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IITK-BMTPC Earthquake Tip 1

What causes Earthquakes? page 2
The sudden slip at the fault causes the earthquake…
a violent shaking of the Earth during which large
elastic strain energy released spreads out in the form
of seismic waves that travel through the body and
along the surface of the Earth. And, after the
earthquake is over, the process of strain build-up at
this modified interface between the tectonic plates
Convergent Boundary starts all over again (Figure 6). Earth scientists know
this as the Elastic Rebound Theory. The collection of
material points at the fault over which slip occurs
Transform Boundary usually constitutes an oblong three-dimensional
volume, with its long dimension often running into
Figure 4: Types of tens of kilometers in case of significant earthquakes.
Divergent Boundary Inter-Plate Boundaries Types of Earthquakes and Faults

Most earthquakes in the world occur along the

The Earthquake boundaries of the tectonic plates as described above
Tectonic plates are made of elastic but brittle rocky and are called Inter-plate Earthquakes (e.g., 1897 Assam
material. And so, elastic strain energy is stored in them (India) earthquake). A number of earthquakes also
during the relative deformations that occur due to the occur within the plate itself but away from the plate
gigantic tectonic plate actions taking place in the Earth. boundaries (e.g., 1993 Latur (India) earthquake); these
But, when the rocky material along the interface of the are called Intra-plate Earthquakes. Here, a tectonic plate
plates in the Earth’s Crust reaches its strength, it breaks in between. In both types of earthquakes, the
fractures and a sudden movement takes place there slip generated at the fault during earthquakes is along
(Figure 5); the interface between the plates where the both vertical and horizontal directions (called Dip Slip)
movement has taken place (called the fault) suddenly and lateral directions (called Strike Slip) (Figure 7),
slips and releases the large elastic strain energy stored with one of them dominating sometimes.

in the rocks at the interface. For example, the energy
released during the 2001 Bhuj (India) earthquake is
about 400 times (or more) that released by the 1945
Atom Bomb dropped on Hiroshima!!

Dip Slip
Faults
Strike Slip
Faults

Stage A
Slip Stage B

Figure 5:
Elastic Strain Build-Up
Stage C and Brittle Rupture Figure 7: Type of Faults
Reading Material
Bolt,B.A., (1999), Earthquakes, Fourth Edition, W. H. Freeman and
EQ EQ Company, New York, USA
EQ http://earthquake.usgs.gov/faq/
Slip C http://neic.usgs.gov/neis/general/handouts/
general_seismicity.html
B http://www.fema.gov/kids/quake.htm
Cumulative Slip A
A C Authored by:
Time C.V.R.Murty
Energy (years) Indian Institute of Technology Kanpur
Strength Build-Up Kanpur, India
Elastic Stress B Energy Sponsored by:
Release
Building Materials and Technology Promotion
C Council, New Delhi, India
A Time This release is a property of IIT Kanpur and BMTPC New

Figure 6: Elastic Rebound Theory (years) Delhi. It may be reproduced without changing its contents
and with due acknowledgement. Suggestions/comments

may be sent to: nicee@iitk.ac.in. Visit www.nicee.org or
www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.
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