The warning said the likelihood of strong shaking and large tsunamis is higher than normal on the Nankai Trough, a subduction zone.
After a 7.1-magnitude earthquake shook southern Japan on August 8,the country’s meteorological agency issued its first-ever “megaquake advisory”.
What is a Megaquake?
A major earthquake — also referred to as a ‘megaquake’ — is an earthquake that reaches a magnitude of eight or above on the Richter scale, which measures the strength of quakes.
Megaquakes generally happen at a subduction zone , that’s where you can have the most stress built up.
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. This accumulates tectonic stress which can cause a megaquake — an earthquake with a magnitude larger than 8.
-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.
-Notably, Thursday’s magnitude-7.1 earthquake occurred on or near the Nankai Trough, according to the United States Geological Survey. As a result, there is worry that the next tremor along the trough could be devastating.
How are Earthquakes measured?
–The earthquake events are measured either according to the magnitude or intensity of the shock.
-The energy from an earthquake travels through Earth in vibrations called seismic waves.
-Scientists can measure these seismic waves on instruments called seismometers.
-A seismometer detects seismic waves below the instrument and records them as a series of zig-zags.
-Scientists can determine the time, location and intensity of an earthquake from the information recorded by a seismometer. This record also provides information about the rocks the seismic waves traveled through.
-The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the quake. The magnitude is expressed in absolute numbers ranging between 0 and 10.
–The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is between 1 and 12.
Prediction of Earthquakes:
1.An accurate prediction of an earthquake needs a precursory signal from within the earth, indicating a big quake is on the way.
2.The signal must also occur only before large earthquakes so that it does not indicate every small movement within the earth’s surface.
3.Currently, there is no equipment to find such precursors.
Distribution of Earthquakes:
Effects of Megaquakes/Earthquakes
Earthquakes are a natural hazard. If a tremor of high magnitude takes place, it can cause heavy damage to the life and property of people. The following are the immediate hazardous effects of earthquake:
-Ground Shaking
-Differential ground settlement
-Land and mudslides
-Fires
-Ground lurching
-Avalanches
-Ground displacement
-Floods from dam and levee failures
-Structural collapse
-Tsunami