週次 時間 授課內容 教師

1 2/26 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) 龔源成

18 6/25 期末考 ( 試卷彙整：魏國彥 )

Field Trip/Discussions96 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

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.

Rupture FrontThe rupture front is the instantaneous boundary between the slipping and locked parts of a fault during an earthquake.

Rupture velocityThe speed at which a rupture front moves across the surface of the fault during an

earthquake. (~ 0.8 Vs)

Figure 10.3

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), orShear 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, orRayleigh 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 )

Dan Russel

Compressional waves are the first waves to be recorded by a seismometer, so they are called P (for “primary”) waves.

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

Dan Russel

SV motions on the vertical plane parallel to the propagation direction

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 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.

3/1

Figure 10.5

Seismic body waves, like light waves and sound waves, can be reflected and refracted by change in material properties.

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.

Figure 10.6

c : reflection at the core-mantle boundary

K: P wave transimission in the outer core

i: P wave reflection at the inner-outer core boundary

I: P wave transmission in the inner core

J:S wave transmission in the inner core

PKIKP

PKP

KI

PKJKP

PKP

K

J

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 10.8Figure 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 MagnitudeAssumptions

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.

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.

Seismic waves in frequency domain

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

Each step in the magnitude scale represents approximately a thirty fold (101.5=31.6) increase in seismic moment (or energy).

10 0

10 0

6.0 (3/ 2) 9.0 1.50

9.0 1.5 120 1.5

9.0 1.5 13.50

(2 / 3) log 6.0

6.0 (3/ 2) log

10 10

For M=2 10 10 10 ~ 32

For M=3 10 10

M M

M

M

M M

M M

M

M

M

Earthquake Magnitude (9)

Magnitude Change

Ground Motion Change(Displacement)

Energy Change

1.0 10.0 times about 32 times

0.5 3.2 timesabout 5.5

times

0.3 2.0 times about 3 times

0.1 1.3 timesabout 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 Mw1. Chile 1960 05 22 9.52. Prince William Sound, Alaska 1964 03 28 9.23. Andreanof Islands, Alaska 1957 03 09 9.1

4. Off the West Coast of Northern Sumatra 2004 12 26 9.0

5. Kamchatka 1952 11 04 9.06. Off the Coast of Ecuador 1906 01 31 8.87. Northern Sumatra, Indonesia 2005 03 28 8.78. Rat Islands, Alaska 1965 02 04 8.79. Assam - Tibet 1950 08 15 8.6

10. Ningxia-Gansu, China 1920 12 16 8.6

9.3

Earthquake Frequency

Magnitude Average Annually

8 and higher 1

7 - 7.9 17

6 - 6.9 134

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

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

E = $ ?

Take a break

A seismic risk map based on maxium horizontal acceleration during an earthquake. [gravity =9.8m/s2 ]

Earthquake Hazard

from USGS

Earthquake Hazard in Taiwan

Peak Ground Acceleration (m/s/s) with 10% probability of exceedance in 50 years.

Wu and Chen, 2006

1990-1999 Earthquakes with M >5

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），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（ intensity)

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。 　 　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。 電燈等懸掛物有小搖晃。

靜止的汽車輕輕搖晃，類似卡車經過，但歷時很短。

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。

靜止的汽車明顯搖動，電線略有搖晃。

4 中震 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。

房屋搖動甚烈，底座不穩物品傾倒，較重傢俱移動，可能有輕微災害。

汽車駕駛人略微有感，電線明顯搖晃，步行中的人也感到搖晃。

5 強震 80~250gal 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。

汽車駕駛人明顯感覺地震，有些牌坊煙囪傾倒。

6 烈震 250~400gal 搖晃劇烈以致站立困難。 部分建築物受損，重傢俱翻倒，門窗扭曲變形。

汽車駕駛人開車困難，出現噴沙噴泥現象。

7 劇震 400gal以上 搖晃劇烈以致無法依意志行動。部分建築物受損嚴重或倒塌，幾乎所有傢俱都大幅移位或摔落地面。

山崩地裂，鐵軌彎曲，地下管線破壞。

註： 1gal = 1cm/sec

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

Figure 10.16

World seismicity:

Circum-Pacific belt ~ 80%

Mediterranean-Himalayan belt ~ 15%

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

SeismicTomography

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

(1936) I. Lehmann (1888-1993)

  (1906-1914) B. Gutenberg (1889-1960)

(1909) A. Mohorovicic (1857-1936)

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.

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）

Images of Subducting slabs （隱沒板塊）

The 2650-km (D”) Seismic Discontinuity

An increase in seismic-wave velocities occurs at a depth of 2650 km.

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.

Shyu et al., 2005

Taipei Basin

Improved Theory for Earthquake Prediction (2)

Weak fault behavior 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 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

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

The P-wave velocity at the top of the mantle is about 8 km/s and it increases to 14 km/s at the core-mantle boundary.

The low-velocity zone can be seen as a small jump in both the P-wave and S-wave velocity curves. 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

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.