Page 2 - EQTips_Eng

Learning

1 Earthquake Design
Earthquake Tip and

Construction

What causes Earthquakes?

The Earth and its Interior
Long time ago, a large collection of material
masses coalesced and formed the Earth. Large amount
of heat was generated by this fusion, and slowly as the
Earth cooled, the heavier and denser materials sank to
the center and the lighter ones rose to the top. The
differentiated Earth consists of the Inner Core (radius
~1290km), the Outer Core (thickness ~2200km), the
Mantle (thickness ~2900km) and the Crust (thickness ~5
to 40km). Figure 1 shows these layers. The Inner Core
is solid and consists of heavy metals (e.g., nickel and
iron), while the Crust consists of light materials (e.g.,
basalts and granites). The Outer Core is liquid in form Figure 2:
and the Mantle has the ability to flow. At the Core, the Local Convective Currents in the Mantle
temperature is estimated to be ~2500°C, the pressure
~4 million atmospheres and density ~13.5 gm/cc; this is
in contrast to ~25°C, 1 atmosphere and 1.5 gm/cc on the Plate Tectonics
surface of the Earth. The convective flows of Mantle material cause the
Crust and some portion of the Mantle, to slide on the
hot molten outer core. This sliding of Earth’s mass
Crust takes place in pieces called Tectonic Plates. The surface
Mantle of the Earth consists of seven major tectonic plates and
many smaller ones (Figure 3). These plates move in
Outer Core different directions and at different speeds from those
Inner Core of the neighbouring ones. Sometimes, the plate in the
front is slower; then, the plate behind it comes and
collides (and mountains are formed). On the other
hand, sometimes two plates move away from one
Figure 1: another (and rifts are created). In another case, two
Inside the Earth plates move side-by-side, along the same direction or
in opposite directions. These three types of inter-plate
interactions are the convergent, divergent and transform
The Circulations boundaries (Figure 4), respectively. The convergent
Convection currents develop in the viscous boundary has a peculiarity (like at the Himalayas) that
Mantle, because of prevailing high temperature and sometimes neither of the colliding plates wants to sink.
pressure gradients between the Crust and the Core, The relative movement of these plate boundaries
like the convective flow of water when heated in a varies across the Earth; on an average, it is of the order
beaker (Figure 2). The energy for the above of a couple to tens of centimeters per year.
circulations is derived from the heat produced from
the incessant decay of radioactive elements in the
rocks throughout the Earth’s interior. These convection Eurasian North American
Plate
Plate
currents result in a circulation of the earth’s mass; hot
molten lava comes out and the cold rock mass goes Pacific
into the Earth. The mass absorbed eventually melts Plate
under high temperature and pressure and becomes a Indo- South African
part of the Mantle, only to come out again from Australian American Plate
Plate
another location, someday. Many such local Plate
circulations are taking place at different regions Antarctic Plate

underneath the Earth’s surface, leading to different Figure 3:

portions of the Earth undergoing different directions Major Tectonic Plates on the Earth’s surface
of movements along the surface.
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