Chapter 10: Earthquakes & The Earth’s Interior

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Transcript Chapter 10: Earthquakes & The Earth’s Interior

Chapter 10: Earthquakes &
The Earth’s Interior
 PowerPoint
Presentation
 Stan Hatfield . SW Illinois College
 Ken Pinzke . SW Illinois College
 Charles Henderson . University of
Calgary
 Tark Hamilton . Camosun College
Copyright (c) 2005 Pearson Education
Canada, Inc.
1
Aug 17, 1999 7.4 Earthquake Izmit Turkey,
>10,000 people died
1988 5.9 Saguenay Earthquake
What is an Earthquake?

An earthquake is the vibration of Earth
produced by the rapid release of energy
 Energy
released radiates in all directions from its
source, the focus (hypocentre)
 Energy radiates in the form of seismic waves
 Sensitive instruments around the world record the
event
 The point on the earth’s surface above the focus is
called the epicentre
What is an Earthquake?
 Earthquakes
 Movements
and faults
that produce earthquakes are
usually associated with large faults in Earth’s
crust
 Most of the motion along faults can be
explained by plate tectonics theory
 Which came first, the big quake or the big
fault?
 What other cause of quakes might there be?
Damage from 1906 San Francisco Quake
Damage from 1906 San Francisco Quake & Fire
What is an Earthquake?

Elastic rebound
 Mechanism
for earthquakes was first
explained by H.F. Reid (early 1900s)
 Rocks
on both sides of an existing fault are
deformed by tectonic forces
 Rocks bend and store elastic energy
 Frictional resistance holding the rocks together
is overcome
 During a quake the stored potential energy is
released as elastic wave vibration
 The strain is released as the fault moves and the
cycle repeats.
Reid’s Elastic Strain
& Rebound Cycle
for a Strike-Slip
Fault like the San
Andreas or the
Queen Charlotte
What is an Earthquake?
 Foreshocks
and aftershocks
Adjustments
that follow a major
earthquake often generate smaller
earthquakes called aftershocks
Small earthquakes, called foreshocks,
often precede a major earthquake by
days or even by as much as several
years
Due to Strain on smaller or adjacent
faults
A Seismograph
is a Mechanical
Amplifier
This one
indicates the
direction of first
motions.
Horizontal Motion
Seismograph
P, L waves
Or
Components of
Motion.
Vertical
Motion
Seismograph
P, S &
Rayleigh
waves
Seismograms & Seismology

2 Types of seismic waves
 Body waves and Surface Waves
Body waves (depend more on rigidity than density)
 Short period, small amplitude
 Travel through Earth’s interior refracting with depth
 Two types based on mode of travel




Primary (P) waves
 Push-pull (compress and expand) motion, changing the volume
of material
 Travel through solids, liquids, and gases
 Arrive first due to fastest mode of travel
Secondary (SV) waves
 Shear motion at right angles to their direction of travel
 Slightly greater amplitude than P waves
 Travel only through solids
Generally, in any solid material, P waves travel about 1.7
times faster than S waves
Seismograms & Seismology

2 Types of seismic waves
 Body waves and Surface Waves
Surface Waves: Long Period, Large Amplitude, Most
damaging
 2 types based on mode of particle motion
 Travel along interfaces, especially the Earth’s surface
 Formed by P & S conversion at Surface
 Dispersive, speed depends on frequency or period
 Two types based on mode of travel

Rayleigh waves (R) waves (slowest ~90% of Shear wave 1 to 5 km/s)
 Retrograde elliptical motion, dampened with depth
 Travel over solids at ~10X sound in air, ~3 km/s
Love waves (SH) waves (2 to 6 km per second)
 shear motion, dampened with depth
 Travel through solids

Elastic
deformation,
distortion &
particle motion
for Body (P, S)
and
Surface (Love &
Rayleigh) waves
Seismogram (Vertical motion vs Time)
Seismic Travel
Time Curves:
Arrival time versus
Distance
VP & VS depend on
rock properties
VL depends on
period
Seismic Wave Velocities

P-waves:
The velocity of P-waves can be calculated from
elastic constants of material through which the
wave is traveling - one formula is:



Vp = (Κ + 4/3 μ) / ρ
where K is the bulk modulus (rigidity, change in
volume), μ is the shear modulus (change in one
linear direction) & ρ is the density
Rocks get stiffer with depth more than they get
denser so Vp increases & the fastest ray path is
curved
Seismic Wave Velocities

S-waves:
The velocity of S-waves can be calculated from
elastic constants of material through which the
wave is traveling - one formula is:




Vs = μ / ρ
where μ is the shear modulus & ρ is the density
Rocks’ shear strength increases more with depth
than density so Vs increases & the fastest ray
path is curved
The shear modulus is Zero in liquids so Vs = 0
Seismic Wave Velocities

Why are P-waves always faster than S-waves?

