Something is going on along the Nankai Trough in Japan! Earthquake swarms are moving along the subduction zone, from Mount Fuji to Kii Channel to Tokara Island

Four days, almost 200 earthquakes and the ground is still shaking. That’s been life lately for residents of the Tokara Islands, a small chain that falls within Kagoshima Prefecture. But officials and experts say the quakes are not necessarily a cause for alarm.

The Meteorological Agency said it doesn’t know what exactly is causing the mostly small quakes but added that they may continue for about a week.

The cluster of temblors began on Saturday. On Sunday, the largest to date — 4.8 magnitude quake measuring 4 on Japan’s shindo intensity scale — was on Akuseki Island, home to 60 people. Monday and Tuesday saw quakes hitting shindo 3, but the remainder have been shindo 1 or 2.

No reports of injuries or damage to property were reported in the village of Toshima, an area hit by the stronger of the quakes.

It feels like (the ground) is constantly shaking, which makes us feel uneasy,” said Miho Kanda, who heads Toshima’s Akuseki Island branch office.

The area saw a similar series of mostly small quakes in April, when 265 temblors ranging from shindo 1 to 4 rattled the nerves of locals.

Sewanose Island, a volcanic island among the Tokara chain — where the Meteorological Agency has issued a level 3 volcanic warning, which bans people from entering the area — is located nearby, but the weather agency has reported no major seismic changes there and said the possibility is low that the quakes are being caused by volcanic activity.

Yukihiro Nakatani, an assistant professor at Kagoshima University, said it is unlikely that the latest quakes would impact any megathrust earthquake centered on the Nankai Trough, a massive ocean-floor trench stretching off the southern coast of Japan from Shizuoka Prefecture to the island of Kyushu.

The quakes we are seeing now seem to be a localized phenomenon, so I don’t think we need to consider the effect it may have on the areas of the Nankai Trough plates,” he said.

A separate cluster of small quakes has hit the Izu-Oshima island region, which is administered by Tokyo, since Saturday. A dozen quakes hit on Monday alone, all measuring shindo 1.

Kagoshima Prefecture has joined Yamanashi and Wakayama prefectures as areas recently shaken by earthquakes that have also rattled nerves among residents.

Although the Japan Meteorological Agency and other seismic experts have tried to alleviate fears of impending doom, concerns are still growing that the frequency of the tremors indicates that a megaquake could occur in the near future.

The quakes were strong enough to sever water supplies to islanders from Dec. 5 to the afternoon of Dec. 6. A portable water tank equipped in a regular-connection ferry was unloaded to secure water for residents, officials said.

A clinic in the island heard from mostly elderly residents that they were anxious because of the quakes and could not sleep.

Village officials are considering dispatching a health nurse to the island to help ease the residents’ fears if the quakes continue.

The area is no stranger to seismic activity

A magnitude 5.9 quake rocked the area in October 2000, causing shaking of an upper 5 intensity on Akusekijima island.

And between April 9 and 30 this year, 265 quakes with intensities of 1 or bigger were recorded in the area. There were also six quakes with intensities of 4.

According to the Fukuoka District Meteorological Observatory, the hypocenters of the recent quakes have mostly been shallow, at 10 to 20 kilometers below the surface, similar to those in April this year.

Yukihiro Nakatani, a special-appointment assistant professor at Kagoshima University’s Research and Education Center for Natural Hazards, noted that the quakes are continuing with the hypocenter located beneath the seabed.

It is difficult to identify a factor at this point,” he said. “But similar to April, the current quakes are a regional activity.

He said it is unlikely that both series of quakes are related to Suwanosejima island, an active volcano located about 50 km away from the epicenter area. He also said the seismic activity was probably not connected to the Nankai Trough, where a potentially catastrophic earthquake has been predicted.

Earthquakes in Wakayama and Mt. Fuji area

Evolution of earthquake swarms along the Nankai Trough: Mount Fuji area, Kii Channel, Tokara Island
Evolution of earthquake swarms along the Nankai Trough: Mount Fuji area, Kii Channel, Tokara Island

The Fuji Lakes area around Mount Fuji in the eastern part of Yamanashi Prefecture has also experienced a number of recent tremors, the strongest being a magnitude 4.8 quake on the morning of Dec. 3. A maximum intensity of lower 5 was recorded in Otsuki in the prefecture. In the wee hours of Dec. 3, two weaker earthquakes struck the same area.

The JMA said the three quakes were caused by the Philippine Sea Plate on the ocean side sinking into the Eurasian Plate on the land side.

Earthquakes frequently occur in the area, and it is not a special seismic activity,” a JMA official said.

The area was hit by a magnitude 5.4 quake in January 2012 that caused a maximum seismic intensity of lower 5. Mount Fuji, an active volcano, is close to the epicenter.

But JMA officials said they have not seen any abnormalities that indicate volcanic activities, such as data from a distortion meter that measures movements of surrounding bedrock.

The biggest quake in Japan on Dec. 3 was a magnitude 5.4 temblor centered in the Kii Channel. A seismic intensity of lower 5 was registered in Gobo, Wakayama Prefecture, in the morning quake.

The focus of the quake was a rather shallow 18 km deep, leading seismologists to believe it occurred inside the Eurasian Plate.

A magnitude 5.5 quake, with a maximum intensity of upper 5, hit the same area in July 2011.

Quakes frequently occurred there in the past, and it is unlikely that each of them are related,” a JMA official said.

Agency officials noted that the epicenters of the strongest Yamanashi and Wakayama quakes are far apart, and that each quake was about magnitude 5, so not that big.

Experts have said there is a 70 to 80 percent probability that a megaquake will occur within the next 30 years in the Nankai Trough, which stretches from the offshore area of Kyushu to the offshore of the Tokai region.

The agency denied the recent quakes were related to such a possibility, saying, “It is unlikely that the probability (for a megaquake) has become higher.

Koshun Yamaoka, a professor at the Earthquake and Volcano Research Center at Nagoya University, said: “They (the Dec. 3 quakes) are far away from each other, and the mechanism of each earthquake differs. There is no mechanism that connects the quakes to each other.

He thinks it was a coincidence that quakes struck around the same time.

The areas where the recent quakes occurred have been seismically active,” he said. “In eastern Yamanashi Prefecture, quakes used to occur on occasion. Wakayama Prefecture also observed quakes quite often back in the days when the Showa Nankai Earthquake occurred in 1946.

However, Yamaoka said it is difficult to know what is going on beneath the Tokara island chain at the moment.

But it is close to a volcano, so it is little wonder that quakes are occurring in such a manner,” he said.

Although the JMA said the recent quakes are unlikely to trigger a huge disastrous event, it is still urging people to prepare.

Japan is a land of earthquakes,” a JMA representative said. “We don’t know when and where an earthquake will occur. We want people to be well-prepared again.” [Japan TimesAsahi]