3. Ground shaking : seismic waves radiate
waves away from source and travel rapidly through
the earth crust. They reach the surface of earth and
cause shaking. Strength , duration of shaking at
particular site depends on size and location of
earth quake and site characteristics.
Structural hazards : 1970 Peru
earthquake , 1985 Mexico earthquake , 1971 sa
funado earth quake are examples to show
structural damage caused .
Lifeline hazards: These include power
plants , telecommunication , transportation , water
and sewage , oil and gas pipelines , burried electric
cables , roads and bridges.
4. Liquefaction :soil liquefaction describes a
phenomenon where by a saturated or partially saturated
soil substancially looses strength and stiffness in
response to an applied stress , usually in earthquake
shaking or any sudden change in stress condition ,
causing it to behave like a liquid
Flow failures : for example , it occurs when
strength of soil drops below the level needed to maintain
stability under static conditions .
Liquifraction was first used by Sir ALLEN HAZEN in
reference to 1918 failure of Calaveras Dam
Tsunami and seiche hazard: Rapid vertical
seafloor movements caused by fault rupture during
earthquake can produce long period seawaves called
tsunami.
shape of the seafloor may amplify the wave.
6. P Waves :
primary , compression or longitudinal waves .
These involve successive compression and
rarefaction.
They pass through solid and fluids .
These are first waves to arrive site.
S waves
secondary , shear or transverse waves .
These waves cause shear deformations.
Monochrovicic discontinuity is boundary marked
between crust and mantle by distinct change in
propagation velocity.
7. Continental Drift :
According to the theory of continental drift, the
world was made up of a single continent through
most of geologic time. That continent eventually
separated and drifted apart, forming into the
seven continents we have today.
Plate techtonics:
a theory explaining the structure of the earth's
crust and many associated phenomena as
resulting from the interaction of rigid
lithospheric plates which move slowly over the
underlying mantle.
8. Seismic hazard analysis involves the
quantitative estimation of ground shaking
hazards at a particular area. The most
important factors affecting seismic hazard at
a location are:
Earthquake magnitude
the source-to-site distance
earthquake rate of occurrence (return period)
duration of ground shaking
9. Earthquake Magnitude
- Magnitude is the most common measure of
an earthquake's size. It is a measure of the size of
the earthquake source and is the same number no
matter where you are or what the shaking feels like
Earthquake rate of occurrence (return period)
- A return period is an estimate of the interval of
time between earthquake. It is a statistical
measurement denoting the average recurrence
interval over an extended period of time, and is
usually required for risk analysis.
11. Geologic Evidence: The theory of plate
tectonics assures us that the occurrence of
earthquake is written in the geologic
record, primarily in the form of offsets, or
relative displacements of various strata.
Study of the geologic record of past
earthquake activity is called
paleoseismology.
Fault activity: the use of the term active fault
to describe a fault that poses a current
earthquake threat and inactive fault to
describe one on which past earthquake
activity is unlikely to be repeated.
12. Magnitude Indicators
Geologic evidence can also be used to estimate the
magnitude of past earthquakes by correlating
observed deformation characteristic with the known
magnitudes of recorded earthquake
Fault rupture has often been used to estimate
earthquake magnitude
Tectonic Evidence :Plate tectonics and elastic
rebound theory tell us that earthquakes occur to
relieve the strain energy that accumulates as plates
move relative to each other. The rate of movement,
therefore, should be related to the rate of strain
energy accumulation and also to the rate of strain
energy release
13. Historical Seismicity
Historical accounts of ground-shaking effects
can be used to confirm the occurrence of past
earthquakes and to estimate their geographic
distribution of intensity. When sufficient data are
available, the maximum intensity can be
determined and used to estimate the location of
the earthquake epicenter and the magnitude of the
event
Instrumental Seismicity Instrumental records from
large earthquakes have been available since about
1900, although many from before 1960 are
incomplete or of uneven quality. Nevertheless,
instrumental recordings represent the best
available information for the identification and
evaluation of earthquake sources.
