The great Tohoku-oki earthquake (Mw 9.0) occurred on 11 March 2011 in the NE Japan forearc region and it was the largest recorded earthquake ever to hit Japan. However a newly released study from the European Geosciences Union, warns Japanese officials and the world that the seismic risk at the Fukushima Daiichi nuclear plant has increased because the magnitude 9 earthquake jolted the plates underneath the area into a more unstable position, and could tip off another severe accident at the already crippled plant.
The March 11th, earthquake was so large it has caused large variations in stress ﬁeld not only near the source zone but also in regions far away from the source area, and so the seismic activity in the crust of the overriding plate west of the source area has increased signiﬁcantly after the Tohoku-oki mainshock that ruptured the megathrust zone beneath the Paciﬁc Ocean. The latest revelations show that unknown forces under the crust may have been reducing the strength of the faults in the Fukushima Daiichi area.
From now the compressional stress regime will 25 continue to build up in the overriding plate in NE Japan, which has potential to reactivate the reverse faults there to generate large crustal earthquakes, such as the 2008 Iwate-Miyagi earthquake that occurred about 200 km north of Fukushima Daiichi.
The paper’s parting words: “Therefore, much attention should be paid to the FNPP (Fukushima nuclear power plant) seismic safety in the near future.”
The real problem is when the fluids forming as a result of the Pacific plate digging under the adjacent Okhotsk plate threaten to swim up toward fault zones, where they can soak into the brittle crust of the earth along the fault line, reducing friction, pulling the fault lines apart and triggering another large earthquake.
Fukushima Daiichi is located above a low-V and high-σ anomaly. But the anomaly under FNPP may be also associated with the ascending ﬂuids from the subducting Paciﬁc slab. The ascending ﬂuids have likely reduced the strength of the faults in the FNPP area, such as the reverse Namie fault.
The fluids are a product of the Pacific plate’s shifting beneath Japan’s northeast region. The plate’s movement raises the temperature and pressure of minerals inside it, causing them to dehydrate. The fluids released are then able to move around the thick rocks toward the upper crust. Japan’s northern region lies directly above the Okhotsk plate.
The March 11 quake centered about 100 miles away from the Fukushima Daiichi plant, the scientists say the next big earth-shaker could be centered much closer.
The scientists concluded it would be wise to strengthen the plant’s infrastructure accordingly.
Source: Tomography of the 2011 Iwaki earthquake (M 7.0) and Fukushima nuclear power plant area P. Tong 1,2 , D. Zhao 1 , and D. Yang 2