Vp / Vs = {(Κ + 4/3 μ) / ρ} / {μ / ρ} , so:

Vp / Vs = Κ/ μ + 4/3

For most rocks the ratio of bulk modulus to shear
modulus is about 0.35 to 0.4 so:
Vp / Vs ≈ 1.7

An
Earthquake
Epicentre
is Located
from 3
or more
Seismic
Stations:
(P-S) gives
Distance to
Source
Locating the Source of an
Earthquake
A circle
with a radius equal to the
distance to the epicentre is drawn
around each station on a globe of Earth
The point where all three circles
intersect is the earthquake epicentre
Since most earthquakes are shallow the
circles usually intersect at a point
More seismic stations permit better
solutions
Locating the Source of an Earthquake

Earthquake belts
 About
95 percent of the energy released by
earthquakes originates in a few relatively narrow
zones that wind around the globe
 Major earthquake zones include the Circum-Pacific
belt, Mediterranean Sea region to the Himalayan
complex, and the oceanic ridge system
 Subduction zones pierce the entire thickness of the
lithosphere and generate quakes up to about 9.5
 Transform faults cut the lithosphere vertically and
generate quakes up to about 8.5
 Ridge systems are warm so the brittle depth is less
and they generate quakes up to about 7
Major Earthquake Belts
Cold Continental Crust can generate large
quakes too!
3 Zones of
Earthquake
Depths (km):
Shallow <70
80< Inter<300
300<Deep<660
Which pose the
greatest risk?
Why none
deeper?
Measuring the Size of an Earthquake


Magnitude scales
 Richter magnitude - concept introduced by Charles
Richter in 1935
 Richter scale
 Based on the amplitude of the largest seismic
wave recorded on a seismogram
 Accounts for the decrease in wave amplitude
with increased distance
Intensity scales
 Modified Mercalli Intensity Scale was developed
using California buildings as its standard
 The drawback of intensity scales is that destruction
may not be a true measure of the earthquakes’
actual severity
The Modified
Mercalli Intensity
Scale:
Turning anecdotal
observations into an
intensity map.
Standardizing Richter Magnitude
(S-P) versus Amplitude
Equals
Richter Magnitude
Measuring the Size of an Earthquake

Moment Magnitude Scale
 Moment
magnitude was developed because
none of the “Richter-like” magnitude scales
adequately estimates the size of very largest
earthquakes
 Derived from the amount of displacement
that occurs along a fault zone (displacement
X surface area)
 Can be determined from offsets and the
length of the aftershock zone
Mr = 9, 1964
Alaska Quake
Relocated
Valdez &
Douched Port
Alberni
US Death Toll
~131
Damage from an Earthquake
1985 Mexico Earthquake, 8.1





10,000 people killed, $4 billion USD damage
412 buildings collapsed, 312 seriously damaged
Severe damage in Mexico City ~400 km from
epicentre
1 strike
3 spares
Why did some buildings
collapse here?
Why did this
building
buckle here?
Earthquake Destruction
Earthquakes don’t kill people, buildings do!
 Destruction from seismic vibrations –
depends on:

 Intensity
and duration of the vibrations
 Nature of the material upon which the
structure rests
 Design of the structure
 All of these factors can amplify surface wave
motion (like a kid on a swing)!
Sand or Mud
Volcanoes above a
fracture are formed
by liquefaction of
saturated beds
during a quake.
1989 Loma Prieta
California
Ground motion depends on substrates!
Can Earthquakes be Predicted?

Short-range predictions
 Research
has concentrated on monitoring possible
precursors – phenomena that precede an
earthquake such as measuring uplift, subsidence,
strain in the rocks, well level or pressure changes,
deformation arrays (Laser ranging, GPS)

Long-range forecasts
 Give
the probability of a certain magnitude
earthquake occurring on a time scale of 30 to 100
years, or more based on strain build up and
possible recurrence intervals
Can Earthquakes be Predicted?

