Earthquakes in Delhi NCR Region, Earthquake Microzoning

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HT | Prelims + Mains | GS1 > Important Geophysical phenomena such as earthquakes, Tsunami, etc.

Context: The National Capital Region (NCR) has witnessed more than 20 earthquakes since April.
Experts say that these low-intensity earthquakes are a sign that a major earthquake is due for the Delhi-NCR.
While others believe that these earthquakes are reducing the chances of a major earthquake in the region.

Why so many earthquakes in Delhi NCR Region?

New Delhi experienced its seventh earthquake in just two months.
They were all minor, with magnitudes of less than 5.
Studies have indicated that the Indian plate is still moving into the Asian plate at 5-6 cm per year.
The consequences of this motion can be observed in the form of ongoing mountain formation.
The crust has deformed under the Himalaya in such a way that there is a tremendous amount of strain.
The strain will be able to release if rocks can find some room to shift, move around or even break apart.
This slow release of tension is what Delhi’s seismic zone is experiencing in the form of minor earthquakes.
If the crust in this part can slowly release all its tension, there would not have to be a megaquake in the future since there is no accumulation of energy/strain.
Also, Delhi is closer to multiple faults (Faults are the regions of seismic activities) such as:
Mahendragarh-Dehradun Fault (MDF),
Sohna Fault (SF) &
Mathura Fault (MF)

A ~500 km long ‘Central Himalayan seismic gap’ in northwest India that has not experienced a very large earthquake in the past 200-500 years.
A seismic gap refers to the portion of a fault that hasn’t experienced quakes in a while though it should have, indicating it’s building strain that it could later release in one large burst.
Since there will be an accumulation of strain/energy the higher magnitude energy is released at once.
The last time the Central Himalayan gap (CHG) experienced a quake was in 1803, & before that in 1505.

National Seismological Network (NSN) is being operated & maintained by National Seismological Centre (NCS), Ministry of Earth Sciences.
The seismic network in Delhi & surroundings was strengthened by deploying 11 temporary additional stations covering the known faults in the region to precisely locate the earthquakes & their aftershocks.
Data from these stations are received in near real-time & used for locating the micro & small earthquakes in the region.
The expanded network has improved the accuracy of the epicentre location to ~2 km.
A geophysical survey, namely, Magneto-telluric (MT) is also being conducted over the Delhi region.
Magneto-telluric (MT) is a geophysical method which uses natural time variation of the Earth’s magnetic & electric fields to understand geological (underground) structure & processes.
Micro zoning has been done in Delhi & the city is divided into nine parts according to the intensity of quakes.
Among them, three areas, including densely populated Yamunapar, are the most dangerous.
Five zones are medium risk, one zone is safe.

Micro zoning means micro classification.
In this, the surface structure of the ground is investigated.
Soil samples are collected by drilling into the ground every 200 to 500 meters in the field & after scientific investigation it is decided how sensitive the location is.
For example, if the building is built on a damp surface, i.e., on the ridge area or on any soil that absorbs water for a long time, then the danger is greater.
On the other hand, where the soil is dry or sandy, or the rocks are below, it has different effects.

According to the risk of earthquake, the country is divided into four parts, zone-2, zone-3, zone-4 & zone five.

Of these, the least threatened is Zone 2 & the highest risked is Zone-5.
North-East, Jammu & Kashmir, Uttarakhand & parts of Himachal Pradesh fall under Zone-5.
Delhi falls in Zone-4, central India falls under relatively low danger zone of Zone-3, while most of the south falls in Zone-2 with limited danger, but it is a thicker classification.
There are some areas in Delhi which can be as dangerous as Zone-5.
Thus, there may be many places in the southern states which may be hazardous like zone-4 or zone-5.