2. Earthquakes: The Science Behind Earthquakes
What is an earthquake?
An earthquake is what happens when two blocks of the
earth suddenly slip past one another. The surface where
they slip is called the fault or fault plane. The location
below the earth’s surface where the earthquake starts is
called the hypocenter, and the location directly above it
on the surface of the earth iscalled the epicenter.
Sometimes an earthquake has foreshocks. These are
smaller earthquakes that happen in the same place asthe
larger earthquake that follows. Scientists can’t tell that
an earthquake is a foreshock until the larger earthquake
happens. The largest, main earthquake is
called the mainshock. Mainshocks always have aftershocks that follow. These are smaller
earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the
mainshock, aftershocks can continue for weeks, months, and even years after the mainshock!
What causes earthquakes and where do they happen?
The earth has four major layers: the inner
core, outer core, mantle and crust. (figure 2)
The crust and the top of the mantle make up
a thin skin on the surface of our planet. But
this skin is not all in one piece –it is made up
of many pieces like a puzzle covering the
surface of the earth. (figure 3) Not only that,
but these puzzle pieces keep slowly moving
around, sliding past one another and
bumping into each other. We call these
puzzle pieces tectonic plates, and the edges
of the plates are called the plate boundaries.
The plate boundaries are made up of many
faults, and most of the
earthquakes around the world occur on these faults. Since the edges of the plates are rough, they
get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far
enough, the edges unstick on one of the faults and there is an earthquake.
Why does the earth shake when there is an
earthquake?
While the edges of faults are stuck together, and the
rest of the block is moving, the energy that would
normally cause the blocks to slide past one another
is being stored up. When the force of the moving
blocks finally overcomes the friction of the jagged
edges of the fault and it unsticks, all that
3. stored up energy is released. The energy radiates outward from the fault in all directions in the
form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move
through it, and when the waves reach the earth’s surface, they shake the ground and anything on
it, like our houses and us!
How are earthquakes recorded?
Earthquakes are recorded by instruments called
seismographs. The recording they make is called a
seismogram. The seismograph has a base that sets
firmly in the ground, and a heavy weight that hangs
free. When an earthquake causes the ground to shake,
the base of the seismograph shakes too, but the hanging
weight does not. Instead the spring or string that it is
hanging from absorbs all the movement. The difference
in position between the shaking part of the seismograph
and the motionless part is what is recorded.
How do scientists measure the size of earthquakes?
The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but
that’s not something scientists can simply measure with a measuring tape since faults are many
kilometers deep beneath the earth’s surface. So how do they measure an earthquake? They use
the seismogram recordings made on the seismographs at the surface of the earth to determine
how large the earthquake was (figure 5). A short wiggly line that doesn’t wiggle very much
means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake.
The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on
the amount of slip.
The size of the earthquake is called its magnitude. There is one magnitude for each earthquake.
Scientists also talk about the intensity of shaking from an earthquake, and this varies depending
on where you are during the earthquake.
How can scientists tell where the earthquake happened?
Seismograms come in handy for locating earthquakes too, and
being able to see the P wave and the S wave is important. You
learned how P & S waves each shake the ground in different
ways as they travel through it. P waves are also faster than S
waves, and this fact is what allows us to tell where an
earthquake was. To understand how this works, let’s compare
P and S waves to lightning and thunder. Light travels faster
than sound, so during a thunderstorm you will first see the
lightning and then you will hear the thunder. If you are close
to the lightning, the thunder will boom right
4. after the lightning, but if you are far away from the lightning, you can count several seconds
before you hear the thunder. The further you are from the storm, the longer it will take
between the lightning and the thunder.
P waves are like the lightning, and S waves are like the thunder. The P waves travel faster and
shake the ground where you are first. Then the S waves follow and shake the ground also. If you
are close to the earthquake, the P and S wave will come one right after the other, but if you are
far away, there will be more time between the two. By looking at the amount of time between the
P and S wave on a seismogram recorded on a seismograph, scientists can tell how far away the
earthquake was from that location. However, they can’t tell in what direction from the
seismograph the earthquake was, only how far away it was. If they draw a circle on a map around
the station where the radius of the circle is the determined distance to the earthquake, they know
the earthquake lies somewhere on the circle. But where?
