Cascadia's Fault - Jerry Thompson [120]
Seventeen months and three weeks after Haicheng, everything China thought it knew about seismic prediction came down like a house of cards. At 3:42 a.m. on July 28, 1976, lightning flashed across the sky and the earth rumbled ominously. Seconds later another major earthquake struck northern China—this time with no prediction and no evacuation. The rupture happened directly underneath the industrial city of Tangshan, roughly 90 miles (140 km) east of Beijing.
The magnitude 7.5 rupture shifted the ground about five feet (1.5 m) horizontally and three feet (1 m) vertically, destroying nearly 100 percent of the living quarters and 80 percent of the industrial buildings in the city. People were jolted from their sleep in total darkness, screaming and choking on thick dust from unreinforced brick buildings that collapsed in piles of rubble. Official reports estimated the death toll at 240,000 with 164,000 more seriously injured. That’s roughly the same number who died from the Sumatra quake and the Indian Ocean tsunami in 2004. Critics claimed the Tangshan estimate was conservative, however, and that the real number of people killed could have been 600,000 according to estimates made by foreign observers. Whatever the count, Tangshan was far and away the most deadly single quake in the twentieth century and one of the great tragedies of all time.
Tangshan was a major center for coal mining, iron and steel production, and the manufacture of cement. Nearly all of it was wrecked. Bridges and highways collapsed, pipelines broke, dams cracked, more than ten thousand large industrial chimneys fell, and twenty-eight trains passing through the city overturned or were derailed. The key question, though, was why nobody saw it coming. Did the lessons of Haicheng not apply here? Apparently not.
In the final two months before Tangshan, not a single foreshock was detected by a regional seismic network capable of measuring tremors as small as magnitude 1.7. Some of the other little twitches and anomalies that had preceded the Haicheng event also preceded Tangshan but apparently the signals were not strong enough to trigger a prediction or evacuation. How different could the geological structures be only three hundred miles (480 km) away from Haicheng? If Haicheng had foreshocks, why not Tangshan? Good and important questions that still need to be answered, said Kelin Wang.
Preliminary answers suggested by Wang and company could be that fault failures are unique, so different from each other that whatever anomaly or precursor helps to predict one event probably won’t work for a different kind of fault in a different physical setting. And once a rupture has happened, it modifies the geological structures and rock properties enough that the precursors then change as well. The symptom that tipped us off to the last quake may not precede the next, even if it happens on the same fault. And then sometimes you get the symptoms but the big temblor never comes.
If that’s the real lesson of Haicheng and Tangshan, why bother with prediction? When I finally got a chance to interview Kelin Wang, he still seemed hopeful about the prospects—a persistent, low-key optimist—although he was wary of putting prediction into practice too soon. “If the Haicheng story was true,” he began, “then earthquake prediction is not impossible.” It did show us “how early in the stage we are still in terms of earthquake prediction. And the difference—the major difference—between these two earthquakes is the Haicheng earthquake had a foreshock sequence and the Tangshan earthquake had no foreshocks at all.” Strange as it may sound this did little to dampen enthusiasm for the dark art of prediction.