断層
This is the sort of worrisome thing I mentioned the other day in relation to the Kobe earthquake and in possible relation to damage from the earthquakes in Niigata over the weekend:
Researchers said the destructive temblors that hit central Niigata Prefecture from early Saturday evening occurred in an area that has usually been considered safe from major seismic activity.
The buried fault lines along which the quakes seem to have occurred are not visible from the surface. That could spell bad news for other regions that have not been too worried about the possibility of a big quake because no fault lines are evident near the surface.
Yet the weekend quakes weren’t the first for Niigata.
According to Tameshige Tsukuda, an associate professor at the Earthquake Research Institute of the University of Tokyo, the area of the epicenters was also the site of the massive 1828 Sanjo Earthquake that killed about 1,400 people. The extent of that quake’s damage led scientists to estimate it had likely registered about a magnitude 6.9 on the Richter scale.
The area from Niigata city through Ojiya and Nagaoka and along the Shinanogawa River toward Nagano city is known as the Shinanogawa River seismic zone because of its deep fault lines.
“There have been very few major earthquakes in recent years in the area from Niigata city to Nagaoka, and the region was considered free from earthquakes,” Tsukuda said.
In fact, on Oct. 13, the governmental Earthquake Research Committee predicted a less than 2 percent chance that a major quake would strike the fault belt at the western end of the Nagaoka plain within the next 30 years.
Ah, yes, probability. There’s no problem with using it to project where quakes are likely to happen, obviously–you have to start with something, or you can’t prepare at all. But given how often seismologists have been reduced to saying things on the order of, “Kobe?! How very extraordinary!” (or, of the Sendai earthquake, “That wasn’t the one we were expecting”) lately, I wonder how well regions that are considered “safe zones” are being provided with just-in-case preparation. It certainly looks as if it could prove useful.