Why earthquakes happen, and how to measure them
While the difference in the magnitude between the two earthquakes is a mere 0.3, the second one is two times bigger than the first, and released 2.8 times more energy, according to an online tool hosted by the US Geological Survey
NEW DELHI: Turkey is known to be prone to earthquakes, lying as it does at the junction of three tectonic plates — Eurasian, Arabian and African. Yet Monday’s two large earthquakes, interspersed with a number of smaller shocks, stand out because of their location.
Turkey lies on the small Anatolian Plate, sandwiched among the three large tectonic plates. Of the two major fault systems running across the Anatolian Plate, the North Anatolian Fault has been at the centre of many previous earthquakes. But it was the East Anatolian Fault that triggered Monday’s devastation.
The East Anatolian Fault had not witnessed any major earthquake over the last several decades. Strain energy, building up in this fault for decades, released on Monday.
Why earthquakes happen
A major cause of earthquakes is a slip along a fault (a junction between two tectonic plates); such earthquakes are called inter-plate earthquakes. Sometimes, earthquakes can occur within tectonic plates; these are intra-plate earthquakes.
The tectonic plates, which lie on the earth’s surface, overlie a layer called the asthenosphere, which behaves like a fluid over long periods. High temperature and pressure at the earth’s core cause the asthenosphere to convect, which in turn causes the tectonic plates to slide. Over long periods, the plates rub against each other, resulting in the accumulation of strain energy within the faults. And when the material crosses a limit, the faults slip against each other, leading to the release of a large amount of strain energy. This sudden release is an earthquake.
Earthquake vs aftershock
An earthquake of large magnitude earthquake is followed by many of smaller magnitudes. Turkey’s earthquake of magnitude 7.8 was followed 9 hours later by one of magnitude 7.5. This was not an aftershock but an earthquake by itself, although one may have led to the other, stressed Srinagesh Davuluri, Professor of Practice at IIT Madras, and formerly head of the Seismology Laboratory at CSIR-National Geophysical Research Institute.
When subsequent earthquakes occur on the same fault, they are called aftershocks. In Turkey, the earthquake of magnitude 7.8 happened on the East Anatolian Fault, and the one measuring 7.5 on a neighbouring sub-fault of the same East Anatolian Fault System.
Measuring an earthquake
It is common to associate the severity of an earthquake with its size, or magnitude. A more accurate indicator of damage, however, is the earthquake’s intensity at each location rather than its magnitude, which is measured at source.
While magnitude and intensity are related to each other, they are distinct measures, and so it is important not to confuse one with the other, said Professor CVR Murty at of the Department of Civil Engineering, IIT Madras.
Magnitude is a measure that reflects the energy released by an earthquake. It takes into account factors such as the amplitude of the wave, the size of the rupture at the source, how much the fault has “slipped”, and the properties of the rock at the rupture zone.
The magnitude scale is logarithmic, so a small difference in magnitude can mean a huge difference in actual effects. Take, for example, the sizes of the two large earthquakes on Monday, measuring 7.5 and 7.8. While the difference in the magnitude is a mere 0.3, the second one is two times bigger than the first, and released 2.8 times more energy, according to an online tool hosted by the US Geological Survey.
‘Observing’ an earthquake
Magnitude is a quantitative measure, one of its purposes being to compare one earthquake with another. Intensity, in contrast, is a qualitative measure, Murty said.
Intensity is a measure of the severity of shaking at a location, as opposed to magnitude that is a measure of the energy released during the entire earthquake. Intensity scales are based on observable data — effects on nature, damage in buildings and perceptions of people.
In India, intensity is measured on 1964 MSK Scale, which ranges from I to XII. Damage to the built environment starts at about intensity VI. The higher the intensity, the larger the damage.
During Monday’s magnitude 7.8 earthquake, the maximum horizontal ground acceleration measured was about 2.5g at a location, implying that structures are shaken horizontally by a force two-and-a-half times their weight, Murty said.