for such a muddy-boot kind of science.
The downside to this idyllic locale was that it nestled cheek by jowl against the main runway at Victoria’s international airport. Walking in to PGC (as it’s known locally), I glanced at David Kaufman, the Toronto-based CBC producer working with me on a new documentary assignment to find out what scientists had learned from the Mexico City disaster. We both winced at the thought of random bursts of ruinous noise from turboprops, passenger jets, and helicopters that would no doubt make on-camera interviews here a challenge.
We were met at the door by seismologist Garry Rogers, who gave us the fifty-cent tour along a convoluted route past a museum-like display of earth science exhibits, presumably designed for public and school tours, to his office deep inside the complex. Once past the main public areas, the work space began to look more familiar, like a thousand other labs and universities.
Arriving at the top of a vast open stairwell, on a landing behind a plate glass wall, I saw a rack-mounted array of fifteen seismographs, each paper drum a furrowed field of squiggly parallel lines, pen strokes that looked like several days’ worth of data—every little twitch of the ground under this mountainous corner of the world. Pinned to corkboards, stapled onto beige walls, and taped to nearly every office door were charts and diagrams, cutaway illustrations of subduction zones and volcanoes, aerial photographs of the coastline, long strips of seismograph paper smeared with erratic ink blots, and reprints of science posters.
We found an empty square table in the coffee nook that looked like the only space big enough to fit several scientists in a group interview. Three of the men we were about to meet were out-of-towners with no office space in the building, visiting members of a team of seismologists and engineers who had just returned from a fact-finding mission to Mexico City. Over the coming days and weeks they would jointly write a science paper for the Canadian government’s committee on earthquake engineering, the panel of experts who make changes to the national building code.
“The most surprising thing for me,” said Dieter Weichert, head of seismology at PGC, “was this two-second period of earth motion, which for an earthquake of this size should have been longer.” He described the tectonic origin of those strong pulses of energy that had pounded through the earth with clockwork regularity every two seconds. Think of a metronome clicking back and forth or a relentless drumbeat.
It seemed to Weichert that a broken plate of oceanic crust ninety miles (145 km) long should have vibrated more slowly, perhaps once every six seconds. One might think the larger the slab of rock the slower the drumbeat, yet for some reason that’s not what happened. The shockwaves in Mexico had come bang, bang, bang—every two seconds.
More fascinating to me, since I’d not heard any of this before, was the discovery that the deep soil of the dried-up lakebed underneath the city’s downtown core had also vibrated every two seconds, which amplified the force of the incoming seismic waves. Who knew that soil had its own frequency and would quiver like a bowl of jelly? I tried to imagine evenly spaced ripples rolling through dirt like waves on the ocean.
It turns out the speed of vibration is determined by the type of soil (what it’s made of) and how hard or loosely packed it is. Weichert explained that structures of