3 min
Why Japan Issued a "Megaquake" Advisory Following Last Week's Tremor
The magnitude 7.1 earthquake that struck Japan's southern islands on August 8 left some residents of the country in panic. Not from the tremor itself, which caused only a handful of minor injuries and quick-expiring tsunami alerts, but rather the unprecedented advisory from the Japanese Meteorological Agency warning of an elevated risk of a "megaquake" in the region over the coming weeks. A "megaquake," short for a megathrust earthquake, is a type of temblor that occurs at a subduction zone, where one tectonic plate slips under another. A release of the tension that forms the thrust fault where the two plates meet can trigger some of the strongest earthquakes on the planet, measuring 9.0 or higher on the Richter scale, and produce large tsunamis. It may sound a bit alarmist, but Isabel Hong, PhD, assistant professor in Villanova University's Department of Geography and the Environment, assures that "even though it is not possible to predict earthquakes, the advisory comes from a place of prior knowledge." "We can't say for certain [when these earthquakes will happen]," she reiterated. "But probability suggests it could be more likely, in part because this smaller earthquake event occurred." The acute event—last week's earthquake—is indeed the root of the alert, which was issued in the following hours. The quake's epicenter was located close to the end of the Nankai Trough, a subduction zone off the coast of Japan where the Philippine Sea Plate slips under the Eurasian Plate. The Nankai Trough has historically produced strong earthquakes, most recently an 8.0 tremor in 1946. "Oftentimes, a large earthquake event can then trigger subsequent earthquakes," Dr. Hong said. "It can transfer stress to other faults that can make it more conducive for other earthquakes to then rupture, and that's the general belief of what's happening with the Nankai Trough right now." To compound the acute disturbance last week, Japanese government officials had already previously warned of a 70-80 percent likelihood of a Nankai Trough earthquake measuring 8-9 on the scale within the next 30 years. That warning was the product of extensive research into the region's seismic history. "All of the data that goes into [an advisory like that] is pulled from the work of dedicated scientists looking at past earthquake and tsunami deposits," said Dr. Hong, who herself studies prehistoric geohazards by analyzing their geologic trails along coasts. "This allows us to refine our understanding of the frequency of such events in a region. In this case, scientists can say, 'These happen about every 100 years, and it's already been over 70. Therefore, there's a higher probability another will occur in the next 30.'" If it does, officials fear that a strong earthquake could trigger a massive tsunami that would reach the coast of Japan within minutes due to its proximity, threatening the lives of hundreds of thousands of individuals. "Tsunamis occur along active subduction zones like the Nankai Trough," Dr. Hong said. "They do have to be generated by a strong earthquake, yes, but more important in their impact to coastal communities is the shape of the coastline offshore. If they go from deep to shallow water very fast, the tsunami builds tall." So, whether it appears alarmist or not, having the ability to study these seismic events in a way that can warn individuals of heightened risks should not be taken for granted, says Dr. Hong. Early warning signs and advisories for potential geohazards can save lives. "One of the reasons we dig into the geologic past is to help inform people what could happen in the future."