What is an Earthquake?
An earthquake is the
vibration of the Earth
caused by a rapid
release of energy.
The focus is point in the
Earth where the release
of energy originates.
The location on the
surface of the Earth
directly above the focus
is called the epicenter.
An earthquake occurred on the Erie Fault 5 km
below Ashtabula. Damage from the earthquake
was greatest in nearby Chardon. The furthest
report of shaking was recorded in Akron. Where
was the earthquake's epicenter?
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Chardon
Akron
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The Elastic Rebound Theory
Earthquakes are
produced by the rapid
release of elastic energy
stored in rock that has
been subjected to
stresses.
Once the strength of the
rock is exceeded, it
suddenly ruptures,
causing the vibrations of
an earthquake.
Fault Movement
A fault is a fracture in
Earth’s crust that
shows evidence of
movement or
displacement.
Movement along faults
can be sideways or
horizontal (strike-slip)
or vertical (dip-slip) as
shown here.
Earthquake Detection
Seismographs are
instruments that
record earthquake
waves.
Essentially, a weight is
suspended from a
support that is
attached to bedrock.
When the bedrock
vibrates, a pen records
the earthquake on a
rotating drum.
Seismic Records
Records of the
ground motion are
called seismograms.
These are the sheets
of paper removed
from the rotating
drum of the
seismograph.
Seismic Waves
Seismograms reveal that there are two types of
seismic waves generated by rock slippage:
• Surface waves travel along the Earth’s outer
layer
• Body waves travel through the Earth’s interior.
Body waves are further divided into primary
and secondary waves.
Seismic Waves
P waves are push-pull waves. P waves compress and expand in the
direction the wave is traveling. S waves move up and down at right
angles to their direction of travel.
Seismic Wave Arrivals
Location: Off the coast
of Central America
Date: January 13, 2001
Time: 17:33:29
Magnitude: 7.1 Mw
P waves arrive at the recording station first, then S waves , and
finally surface waves. P waves travel about 1.7 times faster than
S waves.
Examine the seismogram below that shows a 26-minute
long record of the seismic waves from the 1906 San
Francisco earthquake as it was received by a seismograph
station in Germany. Approximately how much time
elapsed between the arrival of the first P and S waves?
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4 minutes
10 minutes
20 minutes
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Locating Earthquakes
The difference in velocities
of P and S waves provides
a method for locating the
epicenter.
The greater the length of
the time between the first
arriving P and S waves,
the greater the distance the
earthquake is away.
In this example, an interval of 5 minutes indicates the
earthquake was 3400 km away from the recorder.
Locating Earthquakes
From the travel-time
graph we get the
distance to the
earthquake, but how
can we figure out the
direction??
The precise location
can be found if three
seismic stations
recorded the
earthquake.
Earthquake Distributions
Oceanic Ridge
System
Circum-Pacific
Belt
Earthquakes occur in narrow zones marking the boundaries of
the main plates.