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EARTHQUAKE MAGNITUDE AND
INTENSITY
• Magnitude is the measurement of the amount of
energy released during an earthquake.
• The Richter scale is a numerical scale based
on the size of the largest seismic waves
generated by a quake that is used to
describe its magnitude.
– Each successive number in the scale represents
an increase in seismic-wave size, or amplitude,
of a factor of 10.
– Each increase in magnitude corresponds to
about a 32-fold increase in seismic energy.
HOW ARE THE SIZE AND STRENGTH
OF AN EARTHQUAKE MEASURED?
• Magnitude
• Richter scale measures total
amount of energy released
by an earthquake;
independent of intensity
• Amplitude of the largest
wave produced by an event
is corrected for distance and
assigned a value on an
open-ended logarithmic
scale
EARTHQUAKE MAGNITUDE AND
INTENSITY
Modified Mercalli Scale
– The modified Mercalli scale, which measures
the amount of damage done to the structures
involved, is used to determine the intensity of
an earthquake.
– This scale uses the Roman numerals I to XII
to designate the degree of intensity.
– Specific effects or damage correspond to
specific numerals; the higher the numeral, the
worse the damage.
EARTHQUAKE MAGNITUDE AND
INTENSITY
EARTHQUAKE MAGNITUDE AND
INTENSITY
Depth of Focus
– Earthquake intensity is related to earthquake magnitude.
– The depth of the quake’s focus is another factor that
determines the intensity of an earthquake.
– An earthquake can be classified as shallow, intermediate,
or deep, depending on the location of the quake’s focus.
– A deep-focus earthquake produces smaller vibrations at
the epicenter than a shallow-focus quake.
CLASSIFICATION ACCORDING
TO FOCUS DEPTHS
WHAT ARE THE DESTRUCTIVE EFFECTS OF
EARTHQUAKES?
Damage in Oakland, CA, 1989
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•
•
•
Building collapse
Fire
Tsunami
Ground failure
EARTHQUAKE DESTRUCTION
Amplitude,
duration, and
damage
increases in
poorly
consolidated
rocks.
EARTH DESTRUCTION
Liquefaction Unconsolidated materials
saturated with water turn
into a mobile fluid.
Seiches - These are the rhythmic sloshing of
water in lakes, reservoirs, and enclosed
basins. The waves can weaken reservoir
walls and cause destruction.
EARTH DESTRUCTION
• Landslides and ground subsidence – Whenever the
land moves quickly, as with landslides, there is the
potential for a lot of damage and potential loss of
life.
• Fire – Ruptured gas lines from earthquakes is one of
the major hazards.
• Ground shaking versus material type – More ground
shaking occurs in poorly consolidated (loose)
sediments than solid bedrock.
EARTH DESTRUCTION
Ground Rupture ‐ This refers to areas where the land splits apart
causing a rupture. These are often long linear features.
TSUNAMIS
– A tsunami is a large ocean wave generated by vertical
motions of the seafloor during an earthquake.
– These motions displace the entire column of water
overlying the fault, creating bulges and depressions in the
water.
– Long waves that can travel at speed over 800 km/h (500
mph).
– When the waves enter shallow water they may form huge
breakers with heights occasionally exceeding 30 m (19
ft.).
FORMATION OF A TSUNAMI
Tsunami waves can travel at a speed of over 500 mph.
CAN EARTHQUAKES BE
PREDICTED?
Earthquake
Precursors
• changes in elevation
or tilting of land
surface, fluctuations
in groundwater
levels, magnetic
field, electrical
resistance of the
ground
• seismic dilatancy
model
• seismic gaps
CAN EARTHQUAKES BE PREDICTED?
Earthquake Prediction Programs
• include laboratory and field studies of rocks before, during,
and after earthquakes
• monitor activity along major faults
• produce risk assessments