By Donald Prothero, Occidental College
As many of us watch the horrors of the nonstop news coverage from Japan, a lot of misinformation seems to be sweeping through the media and the blogosphere. Since I’m a geologist trained in seismology, and also the author of the new book Catastrophes: Earthquakes, Tsunamis, Tornadoes, and other Earth-Shattering Disasters (Johns Hopkins University Press), I’ve been asked to write up a brief summary of the fact and myths about the earthquake.
First, the basics. The March 11, 2011, Sendai quake, which occurred at 2:46 in the afternoon local time, lasted almost 5 minutes in terms of total shaking. Its epicenter (38.322�N 142.369�E) was about 130 km (81 mi) east of the Oshika Peninsula, near Sendai, on the main island of Honshu. Its moment magnitude (Mw, a modification of the Richter scale used for largest earthquakes) was reported at 8.9 9.0, making it the largest in Japanese history (which has had at least 6 other quakes greater than Mw 8.0 since 1896, and millions of smaller quakes), and the fifth largest quake in world history since the invention of the seismograph. (The largest is the Mw 9.5 Chile quake in 1960, followed by the Mw 9.2 quake in 1964 in Alaska, the Mw 9.1 quake in Sumatra that caused the great Indian Ocean tsunami, and a Mw 9.0 in Russia in 1952; the quake in Chile last year was a Mw 9.8 8.8, just slightly smaller). Some sources place the magnitude of the Sendai quake at Mw 9.0 or 9.1, which would tie it for fourth largest in history.
Like all earthquakes in Japan and most of the countries on the west Pacific rim of the “Ring of Fire,” it was produced in a subduction zone, where one tectonic plate (the Pacific plate) is sliding down beneath another plate (the Eurasian plate). The plates are not sliding smoothly, but grinding past one another at an oblique angle, so a lot of friction and stress builds up over a long time until it is released in huge earthquakes. The rupture zone is reported to be about 480 km (300 mi) long undersea, and 200 km (120 mi) wide, and the overlying plate is reported to have uplifted as much as 10 m (30 ft) once the stress was released (similar to what happened in Alaska in 1964) (see here for some interesting plots). The shaking was so extreme that the upward force on the people and objects was about 0.3 times the acceleration of gravity. Since the initial event, there were over 600 Mw 6.0 aftershocks in the first 24 hours, and thousands of smaller ones, and the area is still feeling them more than 48 hours later.
Tsunami. Ever since consciousness-raising of the 2004 Sumatran disaster, people have gradually stopped mislabeling seismic sea waves as “tidal waves” (since tides are not involved in these waves, which are generated by earthquakes, volcanoes, or landslides). Now they refer to them by their proper name “tsunami,” which means “harbor wave” in Japan, a country that has experienced many of them over the past centuries. As we saw in the Indian Ocean in 2004 and again in Sendai, a tsunami is a wave with very long wavelength generated by a sudden displacement of water in the ocean (earthquake, volcanic eruption, giant submarine landslide). On the open ocean, it is a barely perceptible swell that most boats don’t notice. When it reaches shallow water and the wave base intersects the bottom, however, it turns into a huge wall of water that sweeps a long way inland, then withdraws dramatically (exposing the intertidal and subtidal zone), then may come back with even longer waves every few minutes. The reports of the Sendai tsunamis are that the waves crested at 10 m (33 ft), and washed 10 km (6 mi) inland. If you saw footage of the area on TV, they often showed these huge waves carrying boats, cars, airplanes and other debris far inland or sweeping them out to sea, and advancing across the flat farmland around Sendai in an unstoppable wall of water. Thanks to the excellent earthquake-resistant construction of most buildings in Japan, relatively few people were killed by building collapse. Instead, far more victims (the count is still unknown) were killed by the tsunami.
