On Friday evening tremors were again felt across Delhi and the NCR following an earthquake in Haryana’s Jhajjar. According to the National Center for Seismology, the 3.7- magnitude tremblor occurred at a depth of 10 km. On Thursday morning, a stronger 4.4 magnitude quake had jolted the same region.
Experts say aftershocks are common after an earthquake and they may also be a positive signal indicating the release of built-up tectonic energy thereby reducing the risk of a larger event.
The Delhi-NCR falls in Seismic Zone IV, the most vulnerable and falling under the “fairly high seismicity” category.
Tectonically, the region is influenced by Himalayan movements as also local geological activity sitting on a network of geological fault lines like the Mahendragarh-Dehradun Fault (MDF), the Delhi-Haridwar Ridge (DHR), the Delhi-Sargodha Ridge (DSR), the Delhi-Moradabad Fault, the Sohna Fault, the Mathura Fault and the Reverse Fault (F1).
Several other weak zones and faults are also there like the Great Boundary Fault, Delhi-Sargodha Ridge, Yamuna river lineament and Ganga river lineament, as per the NCS.
A fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements.
Why earthquakes happen
Energy release associated with rapid movement on active faults is the cause of most earthquakes.
Due to friction and the rigidity of the constituent rocks, the two sides of a fault cannot always glide or flow past each other easily, and so occasionally all movement stops.
Regions of higher friction along a fault plane, where it becomes locked, are called asperities.
Stress builds up when a fault is locked, and when it reaches a level that exceeds the strength threshold, the fault ruptures and the accumulated strain energy is released in part as seismic waves, forming an earthquake
A large earthquake happens when built-up tectonic stress on fault lines is too much and has not been released for long in the form of seismic waves.
When the stress exceeds the strength of the rocks, a rupture occurs, causing the ground to shake violently.
An earthquake that follows a large magnitude earthquake called, “main shock‟ and originates in or around the rupture zone of the main shock.
Generally, major earthquakes are followed by a number of aftershocks, which show a decreasing trend in magnitude and frequency with time, explains NCS.
