Chapter 10: Earthquakes and Earth’s Interior

Introduction

When the Earth quakes, the energy stored in elastically strained rocks is suddenly released.

The more energy released, the stronger the quake.

Massive bodies of rock slip along fault surfaces deep underground.

Earthquakes are key indicators of plate motion.

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.

The seismometer measures the electric current needed to make the mass and ground move together.

Figure 10.1

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.

Figure 10.2

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Earthquake Focus And Epicenter

The earthquake focus ( 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.

Fault slippage begins at the focus and spreads across a fault surface in a rupture front.

The rupture front travels at roughly 3 kilometers per second for earthquakes in the crust.

Figure B10.01

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 Potsdam, Germany by E. von Rebeur-Pacshwitz (Nature, 1889). The instrument was a photographically recording horizontal pendulum originally installed for astronomical purposes. The earthquake was in Japan and had a magnitude of about 5.8. Source: http://www.iris.edu

Figure 10.3

Focus

Epicenter

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.

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

P-wave: Longitudinal wave ( 縱波 )Figure 10.4

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

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 (S) waves consist of an alternating series of side-wise movements.

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.

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

When change in material properties results in a change in wave speed, refraction bends the direction of wave travel.

3/1

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.

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 boundaryK: P wave in the outer coreI: P wave in the inner corei: P wave reflection at the inner-outer core boundary

Surface Waves (1)

Surface waves travel along the surface of the ground or just below it, and are slower than than body waves, but are often the largest vibrational signals in a seismogram. The two most important are Rayleigh and Love 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.

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 – particle motions on the vertical plane parallel to the propagation direction

Propagation direction

Propagation direction

Love wave – particle motions on the horizontal plane perpendicular to the propagation direction

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 ).

Chi-Chi Earthquake: a shallow earthquake caused by the collision between the Eurasian and Philippine plate.

Northridge Earthquake: a shallow earthquake occuring along a splay of the San Andreas fault system near San Fernado Valley, just north of Los Angeles.

Izmit Turkey Earthquake: a shallow earthquake occuring along the E-W trend North Anatolian fault. Starting in 1939, the NA fault has produced a sequence of major earthquakes, of which the 1999 event is the 11th with a magnitude M ≥ 6.7.

Bolivia Earthquake: the deepest earthquake of this size (M=8.1, d=670 km) ever recorded, caused by the Cocos plate subducting under the south American continent.

Source: http://www.iris.edu/pub/programs/sel/ibmpc/Seislove and Seiswave

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. The farther 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).

The depth of an earthquake focus below an epicenter can also be determined, using P-S time intervals.

Figure 10.8Figure 10.9

Earthquake Magnitude (1)

In 1935, Charles Richter at Caltech devised a calibration scheme, called Richter scale, for describing the size of an earthquake.

The Richter magnitude ( 芮氏地震規模 ) is based on a logarithmic scaling of energy released by earthquakes and divided into steps called magnitudes with numerical values M from 0 to 10.

A large-sized earthquake occurs over a larger fault, requires more time to rupture, and therefore generate longer-period seismic waves. Thus, the energy released by an earthquake affects both the amplitude X and the oscillation period T of the P wave.

Earthquake Magnitude (2)

Richter’s original magnitude formula depends on the logarithm of the ratio X/T. One unit of Richter magnitude corresponds to a tenfold increase in X/T as measured by a seismometer. A correction factor is added to adjust the different distance between the earthquake and seismometer.

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.

Earthquake Magnitude (3)

Lately, earthquake magnitude has been connected more directly to motions on a fault. In 1977, Hiroo Kanamori at Caltech proposed a relation between Richter magnitude M and seismic moment M0. The moment is directly related to the mechanical energy released by fault motion. The seismic moment is expressed as

M0= μAD

where μ (unit: newtons/m2) is shear stiffness of rock surrounding the fault, A (unit: m2) is the area of the fault, and D (unit: m) is the average slip during the earthquake. Seismic moment has the energy units of newton-m=joules. Kanamori’s relation between moment and magnitude is

M = (2/3) log10 M0 - 6.0

Figure B10.01

Earthquake Magnitude (4)

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 Richter scale, for instance, from magnitude M = 2 to magnitude M = 3, represents approximately a thirty fold (101.5=31.6) increase in seismic moment (or energy).

The largest earthquake in 20th century that occurred in 1960 in the subduction zone beneath Chile has a M=9.5, breaking along an 800-km-long fault. The average fault slip was estimated to be 25 m.

Earthquake Frequency (1)

Each year there are roughly 200 earthquakes worldwide with magnitude M = 6.0 or higher.

Each year on average, there are 20 earthquakes with M = 7.0 or larger.

