REPORT Field visit to Nepal Lessons learnt from of 7.8 M on 25 April 2015 And 7.3 M 12 May 2015 Disastrous Earthquake Submitted to Seismology Division Ministry of Earth Sciences Government of India, New Delhi Submitted by Manipur University, Imphal 795003 July 2015
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REPORT
Field visit to Nepal
Lessons learnt from of 7.8 M on 25 April 2015
And 7.3 M 12 May 2015 Disastrous Earthquake
Submitted to
Seismology Division
Ministry of Earth Sciences
Government of India, New Delhi
Submitted by
Manipur University, Imphal 795003
July 2015
REPORT
Field visit to Nepal
Lessons learnt from Disastrous Earthquake of 7.8 M on 25 April 2015
Background
Large to Great earthquakes are disastrous for the life of people and their properties,
but beside that there are ample opportunities to seismologists, earth scientists and engineers
to understand the type of damages and sustainability of existing infrastructures and
preparedness for future. Every earthquake has its unique characteristics that may be
evidenced by damages to the existing infrastructures. Now a days, we have modern
earthquake instruments network global level to records the various earthquake parameters.
The April 25th and May 12th, 2015 earthquakes in Nepal present themselves as the two
major post-instrumentation era events in the Himalayan region and they provide an
opportunity to study the earthquakes in relation to the seismotectonics of the Himalayan
terrain. In order to carry out field studies and learning from the earthquake effects, the
Ministry of Earth Sciences provided us an opportunity to learn from the actual effects of a
large Nepal Himalayas earthquake.
7.8 M Earthquake
A large earthquake of magnitude Mw7.8 occurred in central Nepal on April 25th,
2015 (Fig 1). The April 25 earthquake has been followed by many aftershocks that
followed the main shock. Most of the aftershocks are close to edges of the ruptured zone
as it may be approximately delineated from seismological observations. Based on
geological cross-sections, crustal structure model, and the preliminary seismological data
available few days after the main shock, it appears that the earthquake has ruptured a piece
of the Main Himalayan Thrust (MHT). The rupture started at the epicentre, about 80 km
to the northwest of Kathmandu and propagated eastward for about 130 km, rupturing the
area directly located under the capital city of Nepal. The focal mechanism derived from
the analyses of seismological data shows a fault plane striking N143°E, with a very shallow
dip of 7° towards the North.
Tectonic Summary
The April 25, 2015 M 7.8 Nepal earthquake occurred as the result of thrust faulting
on or near the main frontal thrust between the subducting India plate and the overriding
Eurasia plate to the north. At the location of this earthquake, approximately 80 km to the
northwest of the Nepalese capital of Kathmandu, the India plate is converging with Eurasia
at a rate of 45 mm/yr towards the north-northeast, driving the uplift of the Himalayan
mountain range. The preliminary location, size and focal mechanism of the April 25
earthquake are consistent with its occurrence on the main sub-duction thrust interface
between the India and Eurasia plates ( Fig 2).
Although a major plate boundary with a history of large-to-great sized earthquakes,
large earthquakes on the Himalayan thrust are rare in the documented historical era. Just
four events of M6 or larger have occurred within 250 km of the April 25, 2015 earthquake
over the past century. One, a M 6.9 earthquake in August 1988, 240 km to the southeast of
the April 25 event, caused close to 1500 fatalities. The largest, an M 8.0 event known as
the 1934 Nepal-Bihar earthquake, occurred in a similar location to the 1988 event. It
severely damaged Kathmandu, and is thought to have caused around 10,600 fatalities.
May 12, 2015 M 7.3 Nepal earthquake
The May 12, 2015 M 7.3 Nepal earthquake (SE of Zham, China) occurred as the
result of thrust faulting on or near the decollément associated with the Main Himalayan
Thrust, which defines the interface between the underthrusting India plate and the
overriding Eurasia plate to the north. At the location of this earthquake, approximately 80
Fig 1 Damaged areas around epicentral of 7.8 M and 7.3 M
earthquakes in Nepal Laprak village (7.8 M Earthquake) and
Singati village (7.3M Earthquake)
km to the east-northeast of the Nepalese capital of Kathmandu, the India plate is
converging with Eurasia at a rate of 45 mm/yr towards the north-northeast a fraction of
which (~18 mm/yr) is driving the uplift of the Himalayan mountain range. The May 12,
2015 event was located 150 km to the west, and which ruptured much of the decollément
between these two earthquakes (Fig 1). Events of the size of the May 12, 2015 earthquake
are typically about 55x30 km in size (length x width). The April 25, 2015 M 7.8 main
shock had approximate dimensions of ~120x80 km, directed from its hypocentre
eastwards, and towards Kathmandu. The May 12, 2015 earthquake is located just
beyond the eastern end of that rupture (NOAA 2015). It is not considered as aftershocks
of 7.8 M.
