A research group from Tohoku University, the University of Tokyo, and their collaborators, has discovered a new seismic zone called the "forearc seismic belt," which extends upward from the subducted Pacific Plate from Hokkaido to the Kanto region. By applying AI deep learning to approximately four years of data from the seafloor seismic observation network, the team detected about six times more earthquakes in the offshore Pacific waters of eastern Japan than were previously known.
In the forearc seismic belt, water expelled from the plate causes slow slip at plate boundaries, preventing the expansion of megathrust earthquakes. However, when water rises closer to the surface, it can potentially trigger shallow crustal earthquakes. Future detailed research into the distribution and other characteristics of seismic activity and the role of water is expected to deepen the understanding of the distribution and mechanisms of both megathrust earthquakes and shallow crustal earthquakes.
Provided by Professor Naoki Uchida of the University of Tokyo
The operation of "S-net," a wide-area permanent seismic observation network connecting 150 seismographs by cable, began in 2016 following the 2011 Tohoku Earthquake, focusing on earthquakes in the offshore Pacific waters of eastern Japan. Because seismic activity can be captured directly above earthquake sources, Professor Naoki Uchida, who studies seismology at the Earthquake Research Institute of the University of Tokyo (formerly an Associate Professor at Tohoku University), thought that AI could automatically perform high-precision earthquake source determination.
The study covered approximately four years from summer 2016 to summer 2020, during which S-net data existed but had not been used for earthquake source determination. Using models obtained through AI deep learning, earthquake sources were determined from east-west, north-south, and vertical seismic waveform data collected at 594 observation points, including onshore locations. The researchers obtained 587,585 pieces of earthquake source information.
Previously known earthquake source information was compared with information covered by S-net. The comparison illustrates that there were 1.2 times more earthquakes in land areas and areas close to land and 5.9 times more earthquakes in offshore areas. The system became capable of detecting smaller earthquakes than before, and the accuracy of earthquake source depth determination in offshore areas improved.
Provided by Professor Naoki Uchida of the University of Tokyo
Analysis of the earthquake distribution revealed by the researchers showed that earthquakes actively occur in areas extending upward from plates at depths of approximately 35-75 kilometers beneath the Pacific coastal waters of Hokkaido, Aomori, Iwate, Miyagi, and Fukushima Prefectures through to beneath the Kanto region. Because seismic activity appeared to concentrate in a belt-like pattern in the "forearc" region between the ocean trench and the volcanic chain of the Japanese archipelago, Uchida and his colleagues named this the "forearc seismic belt."
The earthquake sources comprising this forearc seismic belt were divided into three regions from shallow to deep underground locations: (1) areas shallower than the subducting Pacific Plate, (2) Pacific Plate boundary areas, and (3) the Pacific Plate crust (slab crust). These characteristics can also be confirmed in planar distribution maps that classify earthquake sources by depth.
Provided by Professor Naoki Uchida of the University of Tokyo
When water enters cracks in rock or gaps in faults, it generates forces that cause the gaps to open wider, reducing friction. It could be determined that during the process by which seawater-infiltrated plates subduct from ocean trenches and are carried deep underground, water separates from plates at depths of approximately 35-75 kilometers where the forearc seismic belt exists, based on the maximum water content according to plate depth and temperature.
When water emerges from plates and enters between the plate and overlying rock mass, friction decreases, causing "slow slip" where the plate slides slowly enough to be imperceptible, preventing plate boundary-type megathrust earthquakes from occurring. Conversely, when water from the plate rises further upward, it then enters shallow faults. This water enters gaps in fault planes, reducing friction and potentially making shallow crustal earthquakes more likely to occur.
Uchida describes the newly discovered forearc seismic belt as a "fluid pathway" involved in both megathrust earthquakes and shallow crustal earthquakes, providing clues for predicting the scope and scale of future earthquakes. The research was published in the electronic version of the American journal Science on July 11.
Original article was provided by the Science Portal and has been translated by Science Japan.