The African, Arabian and Indian Plates collide with the southern margin of
the Eurasian Plate along a 12,000-kilometer convergent boundary. This has
resulted in a mountain chain that stretches from the Alps to the Himalayas.
Whereas divergent, transform, and subduction-zone plate boundaries produce
relatively narrow belts of earthquakes, continent-continent collision produces
broad earthquake zones. Let's go back 60 million years to understand the
plate-tectonic history of the Himalayan region. About fifty million years ago the
Indian continent collided with Southern Eurasia and began constructing the
Himalayas and the Tibetan Plateau, the highest mountain range and
the largest continental plateau on earth
Regional compression produces broadly distributed earthquakes north of the
2,900 kilometer Himalayan plate boundary. However the historical earthquake record
indicates that the largest earthquakes, shown by their rupture areas. occur on
the shallow portion of the megathrust boundary. The magnitude 8.6 Assam, Tibet
event in 1950 was the tenth largest earthquake of the twentieth century.
On April 25th 2015 the magnitude 7.8 earthquake started 15 kilometers beneath
the epicenter, northwest of the Nepal capital of Kathmandu, and ruptured 100
kilometers toward the east. To understand the earthquake let's look at a
north-south oriented cross section through the Kathmandu basin.
Geologicalstudies and seismic imaging reveal a complicated history of faulting and
earthquakes in the Himalayas. But here we focus on the active main
Himalayan Thrust Fault. The Indian plate pushes the leading edge of the Eurasian Plate
northward shortening the overriding crust by over two centimeters per year.
On the deeper, low-friction part of the plate boundary displacement occurs by
slow creep with few earthquakes.
In the 15-20 kilometer depth range, frequent magnitude 3 to 6 earthquakes
occur on the mega thrust plate boundary. The shallow part of the fault is locked
by high friction and stress increases during motion on the fault at deeper levels.
During the Gorkha earthquake, that stress overcame friction and the
overriding Eurasian crustal block lurched southward. Maximum displacement
of 3 meters occurred on the mega thrust about 20 kilometres north of
Kathmandu but the fault did not rupture to the surface. This displacement
uplifted Kathmandu over 60 centimeters and moved the city 1.2 metres south
towards India. A GPS station in the Kathmandu basin recorded the ground
motion while video captured people's response. Following ground motion of 1.5
metres to the southeast over the first 10 seconds,
the ground lurched 1.6 metres west in less than three seconds
causing people to stagger and some to fall. Over the next 12 seconds the ground
moved back and forth 3 times, with an average time of 4 seconds for each
oscillation. This motion, with 4-second period and decreasing amplitude,
continued for another minute. The Kathmandu basin is a broad valley in the
foothills leading to the High Himalayas. This vally was formerly the site of a lake
within which up to 600 meters thickness of river delta and
lake sediments accumulated.
Compared to bedrock around and beneath the basin, seismic waves from the
earthquake caused the lake sediments to shake like Jello in a bowl as they reverberated,
or resonated, back and forth with the period of 4 to 5 seconds.
The good news is that well-constructed houses of 1 or 2 stories withstood
this 4-second-period ground shaking with minimal damage. The bad news is that
tall structures or poorly constructed buildings were vulnerable to the
prolonged ground shaking and preferentially collapsed, accounting for
many deaths and injuries. Even more deaths and injuries occurred in the
rural areas north of Kathmandu over the center of the rupture zone primarily due
to very weak building construction practices.
Tens of thousands of landslides ranging, from small rock falls to fatal rock-and-ice
debris avalanches, occurred during the ground shaking from the Gorkha
earthquake and its aftershocks. The most deadly was the Langtang debris avalanche
that slid as much as 1,500 meters down slope, then became airborne and plunged
an additional 500 meters burying the village below and killing two hundred people.
A lateral compressive air blast knocked down houses nearby.
It is noteworthy that this magnitude 7.8 earthquake produced fewer landslides,
less damag, and a smaller number of injuries and fatalities than
seismologists expected for an earthquake of this magnitude in Nepal. In part, the
lesser impact is a result of the slow rupture velocity due to the complex
geology in this continental collision zone. More importantly, rupture during the 2015
Nepal megathrust earthquake did not reach the surface so ground shaking and
associated effects were less severe than would have occurred during a shallow
earthquake of similar magnitude. Because large amounts of water are provided from
Himalayan snow and ice melt, the area south of the mountain range has become
densely-populated. Since 1950, India's population has grown from 360 million,
to 1.3 billion people today. When we overlay the seismic hazard map,
we see a confluence of high population density and countless
vulnerable structures within areas susceptible to strong shaking during earthquakes.
Near boundaries cities, such as Delhi, with a population of 11 million
may be particularly vulnerable to a mega-project earthquake that rushes to
the surface along the thrust zone.
Strong buildings and community planning are essential to hazard mitigation.