Since the devastating Great East Japan Earthquake of March 11, 2011, aftershocks have occurred frequently along Japan's Pacific coast. That may be the reason why people didn't pay a great deal of notice to the M4 earthquake that rattled the northern part of Ibaraki Prefecture on Dec 28.
But in retrospect, that one may be worth remembering. Yukan Fuji (Feb 2) notes that particular quake was the first of more than 16 temblors larger than M3 on the Japanese scale, that occurred between Dec 28 and Feb 1.
The epicenters of these quakes were distributed along the Pacific coast of Japan from Iwate Prefecture in the north to Chiba Prefecture in the south.
The frequency of aftershocks following the March 11 quake has been gradually subsiding, but now seismic activity seems to be picking up once again along the eastern Japan's Pacific coast, raising concerns these may be a harbinger of another major aftershock, possibly in the M8 class.
At 11:53 a.m. on Jan 31, Hitachi City in northern Ibaraki recorded a medium-intensity quake of a low 5 on the Japanese scale. It was estimated to have occurred at a relatively shallow depth of 10 kilometers, and was strong enough to give the capital a sustained shake. Then approximately one hour afterwards, a second, slightly smaller quake occurred in the same area.
On Dec 7, 2012, an M7.3-intensity quake occurred of the coast of Tohoku's Sanriku area. While casualties and damage were minimal, the energy unleashed by that quake was estimated as about the same as the Great Hanshin Earthquake on Jan 17, 1995, in which 6,434 people died.
Toshiyasu Nagao, professor of Geophysics, director of Earthquake Prediction Research, Oceanic Research and Development at Tokai University, has continued to assert his conviction that since the M9.0 quake centered off Miyagi Prefecture in March 2011, conditions along the tectonic plates beneath Japan have changed drastically, and the Japanese archipelago has entered a period of increased seismic activity.
"The 1891 Nobi Earthquake (estimated to have been above M8.0), is said to affect the surrounding region (centered on northwest Aichi and Gifu prefectures) even today," Nagao was quoted as saying. "One can consider that aftershocks from that earthquake will continue for the next 100 years. So another M8-class aftershock occurring any time, even tomorrow, would not be unexpected."
Prof Nagao also expressed concern over the relationship known to exist between earthquakes and volcanic eruptions.
"Since 2004, the world has seen four M9-class earthquakes occur. Of the four, three have also resulted in occurrences of major volcanic eruptions in regions near the quakes," he points out. "The only one of those four that's yet to be followed by a volcanic eruption was the 3/11 earthquake in Japan.
"The frequent aftershocks can be expected to have an effect on magma in the earth's core, so we have to stay vigilant," Nagao adds.
Yukan Fuji interprets this to mean that the possibility of an eruption of its namesake is more than a pipe dream. In a geologically active archipelago such as Japan, almost anything can happen.© Japan Today