Download Seismic Waves

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How Rock Bends, Buckles, and Breaks (Chapter 9)
龔源成
2
3/5
Global Tectonics (Chapter 2)
龔源成
3
3/12
Earthquakes and the Earth's Interior (Chapter 10)
龔源成
4
3/19
The Changing Face of the Land (Chapter 12)
賈儀平
5
3/26
不上課
6
4/2
Groundwater (Chapter 15)
賈儀平
7
4/9
Atmosphere, Winds and Deserts (Chapter 17)
楊燦堯
8
4/16
期中考 (試卷彙整：龔源成)
9
4/23
野外地質準備
魏國彥
10
4/30
Geologic Time and the Rock Record (Chapter 11)
魏國彥
11
5/7
古生物地層野外成果報告
魏國彥
12
5/14
Glaciers and Glaciation (Chapter 16)
魏國彥
13
5/21
不上課
14
5/28
Climate and Our Changing Planet (Chapter 19)
魏國彥
15
6/4
火成岩野外成果報告
楊燦堯
16
6/11
Earth Through Geologic Time (Chapter 20)
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18
6/25
期末考 (試卷彙整：魏國彥)
Field Trip/Discussions
96年4月28日--古生物地層野外地質（苗栗磚場剖面）--魏國彥老師
96年5月26日--火成岩野外地質（觀音山）--楊燦堯老師
教師
More on mantle plumes



Plume distorted by mantle
convection? Rising speed of
plume ?
Diameter of plume conduit
and plume head ?
Plume hypothesis – a vague
theory ?
Chapter 10: Earthquakes and
Earth’s Interior
Seismology

Seismology is the study of the generation,
propagation, and recording of elastic waves in the
Earth (and other celestial bodies) and of the sources
that produce them.
How Earthquakes Are Studied (1)
 Seismometers are used to record the shocks and
vibrations caused by earthquakes.
 All
seismometers make use of inertia (慣性), which is
the resistance of a stationary mass to sudden
movement.

This is the principal used in inertial seismometers.
Figure 10.1
Figure 10.2
How Earthquakes Are Studied (2)
 Three inertial seismometers are commonly used
in one instrument housing to measure up-down,
east-west, north-south motions simultaneously.
Earthquake Focus And Epicenter
 The earthquake focus (or hypocenter) (震源) is the point
where earthquake starts to release the elastic strain of
surrounding rock.
 The epicenter (震央) is the point on Earth’s surface that
lies vertically above the focus of an earthquake.
Figure 10.3
Rupture Front
The rupture front is the instantaneous
boundary between the slipping and locked
parts of a fault during an earthquake.
Rupture velocity
The speed at which a rupture front moves
across the surface of the fault during an
earthquake. (~ 0.8 Vs)
Vel ~ 3 km/s
Finite Source Modeling of 1999
Taiwan Chi-Chi Earthquake and its
Tectonic Implications
Wu-Cheng Chi, Douglas Dreger, and
Anastasia Kaverina
Source rupture process of the 2003 Tokachi-oki earthquake (Yagi, 2004, EPS)
The First Seismogram from a Distant
Earthquake
Seismology (地震學) is a fairly young
science; recordings of earth motion
(seismograms) have only been made for
about 100 years. Shown is what is widely
considered to be the first remote
(teleseismic) seismogram, made on April
17, 1889, in Postdam, Germany by E.
von Rebeur-Pacshwitz (Nature, 1889).
The earthquake was in Japan and had a
magnitude of about 5.8.
Source: http://www.iris.edu
Some facts about seismology

Seismic tomography/Inverse theory

The recorded motions can be viewed as the output response of a sequence of
linear filters with properties we wish to determine.
Source mechanism
Earth structure
instrument
Some facts about seismology

The history of seismological advances is one of the
alternating progress in describing source properties
or in improving models of Earth structure.
Seismic
Source
mechanism
Earth
Structure
Some facts about seismology

Earthquake faulting and its role in global plate
tectonics.

Kinematic and dynamic characteristics of shear faulting
sources, their scales of variation, and measures of energy
release such as seismic magnitudes and earthquake
moment.
Seismic Waves (1)
 Vibrational waves spread outward initially from the
focus of an earthquake, and continue to radiate from
elsewhere on the fault as rupture proceeds.
Seismic Waves (2)
 There are two basic families of seismic waves.
Body waves (體波) can transmit either:
 Compressional motion (P waves), or
 Shear motion (S waves).
 Surface waves (表面波) are vibrations that are trapped
near Earth’s surface. There are two types of surface
waves:
 Love waves, or
 Rayleigh waves.

