On March 11, 2011, at 2:46 p.m. local time, a magnitude-9.0 earthquake ruptured a 500-kilometer-long fault zone off the northeast coast of Japan. Its epicenter was 130 kilometers off Sendai, Honshu; it occurred at a relatively shallow depth of 32 kilometers. The temblor violently shook northeast Honshu for six minutes, and collapsed its coastline by one meter. The thrusting moved Honshu about 2.4 meters eastward, and the seismic waves on the Pacific Ocean floor set off tsunami waves traveling at the speed of a jet plane (about 700 kilometers per hour). Waves 3 to 38 meters tall pounded Honshu’s coastline, destroying towns and villages and flooding areas up to 10 kilometers inland. Tsunami waves also swept across the Pacific, causing damage or disruptions in Hawaii, California, Oregon, Washington and British Columbia.
Casualties from the earthquake and tsunami in Japan may be 30,000. More than 125,000 buildings have been washed away or seriously damaged; property damage is estimated to be more than $310 billion. Japan is used to dealing with seismic hazards, but the 2011 Great East Japan earthquake and tsunami (as it has been officially named) were unusual even for Japan. History will record this event as among the world’s worst natural disasters, but geoscience textbooks will discuss it because of certain rare characteristics.
Words like tsunami, kazan (volcano), danso (fault), jishin (earthquake) and typhoon (hurricane) are basic vocabulary in Japan. The country lies on the circum-Pacific Ring of Fire where Earth’s largest and fastest-moving tectonic plate (the Pacific Plate) is subducting at a rate of several centimeters per year beneath several other plates along deep trenches. This subduction has been producing earthquakes and volcanic eruptions along the Pacific rim for millions of years, since the Cretaceous.
Nevertheless, the March earthquake was unusual in two ways: First, it was the country’s largest recorded quake and the fourth-largest in the world since 1935 when the Richter scale was introduced. Second, it complicated the picture for how we forecast the main shock of a large earthquake.
Two days prior to the massive temblor, a magnitude-7.2 earthquake with three aftershocks greater than magnitude 6.0 hit offshore eastern Honshu. These quakes caused little damage even though the main rupture was only 8 kilometers deep. It also produced a maximum 60-centimeter-high tsunami, which struck the coast half an hour after the quake. This fooled everyone. Given the earthquake’s large magnitude and the smaller aftershocks that occurred as expected over the next day, no one thought that these could be foreshocks of an even larger event. But it now looks like those quakes were all foreshocks for the magnitude-9.0 quake that hit two days later, just 40 kilometers north of the magnitude-7.2 event.
As we have seen, it is indeed difficult to define any earth tremor as the main quake until after the whole sequence of earth shocks has occurred.