Prediction. After any major earthquake, there are always people asking why geologists didn’t predict the quake. Actually, this quake had been predicted as long overdue, since it was in a “seismic gap,” a part of a known fault zone that has fewer than the expected number of quakes and is thought to be building up stress for a big one. But what most people want is a short-term warning that allows them to evacuate and seek shelter. The Japanese have hundreds of seismographs in place, so they were able to issue warnings about a minute before the quake, although that would not have made much difference to people who were too close to the quake to find protection in less than a minute. That’s about as good as we can expect in terms of a short-term warning. Seismologists have been seeking the “holy grail” of short-term earthquake prediction for decades, but without much luck. After some spectacular successes and failures in the 1970s with the dilatancy model, most seismologists now realize that earthquakes have many different causes and behaviors. Some appear to have precursors, while others have none whatsoever. Thus, geologists have become more and more cautious over the decades that we will ever be able to predict most earthquakes except over the very long term. If you hear some psychic or astrologer say otherwise, it’s a fake. As Charles Richter himself said, “Only fools, charlatans, and liars predict earthquakes.”
Are we seeing more big quakes than normal? This is another question buzzing over the Internet and the media. With our short attention spans, it sure seems like the events in Japan, Haiti, New Zealand and Chile add up to a lot more than average. However, if you do the statistics carefully, the quakes of this past few years are about normal for a given period of time. In any given year, we average about three huge quakes worldwide that are bigger than Mw 6.0 or greater, and thousands of smaller ones; earthquakes are happening every second somewhere around the world. And if we look over enough decades, we see that this current crop of big events is not even the biggest in the past 50 years. The 1960s, with the biggest earthquake on record (1960 Chile) and the second biggest (1964 Alaska), had far more giant quakes than we have had in the past decade.
The myth probably arises because we have short memory spans, and most of us were not even born then, let alone adults paying attention the news in 1960 or 1964. In addition, we now have worldwide instant media coverage of a big quake, especially those in countries like Japan where there are cameras everywhere. By contrast, there was almost no film coverage of the 1960 disaster in southern Chile, and only a few films were made of the 1964 Alaska quake. Most people learned of those quakes by the newspaper days later, and saw little or no film footage on TV.
The “supermoon” theory. The media are buzzing with the predictions of an astrologer who claims that this quake was caused by the unusual perigee (closest point of its orbit) of the moon (the “supermoon”) on March 19, when it will be closer than it has been in 18 years. But this, like all of astrology, is pure garbage. As Hank Campbell, Steve Shimmrich, Phil Plait and others have pointed out in their blogs, the “supermoon” idea is ridiculous. The Sendai quake happened on March 11, a week too early, when the moon was nowhere near perigee. Likewise, it is absurd to link the moon’s perigee to last month’s quake in New Zealand. That time framework is ridiculously broad, so that ANY event occurring anywhere near the “supermoon” of March 19 could be claimed to be related. In fact the moon reaches perigee every 29.5 days, and none of the other perigees over the past few decades can be statistically associated with earthquakes, either, or any of the many “supermoons” of the past 30 years. Finally, the physics of the situation rules the “supermoon” prediction impossible. The distance is only 15 km closer for an object over 360,000 km away, so the difference between the gravitational attraction on March 19 and any other perigee of the moon is minuscule. It will make higher and lower tides than a normal full moon, but it has no effect whatsoever on something more massive like the earth’s crust.
All this attempt to stir up a media frenzy and put out crazy notions about supermoons affecting earthquakes, volcanoes, and weather is pure bunk (see Phil Plait’s links on the topic. Those who do so are trying to get free media attention, since the media these days can’t tell what is news and what is garbage. They are relying on our usual psychological blind spots of “confirmation bias” (remember the hits, forget the misses) and “correlation does not equal causation” to get people to buy into their crazy ideas and garner attention. And when, after March 19, nothing really happens, no one will go back to these astrologers and demand an explanation for their failures.
And of course, nothing brings out the “end of the world” crazies like a big earthquake. As always, this kind of talk is purely in the realm of religion and supernaturalism, not science.