TOKYO -- There was something unusual about the series of strong earthquakes that wreaked havoc across the Japanese island of Kyushu this month: The epicenters spread across a distance of more than 100km, causing experts to ponder whether the chain reaction could extend even farther -- potentially reaching Japan's biggest fault line.
The first temblor hit near Kumamoto Prefecture's capital city on the night of April 14. A more powerful earthquake struck early in the morning of April 16. That triggered a series of tremors with a magnitude of around 5 along a line from Mount Aso, east of Kumamoto city, to neighboring Oita Prefecture.
The earthquakes were caused by slips of adjacent faults, and it is not uncommon for such quakes to trigger another temblor along an active fault. However, it is "very unusual" for earthquakes of this magnitude to spread across a distance of more than 100km, according to Teruyuki Kato, president of the Seismological Society of Japan.
"The most worrying thing is that [the course of the chain reaction] eventually leads to the Japan Median Tectonic Line," he said at an emergency news conference on April 18.
The Median Tectonic Line is a huge fault structure bisecting western Japan like a backbone. Many active faults run along it. Since this month's string of earthquakes struck at the western end of the line -- along an expansive valley known as the Beppu-Shimabara graben, which is sandwiched between faults and cuts across Kyushu horizontally -- many worry that the chain reaction may continue east along the tectonic line.
From the graben, the line stretches more than 1,000km along Japan's main island. It hits the Fossa Magna, a large graben intersecting the main island, in Nagano Prefecture, west of Tokyo. At the west end of the Fossa Magna lies the Itoigawa-Shizuoka Tectonic Line, another big, active fault zone.
There have not been many cases of large-scale earthquakes triggered by other quakes at different faults since modern observation started. But scientific literature suggests it is not impossible. For instance, in 1596 three earthquakes, each with a magnitude of about 7, happened in the space of five days. The first one occurred along the Median Tectonic Line in Ehime Prefecture, western Japan, on Sept. 1. Three days later, a similarly strong earthquake took place in Oita, about 200km away from the first one. The third one, though its connection to the first two is still contested, occurred in Hyogo Prefecture, also in western Japan.
Scientists are divided over whether the recent wave of earthquakes could spread beyond Oita to the east. "The Great Hanshin-Awaji Earthquake occurred near the Median Tectonic Line in 1995, so we cannot rule out the possibility that more earthquakes get triggered along the line this time," said Kyushu University associate professor Takeshi Matsushima. Tomotaka Iwata, a professor at Kyoto University's Disaster Prevention Research Institute, is more optimistic. "It's been only about 400 years since the 1596 earthquakes," Iwata said. "There is not enough buildup of strain [along the Median Tectonic Line] to make it move immediately."
There is, however, a concentration of strain along the line. In addition, along the Beppu-Shimabara graben lies the Hinagu fault zone, which triggered the initial Kumamoto earthquakes, the Futagawa fault zone, and the Beppu-Haneyama fault zone -- the suspected culprit of the subsequent earthquake in Oita. Also, at the heart of Kyushu stands Mount Aso, a volcano. The area between Yatsushiro, a coastal city in Kumamoto, and the Shimabara Peninsula across the bay is also crowded with active faults.
The latest earthquakes were a reminder that distant faults can move together. With no reliable tools available to predict exactly what is going on underground, the only thing to do is to stay alert.