Body Waves (1)
 Body waves travel outward in all directions from
their point of origin.
 The first kind of body waves, a compressional
wave, deforms rocks largely by change of
volume and consists of alternating pulses of
contraction and expansion acting in the direction
of wave travel.
P-wave: Longitudinal wave (縱波)
Compressional waves are the first waves to be recorded by
a seismometer, so they are called P (for “primary”) waves.
 Dan Russel
Body Waves (2)
 The second kind of body waves is a shear wave (剪
力波).
 Shear
waves deform materials by change of shape,
 Because shear waves are slower than P waves and reach
a seismometer some time after P waves arrives, they are
called S (for “secondary”) waves.
S-wave: Transverse wave (橫波)
Figure 10.4
SV motions on the vertical plane parallel to the propagation direction
 Dan Russel
Body Waves (3)
 Compressional (P) waves can pass through solids,
liquid, or gases.
 P waves move more rapidly than other seismic
waves:


6 km/s is typical for the crust.
8 km/s is typical for the uppermost mantle.
Body Waves (4)

Shear waves can travel only within solid matter.

The speed of a S wave is 1/ 3 times that of a P wave.
A typical speed for a S wave in the crust is 3.5 km/s,
5 km/s in the uppermost mantle.
Seismic body waves, like light waves and sound waves, can be
reflected and refracted by change in material properties.
Figure 10.5
Body Waves (5)
 For seismic waves within Earth, the changes in
wave speed and wave direction can be either
gradual or abrupt, depending on changes in
chemical composition, pressure, and mineralogy.
Body Waves (6)
 If Earth had a homogeneous composition and
mineralogy, rock density and wave speed would
increase steadily with depth as a result of increasing
pressure (gradual refraction).

Measurements reveal that the seismic waves are refracted
and reflected by several abrupt changes in wave speed.
c : reflection at the core-mantle
boundary
K: P wave transimission in the
outer core
i: P wave reflection at the innerouter core boundary
I: P wave transmission in the
inner core
J:S wave transmission in the
inner core
Figure 10.6
PKIKP
P
K
I
K
P
P
K
PKJKP
J
K
P
Surface Waves (1)

Surface waves travel along the surface of the ground
or just below it, and are slower than body waves, but
are often the largest vibrational signals in a
seismogram. The two most important are Rayleigh
(雷利波) and Love (洛夫/勞夫/樂夫 … 波) waves,
named after British scientists, Lord Rayleigh and
A.E.H. Love who discovered them.
Surface Waves (2)

Rayleigh waves combine shear and compressional
vibration types, and involve motion in both the vertical
and horizontal directions. The velocity of Rayleigh
waves is about 0.92 times that of S waves..
Surface Waves (3)
Love waves consist entirely of shear wave vibrations in the
horizontal plane, analogous to an S wave that travels horizontally,
so they only appear in the horizontal component of a seismogram.
The velocity of Love waves is approximately equal to that of S
waves, so they arrive earlier than Rayleigh wave.
Rayleigh wave retrograde (counter-clockwise) motions
P > S > Rayleigh wave velocity
Surface Waves (2)

The longer the wave length of a
surface wave, the deeper the
wave motion penetrates Earth.

Surface waves of different wave
lengths develop different
velocities. This behavior is called
Dispersion (色散).
Figure 10.7
Determining The Epicenter (1)

An earthquake’s epicenter can be calculated from
the arrival times of the P and S waves at a
seismometer. (P-S differential travel time)

The further a seismometer is away from an epicenter, the
greatest the time difference between the arrival of the P
and S waves.
Determining The Epicenter (2)

The epicenter can be determined when data from
three or more seismometers are available.

It lies where the circles intersect (radius = calculated
distance to the epicenter).
Figure 10.8
Figure 10.9
Earthquake Magnitude (1)
 In 1931 Kiyoo Wadati constructed a chart of
maximum ground motion v.s. distance for a
number of earthquakes
 In 1935 C. Richter constructed a similar diagram of
peak ground motion versus distance and used it to
create the first earthquake magnitude scale. Richter magnitude (芮氏地震規模)
Earthquake Magnitude (2)
What do you observe from the figure ?
Earthquake Magnitude (3)
In 1935 C. Richter constructed a similar diagram of peak
ground motion versus distance and used it to create the first
earthquake magnitude scale.
Earthquake Magnitude (4)
Richard Magnitude
Assumptions
1. Given the same source-receiver geometry and two earthquake of
different size, the “larger” event will “on average” generate larger
amplitude arrivals.
2. The amplitudes of arrivals behave in a “predictable” fashion. i.e.
the effects of propagation are known in a statistical fashion.
Earthquake Magnitude (5)
ML=log(A) + f(Δ,h) + Cs + Cr
A – Ground displacement of the measured reference phase
f – a correction for epicentral distance
Cs – correction for the sitting of a station. (site effects)
Cr – correction for the source region.
Multiple stations are used to reduce the amplitude biases caused by
radiation pattern, directivity, and anomalous path properties.
Earthquake Magnitude (6)

