Nankai Trough Earthquake: 80% Risk Linked to Mysterious Trigger Fluid

Significant omens

The “X-Day” of an unprecedented disaster, when a massive earthquake will occur, is undoubtedly approaching.

This refers to the Nankai Trough Mega Earthquake, which could result in up to 320,000 deaths. Earlier this year, the government’s Earthquake Research Committee raised the probability of it occurring within the next 30 years from “70-80%” to “around 80%.” The cycle is said to be every 90 to 100 years, and more than 80 years have already passed since the last one (the 1944 Showa Tonankai Earthquake).

Yoshiaki Kawata, a Kansai University specially appointed professor and expert in natural disasters, commented:

“The Nankai Trough Earthquake could happen at any moment. If it occurs within the next 30 years, it’s only natural that the probability increases with time.”

According to Kawata, there are significant “precursors” to the Nankai Trough Earthquake.

“An examination of past Nankai Trough earthquakes reveals that about 40 years before the event, around ten magnitude (M) 6-class earthquakes occurred in the Kansai region. Since the Great Hanshin-Awaji Earthquake in January 1995, seven M6 or higher earthquakes have already occurred. We’re already in a dangerous state, but if two more occur, we should be on high alert. What I pay close attention to are the Nara Eastern Edge Fault and the Hanaore Fault in Kyoto. Neither has moved for more than 500 years, and the accumulated stress could lead to significant shaking.”

Something is wrong with the hot springs

There are also experts paying attention to the eerie groundwater. One such expert is Junichi Nakajima, a professor at the School of Science at Tokyo University of Science, who studies seismology.

“It’s the ‘fluid,’ such as water, within plates about 100 kilometers underground (refer to the above diagram for the mechanism of fluid and large earthquakes). When fluid enters the boundary between plates, it can become a factor that triggers a massive earthquake. It acts like lubricating oil, causing a slow slip where plates slowly slide. When it enters active faults, it can also cause major earthquakes by widening the fault with water pressure and activating its movement. It’s been found that fluid was closely related to the 2011 Great East Japan Earthquake and the Noto Peninsula Earthquake last January. During the Matsushiro earthquake swarm in Nagano in the 1960s, groundwater erupted in many locations. The fluid underground likely overflowed to the surface. Fluid is expected to influence the occurrence of the Nankai Trough Earthquake as well.”

Though still in the research phase, the activity of fluid that triggers large earthquakes may be predicted from anomalies in hot springs. Tsutomu Yamanaka, a professor specializing in hydrology at the Life and Environmental Sciences department of the University of Tsukuba, explains:

“It has been found that a considerable amount of groundwater originating from plates is contained in some of Japan’s hot springs. For example, the water in Arima Onsen (Kobe, Hyogo Prefecture) was found to have nearly the same water quality as water originating from the Philippine Sea Plate. It’s believed that the Great Hanshin-Awaji Earthquake occurred because water accumulated in the Nojima Fault on Awaji Island, which was the epicenter. At Arima Onsen, anomalies believed to be ‘precursors’ were observed. Just before the Great Hanshin-Awaji Earthquake, the proportion of water from the plate increased, and radon levels rose.”

Hot springs containing components derived from plates, like Arima Onsen, can be found scattered across Japan.

“There are several places in potential epicenters of the Nankai Trough Earthquake, such as Wakayama and Shizuoka. If fluid enters the areas within the Nankai Trough where strain has accumulated, it could act as ‘lubricating oil’ and trigger an earthquake. If the temperature of the hot spring water rises, its color deepens, or if the texture changes from smooth to slippery, it may be a precursor to the Nankai Trough Earthquake,” says Yamanaka.

Since the beginning of the year, ominous tremors have repeatedly occurred, with a magnitude 5 earthquake in Miyazaki, a magnitude 3 in Ehime, and a magnitude 6 in Taiwan. After showing some precursors, the Nankai Trough Earthquake will strike Japan with a massive tsunami.

From the February 21-28, 2025 issue of “FRIDAY”