Cascadia's Fault - Jerry Thompson [23]
Exactly how long the violent tremors lasted was hard to tell because “every seismic instrument within a radius of several hundred kilometers was thrown off scale” after the first few seconds, according to a U.S. federal study. How long it seemed to last depended upon who was telling the story and what kind of ground they were standing on at the time. If they were anywhere near the epicentral region, the lurching and jolting went on for at least four minutes and possibly as long as six. It must have felt like forever.
Measurements were made by a network of seismic stations around the globe, but again—because none of the instruments on the ground in Alaska had survived long enough to capture an accurate local record of the energy—there were discrepancies in Richter scale calculations as well. A report prepared by the U.S. National Research Council lamented that the seismographic record of this earthquake was “woefully incomplete.” Some seismologists figured the magnitude was 8.3; others pegged it at 8.6. Part of the problem was the Richter scale itself.
As one geologist explained it to me, no seismograph in the world at that time could get a correct magnitude because—until digital seismographs came along—the entire earthquake spectrum could not be accurately recorded for any event that lasted longer than about a hundred seconds. Great earthquakes like this one can take as long as three hundred seconds, sometimes even more, to finish rupturing. At an average velocity of just over 1.8 miles (3 km) per second, that’s how long it would take for a fracture like this to unzip over such a long distance.
Basically, the old Richter scale was good only for shocks up to about magnitude 8; anything bigger (or longer lasting) was off the scale, so to speak. So an extension of the scale—the moment magnitude scale—had to be devised for measuring the relative sizes of the 8+ events. Thus the Alaska temblor was eventually assigned a magnitude of 9.2 while assessment of the Chile quake of 1960 shifted from 8.9 to 9.5, the largest earthquake ever recorded with modern equipment. The point being that no matter how the numbers are crunched, the Good Friday earthquake was then and remains the largest scientifically documented seismic shock to hit continental North America and the second largest in the world.
Piecing together a sequence of events in the days and weeks after the Easter weekend, scientists concluded that the zone of significant damage covered 50,000 square miles (130,000 km2). The vibrations were felt over an area of 500,000 square miles (1.3 million km2). The pulses of energy had traveled roughly 1,200 miles (1,930 km) to the southeast, when a group of scientists attending the annual conference of the Geological Society of America, who were enjoying dinner in the revolving restaurant atop Seattle’s Space Needle, felt the tower vibrate slightly.
Barometers in La Jolla, California, roughly two thousand miles (3,200 km) from Anchorage, detected an atmospheric pressure wave generated by the quake. Water levels in 650 wells across North America, in Hawaii, and as far away as South Africa jumped abruptly, one as much as seventeen feet (5 m). The Council report said “probably twice as much energy was released by the Alaska earthquake as by the one that rocked San Francisco in 1906.” Measured by the newer moment magnitude scale, Alaska 1964 was 160 times larger than San Francisco 1906.
In the immediate aftermath of Good Friday, however, reports of casualties and property damage were slow in reaching the outside world because power and telephone lines were down all over south-central Alaska. Only those living through the disaster knew how bad things really were. The statewide death toll, at 115, would be described officially as “very small for an earthquake of this magnitude.” Of those who died, 106 were killed by the tsunamis. But in some ways, the people of Alaska