Later seismologists devised more general
magnitude estimate based on either on P wave (T~1
s), called mb, surface wave trapped in the crust
(T~20 s), or surface trapped in the upper mantle
(T~200 s), called Ms.
Seismic waves in frequency domain
Estimates of Stress Drop of
the Chi-Chi, Taiwan,
Earthquake of 20 September
1999 from Near-Field
Seismograms Ruey-Der
Hwang et al. BSSA, 2000.
Magnitude
saturation
 A large-sized earthquake occurs over a larger fault, requires more
time to rupture.
 Measures of earthquake size based on the maximum ground shaking do
not account for an important characteristic of large earthquakes - they
shake the ground longer.
Earthquake Magnitude (7)

In 1977, Hiroo Kanamori at Caltech proposed a relation between
Earthquake magnitude M and seismic moment M0. The seismic
moment is expressed as
M0= μAD
μ (unit: newtons/m2) is shear stiffness of rock surrounding the fault.
A (unit: m2) is the area of the fault
D (unit: m) is the average slip during the earthquake.
Kanamori’s relation between moment and magnitude is
M = (2/3) log10 M0 - 6.0
(M: joules [nt-m])
Earthquake Magnitude (8)

Most crustal rocks have shear stiffness μ=3x1010 nt/m2. If an
earthquake slips 3 km on a vertical fault 50 km long that
extends from the surface to 15 km depth, what is the magnitude
of this earthquake?
M0 =(3x1010 nt/m2)(50,000m)(15,000m)(3m)=6.75x1019 joules
M=(2/3)log10 M0 - 6.0=7.2
Earthquake Magnitude (9)
Each step in the magnitude scale represents approximately a thirty
fold (101.5=31.6) increase in seismic moment (or energy).
M  (2 / 3)log10 M 0  6.0
 M  6.0  (3/ 2)  log10 M 0
 M 0  10 M  6.0(3/ 2)  109.0 1.5 M 
For M=2 M 0  109.0 1.5 M   1012 
1.5
10
~ 32

 9.0 1.5 M 
For M=3 M 0  10
 1013.5 
Magnitude Ground Motion Change
Change
(Displacement)
Energy
Change
1.0
10.0 times
about 32 times
0.5
0.3
0.1
3.2 times
2.0 times
1.3 times
about 5.5 times
about 3 times
about 1.4 times
Earthquake Magnitude (10)

The nuclear bomb dropped in 1945 on the Japanese
city of Hiroshima was equal to an earthquake of
magnitude M = 5.3.

The most destructive man-made devices are small in
comparison with the large earthquakes.
Location
Date
1. Chile
1960 05 22
2. Prince William Sound, Alaska
1964 03 28
3. Andreanof Islands, Alaska
1957 03 09
4. Off the West Coast of Northern Sumatra
2004 12 26
5. Kamchatka
1952 11 04
6. Off the Coast of Ecuador
1906 01 31
7. Northern Sumatra, Indonesia
2005 03 28
8. Rat Islands, Alaska
1965 02 04
9. Assam - Tibet
1950 08 15
10. Ningxia-Gansu, China
1920 12 16
Mw
9.5
9.2
9.1
9.3
9.0
9.0
8.8
8.7
8.7
8.6
8.6
Earthquake Frequency
Magnitude
8 and higher
7 - 7.9
6 - 6.9
Average Annually
1
17
134
5 - 5.9
1319
4 - 4.9
13,000
3 - 3.9
130,000
2 - 2.9
1,300,000
http://neic.usgs.gov/neis/eqlists/eqstats.html
E=$?
一度電是1000W*1hr=3600000J，台電非夏令時間一度電是收2元。
所以廣島原子彈的能量透過台電我們可以賺新台幣約為3.5*107元(3千5百
萬元)，以1:35換算成美金大約是1百萬。
集集地震則約是1.2*1010新台幣(1百20億)，約為3.6*108美金(3億6千萬)。
美國富比士雜誌2005年12月9日做出好萊塢最貴的電影統計
1.埃及豔后(Cleopatra) 2億8640萬美元
2.鐵達尼(Titanic) 2億4700萬美元
3.水世界(Waterworld) 2億2900萬美元
4.魔鬼終結者3(Terminator 3: Rise of the Machines) 2億1600萬美元
5.蜘蛛人2(Spider-Man 2) 2億1000萬美元
Take a break
Earthquake Hazard
A seismic risk map based on maxium horizontal acceleration during
an earthquake. [gravity =9.8m/s2 ]
Earthquake Hazard in Taiwan
Peak Ground Acceleration (m/s/s) with 10% probability of
exceedance in 50 years.
from USGS
1990-1999 Earthquakes with M >5
Wu and Chen, 2006
Earthquake Disasters

