Japan's megaquake and killer tsunami: How did this happen?

by Rasoul Sorkhabi
Thursday, January 5, 2012

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. Furthermore, despite advances in our knowledge of how and where earthquakes happen, our capability to predict exactly where and when the next earthquake will hit is in its infancy. Scientists may monitor geophysical or geochemical signatures (“precursors”) at critical fault zones, but it’s only with luck that they may forecast a Big One shortly before it hits.

Over the long term, in determining when a major fault is “ripe” for an earthquake, scientists use the concepts of the earthquake cycle and seismic gaps. In terms of the cycle, we know that large earthquakes strike a given area periodically as a fault system releases accumulated stress. A seismic gap is part of a fault that has not experienced a large earthquake in a very long time. As the time between earthquakes grows longer, a Big One occurring on the gap becomes more likely, so the theory goes. The problem with relying on these methods is that we do not have a truly long-term scientific record of earthquakes for any given area on Earth. Therefore, our forecasts of earthquake cycles and seismic gaps are far from precise.

In Japan, for example, in the early 1990s, many seismologists suggested that the Great Kanto earthquake of 1923 (a magnitude-8.2 quake that killed more than 140,000 people in Tokyo and Yokohama) would repeat itself sometime in the mid-1990s to the early-2000s based on a 70- to 80-year cycle. But it didn’t strike. Instead, a megaquake hit Awaji Island and the nearby populous city of Kobe, killing 6,400 people in 1995.

Nevertheless, it is important to collect and analyze as much data as possible about past and present earthquakes. Some of Japan’s other largest quakes that have hit off Honshu and produced tsunamis include an estimated magnitude 8.6 in July A.D. 869, an estimated magnitude 8.5 in June 1896, a magnitude 8.4 in March 1933 and a magnitude 7.4 in June 1978. The seismo-tectonic relationship between these past earthquakes and the March 2011 megaquakes off northeast Honshu will be of particular interest to geoscientists. But it will likely be awhile before we have a big picture of how these events were related.

Meanwhile, the risk of the Big One hitting the populous Kanto region, where Tokyo is located, is far from over. Recent fault stress modeling by Ross Stein of the U.S. Geological Survey in Menlo Park, Calif., and Shinji Toda of Kyoto University in Japan suggests that after the March 11 megaquake, the crustal stress off Honshu may have transferred to adjacent segments along the subduction zone and thus loaded the existing faults in the upper plate with increased stress.

Although we may still have a lot of questions about how and when megaquakes occur, one thing is becoming clearer with each passing earthquake disaster: No single nation can be fully prepared against natural disasters, and international sharing of science and technology on geohazards is crucial for humanity.

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