Long-range forecasts
 Based
on the premise that earthquakes are
repetitive or cyclic
Using historic records or paleoseismology to show quiet
zones or seismic gaps
 Gaps could indicate strain building or mapping errors

 Are
important because they provide information
used to
Develop construction standards for buildings, dams,
bridges, pipelines, mines etc.
 Assist in land-use planning
 National Building code is revised every 5 years
 Buildings over 4 stories require earthquake engineering

Can Earthquakes be Predicted?
Can Earthquakes be Predicted?
No one knew about this fault prior to 1994!
Can Earthquakes be Predicted?
Activity along the Queen Charlotte Fault &
Aleutian Trench 1930-1979: Seismic Gaps
Quake Probabilities
From California
Recurrences:
Where would you want to
live if you were a
renovation contractor?
Faults of the
Turkish
Microplate
The squeeze
play
between
Africa &
Eurasia
Can Earthquakes be Predicted?
Activity along the North Anatolian Fault
1939-1999: Westerly Moving Seismicity
Hey Meester, you want to buy time share in Istanbul?
20 years
worth of
earthquakes
in Canada
1980 to 2000
See NRCan
Website
Western Canada’s Plate Boundaries
& Significant Historic Earthquakes
2009
2001
Can Earthquakes be Predicted?
Epicentres and seismic
gap near the Queen
Charlotte Islands. The
seismic gap is the most
likely location for future
earthquakes.
Tues Nov 17, 2009
M6.5, 7:30 AM
Felt to Burns Lake!
Damage to a school
in Courtenay, B.C.
June 23, 1946 Vancouver Island 7.3 Quake
1 death, roads, docks & chimneys collapsed
Tsunamis

Tsunami (“Harbour Wave” in Japanese)
 In the open ocean, height is usually <1 metre
 In shallower coastal waters the water piles up to
heights that can exceed 30 metres
 Displaces the entire water column & can cause
major damage to low lying coastal areas
 Results from vertical displacement along a fault
which moves the ocean floor or a
 Large submarine landslide or a large
 Submarine volcanic eruption
People were unprepared
as a tsunami wave hit a
beach on Thailand (A)
Dec 26, 2004
resulting in extensive
damage (B) from an
earthquake off the coast
of Sumatra (C)
Banda Aceh before 12/26/04 Tsunami
Banda Aceh during Tsunami withdrawal
Banda Aceh before 12/26/04 Tsunami
Banda Aceh after 12/26/04 Tsunami
Tsunamis: gravity displacement waves
Burin Peninsula NF after 1929 Grand Banks
Earthquake & Tsunami
Port Alberni after 1964 Good Friday Tsunami
Tsunami warning
systems are now
being built for
Indian & Atlantic
Probing Earth’s Interior: The MOHO
Discovery of the
Moho using P wave
travel to three seismic
stations. After
Andrija Mohorovicic
1909
P-waves arrive
earlier at more
distant stations due
to velocity increase
with depth &
refraction.
Probing Earth’s Interior



Most of our knowledge of Earth’s interior comes from
the study of earthquake waves
Most of this happened post WWII due to seismic
monitoring during the Cold War
Nuclear blasts are pure compressional and have no S
waves!
 Travel times of P and S waves through the Earth vary
depending on the properties of the materials
 Variations in the travel times correspond to changes in
the materials encountered
 Analyzing seismograms created tomography of Earth
 Seismic Tomography was co-opted for Ultrasound
imaging, CAT Scans and MRI’s (Serendipity: Fund all
research and good ideas spread like wildfire!)
Wave fronts are spheres in a uniform solid.
Rays are wave front normals.
Probing Earth’s Interior

The nature of seismic waves

P waves can travel through solids and liquids
S
waves cannot travel through liquids
 In all materials, P waves travel faster than S
waves
 Body waves increase in velocity with depth and
pressure as rocks get stiffer faster than they get
denser
 When seismic waves pass from one material to
another, the path of the wave is refracted (bent)
 Mineral/rock boundaries, different layers are
discontinuities
Where
velocity
increases
versus
depth, ray
paths curve
A few
possible ray
paths in a
spherically
layered
Earth.
(Seismic
arrival
times map
velocity
variations.)
Probing Earth’s Interior

The core-mantle boundary
 Discovered
in 1914 by Beno Gutenberg
 Based on the observation that P waves die out at
105 degrees (angular distance) from the earthquake
and reappear at about 140 degrees - this 35 degree
wide belt is called the P-wave shadow zone
Probing Earth’s Interior
The P-wave shadow zone.
Discovering Earth’s Major Boundaries
The P-wave shadow zone.
Probing Earth’s Interior

Discovery of the inner core
 Predicted by Inge Lehmann in 1936
 P waves passing through the inner core from an
atomic blast in the 1960’s show increased
velocity suggesting that the inner core is solid
C’est Tout!