Nations with urban areas that are known to be
earthquake-prone have special building codes that
require structures to resist earthquake damage.

The most disastrous earthquake on record occurred
in 1556, in Shaanxi province, China, where in
estimated 830,000 people died.
Earthquake Damage (1)


Earthquakes have six kinds of destructive effects.
Primary effects:


Ground motion results from the movement of seismic
waves.
The Fault may break the ground surface itself.
Earthquake Damage (2)

Secondary effects:

Ground movement displaces stoves, breaks gas lines, and
loosens electrical wires, thereby starting fires .

In regions of steep slopes, earthquake vibrations may cause
regolith (表土) to slip and cliffs to collapse.

The sudden shaking and disturbance of water-saturated
sediment and regolith can turn solid ground to a liquid
mass similar to quicksand (流沙) (liquefaction, 液化)

Earthquakes generate seismic sea waves, called tsunami.
Modified Mercalli Scale (修正麥卡利震
度階級 ﹐簡稱為MM震度階級)

This scale is based on the amount of vibration people feel
during low-magnitude quakes, and the extent of building
damage during high-magnitude quakes.

There are 12 degrees of intensity in the modified Mercalli
scale.
震度（intensity)
震度（intensity），是表示地震時地面上的人所感受到振動的
激烈程度，或物體因受振動所遭受的破壞程度。
現今地震儀器已能詳細描述地震的加速度，所以震度亦可由
加速度值來劃分。震度級以正的整數表示之（見交通部中央
氣象局地震震度分級表）。
「交通部中央氣象局地震震度分級表」
(89年8月1日公告修訂)
震度分級
地動加
速度
人的感受
屋內情形
屋外情形
0
無感
0.8gal以下
人無感覺。
1
微震
0.8~2.5gal
人靜止時可感覺微小搖晃。
靜止的汽車輕輕搖
電燈等懸掛物有小搖晃。 晃，類似卡車經過，
但歷時很短。
2
輕震
2.5~8.0gal
大多數的人可感到搖晃，睡眠
中的人有部分會醒來。
3
弱震
8~25gal
幾乎所有的人都感覺搖晃，有
的人會有恐懼感。
房屋震動，碗盤門窗發
出聲音，懸掛物搖擺。
25~80gal
有相當程度的恐懼感，部分的
人會尋求躲避的地方，睡眠中
的人幾乎都會驚醒。
汽車駕駛人略微有
房屋搖動甚烈，底座不
感，電線明顯搖晃，
穩物品傾倒，較重傢俱
步行中的人也感到
移動，可能有輕微災害。
搖晃。
大多數人會感到驚嚇恐慌。
部分牆壁產生裂痕，重
傢俱可能翻倒。
250~400gal
搖晃劇烈以致站立困難。
汽車駕駛人開車困
部分建築物受損，重傢
難，出現噴沙噴泥
俱翻倒，門窗扭曲變形。
現象。
400gal以上
部分建築物受損嚴重或
搖晃劇烈以致無法依意志行動。 倒塌，幾乎所有傢俱都
大幅移位或摔落地面。
4
5
6
7
中震
強震
烈震
劇震
80~250gal
註：1gal = 1cm/sec
靜止的汽車明顯搖
動，電線略有搖晃。
汽車駕駛人明顯感
覺地震，有些牌坊
煙囪傾倒。
山崩地裂，鐵軌彎
曲，地下管線破壞。
Depth of Earthquake Foci

Most foci are no deeper than 100 km. down in the Benioff
zone, that extends from the surface to as deep as 700 km.

