Download Earthquakes

What is an Earthquake?
Tangshan, China 1976
Lisbon 1755
Earthquake is a sudden slip (rupture) on a fault
and the resulting ground shaking caused by the
radiated seismic energy generated by the slip
(rupture)
Earthquakes
Alternate definition:
 A natural vibration of the ground caused when the
stress built up between tectonic plates is suddenly
released.
 There are three types of stress:
 Compression – caused by two plates being push
together.
 Tension – Pulls plates apart.
 Shear – causes plates to twist.
Types of faults
 There are three types of Faults
1. Normal fault

caused by tension.
Reverse fault
2.

caused by compression.
Strike-slip fault
3.

caused by shearing.
Three Types of Seismic Waves
1.
Primary waves (p-waves)

Longitudinal waves – these vibrate by the compression
of material in the direction of the wave’s motion.
These can travel through the core.
2. Secondary waves (s-waves)
 Transverse waves (Like in water) – these vibrate by
moving the particles perpendicular to the direction of
wave’s motion. These cannot travel through the core.
3. Surface waves (Love and Rayleigh waves)
 Vibrate in all directions.
Body Waves
• P waves are compressional (or
longitudinal) body waves in which
rock vibrates back and forth parallel
to the direction of wave propagation
– Fast (4 to 7 kilometers per
second) wave that is the first or
primary wave to arrive at a
recording station following an
earthquake
– Can pass through solids and
fluids (liquids or gases)
Fig 10.7
Body Waves
• S waves are shearing (or
transverse) body waves in which
rock vibrates back and forth
perpendicular to the direction of
wave propagation
– Slower (2 to 5 kilometers per
second) wave that is the
secondary wave to arrive at a
recording station following an
earthquake
– Can pass only through solids
Fig 10.7
Surface Waves
• Slowest type of seismic waves set
off by earthquakes
• Love waves involve only side-toside motion of the ground surface
– Can’t travel through fluids
• Rayleigh waves behave like
ocean waves, and cause the
ground to move in an elliptical
path opposite the direction of
wave motion
– Extremely destructive to buildings
Fig 10.7
The Earth’s Interior
Earthquakes (P and S waves)
Were instrumental in the
Study of the Earth’s Core
S-waves can only propagate
Through the Mantle,
P-waves can propagate
Through the core.
This shows a density
Difference inside the Earth
Figs 10.23 & 10.24
& 10.26
Measuring Earthquakes
 Richter Scale
 Based on size of the largest seismic waves generated by
the Earthquake.
 Each level increase represents an increase of energy by a
factor of 32 and a wave size of 10 times.
Anatomy…
Worst damage is
not necessarily
nearest to focus:
it depends on
ground type,
time, magnitude,
terrain, nature of
structures…
Fig 10.2
Measuring Earthquakes
 Moment of Magnitude Scale
 Based on the magnitude of seismic waves as well as the
size of the fault rupture, amount of movement, and rock
characteristics.
 More accurate than Richter scale.
 Modified Mercalli Scale
 Measures the amount of damage caused by the
Earthquake.
Strongest Earthquakes of 2010
1.
2.
3.
4.
5.
6.
7.
8.
Chile (Feb 27th) Mag. 8.8
Indonesia (April 6th) Mag. 7.7
Vanuatu (May 27th) Mag. 7.7
Indonesia (May 9th) Mag. 7.2
Mexcali Valley, California (April4th) Mag. 7.2
Solomon Islands (Jan 3rd) Mag. 7.1
Haiti (Jan 12th) Mag. 7.0
Ryukyu, Japan (Feb 27th) Mag. 7.0
There have been 7460 earthquakes so far this year. Most are
below a magnitude of 4.9.
Chile
 7th Strongest earthquake in History
 130 after shocks over the next week.
 The focus ground at epicenter moved
3.0 up and 10 m west.
 500 times more powerful than Haiti
 Called a megathrust quake because it
is in a subduction zone
Earthquake Size and Strength…
•
Los
Angeles
area…
Modified Mercalli Intensity Map
– 1994 Northridge, CA earthquake, Richter
magnitude 6.7
Collapsed Cypress Freeway
Source: Richards/Sipa Press
Copyright © Houghton Mifflin Company. All rights
reserved.
Japan (Tohoku) Earthquake and Tsunami
 CBC News Reports
 http://www.cbc.ca/ne
ws/world/story/2011/0
3/11/japan-quaketsunami.html
 The Guardian
 http://www.guardian.c
o.uk/world/video/2011
/mar/14/japantsunami-amateurfootage-video
Tsunami
 The 9.0 magnitude
earthquake and
subsequent 40 m
tsunami in Japan in
March 2011, took
nearly 20,000 lives and
threatened many more
with concerns over
radiation from the
damaged Fukushima
Nuclear Power Plant.
The Fukushima
evacuation zone
continues to displace
thousands of residents
of the region.
Copyright © Houghton Mifflin Company. All rights reserved.
Generation of a Tsunami from EQ
Location of the Earthquake
 Focus
 The location in the Earth’s crust where the earthquake
occurs.
 It can occur anywhere between the surface and a depth
of 700 km. The closer to the surface it occurs, the more
powerful the earthquake is.
 Epicenter
 The location on the Earth’s surface above the focus.
Finding the Epicenter
 Epicenters are found using triangulation from three or
more seismograph stations.
 P-waves travel a lot faster than s-waves. The difference
between the arrival time of each type of wave tells you
the distance to the epicenter.
 The speed of each wave is a constant factor.
Steps for finding the Epicenter
1.
Determine the difference in time between the
arrival of the s & p waves.
Steps for finding the Epicenter
Use the travel-time graph at
which the p-curve and scurve have the same
separation.
2. Record this distance.
1.
Steps for finding the Epicenter
1.
Repeat the steps #1-3 for two other stations
Steps for finding the Epicenter
Use a compass to draw a circle around each station
with the radius in step #3.
2. The epicenter is located where all three circles
intersect.
1.
Locating Earthquakes
•
•
•
P- and S-waves start out from the focus
of an earthquake at same time
P-wave gets farther and farther ahead of
the S-wave with distance and time from
the earthquake
Travel-time curve can be used to
determine the distance to the focus
based on the time gap between first Pand S-wave arrivals
– Plotting distances from 3 stations on
a map, as circles with radii equaling
the distance from the quake, will
show the location of the epicenter
TRIANGULATION OF DATA
Fig 10.10
Earthquake Location Activity
 Class Activity:
 http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/
swf_earthquake_triangulation/p_activity_eqtriangulat
ion.html
 Class lab Activity
 http://www.sciencecourseware.org/eec/earthquake/
Homework
 P. 499 #1-4
 p. 504 #2, 4
 P. 510 #1-4