Each year on average, there is one “great” earthquake with M = 8.0 or larger.

Earthquake Frequency (2)

Four earthquakes in the twentieth century met or exceeded magnitude 9.0. 1952 in Kamchatka (M = 9.0). 1957 in the Aleutian Island (M = 9.1). 1964 in Alaska (M = 9.2). 1960 in Chile (M = 9.5).

Earthquake Frequency (3)

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 largest earthquakes.

Earthquake Hazard

Seismic events are most common along plate boundaries.

Earthquakes associated with hot spot volcanism pose a hazard to Hawaii.

Earthquakes are common in much of the intermontane western United States (Nevada, Utah, and Idaho).

Several large earthquakes jolted central and eastern North America in the nineteenth century (New Madrid, Missouri, 1811 and 1812).

Figure 10.10 A seismic risk map based on maxium horizontal acceleration during an earthquake, expressed as the percentage of the acceleration due to gravity (9.8m/s2).

Earthquake Disasters (1)

In Western nations, urban areas that are known to be earthquake-prone have special building codes that require structures to resist earthquake damage. However, building codes are absent or ignored in many

developing nations.

In the 1976 T’ang Shan earthquake in China, 240,000 people lost their lives.

Earthquake Disasters (2)

Eighteen earthquakes are known to have caused 50,000 or more deaths apiece.

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.

Where a fault breaks the ground surface itself, buildings can be split or roads disrupted.

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 seemingly solid ground to a liquid mass similar to quicksand ( 流沙 ) (liquefaction, 液化 ).

Earthquakes generate seismic sea waves, called tsunami ( 海嘯 ), which have been particularly destructive in the Pacific Ocean.

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 (see Table 10.2)

World Distribution of Earthquakes

Subduction zones have the largest quakes. The circum-Pacific belt, where about 80 percent of

all recorded earthquakes originate, follows the subduction zones of the Pacific Ocean.

The Mediterranean-Himalayan belt is responsible for 15 percent of all earthquakes.

Figure 10.15

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 10.16

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.

S-waves and surface waves also carry the signature of earthquake slip and fault orientation and can provide independent estimates of motion at the earthquake focus.

Figure 10.17

Figure 10.18. Focal mechanism of earthquakes. Black quadrants indicate compressional first motion, while white quadrants tensional first motion.

Earthquake Forecasting And Prediction (1)

Forecasting ( 預報 ) identifies both earthquake-prone areas and man-made structures that are especially vulnerable to damage from shaking.

Earthquake prediction ( 預測 ) refers to attempts to estimate precisely when the next earthquake on a particular fault is likely to occur.

Earthquake Forecasting And Prediction (2)

Earthquake forecasting is based largely on elastic rebound theory and plate tectonics.

The elastic rebound theory ( 彈性回復理論 ) suggests that if fault surfaces do not slip easily past one another, energy will be stored in elastically deformed rock, just as in a steel spring that is compressed. The theory can only explain long-term risk of future earthquakes.

Currently, seismologists use plate tectonic motions and Global positioning System (GPS) measurements to monitor the accumulation of strain in rocks near active faults.

Figure 10.19

Earthquake Forecasting And Prediction (3)

Earthquake prediction has had few successes. Earthquake precursors:

Suspicious animal behavior. Unusual electrical signals (1989 Loma Prieta earthquake). Many large earthquakes are preceded by small earthquakes

called foreshocks (Chinese authorities used an ominous series of foreshocks to anticipate (the Haicheng earthquake M=7.3 in 1975).

Not all the large earthquakes are preceded by strong foreshocks. In 1976, a stronger earthquake struck Tangshan without warning and killed 240,000 people.

Figure 10.20

Figure B02

Red circles represent the places where marshes and swamps suddenly become tidal flats in a recent earthquake catastrophe causing abrupt elevation changes along coastal lines.

megathrust

Earthquake Prediction (4)

One hypothesis for earthquake prediction was based on where earthquakes occur at regular intervals. Elastic rebound theory could explain short-term risk if the following conditions were satisfied:1. The rocks along a fault have a well-defined critical threshold for strain, above which the fault will slip.2. Strain accumulates in the fault zone in a steady manner.

If the strain accumulation rate is known from the motions associated with plate tectonics, one can project when the next earthquake will occur on the fault.

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 fairly regular intervals - in 1857, 1881, 1901, 1922, 1934, and 1966. The first, in 1857, was a foreshock to the great Fort Tejon earthquake which ruptured the fault from Parkfield to the southeast for over 300 km. Available data suggest that all six moderate-sized Parkfield earthquakes may have been "characteristic" in the sense that they all ruptured the same area on the fault. If such characteristic ruptures occur regularly, then the next quake would have been due before 1993. However, the predicted earthquake still has not occurred.