No earthquakes have been detected at depths below 700 km.
Two hypotheses may explain this.


Sinking lithosphere warms sufficiently to become entirely ductile at
700 km depth.
The slab undergoes a mineral phase change near 670 km depth and
loses its tendency to fracture.
Figure 10.15
World seismicity:
Circum-Pacific belt ~ 80%
Mediterranean-Himalayan belt ~ 15%
Figure 10.16
First-Motion Studies of the Earthquake
Source

If the first motion of the arriving P wave pushes the
seismometer upward, then fault motion at the
earthquake focus is toward the seismometer.

If the first motion of the P wave is downward, the
fault motion must be away from the seismometer.
Figure 10.18. Focal mechanism of
earthquakes. Black quadrants indicate
compressional first motion, while white
quadrants tensional first motion.
From “An introduction to seismology, earthquakes and
earth structure” (by Seth Stein & Michael Wysession)
http://epscx.wustl.edu/seismology/book/
Earthquake Forecasting And Prediction
(1)

Earthquake forecasting is based largely on elastic
rebound theory and plate tectonics.

Currently, seismologists use plate tectonic motions
and Global positioning System (GPS)
measurements to monitor the accumulation of strain
in rocks near active faults.
Elastic rebound theory
Earthquake Forecasting And Prediction
(2)


Earthquake prediction has had few successes.
Earthquake precursors:




Suspicious animal behavior.
Unusual electrical signals.
Many large earthquakes are preceded by small earthquakes
called foreshocks
Not all the large earthquakes are preceded by strong
foreshocks. In 1976, a stronger earthquake struck
Tangshan without warning and killed 240,000 people.
Earthquake Prediction from Parkfield
Experiment in California

Moderate-size earthquakes of M~6 have
occurred on the Parkfield section of the San
Andreas Fault at regular intervals of 22 years in 1857, 1881, 1901, 1922, 1934, and 1966.

 the next quake would have been due before
1993 (1988 in textbook).

However, the predicted earthquake didn’t
occur until Sept. 28, 2004, over a
decade later than predicted.
http://quake.usgs.gov/research/parkfield/index.html
Improved Theory for Earthquake
Prediction (1) – Earthquake Triggering

Fault interaction

After a fault slips during an earthquake, the stresses on all neighboring
faults are affected.

Every large earthquake is followed by numerous aftershocks, which are
smaller earthquakes that occur in response to the sudden release of strain
in surrounding rock.

aftershocks concentrate in area of rock where the calculated stresses
increased. Moreover, some cases showed that the next large earthquake
occurs, sometimes decades later, in the region where the last earthquake
has increased the local stress.
Figure 10.21 Aftershocks
induced by earthquake stress.
A case where aftershocks
concentrate on areas with
stress increase induced by a
large earthquake. Red and
yellow indicate areas where the
calculated stress increased
slightly after a main shock.
Changes in stress are small, up
to three bars, comparable to a
pressure change of 3
atomspheres.
Using Seismic Waves As Earth Probes
Medical Tomography
CT (Computerized tomography) Scan
Seismic
Tomography
data
Using Seismic Waves As Earth Probes

Early in the twentieth century, the boundary
between Earth’s crust and mantle was demonstrated
by a Croatian scientist named Mohorovicic -- the
Mohorovicic discontinuity
Using Seismic Waves As Earth Probes

The thickness and composition of continental crust
vary greatly from place to place.


Thickness ranges from 20 to nearly 70 km and tends to be
thickest beneath major continental collision zones, such
as Tibet.
P-wave speeds in the crust range between 6 and 7
km/s. Beneath the Moho, speeds are greater than 8
km/s.
This map shows the superimposition of the topography of the world (warm colours =
high topography) with isopach lines depicting the thickness of the continental
crust. Regions with a thick continental crust such as Tibet and the South American
Andes correspond to regions with elevated topography.
http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-1002/HTML.Lect2/sld005.htm
Using Seismic Waves As Earth Probes

Laboratory tests show that rocks common in the crust, such
as granite, gabbro (輝長岩), and basalt, all have P-wave
speeds of 6 to 7 km/s.

Rocks that are rich in dense minerals, such as olivine,
pyroxene, and garnet, have speeds greater than 8 km/s.

