Following a magnitude 7.5 earthquake in north-eastern Japan, authorities have reiterated warnings regarding the potential for a future megaquake.
In September, Japan's earthquake investigation panel reported a 60-90% probability of a megaquake occurring in the Nankai Trough within the next 30 years. The Nankai Trough is an area of seismic activity located along Japan's Pacific coast.
In April, authorities issued a warning indicating that a megaquake could trigger a tsunami exceeding 20 meters (66 feet) in height, potentially impacting parts of Tokyo and other prefectures. Projections included approximately 300,000 fatalities and economic damage amounting to trillions of dollars.
Recent Alert Details
Officials advised residents in seven prefectures, spanning from Hokkaido in the north to Chiba in central Japan, to maintain high alert for a potential megaquake. This extensive area includes millions of inhabitants.
A government official stated the possibility of a "large-scale earthquake with a magnitude of 8 or higher occurring as a follow-up earthquake" in the region.
Authorities recommended checking evacuation routes, securing furniture, and preparing emergency supplies such as food, water, and portable toilets. An evacuation order was not issued.
Japan's director for disaster management indicated at a press conference that global earthquake data suggests a possibility, rather than a prediction, of a larger tremor. Officials estimated the probability of a larger quake at approximately one in 100.
Understanding Megaquakes
Japan, situated on the Ring of Fire, experiences approximately 1,500 earthquakes annually. While most cause minimal damage, some events, such as the 2011 magnitude 9.0 earthquake, resulted in a tsunami affecting the north-east coast and over 18,000 deaths.
Authorities have expressed concern regarding the potential for a similarly powerful or more devastating earthquake in the densely populated southern regions.
Earthquakes along the Nankai Trough have historically caused significant loss of life. For instance, a 1707 rupture along its 600-kilometer length produced Japan's second-largest recorded earthquake, which was followed by the eruption of Mount Fuji.
The Nankai Trough extends between Suruga Bay in central Japan and the Hyuganada Sea in Kyushu.
These "megathrust" earthquakes typically occur at intervals of roughly a century, often appearing in pairs. The most recent recorded events were in 1944 and 1946. Geologists Kyle Bradley and Judith A Hubbard have identified this anticipated event as the "original definition of the 'Big One'."
Earthquake Prediction Limitations
Robert Geller, professor emeritus of seismology at the University of Tokyo, asserts that earthquakes cannot be predicted.
Regarding previous megaquake warnings, Professor Geller stated that such advisories had "almost nothing to do with science" and were "not a useful piece of information."
Geller further explained that while earthquakes are a "clustered phenomenon," it is not possible to distinguish in advance whether a seismic event is a foreshock or an aftershock.
Bradley and Hubbard note that approximately 5% of earthquakes are foreshocks. They also highlighted that a magnitude 7.2 foreshock preceded the 2011 earthquake but was largely disregarded.
A warning system was developed following the 2011 disaster to mitigate similar future events. The Japan Meteorological Agency (JMA) recently utilized this system for the first time.