Dolakha Earthquake of May 12, 2015

Tuesday May 12, 2015

I distinctly remember having experienced the violent shaking of Dolakha Earthquake. It was Tuesday May 12, 2015, and preparedness of the state governments to face an earthquake were being reviewed by the Ministry of Home Affairs through video conferencing. A routine exercise often undertaken ritualistically, particularly after major disaster events in the region. This was on the aftermath of the devastating 7.8 magnitude Gorkha Earthquake of April 25, 2015 that had killed more than 9000 persons in Nepal. It was already around 1230 hrs, and there seemed no end to the meeting. Then all of a sudden, everyone attending the meeting together with N. Ravishankar, Chief Secretary, Uttarakhand noticed swaying of lights, and other hanging fixtures of the Lucknow office. With the movement becoming stronger, and chaos overwhelming the Lucknow office, we could guess that an earthquake has struck yet again. We were yet not over thinking about the epicenter, and magnitude, and the ground below us started to shake. On the first floor shaking was felt quite violently, and I remember it having lasted for quite some time.

Soon thereafter it was communicated that Nepal has witnessed yet another major earthquake. This 7.3 magnitude earthquake was designated Dolakha Earthquake that killed 153 persons, and injured 3200. Minutes later, this quake was followed by another earthquake of 6.3 magnitude that had epicenter around Ramechhap, east of Kathmandu.

Earthquake prone Himalayas

We all know that ongoing subduction of the India Plate beneath the Eurasian Plate causes built up of strain making the Himalayan region seismically vulnerable. It is the release of this energy that causes earthquakes in this region. In the previous 120 yearsthe Himalayan region has witnessed 06 major earthquakes; 8.0 magnitude Shillong Earthquake of June 12, 1897, 7.8 magnitude Kangara Earthquake of April 4, 1905, 8.2 magnitude Bihar–Nepal Earthquake of January 15, 1934, 8.6 magnitude Assam now Arunachal Earthquake of August 15, 1950, 7.6 magnitude Kashmir Earthquake of  October 8, 2005, and 7.8 magnitude Gorkha Earthquake of April 25, 2015.

Uttarakhand: Hotspot for next big one

It is logical to postulate that the stress getting accumulated on a regular basis due to the Plate movement have not yet been released in the regions that have not witnessed a major earthquake for a long time. Scientists call such geographical stretches “seismic gap” and located to the west of Nepal the state of  Uttarakhand is located in the seismic gap of 1905 Kangara, and 1934 Bihar–Nepal earthquakes. Based on this scientists have long been expressing possibility of a major earthquake in this region in near future, and this future earthquake is assessed to be particularly devastating for the entire region. Magnitude 7.5 Garhwal Earthquake of September 1, 1803 is considered the last major earthquake to hit this region.

1803 Garhwal Earthquake

Garhwal Earthquake of September 1, 1803 was felt as far as in Calcutta, and the damage caused by it in the Garhwal region is assessed on the basis of a report of Capt. Raper, who visited the area in 1807-08.

This earthquake reportedly caused damage to (i) most houses in Uttarkashi where around 300 persons died, (ii) temple, and 20-30 houses in Badrinath, (iii) 100-150 houses in Joshimath, (iv) palace, and 1000 houses in Srinagar, and (v) fort at Ojha Ghor on Yamuna.

Apart from Badrinath, Kedarnath, and Gangotri temples this earthquake caused damage to Gopinath temple at Gopeshwar, Raghunath temple at Devprayag, Vishwanath temple at Uttarkashi, Chandreshwar temple in Yamuna valley,  Bhaskareshwar temple at Bhatwari, Tunganath temple at Makkumath, and Mahamaya, and Daksheshwar temples at Haridwar.

According to an account by Paddington this earthquake damaged 11^thcentury main Masjid, and stone houses in Mathura where ground fissures, and water coming out of these was also observed which is considered a sign of liquefaction of ground.

Damage to structures is also recorded at Aligarh, and the success of the English Army on siege of the Fort at Aligarh is attribute to the damage caused by this earthquake.

This earthquake also caused damage to the façade on the top floor of the 13^thcentury Qutub Minar at Delhi.

