The apparent triumph of the Haicheng prediction
was overshadowed by disaster on July 28, 1976, when a devastating
(M 7.8) earthquake struck the Chinese city of Tangshan, located
150 km east of Beijing, and home to over one million people.
The hypocenter of the earthquake was located directly beneath the city,
at a depth of 11 kilometers, on a right-lateral strike-slip fault.
About 93 percent of all residential buildings in the city were
destroyed, and at least 240,000 lives were lost as a result --
one of the highest earthquake death tolls ever. Though this area
was also being extensively monitored, there were few precursory
signals before the mainshock, and so the earthquake came as a total
surprise to Chinese scientists. No prediction for such an event
was made; the residents of Tangshan were caught completely off-guard.
A much less tragic but similarly discouraging result came from the only official earthquake prediction attempt ever made in the United States. That prediction was made by the United States Geological Survey (USGS) in April 1985, and it stated that an earthquake of roughly magnitude 6.0 would occur along the San Andreas fault near the small town of Parkfield, California, sometime between 1987 and 1993.
The USGS prediction clearly didn't have the immediacy of the Chinese prediction at Haicheng (and was thus more of a long-range forecast), but it was backed with high confidence, and did contain two precise details essential for useful earthquake prediction: location and magnitude. This forecast, called the Parkfield Prediction Experiment, was based primarily upon the rather reliable recurrence interval of the San Andreas fault's Parkfield segment, which was thought to rupture about every 22 years, according to seismograph records and historical accounts. Each of those ruptures seemed to be roughly the same -- about magnitude 6.
In preparation for the expected event, the countryside around Parkfield was peppered with seismic monitoring equipment, and all sorts of detectors. The idea was that since scientists knew where and roughly when a magnitude 6 earthquake would occur, they could be ready to detect not only the changes caused by the earthquake itself, but also any possible precursors to the mainshock, which might prove a valuable asset to our ability to forecast other earthquakes.
