Kyodo, AP, AFP-JIJI, JIJI
The powerful earthquake that struck near Japan’s remote southern islands on Saturday is thought to have taken place deep inside the Pacific tectonic plate, which moves under the Philippine sea plate, allowing its tremors to travel through the Pacific plate and shake areas far away, an expert says.
The quake, felt throughout most of Japan, likely occurred at a depth of around 600 km, which corresponds to the plates’ deepest reaches, said Kobe University professor emeritus Katsuhiko Ishibashi, a seismology expert.
The quake’s focus was at the “pivot of a folding fan,” Ishibashi added, which played a role in extending its reach.
The Great East Japan Earthquake in March 2011 had a shallow focus — just 24 km below the surface — allowing it to spawn massive tsunami. No tsunami were triggered by Saturday’s quake, however, since the unusually deep focus prevented it from touching off the sea bed distortion needed to get seawater moving, Ishibashi said.
“In a most general sense, I would think, the quake this time may have been affected by the Great East Japan Earthquake, and a sign of more active seismic activity in the Japanese archipelago,” Ishibashi added.
In Saturday’s quake, several people suffered non-life-threatening injuries, but there were no reports of deaths or major damage.
The Japan Meteorological Agency did not issue a tsunami warning because the quake struck so far beneath the Earth’s surface. Deep offshore earthquakes usually do not cause tsunami, and generally cause less damage than shallow ones.
The magnitude-8.1 offshore quake struck off the Ogasawara Islands at a depth of 682 km, the Meteorological Agency said. The U.S. Geological Survey said the quake had a magnitude of 7.8 and a depth of 678 km (421 miles).
Tokyo’s Otemachi business district logged an intensity of 4 on the Japanese scale to 7, and many areas in neighboring Kanagawa Prefecture were rocked more violently, according to data released by the agency.
In the town of Ninomiya, facing Sagami Bay, the quake was logged as upper-5.
The temblor was powerful enough to rattle the entire country from Okinawa to Hokkaido. It caused buildings to sway in Tokyo — about 1,000 km (620 miles) north — and temporarily disrupted train services in the capital.
About 400 houses in Saitama Prefecture, just north of the capital, were deprived of power by the quake, Tokyo Electric Power Co. said.
There were no anomalies reported at the region’s mothballed nuclear power plants.
On Sunday morning, a strong magnitude-6.4 earthquake struck off of the Izu Islands south of Tokyo, the USGS said. It struck at a depth of 13 km (8 miles) and had an epicenter 630 km (390 miles) south-southeast of Tokyo.
The quake was not strong enough to generate a tsunami or close enough to the islands to cause any significant damage or injuries, said John Bellini, a geophysicist with the USGS in Golden, Colorado. He said it is considered a separate seismic event and not an aftershock to the magnitude-8.1 quake that struck hours earlier.
Late Saturday, at Tokyo’s Roppongi Hills shopping and business complex, elevators stopped soon after Saturday’s quake, forcing hundreds of visitors to climb down the stairs. Among them were about 200 people who came to see the “Star Wars” exhibit on the 52nd floor of Mori Tower.
In Saitama, a woman in her 70s sustained a minor head injury when a ceramic plate fell from a cupboard, local police said. In Kawasaki, a 56-year-old office worker fell when the quake caught him by surprise and injured his ribs, according to NHK.
Some experts warn that the nation’s recent quakes and volcanic eruptions may be signs that the surrounding areas are entering “an active phase of crustal changes.”
“I can say Japan is in an active stage now,” said Toshiyasu Nagao, head of the Earthquake Prediction Research Center at Tokai University.
“Considering the geographic location of Japan, we can say the current activities are rather normal and it was too quiet” before the 2011 jolt, Nagao said.
“We should be vigilant by knowing that it is no wonder that an earthquake sizable enough to affect our society can occur anytime in the future,” he said.
Kazuki Koketsu, professor with the Earthquake Research Institute at the University of Tokyo, said the latest tremor was unlikely to be a sign that the Big One is due to hit the capital, which was devastated by an massive earthquake in 1923.
Yoshiyuki Sasamoto, a municipal official on Chichijima Island, which is part of the Ogasawara group, told NHK that he initially felt a mild tremor, but when he thought it was over “there was a violent shaking and I couldn’t even stand on my feet.”
At an inn on the Ogasawara island of Hahajima, furniture shook violently, although nothing fell or broke, innkeeper Michiko Orita told NHK. “It was so frightening. The entire house shook and a Buddhist altar violently swayed like I have never experienced before,” she said, adding that all her guests were safe.
Saturday’s rattle was the second sizable shake Tokyo had last week, after a much less powerful — but far shallower — quake hit close to the capital Monday.
Japan sits at the meeting place of four tectonic plates and experiences around 20 percent of the world’s most powerful earthquakes every year.
The magnitude-9.0 March 2011 earthquake that hit the Tohoku region triggered a tsunami that killed more than 18,500 people and ravaged much of the northern Pacific coast.