30 terms

Geol 101: Earthquakes

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An earthquake is
the vibration of Earth produced by the rapid release of energy
Energy released from an earthquake radiates in all directions from its source,
the focus
Epicenter is
the location of the focus of the earthquake projected on the Earth's surface
Energy is released from earthquakes in the form
of waves
Elastic rebound
-Rocks on both sides of an existing fault are
deformed by tectonic forces
-Rocks bend, thus store elastic energy
-Frictional resistance holding the rocks together is
overcome when enough energy is stored
-Energy breaks the rock, rupture propagates and
sends EQ waves into surrounding rock
Seismographs are
instruments that record seismic waves
-Records the movement of Earth relative to a
stationary mass
-Shaking of Earth produces waves on a rotating
drum or magnetic tape
Seismic waves travel
through Earth and over its surface
Types of Seismic Waves
-Body Waves: Move through the interior of the Earth
-P-waves (Primary Waves - Compressional Wave)
-S-waves (Secondary Waves - Shear Waves)
-Surface Waves: Move across the surface of the Earth
Body Waves
-Travel through Earth's interior
-Two types based on mode of travel
-Useful for Locating Earthquakes
-Useful to determine Earthquake magnitude
P-waves (Primary Waves - Compressional Wave)
Generally, in any solid material, travel
about 1.7 times faster than S waves
-type of body wave
S-waves (Secondary Waves - Shear Waves)
"Shake" motion at right angles to their direction
of travel
-Travel only through solids
-Greater amplitude than P waves
-type of body wave
Surface Waves
-Travel along Earth's surface
-Complex motions
The waves travel at
different speeds and arrive at the seismograph at different times
P waves are compressional
waves that travel quickly
through
rock
P waves push and pull
particles in the direction
of their path of travel
The basis for locating an earthquake, is the difference in
the arrival time between the first P-wave and the first S-wave
The greater the distance between the hypocenter or epicenter and the station, the greater
the difference between the arrival times of the first P-wave and the first S-wave
Two measurements that describe the size of an
earthquake are
-Magnitude
-Intensity
Magnitude
estimates the amount of energy
released at the source of the earthquake
Intensity
a measure of the degree of earthquake shaking at a given locale based on the amount of damage
Magnitude scales
-Richter magnitude
-Richter scale
-Moment magnitude
Richter magnitude
concept introduced by Charles Richter in 1935
Richter scale
-Based on the amplitude of the largest seismic wave recorded in Southern California
-Accounts for the decrease in wave amplitude with increased distance
-Scaled to the Wood-Anderson type seismograph
Moment magnitude
was developed because none of the "Richter-like" magnitude scales adequately estimates very large earthquakes
-Derived from the amount of displacement that occurs along a fault
Modified Mercalli Intensity Scale
was developed using California buildings as its standard
-Drawback: destruction may not be a true
measure of the earthquake's actual energy
release
Amount of structural damage attributable to
earthquake vibrations depends on
-Intensity and duration of the vibrations
-Nature of the material upon which the structure
rests
-Design of the structure
Ground shaking
-Regions within 20 to 50 kilometers of the
epicenter will experience about the same
intensity of ground shaking
-However, destruction varies considerably
mainly due to the nature of the ground on
which the structures are built
What Causes Intraplate Earthquakes?
Old faults are weak zones that concentrate stress
Short-range predictions
-Goal is to provide a warning of the location and magnitude of a large earthquake within a narrow time frame
-Research has concentrated on monitoring possible precursors - phenomena that precede a forthcoming earthquake such as measuring uplift, subsidence, and strain in the rocks
-Currently, no reliable method exists for making short-range earthquake predictions
Long-range forecasts
-Give the probability of a certain magnitude earthquake occurring within 30 to 100 years, or more
-Based on the premise that earthquakes are repetitive or cyclical
-Using historical records or paleoseismology
-Are important because they provide information used to:
*Develop the Uniform Building Code
*Assist in land-use planning