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. Seismologists have found cases where 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. For instance, the focus of the 1994 Northridge earthquake in LA (M=6.8) was located where stress had been increased by a 1971 earthquake in nearby San Fernando.

Figure 10.21

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.

Fig. 1. Overlapping aftershocks of the 1971 San Fernando (blue; first year, M > 2) and 1994 Northridge (red; first 24 days, M > 3) earthquakes (Stein et al., 1994)

Red and yellow are the areas where the stress increase after the major shocks 1971 San Fernando earthquake and 1994 Northridge earthquake.

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 (1)

Seismic waves are the most sensitive probes we have to measure the properties of the unseen parts of the crust, mantle, and core.

Distinct boundaries (or discontinuities) can be readily detected by refraction and reflection of body waves deep within Earth.

Using Seismic Waves As Earth Probes (2)

Early in the twentieth century, the boundary between Earth’s crust and mantle was demonstrated by a Croatian scientist named Mohorovicic.

A distinct compositional boundary separated the crust from this underlying zone of different composition (the Mohorovicic discontinuity).

Seismic wave speeds can be measured for different rock types in both the laboratory and the field.

Figure 10.22

Figure 10.23 The crustal thickness in NA. This map is compiled from seismic wave and gravity measurements. The gravity is lower over regions of thickened crust because the density of crustal rocks is lower than that of mantle rocks.

Using Seismic Waves As Earth Probes (3)

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.

Using Seismic Waves As Earth Probes (4)

Laboratory tests show that rocks common in the crust, such as granite, gabbro, 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,

must be among the principal materials of the mantle.

Using Seismic Waves As Earth Probes (5)

Some evidence can be obtained from rare samples of mantle rocks found in kimberlite pipes—narrow pipe-like masses of intrusive igneous rock, sometimes containing diamonds, that intrude the crust but originate deep in the mantle.

Using Seismic Waves As Earth Probes (6)

Both P and S waves are strongly influenced by a pronounced boundary at a depth of 2900 km. Geologists infer that it is the boundary between the

mantle and the core.

Seismic-wave speeds calculated from travel times indicate that rock density increases from about 3.3 g/cm3 at the top of the mantle to about 5.5 g/cm3 at the base of the mantle.

Figure 10.25

Using Seismic Waves As Earth Probes (7)

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.

Using Seismic Waves As Earth Probes (8)

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; 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 blip 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.

Figure 10.26

Pd: diffractive wave along the CMB

PKIKP: P wave entering the inner corePKiKP: P reflected at the inner-outer core boundary

The 400-km Seismic Discontinuity

From the P-and S-wave curves, velocities of both P and S waves increase in a small jump at about 400 km.

When olivine is squeezed at a pressure equal to that at a depth of 400 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.

Seismic Waves and Heat (1)

Seismic wave speed is affected by temperature. Seismologists translate travel-time discrepancies

into maps of ‘fast” and “slow” regions of Earth’s interior using seismic tomography. The principle is similar to CAT scan (X-ray tomography) of the human body in medical science.

Figure 10.27

Seismic Waves and Heat (2)

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.

Figure 10.28

Earthquakes Influence Geochemical Cycles (1)

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.

Earthquakes Influence Geochemical Cycles (2)

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.

Homework # 2 Due 3/8/20041. (25pts)(a) Explain how seismologists use the seismic moment to estimate the earthquake size. (b) Suppose a crustal fault 3 km long and 2 km wide slips 10 cm? What is the seismic

moment? (use the shear stiffness of the surrounding rock in the textbook).(c) What is the Richter magnitude? (d) How much will seismic moment and magnitude increase if the length, width, and slip of

the fault each grows by a factor of 3.16?

2. (20pts) What are the two types of seismic body waves? What are their main distinguishing features? What are surface waves?

3. (25pts) Explain the following seismic discontinuities in the earth’s interior and what causes the change in seismic velocity across them ： (a) M-Discontinuity (b) Low-Velocity Zone (c) 670-km Discontinuity (d) Core-Mantle Boundary.

4. (30pts) 恰好位在震源位置的地震儀測站記錄到 PcP ， ScS 和 PKiKP 三個波相的走時分別為 8 分 31 秒， 15 分 36 秒和 16 分 35 秒。另外在地球相反的一側離震源距離 180 度的測站記錄到 PKIKP 波的到時為 20 分 12 秒。繪出這些波相在地球內部傳遞的路徑，並根據地球的分層，利用這些波相的走時估算地函 (Mantle) P和 S 波的平均速度以及外核 (Outer Core) 和內核 (Inner Core) P 波的平均速度。