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Predicting a major earthquake not an exact science

July 22, 2012

By SHIGEKO SEGAWA/ The Asahi Shimbun Science and Medical News Section

The government's Headquarters for Earthquake Research Promotion says that there is an 88 percent probability that a magnitude 8.4 earthquake will occur in the Tokai region within the next 30 years. This is a forecast, not a prediction.

Numerical data pointing to a 70 percent chance of a magnitude 7 earthquake centered on the nation's capital within the next four years is another example of forecasting.

The disciplines of predicting and forecasting earthquakes overlap somewhat and their usage differs, even from expert to expert. However, both are increasingly being clearly differentiated from one another and used selectively.

Prediction involves ascertaining early signs of some kind and issuing a prior warning as to the time, location and scale of an earthquake. Forecasting uses historical data to estimate the probability of a future earthquake within a certain period of time.

Prediction techniques are being put to active use in efforts to catch early signs of an earthquake in the Tokai region.

Major temblors occur when pressure built up on the borders of tectonic plates is released. The Japan Meteorological Agency (JMA) has expanded its observation network of seismographs, clinometers and strain gauges, and is monitoring them on a 24-hour basis in an attempt to detect and predict "pre-slip," when the tectonic border slowly begins to move. When an anomaly is discovered, seismologists and other specialists are assembled to determine whether it represents a precursor to an earthquake. If it is judged to be so, the prime minister issues a warning. Citizens are evacuated and economic activity grinds to a halt. Bullet trains and other modes of public transport are restricted, and the overall effect on society is immense.

Forecasts can be short term or long term.

Long-term forecasts are mainly used for major earthquakes, which only occur rarely. One typical technique estimates earthquake cycles by retracing hundreds and thousands of years of history on the assumption that they occur periodically in the same location, and their probability is calculated based on the passage of time since the last occurrence.

A prime example of short-term forecasting is the information provided on aftershocks following a major earthquake. The JMA makes statistical calculations based on factors such as the tendency of aftershock frequency to decrease dramatically with time and releases the results in a form similar to this: "the probability of an aftershock with a maximum intensity exceeding 5 in the next three days is 20 percent."

* * *

Earthquakes are a complicated natural phenomenon involving the movement of subterranean strata. Most monitoring efforts in various countries could be described as basic research to grasp their overall nature, rather than for the functional purpose of predicting them in advance.

It is difficult to predict earthquakes, so increasing the accuracy of forecasts is more pragmatic. ... An event emblematic of this global trend occurred in April 2009 in the central Italian city of L'Aguila.

A series of earthquakes had been ongoing since the year before, and anxiety grew as the pseudo-predictive view spread that a major one was on the way. One week after seismologists and other experts on a government committee had released a statement saying there was no proof that a large earthquake would occur, a magnitude-6.3 temblor struck and around 300 people died. On the grounds that injuries and fatalities were exacerbated because their statement had been interpreted as a declaration of safety by citizens who subsequently did not leave the area, the committee members were accused of negligence resulting in death and taken to court.

How should prediction and forecasting techniques be put to use? The Italian government's Institute for the Protection and Security of the Citizen (IPSC) invited seismologists from nine countries to consider the matter. Their report stated that it is currently impossible to make reliable predictions of large-scale earthquakes within a short period rather than several years and recommended that the pursuit of successful prediction techniques should continue as basic research while practical forecasting methods are developed.

* * *

As in other countries, the view that earthquake prediction is difficult has also taken hold in Japan.

The Great Hanshin Earthquake of 1995 occurred in the Nojima Fault, which had previously been largely disregarded. Even if effort is put into developing prediction techniques, the damage from an earthquake cannot be kept under control. This reconsideration led to the scrapping of the Headquarters for Earthquake Prediction Promotion, and the Headquarters for Earthquake Research Promotion was installed in its place to advance a wide range of scientific study.

The headquarters has subsequently made forecasts regarding the scale and probability of earthquakes in active faults and ocean trenches around the country based on historical data. This forms the basis for the disaster prevention plans of the Japanese government and local authorities.

Even the single remaining prediction system for earthquakes in the Tokai region is in question. It was unable to confirm pre-slips leading up to the Great East Japan Earthquake, which increased uncertainty over whether it would be able to detect similar phenomena in the Tokai region.

At the same time, the Great East Japan Earthquake has brought earthquake forecasting into doubt.

Before it struck, the headquarters forecast that there was a 99 percent possibility of a magnitude-7 earthquake off the coast of Miyagi Prefecture in the next 30 years. However, the epicenter of the actual temblor was broader than forecast, resulting in a colossal magnitude of 9 and wreaking unprecedented damage.

How can we gain prior knowledge of massive earthquakes that occur once in a hundred, or a thousand years? Japan's earthquake research is approaching a major turning point.

* * *

Memo: The mechanism of earthquakes

The earth's crust is formed from several sheets of bedrock called tectonic plates. There are sea plates and land plates, and they collide along their borders causing one to rise and the other to sink.

In areas along tectonic borders where a sea plate slides beneath a land plate, pressure builds with the passage of time, and when it cannot be contained any more, part of the plate breaks and suddenly begins to move. This causes a massive disturbance referred to as an interplate earthquake (or trench-type earthquake), a category that includes the Great East Japan Earthquake, the Great Kanto Earthquake and the hypothetical Tokai Earthquake. The wider the scope (or epicenter), the greater the scale. Additionally, when the seabed is lifted up in an earthquake, it creates a tsunami that can threaten coastal regions.

Earthquakes even occur within sunken sea plates. They are low in frequency, so studying and forecasting their cycle is difficult.

What's more, earthquakes can also occur when strain accumulates within the earth's crust above land plates. Places that have caused earthquakes repeatedly for several hundred thousands of years are called active faults. Even if the scale of a temblor is not major, as is the case with trench-type earthquakes, the location of a fault directly below causes powerful tremors that can result in massive damage. The Great Hanshin Earthquake falls under this category.

By SHIGEKO SEGAWA/ The Asahi Shimbun Science and Medical News Section
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The Great Hanshin Earthquake of 1995 occurred in the Nojima Fault, which had previously been largely disregarded. A profile of the Fault can be seen at the Nojima Fault preservation museum. (Asahi Shimbun file photo)

The Great Hanshin Earthquake of 1995 occurred in the Nojima Fault, which had previously been largely disregarded. A profile of the Fault can be seen at the Nojima Fault preservation museum. (Asahi Shimbun file photo)

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  • The Great Hanshin Earthquake of 1995 occurred in the Nojima Fault, which had previously been largely disregarded. A profile of the Fault can be seen at the Nojima Fault preservation museum. (Asahi Shimbun file photo)

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