Weak zone underneath Jheel Park in Dhaula Kuan led to earthquake: NCS

The rocks at the epicentre crumbled, or fractured, having been weakened over centuries due to ancient waterways, which could have been a rivers, stream, or other water body.

The magnitude 4.0 earthquake that jolted Delhi awake early on Monday had its epicentre under the Jheel Park in southwest Delhi’s Dhaula Kuan and experts believe the nature of the rocks deep inside the ground – roughly 5km, where the temblor originated – hold the key to understanding the unusual occurrence.

A 20-year-old tree at Jheel Park gets uprooted following the earthquake. (ANI)

A preliminary analysis by National Center for Seismology (NCS) said at fault was a weak zone in the earth’s crust, a feature known as “lineament”, different – and notably, less worrying – than a tectonic fault line.

“A lineament is basically a weak zone. These lineaments can become fault lines in future but at the moment it is a weak zone,” said OP Mishra, director at the NCS.

Mishra added that the origin of the weak zone likely lay in several “paleo water channels and hence porous rocks”. “Hydrostatic fracturing may have occurred. This may have resulted in the earthquake. We can say the earthquake did not occur at the boundary of any fault. Delhi may have historically had these water channels and hence such rocks with pores,” he added.

In other words, the rocks at the epicentre crumbled, or fractured, having been weakened over centuries due to ancient waterways, which could have been a rivers, stream, or other water body.

While the water channel may no longer exist, the more porous structures they left behind likely filled with ground water. This water, under gravity, eventually accumulated what Mishra noted as hydrostatic pressure, leading to “hydrostatic fracturing” of the rocks that manifested in the earthquake.

“The shaking was vigorous mainly because this was a shallow depth earthquake,” he added.

The analysis found that the geological features beneath the surface in the surrounding area is varied, which likely contributed to the sort of stress that led the rocks to fracture. In fact, a 4.6 magnitude earthquake took place on December 2007 a mere 6km from the epicentre of Monday’s quake.

The analysis said this reveals “a normal pattern of seismogenesis” linked to the area’s unique underground structure, which the report describes as “sub-surface heterogeneities of weaker category” - essentially, zones where ancient water-carved rocks create natural weak points in Delhi’s geological foundation.

The NCS experts saw positive signs in the readings of Monday’s earthquake. “The signature of today’s earthquake M4.0 is a good signature as the rock materials of source zone released energy prior to reaching its maximum credible past earthquake [intensity, of M4.6 of 2007]…” the report concluded.

Part of the positive sign was the sound that was heard, which implied the seismic waves had reached the surface and interacted with air.

There are, however, two well known regional faults around the Capital, which can potentially be the source of more serious seismic activity: the Mathura Fault and the Sohna Fault.

And experts said while earthquakes cannot be predicted, a greater threat for the national capital lies in the potential of Himalayan earthquakes. “The Himalayan region is zone 5 and Delhi is zone 4 mainly because of proximity to Himalayas and high risk of Himalayan earthquakes. Those can be very intense because of the energy involved with likelihood of very largescale damage. In the past, earthquakes of M5.5 to 6.7 are known to have occurred in the UT of Delhi,” said JL Gautam, former head of office, NCS.

Delhi is in an active seismic zone both because of the faults around the Delhi NCR region and its proximity to the Himalayas, which are seismically very active.

Experts said Delhi, categorised in seismic zone IV — the second highest risk category -- needs to be on alert due to the many faults and weak zones and the proximity to the Himalayas.

“When we have new constructions, their building and design should be earthquake resilient. For the existing stock of buildings, retrofitting needs to be carried out after assessment. We also need to assess what constructions can be allowed in earthquake vulnerable areas for which micro-zonations maps are available. More importantly, strict implementation of earthquake resilience norms is needed. We have to live with earthquakes and we have to find a way to do that,” said Kalachand Sain, former director, Wadia Institute of Himalayan Geology (Dehradun) last year following the Nepal earthquake in October.