Transcript Earthquakes What is an earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy • Energy radiates in.

Earthquakes
What is an earthquake?
An earthquake is the vibration of Earth
produced by the rapid release of energy
• Energy radiates in all directions from its
source, the focus
• Energy moves like waves
• Seismographs record the event
Deformation
Deformation is a general term that
refers to all changes in the original
form and/or size of a rock body
Most crustal deformation occurs
along plate margins
Deformation involves
• Stress—Force applied to a given
area
Deformation
How rocks deform
• General characteristics of rock
deformation
– Elastic deformation—The rock returns
to nearly its original size and shape
when the stress is removed
– Once the elastic limit (strength) of a
rock is surpassed, it either flows
(ductile deformation) or fractures
(brittle deformation)
Folds
During crustal deformation rocks
are often bent into a series of
wave-like undulations called folds
Characteristics of folds
• Most folds result from
compressional stresses which
shorten and thicken the crust
Folds
Common types of folds
• Anticline—Upfolded or arched rock
layers
• Syncline—Downfolds or troughs of
rock layers
• Depending on their orientation,
anticlines and synclines can be
described as
– Symmetrical, asymmetrical, or
recumbent (an overturned fold)
Anticlines and Synclines
Figure 6.20
Faults
Faults are fractures in rocks along
which appreciable displacement
has taken place
Sudden movements along faults
are the cause of most earthquakes
Classified by their relative
movement which can be
• Horizontal, vertical, or oblique
Normal Fault
Figure 6.24 A
Reverse Fault
Figure 6.24 B
Strike-Slip fault
Figure 6.24 D
Earthquakes and faults
• Earthquakes are associated with faults
• Motion along faults can be explained by
plate tectonics
Elastic rebound
• Mechanism for EQ’s explained by H. Reid
– Rocks on sides of fault are deformed by
tectonic forces
– Rocks bend and store elastic energy
– Frictional resistance holding the rocks together
is overcome by tectonic forces
Elastic rebound
• Earthquake mechanism
– Slips starts at the weakest point (the
focus) occurs
– Earthquakes occur as the deformed
rock “springs back” to its original
shape (elastic rebound)
Elastic Rebound Movie in Mapview
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Reid elastic rebound cartoon
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Aftershocks
The change in stress that follows
a mainshock creates smaller
earthquakes called aftershocks
The aftershocks
“illuminate” the
that ruptured in the mainshock
Red dots show location of
aftershocks formed by 3
earthquakes in Missouri
and Tennessee in 1811/1812
Strike Slip Fault Quake - Japan
Strike Slip Fault Quake - California
Normal Fault Quake - Nevada
San Andreas: An active
earthquake zone
San Andreas is the most studied fault system in
the world
Displacement occurs along discrete segments
100 to 200 kilometers long
• Most segments slip every 100-200 years
producing large earthquakes
• Some portions exhibit slow, gradual displacement
known as fault creep
Fence offset by the 1906 San
Francisco earthquake
Seismology
Seismometers - instruments that record
seismic waves
• Records the movement of Earth in
relation to a stationary mass on a rotating
drum or magnetic tape
A seismograph designed to
record vertical ground motion
Motion of a seismograph
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Types of seismic waves
• Surface waves
– Complex motion, great destruction
– High amplitude and low velocity
– Longest periods (interval between
crests)
– Termed long, or L waves
Types of seismic waves
• Body waves
– Travel through Earth’s interior
– Two types based on mode of travel
– Primary (P) waves
– Push-pull motion
– Travel thru solids, liquids & gases
– Secondary (S) waves
– Moves at right angles to their
direction of travel
– Travels only through solids
P-Wave Motion
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S-Wave Motion
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Locating the source of earthquakes
Focus - the place within Earth where
earthquake waves originate
Epicenter – location on the surface
directly above the focus
Epicenter is located using the difference
in velocities of P and S waves
Earthquake focus and epicenter
Locating the epicenter of an earthquake
• Three seismographs needed to locate an
epicenter
• Each station determines the time interval
between the arrival of the first P wave and
the first S wave at their location
• A travel-time graph then determines each
station’s distance to the epicenter
Graph used to find distance to epicenter
Locating the epicenter of an earthquake
• A circle with radius equal to distance to the
epicenter is drawn around each station
• The point where all three circles intersect is
the earthquake epicenter
Epicenter located using three seismographs
Earthquake belts
• 95% of energy released by earthquakes
originates in narrow zones that wind
around the Earth
• These zones mark of edges of tectonic
plates
Locations of earthquakes
from 1980 to 1990
Depths of Earthquakes
• Earthquakes originate at depths ranging
from 5 to nearly 700 kilometers
• Definite patterns exist
– Shallow focus occur along oceanic ridges
– Deep earthquakes occur in western
Pacific east of oceanic trenches
Earthquake in subduction zones
Measuring the size of earthquakes
Two measurements describe the size of an
earthquake
• Intensity – a measure of earthquake
shaking at a given location based on
amount of damage
• Magnitude – estimates the amount of
energy released by the earthquake
Intensity scales
• Modified Mercalli Intensity Scale was
developed using California buildings as its
standard
• Drawback is that destruction may not be
true measure of earthquakes actual severity
Magnitude scales
• Richter magnitude - concept introduced
by Charles Richter in 1935
• Richter scale
– Based on amplitude of largest seismic
wave recorded
– Each unit of Richter magnitude
corresponds to 10X increase in wave
amplitude and 32X increase in energy
Magnitudes scales
• Moment magnitude was developed
because Richter magnitude does not
closely estimate the size of very large
earthquakes
– Derived from the amount of
displacement that occurs along a fault
and the area of the fault that slips
Earthquake destruction
Amount of structural damage depends on
• Intensity and duration of vibrations
• Nature of the material upon which the
structure rests (hard rock good, soft bad)
• Design of the structure
Tsunamis, or seismic sea waves
• Destructive waves called “tidal waves”
• Result from “push” of fault block or
undersea landslide on water
• In open ocean height is > 1 meter
• In shallow coast water wave can be > 30
meters
• Very destructive
Formation of a tsunami
Tsunami 1960, Hilo Hawaii
Tsunami Model, Japan Earthquake
Tsunami Model,
Alaska Quake
Can earthquakes be predicted
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
Earthquakes cannot be predicted
Short-range predictions
• Currently, no method exists for making
short-range earthquake predictions
Long-range forecasts
• Calculates probability of a certain
magnitude earthquake occurring over a
given time period