The term “active fault” was coined in the United States in 1923, appearing for the first time on a fault map of earthquake-prone California to refer to a geological fault that has a possibility of causing a quake.
Since that terminology was translated into Japanese, it has been interpreted as meaning a fault that is active enough to cause a quake. Neither seismologists nor mass media have attempted to rectify this misinterpretation, apparently because it serves their interests.
On the morning of June 18, a powerful quake of magnitude 6.1 struck northern Osaka Prefecture, killing five people. Although the overall damage was not as serious as feared, concerns remain as to what would have happened had a stronger temblor directly hit a more heavily populated area, and when such a quake might come. Academia and the media have fanned this public uneasiness.
During a news conference on the day of the Osaka earthquake, Naoshi Hirata, head of the government’s Earthquake Research Committee, pointed to the possibility that the quake was related to three active fault belts located near the hypocenter. Following up on his comment, the Yomiuri Shimbun ran a headline in its June 19 issue saying in effect that three fault belts may have had something to do with the quake.
But a meeting of the Earthquake Research Committee held the same day reached no conclusion concerning the source of the quake. A paper released by the committee said that near the hypocenter, there exist the Arima-Takatsuki fault belt running east-west, and the Ikoma and Uemachi fault belts running north-south, adding that while there is a possibility that the latest quake was related to these active fault belts, further study is needed to ascertain all the details.
A professor of seismology at a private university says that even this statement represents a jump of scientific logic. He insists that the committee should have gone no further than stating that while the hypocenter was near these three fault belts, nothing else was known.
The paper gives the false impression that further study could determine what actually caused the quake. Hirata’s hint at the possibility of the quake being related to the three fault belts helped seismologists almost fulfill their goal.
As if looking to fan fears, newspapers and TV networks followed with reports with questions like “Will the three fault belts cluster and move together?” and “Are the three fault belts more dangerous than one fault belt?” On television programs, seismologists propagated a theory that active faults are dreadful.
Another seismologist says that such arguments are utterly deceptive. The 1995 Great Hanshin-Awaji Earthquake brought about a drastic change in the direction of seismological studies in Japan, but the mechanism of that quake has not yet been unraveled. Although it is presumed that a fault moved directly below the surface, no consensus has been reached on the precise mechanism.
The 1995 quake led to major changes in Japan’s earthquake protective programs and seismological studies, both of which until then had been disproportionately directed to plate-type temblors like the Tonankai and Nankai trough earthquakes. The government’s budget for seismological research, which had been about ¥10.5 billion a year until the previous fiscal year, was raised to over ¥60 billion in a supplementary budget adopted following the quake, with a large part of that sum going into research of active faults. The Ministry of Economy, Trade and Industry earmarked ¥2 billion to institutes studying active faults.
And thus the study of active faults, which had taken a back seat in seismology in the past, suddenly came into the spotlight. The community of seismologists did not complain about the change because as a whole they benefited financially from it. According to the aforementioned seismologist, this situation led some seismologists to start instigating undue fear about active faults. These people should be called “active fault salespeople,” the seismologist said.
The limit of active fault specialists’ ability came to be known two years ago when two disastrous quakes struck Kumamoto Prefecture. After the first strong quake, the Meteorological Agency issued a warning against “aftershocks.” But what came next was an earthquake even more powerful than the first.
Until then, there was an influential theory that a big active fault quake would be followed by less-powerful shaking, despite the fact that the theory was nothing more than a hypothesis. The agency issued the warning based on this hypothesis. The Seismological Society of Japan conceded “defeat” following the 2011 Great East Japan Earthquake. It suffered a crushing defeat again in the Kumamoto quake, but it has not reflected on this failure.
Still another seismologist says that although faults are found all over Japan, it is not known whether they are the causes of earthquakes or the results of earthquakes.
A fault is created when a stratum or rock bed moves for one reason or another. But it is not under consideration whether the force behind such a movement is a powerful one like an earthquake or a slow crustal movement. Even if a movement far away from the hypocenter of an earthquake causes a planar fracture (discontinuity in the volume of rock), it is still called a fault.
An active fault only means a fault that has moved within the past 100,000 to 200,000 years and does not mean a fault that will cause an earthquake. In fact, among the faults that have been labeled as “active,” there are many that are considered unlikely to be hypocenters.
The aforementioned seismologist says there are faults in which a planar fracture disappears at a point several hundred meters below the Earth’s surface. It is highly unlikely that such shallow active faults are the causes of earthquakes because the hypocenter of a quake that occurs directly below the Earth’s surface is between 10 and several dozen kilometers deep. Yet the term “active fault” creates fear among people living nearby and can even drive down property values.
As is clear from the observations described above, research in active faults is still in its rudimentary stage. In 2013, the Earthquake Research Institute of the University of Tokyo, regarded as the most prestigious institution in seismological studies, made a fool of itself by mistaking pieces of concrete buried underground for the fracture zone of an active fault.
Moreover, the professor of seismology quoted above points out that seismological researchers engaged in the task of determining the feasibility of restarting nuclear power stations that have been idled since the 2011 earthquake are researchers in tectonic geomorphology, a tributary of fault studies, the latter themselves being somewhat outside the mainstream of seismological studies. They wield decisive influence over decisions concerning the restart of nuclear power plants even though they have little or no knowledge about active faults.
Both power companies and anti-nuclear power groups will not be convinced of their conclusions. One thing clear is that it is ridiculous to leave the judgment of whether to restart nuclear power plants to such researchers.
It is incumbent upon those of us who live in a country beset by earthquakes to maintain steady efforts to minimize damage from temblors. It is also incumbent upon the community of seismologists to reflect on their past mistakes and do away with the misleading term “active fault.”
This is an abridged translation of an article from the July issue of Sentaku, a monthly magazine covering political, social and economic scenes. More English articles can be read at www.sentaku-en.com .
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