Earthquake Slides
By Dr. S. A. Isiorho
Copyright © by Isiorho
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Earthquakes
Definitions
 Earthquake- the vibration of the ground due to the sudden
release of energy accumulated in a deformed rock
 Focus (Hypocenter)- spot underground where the rock
begins to break- point at which slip initiates
 Epicenter- the point on the land surface directly above the
focus
 Aftershock- tremors that occur as rocks adjust to their new
position
 Seismology- the study of earthquake
Copyright © by Isiorho
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Seismic Waves
Earthquake’s energy is transmitted through the earth as seismic waves


Two types of seismic waves radiate from the focus
Body waves- transmit energy through earth’s interior
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Primary (P) wave- rocks vibrate parallel to direction of wave (v=47km/sec)
 Compression and expansion (slinky example)
Secondary (S) wave- rocks move perpendicular to wave direction (v=2-5
km/sec)
 Rock shearing (rope-like or ‘wave’ in a stadium) S-wave cannot travel
through liquid
Surface waves- transmit energy along earth’s surface
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
Love (L) wave- Rock moves from side to side like snake
Raleigh ® wave- Rolling pattern like ocean wave
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Locating & Measuring Earthquake
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Seismometer- instruments that detect seismic waves
Seismograph- device that measures the magnitude of
earthquake
seismogram is visual record of arrival time and magnitude of
shaking associated with seismic wave
Mercalli Intensity scale


Measured by the amount of damage caused in human terms- I (low) to XII
(high)
Richter Scale- (logarithmic scale)
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Magnitude- based on amplitude of the waves
Earthquake total energy- uses moment magnitude scale
Copyright © by Isiorho
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Locating Epicenter &
Focus Depth (EQ classification)

Use Arrival time at a recording station (time lag
between P & S waves) to locate the epicenter of an
earth quake
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Need three stations to determine the epicenter
Maximum Depth of Focus
Shallow focus EQ < 70 km (45 mi) most earthquakes
 Intermediate focus EQ- 70-300 km (45- 180 mi)
 Deep focus EQ- > 300 km (> 180 mi)

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Richter Scale

Richter scale is based on a log scale, meaning that each
subsequent number is ten times more in amplitude of
vibration- this translates to about 30 times more energy
than the previous number.

Example: an EQ of 5.0 is 10 times greater than an EQ of
4.0 on the Richter scale and is 30 times more in energy.
An EQ of 5.0 is 100 times greater in amplitude than an EQ
with 3.0 reading on the Richter scale
Copyright © by Isiorho
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Earthquake Locations
 Most
EQs occur in the circum pacific region
 80%
 Most
of shallow focus EQ; 100% of deep focus EQ
EQs occur along plate boundaries
 Oceanic
trenches, Benioff zones, MediterraneanHimalayan
 Most
EQs in US occur near the west coast
 San-Andreas
Fault
Copyright © by Isiorho
7
Effects of Earthquakes

Ground Displacement

Lateral and vertical
Landslides
 Liquefaction
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Seiches
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Conversion of formally stable fine grain materials to a fluid mass
The back & forth movement of water in a semi-closed/closed body of watercould cause flooding
Tsunamis more from submarine landslide

Fire
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Coping with Earthquake
 Earthquake
zone
 Plate boundaries
 Assessing local seismic history and future risks
 Land use planning
 Quake reinforcement of building/structures
 Short term and Long term forecast
 Contingency plan
Copyright © by Isiorho
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