Therefore, the most common such rock, called peridotite (橄欖岩),
must be among the principal materials of the mantle.
Figure 10.25
Stratified Earth
from Garnero
Figure 10.26
(1909) A. Mohorovicic (1857-1936)
(1906-1914) B. Gutenberg (1889-1960)
(1936) I. Lehmann (1888-1993)
The 410-km Seismic Discontinuity


From the P-and S-wave curves, velocities of both P
and S waves increase in a small jump at about 410
km.
When olivine is squeezed at a pressure equal to that
at a depth of 410 km, the atoms rearrange
themselves into a denser polymorph (polymorphic
transition).
The 670-km Seismic Discontinuity

An increase in seismic-wave velocities occurs at a
depth of 670 km.

The 670-km discontinuity may correspond to a
polymorphic change affecting all silicate minerals
present.
Images of Subducting slabs（隱沒板塊）
Grand S.P., van der Hilst R.D., and Widiyantoro S., 1997. Global seismic tomography a
snapshot of convection in the Earth, GSA Today.
Images of Subducting slabs（隱沒板塊）
The 2650-km (D”) Seismic Discontinuity
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An increase in seismic-wave velocities occurs at a
depth of 2650 km.
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The D” discontinuity may correspond to a
polymorphic change of perovskite (MgSiO3) to
post-perovskite.
Seismic Waves and Heat

Researchers hypothesize that these “slow’’ regions
are the hot source rocks of most mantle plumes.

Near active volcanoes, seismologists have
interpreted travel-time discrepancies to reconstruct
the location of hot and partially molten rock that
supplies lava for eruptions.
Depth 100 km
Depth 200 km
Figure 10.28
Figure 10.20 Where the earthquakes M>=7 are expected
Subduction zones have the largest quakes.
Taipei Basin
Shyu et al., 2005
Improved Theory for Earthquake
Prediction (2)

Weak fault behavior
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Fault zones are weak surfaces within subsurface rock.
Friction on the fault prevents slip as strain and stress accumulate in
surrounding rock.
In the laboratory, geologists have observed that friction on many rock
surfaces decreases greatly once the surfaces start to slip. This effect,
called velocity-weakening behavior, allows slip to accelerate and to
release all the strain of the rock.
Incorporating this rock behavior into computer simulations of fault slips,
the simulations show that small patches of the fault surface can be
stressed by slip on neighboring patches, sometimes causing large
portions of the fault to slip simultaneously. The simulations match the
behavior of real faults, displaying earthquakes of all sizes at irregular
intervals.
Improved Theory for Earthquake
Prediction (3)

Fluid in faults
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Subsurface faults form a network of pathways for water, CO2, and other
volatiles in the brittle upper crust.
The volatiles come from (1) rainwater, which percolates downward
through surface fractures and porous rocks, (2) mantle outgassing, a
byproduct of magma migration, eruption, and emplacement, and (3)
metamorphic dehydration reactions.
Fluids in the fault will decrease the friction. Many studies suggest that
water well levels have risen or fallen just before earthquakes, some
open faults have gushed water after an earthquake, and small
earthquakes tend to occur near newly filled reservoirs.
Seismologists have hypothesized that many earthquakes deeper than
100 km in subducting slabs are induced by the release of water from
hydrated minerals.
Using Seismic Waves As Earth Probes
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To balance the less dense crust and the mantle, the core must be
composed of material with a density of at least 10 to 13 g/cm3.
The only common substance that comes close to fitting this
requirement is iron.
Iron meteorites are samples of material believed to have come
from the core of ancient, tiny planets, now disintegrated.
All iron meteorites contain a little nickel (Ni); thus, Earth’s core
presumably does too.
P-wave reflections indicate the presence of a solid inner core
enclosed within the molten outer core.
Layers of Different Physical Properties
in the Mantle
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The P-wave velocity at the top of the mantle is
about 8 km/s and it increases to 14 km/s at the coremantle boundary.
The low-velocity zone can be seen as a small jump
in both the P-wave and S-wave velocity curves.
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An integral part of the theory of plate tectonics is the idea
that stiff plates of lithosphere slide over a weaker zone in
the mantle called the asthenosphere.
In the low velocity zone rocks are closer to their melting
point than the rock above or below it.
Earthquakes Influence Geochemical
Cycles
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Earthquakes play an important role in the transport of
volatiles through Earth’s solid interior.
Earthquakes facilitate the concentration of many important
metals into ore deposits.
In the mantle, the carbon and hydrologic cycles are fed
when the subducting slab releases water, CO2, and other
volatiles at roughly 100-km depth beneath the overriding
plate.
Some seismologists speculate that water released from the
slab helps cause brittle fracture in the slab itself, and that
water may be necessary for deep earthquakes to occur in
the Benioff zone.