Based on the record of the damages intensity of September 1, 1803 Garhwal earthquake is assessed as IX-X on Mercalli Scale at Srinagar, and Devprayag, and VII at Mathura while its magnitude is assessed as 7.5 on Richter Scale.

Earthquake swarm or quiescence: No place is earthquake safe

You might wonder why even after witnessing October 20, 1991 Uttarkashi and March , 1999 Chamoli earthquakes Uttarakhand falls in seismic gap zone?

On Richter Scale the magnitude of these earthquakes was 6.8, and 6.6 respectively. These were thus moderate magnitude earthquakes that do not release significant energy as compared to the ones designated “great earthquakes” whose magnitude is around 8.0 on Richter Scale.

Unlike various scales with which we are familiar Richter Scale is a logarithmic scale; 1, 2, and 3 on it thus represent 10, 100, and 1000. It implies that the energy released in a “x.1” magnitude earthquake is twice that released in “x.0” magnitude earthquake. Around 1000 earthquakes of 6.0 magnitude are thus required to release the energy that is likely to cause an 8.0 magnitude earthquake.

This relationship of the earthquake magnitude and energy released therein has two important connotations.

• Firstly, every earthquake, small or big releases the stresses. However small earthquakes even though frequent do not rule out possibility of the area being affected by large magnitude earthquake, as energy released by these is relatively insignificant. So the “Safety Valve” theory is not valid for earthquakes. Smaller earthquakes no doubt are frequent but the energy released by them is insignificant.
• Secondly, immense stresses are being accumulated due to the movement of massive Plates of continental dimensions. So, even a large magnitude earthquake is no guarantee of safety from yet another large magnitude earthquake. 7.3 magnitude Dolakha Earthquake that followed 7.8 magnitude Gorkha Earthquake of April 25, 2015 on May 12, 2015 validates this fact.

Aftershock: Earthquake that follows a major earthquake, and is associated with the same fracture plane is called an “aftershocks”. According to Bath’s Law magnitude of an aftershock is 1.2 less than the main shock.

• 7.5-7.9 magnitude New Madrid Earthquake of December 16, 1811 was followed the same day by a 7.4 magnitude earthquake.
• 7.8 magnitude Kaikoura Earthquake of November 14, 2016 was followed by major earthquakes of 5.6, 5.9, and 6.5 magnitude.
• 7.8 magnitude Gorkha Earthquake of April 25 was followed by 7.3 magnitude Dolakha Earthquake of May 12, 2015.

Foreshock: An earthquake of lesser magnitude that precedes the main shock, and is associated with the same fracture plane as the main shock is called a “foreshock”. Foreshock is generally experienced  within one day prior to the main earthquake.

• Two earthquakes of magnitude 4.6, and 3.3 were experienced 2 hours before 6.1 magnitude Joshua Tree Earthquake of April 23, 1992.

It is not possible to say immediately, and with confidence if an earthquake is a foreshock, main shock, or an aftershock; it really depends on the magnitude of the earthquakes that follow. If a region experiences a number of earthquakes associated with the same fracture plane in a series, the highest magnitude earthquake would be called the main shock while the ones coming before it would be called “foreshock”, and  the ones following it would be called “aftershock”.

• Had the magnitude of Dolakha Earthquake of May 12, 2015 been more than 7.8 on Richter Scale, Gorkha Earthquake would have been termed its “foreshock”.

Foreshock pattern and earthquake forecasting: It is important to note that not all earthquakes are preceded by foreshocks. The pattern of foreshocks is however being actively studied by researchers across the world in a bid to make earthquake forecasting possible.

Apart from abnormal animal behaviour it was peculiar pattern of the foreshocks that made  warning of 7.3 magnitude Haicheng Earthquake of February 4, 1975 possible. Despite issuing public warning 5.30 hours before the main shock, and successfully evacuating 90000 persons this earthquake devastated the Liaoning province of China, and killed 10000 persons.

Neither swarm of low magnitude earthquakes, nor a big one is a guarantee of earthquake safety in future.

So don’t let your guard down; always keep track of your seismic vulnerability, and do everything possible to lower it.

Risk reduction, and preparedness are thus key to ensuring safety of your near and dear ones.