Scientists then use a method called triangulation to determine exactly where the earthquake was
(figure 6). It is called triangulation because a triangle has three sides, and it takes three
seismographs to locate an earthquake. If you draw a circle on a map around three different
seismographs where the radius of each is the distance from that station to the earthquake, the
intersection of those
three circles is the
epicenter!
Can scientists
predictearthquakes?
No, and it is unlikely
they will ever be able to
predict them. Scientists
have tried many
different ways of
predicting earthquakes,
but none have been
successful. On any
particular fault,
scientists know there
will be another earthquake sometime in the future, but they have no way of telling when it will
happen.
Is there such a thing as earthquake weather? Can some animals or people tell when
anearthquake is about to hit?
These are two questions that do not yet have definite answers. If weather does affect earthquake
occurrence, or if some animals or people can tell when an earthquake is coming, we do not yet
understand how it works.
CASE STUDY - 01:
Consequences Of Earthquakes
The hazardous effect of Nepal earthquakes depends not only on their magnitude of Richter scale or intensity alone, but
also on so many factors, such as geology of the earth crust (lithology, elasticity, soil condition, permissible stress, rock
structure, etc.), design of buildings, quality of construction, population pressure, etc. Several villages, towns, human
constructions and their properties, lives were completely damaged. The urban area of Kathmandu, the capital of Nepal,
was completely destroyed by April earthquake which hit Nepal. Several buildings collapsed like a pack of cards as if
they were mud huts.
5. DHARAHARA TOWER
Affected area of Nepal earthquake.
Several buildings, most of them old, collapsed in the densely populated Kathmandu valley. Kathmandu‟s
Darbar square, a UNESCO world heritage site, was totally damaged in the quake. Harsh damage took place to the
parts of the palace complex in Vasanthapura Square. The site of palaces and temples of the city were warren of
narrow lanes and historic structure. Kathmandu residents ran onto the streets and other open spaces, throwing up
clouds of dust and wide creaks opened on paved streets and the building‟s wall.
Effect of Nepal earthquake.
6. km
km
Earthquakes are often followed by landslides and rock avalanches and glacier avalanches in Himalayan
hilly areas. The quake caused avalanches on Mount Everest, making the climbers running for cover and killing at
least 18 people at the start of the main climbing season. At least 1000 climbers had been at the base camp of the
Everest when the earthquake struck. The base camp had been severely damaged and the teams were trapped.
Because of poor phone network coverage the government of India and Nepal were struggling to assess the
damage on the Everest.
At Langtang valley in Rasuwa district, a major tourist destination in Nepal, also famous for trekking, it is
said that over 100 people were killed and 120 people were injured or missing following the massive avalanche
which swept away the entire village in the wake of the powerful quake that hit Rasuwa district. . Over 329 people
were reported missing after an avalanche struck Ghodatabela and the Langtang villages. The avalanche was
estimated to be 3 kilometres wide.
Continuous aftershocks caused a landslide on the Koshi Highway which blocked the section of the road
between Bhedetar and Mulghat. Many smaller landslides occurred in the Trishuli River Valley with reports of
significant damage at Mailung, Simle, and Archale areas. As a result of landslide the transport system mainly
highways was completely damaged or blocked, which may stop various activities needed for earthquake relief
and rescue. Landslides also blocked the river channels, sewage and other communication system in Nepal and
now in India specially in Darjeeling district.
Bihar, West Bengal and Uttar Pradesh were the worst affected parts of India. The tremors were felt the
strongest in the districts along the Indo-Nepal border, which lie in the seismic zone V (very high seismic zone).
Mud wall, buildings and boundary walls in different locations of Siliguri and Jalpaiguri had collapsed because
of the tremors. At least 52 people were killed and 237 were injured in India in a powerful earthquake, with
epicentre in Nepal, which destroyed or damaged several houses and buildings in Bihar, Uttar Pradesh, West
Bengal and surrounding area. According to Union Home Ministry, 38 people died in Bihar, 11 in Uttar Pradesh
and 3 in West Bengal, where 43 school children were injured. The majority of the deaths were reported from the
northern districts bordering Nepal. According to Bihar Disaster Management Department report, 8 people died in
East Champaran district, 6 in Sitamarchi and other 24 death had been reported from Darbhanga, Supual, Saran,
Lakhisarai, Madhubani, West Champaran, Araria and Sheohar districts. Huge damage was caused to the property
and the lives of the people in north India.
