Seismology & earthquake md. yousuf gazi

Seismology & Earthquakes
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

Seismology
❑ Seismology is the branch of Geophysics concerned with the study and analysis of Earthquakes and
the science of energy propagation through the Earth's crust.
▪ Seismology is extremely important because:
✓ Study of earthquakes gives us important clues about the earth’s interior.
✓ Understanding earthquakes allows us to minimize the damage and loss of life.

What is an earthquake?
◼ An earthquake is the vibration of Earth produced by the rapid release of accumulated energy in
elastically strained rocks.
▪ Energy released radiates in all directions from its source, the focus
▪ Energy propagates in the form of seismic waves
▪ Sensitive instruments around the world record the event

Fault & Earthquake Terminology
• Focus: Location within the crust where the rocks failed under stress. Also called “hypocenter”
• Epicenter: Point on the earth’s surface directly above the focus.
• Fault plane: Surface of the fault that slipped during the earthquake.
• Surface rupture: The intersection of the fault plane and Earth’s surface. The crack on the ground.
• Scarp: A raised “step” due to vertical motion (dip-slip separation) along the surface rupture.
Horizontal offset also occurs (strike-slip separation).

Elastic Rebound Theory
• This theory was discovered by making measurements at a number of points across a fault.
• Prior to an earthquake it was noted that the rocks adjacent to the fault were bending. These bends
disappeared after an earthquake suggesting that the energy stored in bending the rocks was
suddenly released during the earthquake.

Stress and Strain
• Rocks far from a fault move steadily (plate motion).
• Near the fault, friction “locks” the fault so nearby material is strained.
• Eventually strain accumulates and stresses exceed rock strength.
• Earthquake relieves strain at the fault and allows the fault to “catch up” with the rest of the plate.

Foreshocks and aftershocks
▪ Adjustments that follow a major earthquake often generate smaller earthquakes called
aftershocks
▪ Small earthquakes, called foreshocks, often precede a major earthquake by days or, in
some cases, by as much as several years

Causes of Earthquakes
• Magma movement
• Human activity (mining, injection, Examine weapons)
• Faults – tectonic stresses build up and release along fractures in the crust.
• Movement of Tectonic Plates
• Rupture of rocks along a fault

Magma movement

Movement of Tectonic Plates

Human activity (mining, injection, Examine weapons)

Rupture of rocks along a fault

• Duration of an earthquake is very important parameter that influences the amount of
damage due to earthquake.
• A strong motion of very high amplitude of short duration may not cause as much damage to
a structure as a motion with moderate amplitude with long duration can cause.
Duration of earthquakes

▪ Earthquakes usually occur at some depth below the ground Surface. The depth can
also be calculated from seismograph records.
• Earthquake foci are described as:
o Shallow: less than 70 km depth
o Intermediate: 70 - 300 km depth
o Deep: 300 - 700 km depth
• 90% of earthquake foci are less than 100 km deep
• Large earthquakes are mostly at < 60 km depth
• No earthquakes occur deeper than 700 km
Earthquake Depth

EQ Distribution and Depth

Studying Earthquakes
• Seismograph: device used to detect and record earthquake waves. Also called seismometer.
• Seismogram: the record made by a seismometer.
• Seismographic stations around the World work together to
▪ Record earthquake location
▪ Determine earthquake strength

The ancient Chinese seismograph consist of a special vase that had eight sculpted dragons mounted
around the vase in eight primary directions. Each dragon held in its mouth a metal ball. When the
ground shook, some of the balls would fall from the mouths of the dragons into the waiting mouths
of the sculpted frogs to show how the ground had moved.
Ancient Chinese Seismograph

Locating Earthquakes
• Seismic waves travel at different speeds and arrive at seismometers at different times.
• Seismologists have figured out the average travel times of waves over various
distances and have made time-distance graphs.
• Time-distance graph shows distance to an seismic wave source vs. difference in arrival
time of P- and S-waves.
• The epicenter of an earthquake can be found by using the time-distance graph and the
arrival times of the P- and S- waves at a given station.
• If this is done for at least 3 different stations, the epicenter location can be
triangulated.
• If this is done for 4 different stations, depth can also be measured.

Earthquake Damage depends on many factors:
• The size of the Earthquake
• The distance from the focus of the earthquake
• The properties of the materials at the site
• The nature of the structures in the area
Damage due to Earthquakes
Damage decreases with distance

Major Hazards of Earthquakes
◼ Ground Motion: Shakes structures causing them to collapse
◼ Liquefaction: Conversion of formally stable cohesionless soils to a fluid mass, causing damage to
the structures
◼ Landslides: Triggered by the vibrations
◼ Fire : Indirect result of earthquakes triggered by broken gas and power lines
◼ Tsunamis: large waves created by the instantaneous displacement of the sea floor during
submarine faulting

❑ Ground Motion

Earthquake Destruction: Liquefaction
• Sand Boil: Ground water rushing to the surface due to liquefaction

Earthquake Destruction: Landslides

Earthquake Destruction: Tsunamis

Tsunami Movement: ~600 mph in deep water
~250 mph in medium depth water
~35 mph in shallow water

Broken gas lines ignite
Earthquake Destruction: Fires

How Strong Was It?
• Quantitative – a number on the Richter or moment magnitude scale. Direct measurement of
amount of energy released.
• Qualitative – a descriptive estimate, using the Mercalli intensity scale, of the amount of damage
done. Indirect measurement of energy released.
• Richter Scale- (logarithmic scale)
◆Magnitude- based on amplitude of the waves
◆Related to earthquake total energy
• Mercalli Intensity scale
◆Measured by the amount of damage caused in human terms- I (low) to
XII (high); drawback: inefficient in uninhabited area

Richter Scale: Related to intensity
• M=1 to 3: Recorded on local seismographs, but generally not felt
• M= 3 to 4: Often felt, no damage
• M=5: Felt widely, slight damage near epicenter
• M=6: Damage to poorly constructed buildings and other structures within 10's km
• M=7: "Major" earthquake, causes serious damage up to ~100 km (recent Gujarat earthquake).
• M=8: "Great" earthquake, great destruction, loss of life over several 100 km
• M=9: Rare great earthquake, major damage over a large region over 1000 km
Charles Francis Richter

Earthquakes around Bengal Basin

Can We Predict Earthquakes?
• Prediction requires knowing the time (to within useful range) and location, as well as strength.
• In general, not really possible…yet.
• We can define seismic risk, however. Probabilities for (1) occurrence, (2) strength, and (3) damage.

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