Historical Earthquakes in Nepal
The Himalayan front has been the location of many large earthquakes in the past.
Although they are not all well documented, recent paleoseismological investigations
provided a better idea of the past seismicity of the area, and more precisely of the lateral
extent of the Mw8.2 event that devastated Kathmandu in 1934. In the same study, the
penultimate event has also been identified, in 1255AD, and the series of previous events,
yielding a return time close to 700 – 800 yrs (Sapkota et al., 2013, Bollinger et al., 2014).
The most recent work by the international team formed of French researchers (L. Bollinger
and Y. Klinger for CEA and IPGP respectively), Singaporian researchers (P. Tapponnier
and his team, EOS) and Nepalese researchers (S. Sapkota and his team, DMG) has shown,
based on the balance between the energy dissipated by past
earthquakes and the long term tectonic loading, that the area located between the
Fig.2 Epicenters of significant earthquakes in central Himalayan region based on data from
NCEI (2015), Rao et al. (2006) and Gahalaut (2009)
earthquake of 1934, to the east, and the event of 1505, to the west, that did rupture last in
1344 AD, was very close to rupture, as proved by the event of April 25th, 2015 (Fig 2).
This earthquake induced many mass movements in mountainous areas and resulted
in landslide lakes, which could be another cause of secondary disasters. The mass
movements and deformation of weathered soft soil cover are the main causes of the
collapse or heavy damage to buildings and heavy casualties in mountainous areas. In
addition, the earthquake also triggered a major avalanche on the south slopes of Mt.
Everest, located approximately 160 km east-northeast of the epicentre. The avalanche
destroyed the base camp of climbers.
Aftershocks on 7.8 M Earthquake
The aftershocks are distributed in an area roughly 150 km long and 50 km wide,
with the majority of the aftershocks located in the eastern part of the ruptured area. At first
order, the area defined by the bulk of aftershocks corresponds to the ruptured area on the
fault plane. The location of the ruptured area, together with the shallow dip of the focal
mechanism, suggest that it is most probably the sub-horizontal part of the MHT that has
ruptured, mostly the deeper part of it. The absence of aftershocks to the south of the valley
of Kathmandu suggests that the rupture has not propagated southward to reach the surface
along the Main Frontal Thrust. The aftershocks of 7.8 M earthquake continued up to 11
May 2015 and 7.3 M earthquake continued up to 29 June 2015. The rapture of the 7.8 M
earthquake was extended towards east which is evident from the 158 aftershocks (Fig 4
and Fig 5). 7.3 M earthquake on 12 May 2015 was the new rapture and after continued
further SW and NE direction (Fig 4). 54 aftershocks recorded up to 19 June 2015. It seems
that rapture of both the earthquake was on HFT and MBT in Kathmandu Himalaya but the
7.3 M earthquake was not the aftershock of 7.8 M earthquake.
Fig. 3 Epicenters of main shock and aftershocks of 2015 Nepal earthquake series (USGS
Earthquake Hazards Program)
Fig 4 158 aftershocks after the main shock and 54 events after the 7.3 shock
Fig 5 Number of aftershocks observed per day since the main shock (7.8M)
Brief Geology of Nepal:
Nepal is located in the centre of the Himalayan mountain belt, and is almost
rectangular in shape with about 870 km length in the NWW-SEE and 130-260 km in N-S
direction. The Main Frontal Thrust (MFT) system consists of two or three thrust sheets
composed entirely of Siwalik rocks, from bottom to top mudstone, multi-storied sandstone
and conglomerate (Chamlagain and Gautam, 2015). These sedimentary foreland basin
deposits form an archive of the final stage of the Himalayan upheaval and record the most
recent tectonic events in the entire history of Himalayan evolution since ~14 Ma. The
northernmost thrust sheet of the MFT is truncated by the Lesser Himalayan sequence and
overlain by unmetamorphosed to weakly metamorphosed rocks of the Lesser Himalaya,
where the Lesser Himalayan rock package is thrust over the Siwalik Group along Main
Boundary Thrust (MBT). In western Himalaya crystalline thrust sheets are frequently
observed within the Lesser Himalaya (LH). The Lesser Himalayan zone generally forms a
duplex above the mid crustal ramp (Schelling and Arita, 1991; Srivastava and Mitra, 1994;
Decelles et al., 2001). The Main Central Thrust (MCT) system overlies the Lesser
Himalayan MBT system and was formed in ca. 24 Ma. This MCT system consists of high-
7.8 (25th April 2015)
7.3 (12th May 2015)
7.8 (25th April
2015)
7.3 (12th May
2015)
grade rocks, e.g. kyanite-sillimanite gneiss, schist and quartzite and is mostly characterized