Seismic zones of India and Nepal.
7. April 25 quake left 8,699 people dead, 22,220 people injured and flattened thousands of buildings
besides destroying whole villages in affected area. In Nepal, Over 5, 00,000 houses were destroyed and 2,
69,000 were damaged by the earthquakes and hundreds of thousands of people were still staying in temporary
shelters. More than 8, 64,000 affected people, who lived in remote mountainous area, have lost their relatives and
houses. (UN report)
At least 117 people died in Nepal and 17 people in India as a result of the earthquake of 12 May and
about 2,500 people were injured. This earthquake created cracks on a pillar on the top floor of „Writers‟
Buildings and a pillar inside the Shaheed Minar in Kolkata. At Colleges of North Bengal University, over 1100
students had to write their examination paper on college ground after the second quake.
A number of old temples, pagodas and churches in the Kathmandu valley were razed. Several temples
including Manakamana Temple at Gorkha, Janaki Mandir at Janakpur; Kasthamandap, Panchtale temple,
Basantapur Durbar, the Dasa Avtar temple and the Shiva Parvati temple were demolished. The Kumari Temple
and the Taleju Bhawani Temple partially collapsed. Some portion of the Jaya Bageshwari Temple at Gaushala,
Pashupatinath Temple, Swyambhunath, Boudhanath Stupa, Ratna Mandir and Rani Pokhari had been destroyed.
The Char Narayan Mandir, the statue of Yog Narendra Malla, the Taleju Temple, the Hari Shankar, Uma
Maheshwar Temple and the Machhindranath Temple in Bungamati were destroyed. In Tripureshwar, the Kal
Mochan Ghat, a temple inspired by Mughal architecture, was completely destroyed and the nearby Tripura
Sundari also suffered significant damage. In Bhaktapur, several monuments and Temples were totally destroyed.
The poor communities of Nepal were most affected, who lost their houses and properties. The key reason for the
very high Nepalese casualties was the construction of non-seismically engineered buildings.
The U.S. Geological Survey initially estimated economic losses from this quake at 9 percent to 50
percent of gross domestic product, with an estimate of 35 percent.
LANDSLIDE
Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground
movements, such as rock falls, deep-seated slope failures, mudflows, and debris flows.
A landslide near Cusco, Peru, in 2018
Types of landslides
Landslides are generally classified by type of movement (slides, flows, spreads, topples, or falls) and type of material
(rock, debris, or earth).
8. CASE STUDY – 02:
Gangtok, geographically bounded by 27.3325°N latitude to 88.6140°E, longitude is situated at an altitude
of about 1800 m above sea level encompassing the third highest mountain (Mount Kanchenjunga) in the World.
Gangtok city is flanked by two rivers, i.e. Rorochu in the eastward side and Ranikhola on the west. Both the
rivers meet at Ranipool and flow towards the south before they again join the Teesta at Singtam. The city is
located at the south-facing slope of Eastern Himalaya and has erosional topography bounded and dissected by a
large number of perennial and seasonal springs. The study area experiences a high variation of rainfall
throughout the year.
For the city, 7 June 1997 is often referred as a ‘black day’ since in this particular date, the city had
witnessed a number of landslides due to heavy rainfall (approximately 224 mm) causing more than 50 fatalities,
60 people injuries, more than 5000 people lost their home and disruption of National Highway 31A (near Rangpo
town).
9. Annual rainfall in Gangtok for the period of 2006–2017 (Source: Indian meteorological department)
(a) Geological map of the Gangtok and its surrounding area,
(b) (b) cross-section along A-B line, (c) cross-section along C-D line
Showing landslide scars in study area detected by LISS IV (left side) and google earth pro (right side)
10. Landslide inventory map of the study area.
The study area mostly consists of gneissic rocks and schistose rocks type
due to high rainfall, rocks are highly weathered and eroded at many places which causes frequent landslides.
