Earthquakes in Delhi NCR Region, Earthquake Microzoning

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

Basics: ‘Fault’ in Geology | Earthquakes | Richter Magnitude Scale

• 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)

Is it an Indication of Higher magnitude Earthquake?

• 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.

Source & Credits

Delhi standing on danger line due to haphazard development

• Delhi has developed in a very clumsy way.
• The construction of colonies here is quite dense.
• Also, there is a lot of illegal construction in Delhi.
• Two-thirds of the capital’s population live in illegally constructed houses.
• Areas on the banks of the Yamuna in Delhi come under the maximum danger.

Monitoring and Preparation?

• 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.

What is Earthquake Microzoning?

• 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.

Earthquake Zones (Seismic Zones) of India

• 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.