7.1-magnitude earthquake shook southern Japan on Thursday (August 8), the country’s meteorological agency issued its first-ever “megaquake advisory”. The warning said the likelihood of strong shaking and large tsunamis is higher than normal on the Nankai Trough, a subduction zone (a region where tectonic plates collide with each other, and the heavier one slides under another) along Japan’s southwest Pacific coast. Japan's Advisory For Megaquake explained for UPSC WHAT IS A MEGAQUAKE? A megaquake is a term used to describe an earthquake of exceptional destructive power, typically with a magnitude of 8 or greater. These seismic events are capable of causing widespread devastation, including massive structural damage, tsunamis, and loss of life. Key characteristics of a megaquake:
  • Magnitude: Exceeds 8 on the Richter scale.
  • Destructive power: Can cause catastrophic damage over a large area.
  • Tsunami potential: Often triggers massive and destructive waves.
ABOUT THE NANKAI TROUGH The Nankai Trough is an underwater subduction zone (nearly 900 km long) where the Eurasian Plate collides with the Philippine Sea Plate, pushing the latter under the former and into the Earth’s mantle. Japan's Advisory For Megaquake explained for UPSC This accumulates tectonic stress which can cause a megaquake — an earthquake with a magnitude larger than 8. HAS NANKAI TROUGH PRODUCED MEGAQUAKES EARLIER? The trough has produced large earthquakes roughly every 100 to 150 years, according to the 2023 study, ‘High probability of successive occurrence of Nankai megathrust earthquakes’, published in the journal Nature. These tremors usually come in pairs, with the second often rupturing in the subsequent two years — the most recent “twin” earthquakes took place in 1944 and 1946.

POSSIBLE IMPACT OF THE MEGAQUAKE

Japan's Advisory For Megaquake explained for UPSC
  • Such a megaquake could send tremors to areas from central Shizuoka — about 150 km south of Tokyo — to southwestern Miyazaki.
  • Tsunami waves of up to 98 feet may reach Japan’s Pacific coasts within minutes.
  • A 2013 government report found that a major Nankai Trough earthquake could impact an area that covers about a third of Japan and where about half the country’s population of more than 120 million people lives.
  • The economic damage due to the disaster could go up to $1.50 trillion, or more than a third of Japan’s annual gross domestic product.

CAN EARTHQUAKES BE PREDICTED?

No. An accurate prediction of an earthquake needs a precursory signal from within the earth, indicating a big quake is on the way. The signal must also occur only before large earthquakes so that it does not indicate every small movement within the earth’s surface. Currently, there is no equipment to find such precursors. THEN HOW DID JAPAN PREDICT IT? Thursday’s advisory by Japan’s meteorological agency was just a warning not a prediction — it had nothing to do with science, Robert Geller, professor emeritus of seismology at the University of Tokyo, told the BBC. The advisory asked residents to prepare, review evacuation routes, and consider potential future warnings.