23-11-2012, 12:17 PM
SEISMOLOGY
SEISMOLOGY.ppt (Size: 10.67 MB / Downloads: 307)
Introduction
It is a big subject and mainly deals with earthquake as a geological process.
However, some portions of seismology are of great interest to earthquake engineers.
They include causes of earthquake, earthquake waves, measurement of earthquake, effect of soil condition on earthquake, earthquake pre- diction and earthquake hazard analysis
Understanding of these topics help earthquake engineers in dealing seismic effects on structures in a better way.
Further knowledge of seismology is helpful in describing earthquake inputs for structures where enough recorded data is not available.
Plate tectonics
At mid oceanic ridges, two
continents which were joined
together drifted apart due to
flow of hot mantle upward.
Flow takes place because of
convective circulation of
earth's mantle; energy comes
from radioactivity inside the
earth.
Hot material cools as it comes
up; additional crust is formed
which moves outward.
New crust sinks beneath sea surface; spreading
continues until lithosphere reaches deep sea
trenches where subduction takes place.
Continental motions are associated with a variety
of circulation patterns.
As a result, motions take place through sliding of
lithosphere in pieces- called tectonic plates.
There are seven such major tectonic plates and
many smaller ones.
They move in different directions at different
speeds.
Causes of earthquake
There are many theories to explain causes of
earthquake.
Out of them, tectonic theory of earthquake is
popular.
The tectonic theory stipulates that movements
of tectonic plates relative to each other lead to
accumulation of stresses at the plate boundar-
ies & inside the plate.
This accumulation of stresses finally results in
inter plate or intra plate earthquakes.
In inter- plate earthquake the existing fault
lines are affected while intra-plate earthquake
new faults are created.
Seismic waves
Large strain energy released during earthquake
propagates in all directions within earth as elastic
medium.
These waves, called seismic waves, transmit
energy from one point to the other & finally carry
it to the surface.
Within earth, waves travel in almost homogeno-
us elastic unbounded medium as body waves.
On the surface, they move as surface waves.
Reflection & refraction of waves take place near
the surface at every layer; as a result waves get
modified.
Earthquake measurement parameters
They refer to quantities by which size & energy
of earthquakes are described.
There are many measurement parameters; some
of them are directly measured; some are
indirectly derived from the measured ones.
There are many empirical relationships that are
developed to relate one parameter to the other.
Many of those empirical relationships and the
parameters are used as inputs for seismic
analysis of structures; so they are described
along with the seismic inputs.