Context – After a 7.1-magnitude earthquake shook southern Japan on Thursday (August 8), the country’s meteorological agency issued its first-ever “megaquake advisory”
What is Megaquake?
- Earthquakes having magnitude larger than 8 are known as ‘Megaquakes’.
What is 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.
- 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.
- 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.
What is a Richter scale?
- The Richter scale is a logarithmic scale used to measure the magnitude of earthquakes.
- The Richter scale is logarithmic, meaning that each whole number increase on the scale corresponds to a tenfold increase in the amplitude of seismic waves recorded by seismographs. For instance, an earthquake with a magnitude of 5.0 has seismic waves with amplitudes 10 times larger than those of a 4.0 earthquake.
- The Richter scale measures the magnitude of an earthquake, which is the total energy released at the source. This differs from intensity, which describes the effects of an earthquake at specific locations, often measured by the Mercalli intensity scale.
- In theory, the Richter scale has no upper limit, though the largest recorded earthquakes have magnitudes of around 9.0. However, the scale is less commonly used for very large earthquakes, as more advanced scales like the Moment Magnitude Scale (Mw) provide better accuracy for these events.
Earthquakes of magnitude 10 or larger:
Earthquakes of magnitude 10 or larger cannot happen. The magnitude of an earthquake is related to the length of the fault on which it occurs. That is, the longer the fault, the larger the earthquake. A fault is a break in the rocks that make up the Earth's crust, along which rocks on either side have moved past each other. No fault long enough to generate a magnitude 10 earthquake is known to exist, and if it did, it would extend around most of the planet.
The largest earthquake ever recorded was a magnitude 9.5 on May 22, 1960 in Chile on a fault that is almost 1,000 miles long…a “megaquake” in its own right.
Can we predict Earthquakes?
An earthquake prediction must define 3 elements: 1) the date and time, 2) the location, and 3) the magnitude.
- No definite pattern: scientists have attempted to link multiple natural factors that have preceded earthquakes in the past with the earthquake itself, including increased amounts of radon in local water sources, rising levels of ground water, changes in electromagnetic activity and even odd animal behavior. For example, before the main rupture that ultimately causes a quake, smaller breaks called micro-fissures will form in subsurface rock. These smaller cracks change the rock’s permeability, or, in other words, they allow water to more easily pass through the rock. The more permeable rock might then lead to changes in ground water levels. This same change in permeability could also lead to the escape of radon which forms by radioactive decay of elements in certain minerals.
Sometimes these events occur without an ensuing earthquake, and other times earthquakes occur without any of these precursor events. - Huge depths: Earthquakes originate several miles below the Earth’s surface so it is of course possible that other early indicators occur, but we can’t readily detect them here on the surface, especially when we are not sure what it is we are looking for.
- Similar beginning: small and large quakes are thought to start in similar ways: ruptures often occur suddenly.
- Lack of data: More recently, researchers have turned to machine learning to develop nowcasting models by taking advantage of large amounts of data, such as seismology readings and data on Earth’s surface deformation. But challenges include a lack of data on the early warning signs to feed into the machine learning pipelines — given that these warning signs are not yet entirely understood
World distribution of Earthquakes:
Circum-Pacific Belt: Coastal Margins of North and South America and East Asia representing the margins of Pacific. 65% of the global Earthquakes occur in this region. It has ideal conditions of (i)C-O convergence and subduction zones, (ii)zone of young folded mountains and (iii) zone of active volcanoes.
Mid-Atlantic Belt: Epicentres located along mid Atlantic ridge. Earthquakes are because of transform faults and fractures because of splitting of plates
Mid-Continental Belt: Alpine-Himalayan belt representing collision of continental plates. 21% of the global earthquakes occur in this region. They represent weaker zones of folded mountains and fault induced earthquakes.
