REVIEW QUESTIONS
1. What is an earthquake?
2. What are 4 reasons for an earthquake?
3. What is the difference between the epicenter and focus?
4. What are the three types of seismic waves? What type of
shaking does each demonstrate?
5. What waves are recorded first, second and third by a
seismograph?
6. What is meant by the magnitude of an earthquake?
EARTHQUAKES
AND
DESTRUCTION
EARTH SCIENCE 11. CORPUZ
MODIFIED MERCALLI
SCALE
• Measures the intensity and amount of damage done to
structures.
• Using Roman numerals I to XII, Mercalli scale rates the types
of damage and other effects of an earthquake noted by
observers.
• Ex. III – Quite noticeable indoors. Vibrations are like passing
of a truck
• Ex. X – Most ordinary structures are destroyed. Rails are
bent. Landslides are common.
EARTHQUAKE
INTENSITY
• Intensity depends primarily on the epicenter of where surface
waves are generated.
• Surface and body waves gradually decrease in size with
distance from the focus.
• Maximum intensity will be observed ain the region near the
epicenter.
• Mercalli values decrease as we go farther away from the
epicenter.
DEPTH OF FOCUS
• The depth of the quake’s focus influences the quakes intensity.
• A earthquake can be classified as shallow, intermediate, or
deep.
• Deep produce smaller vibrations at the epicenter
• Shallow produce moderate vibration at the epicenter
• Ex. A shallow magnitude 6 earthquake will demonstrate
greater intensity than a deep magnitude 8.
DAMAGE
Ground Shaking:
• The result of the waves set in motion by the earthquake.
• Some vibrations move up and down, while others move side
to side
• Most building can withstand large up and down vibrations, but
not side to side (cause collapse)
Foundation failure:
• The soils under a building may settle from serve shaking,
causing collapse
• Buildings built on solid rock experience little damage
• Buildings located on bog muds or soil fill suffer severe
damage (liquefaction)
RISK AND PREDICTION
•
No location is without some earthquake risk.
•
Most prediction efforts are based on the assumption that earthquakes are
periodic events.
Seismic gap method:
•
Plotting the location of earthquakes foci along a branch of the San
Andreas fault.
•
Scientists can sometimes find a place where the fault has not moved over
a period of time.
Found to proceed earthquakes:
•
Micro quakes
•
Slight increase in the elevation of the land
•
Decreases in the electrical resistance of the ground
•
Increase in the amount of radon
•
P-waves slow by 10-15% for a period of time preceding an earthquake.
EARTHQUAKE WAVES
INSIDE EARTH
P and S-Wave velocities:
• As waves travel through
earth, they change velocity,
bend or get reflected.
At 2900 km:
• Boundary between mantle
(solid) and outer core
(liquid)
• P-waves slow down
• S-waves stop (don’ travel
through liquids)
At 5200 km:
• P-wave velocity increases
(approaches inner core –
solid)
THE MOHO
•
Another abrupt change in P and S-wave velocities occurs at the
boundary between the crust and the mantle.
Discovered in 1909 by Andrila Mohorovicic:
• Found that seismograms showed two distinct groups of P and Swaves.
• One of the groups traveled at an average velocity of 7km/s
• The other group of waves travelled at 8 km/s
• Mohorovicic reasoned that the faster wave group had gone
through denser material below the crust (50km)
Mohorovicic discontinuity (Moho):
• Boundary between the crust and the mantle
• The Moho is about 32km deep under continents but only 8km
deep below the oceans
• This is why continents stand higher on the crust
THE MOHO
• The Moho is the
boundary
between the
crust and the
mantle in the
earth.
• The depth
where seismic
waves change
velocity.
SHADOW ZONE
•
Even though an earthquake sends
waves throughout Earth’s interior,
not all seismograph stations receive
information
•
Seismic stations that receive neither
P nor S-waves are said to be in the
shadow zone
Shadow Zone:
• A wide belt around Earth on the
side opposite the focus of the
earthquake.
Causes:
• Earth’s outer core
• P-waves are refracted (bent) when
they pass through the mantle
• S-waves cannot travel through the
outer core (liquid)