Earthquakes/Earth's Interior


Introduction: Earthquakes are one of the most visible and important Earth processes known to man. Earthquakes frequently cause significant destruction, injury, and loss of life. The more geologists know about how and why earthquakes occur, the better equipped we are to prepare the public for their effects. But, not all aspects of earthquakes are negative. Earthquakes release tremendous amounts of seismic energy which travel throughout the Earth's interior. Study of how this energy changes as it travels throughout the Earth has given geologists a tremendous amount of information about the Earth's interior. After reading and studying these notes, the text,  and suggested web sites, you should have a complete understanding of a) the basic causes and mechanisms of earthquakes, b) the basic types  and properties of seismic waves, c) how geologists determine the magnitude and epicenter of an earthquake, d) a general understanding of the magnitude scale, e) how changes in seismic wave velocity are used to construct our model of the Earth's interior.

The following web sites are helpful as study aids for this topic:

I. Definitions

Earthquake - shaking of the Earth caused by the release of elastic energy.

Elastic Energy - energy stored by a body of rock when a stress is exerted upon the body of rock.

Stress - a force exerted against a body of rock. (Fig 8.5)

Strain - the change in shape and/or size of a body of rock that results from stress applied to the rock. (Fig 8.5)

Elastic Deformation - change in shape and/or size of a body of rock, such that when the stress is removed the rock returns to its original shape and size. (Graph below: This is shown as the region over which the relationship between stress and strain is a straight line.)

Plastic Deformation - change in shape and/or size of a body of rock such that when the stress is removed the rock does not return to its original shape and size. (Graph below: This is shown as the region over which the relationship between stress and strain is a curved line.)

Elastic Limit - The maximum stress that a body of rock can withstand without permanent change in shape and/or size.

Brittle Fracture - Rupture that occurs in a body of rock when the stress applied exceeds the elastic limit. (Graph below: This is shown as the point marked as an X). When brittle fracture occurs, the elastic energy is suddenly released as seismic energy, which causes the shaking of the ground during an earthquake.

Seismic Energy - Elastic energy which is released when a body of rock undergoes brittle fracture. Travels in all directions away from the site of brittle failure.

Fault - A fracture within the earth on either side of which movement of rock has occurred (Fig 8.2)

Focus - The initial rupture point along a fault, from which seismic energy is released. (Fig 8.2)

Epicenter - That point on the Earth's surface directly above the focus. (Fig 8.2)

Seismic Wave - A wave of elastic (seismic) energy that travels through the earth. (Fig 8.2).

Note: A 'wave' can be thought of as a simple picture of energy in motion. A good example is a wave in the open ocean, which primarily consists of energy moving through the surface of the water. The  surface water itself does not move appreciably, but does constantly change its shape as the crests and troughs of the wave of energy pass through it. Seismic energy also causes change in the shape and volume of the earth material through which it passes. Basic descriptions of seismic waves are given below.

Seismology - The study of earthquakes and seismic waves.

 

II. Seismic Waves

Surface Waves - Radiate from the epicenter and travel along the Earth's surface. Have up and down rolling motion, and side to side vibrational motion. During an earthquake, the Earth's surface 'rolls' like waves on the open ocean and moves from side to side.

Body Waves - Radiate from the focus and travel throughout the earth's interior. There are two types of body waves: P-waves and S-waves.

P-wave (primary wave) - A compressional wave that causes alternate compression-expansion of the rock. Move through both solids and liquids (Fig 8.8 A).

S-wave (secondary wave) - A shear wave that moves particles of rock at right angles to the direction of wave travel. Move through solids only (Fig 8.8 B).

Seismograph - An instrument that records seismic waves. (Fig 8.7 )

Seismogram - Written record made by a seismograph. (Fig 8.9)

Seismic Wave Velocity Information:

1.  P-wave velocity > S - wave velocity > Surface Wave Velocity



2. General rule: seismic wave velocity is directly proportional to rock density


III. Earthquake Magnitude/Intensity

Earthquake Magnitude - a measure of the energy released during an earthquake. Is calculated from:
1. The amount of movement along a fault during an earthquake
2. The surface area along the fault plane along which there was movement

Earthquake Magnitude is expressed in terms of a logarithmic scale: Each increment of 1.0 is approximately equivalent to a 30-fold increase in the amount of seismic energy released at the focus. Each increment of 1.0 is approximately equivalent to a 10-fold increase in the maximum wave amplitude recorded on the seismogram. (Fig 8.14)


Example: A magnitude 6.5 earthquake releases approximately 30 x 30 = 900 times more seismic energy than a magnitude 4.5 earthquake. The maximum wave amplitude recorded during a magnitude 7.5 earthquake is approximately 10 x 10 x 10 = 1000 times greater than the maximum amplitude recorded during a magnitude 4.5 earthquake.

Earthquake Intensity - a measure of 'how bad' an earthquake is. Is dependent upon distance from focus, material (rock or soil) upon which buildings are constructed, building design, and earthquake magnitude. Usually expressed as a roman numeral on the Mercalli intensity scale. (Table 8.1)

IV. Locating the Epicenter of an Earthquake

1. Determine difference in arrival times between first P-wave and first S-wave at three different seismographs (Fig. 8.10).

2. Use travel-time curve to determine distance to epicenter from the same three seismographs (Fig 8.11).

3. Intersection of three arcs, each with radius equal to distance from epicenter, defines the epicenter (Fig 8.12).

 

V. Earthquake Destruction

1. Seismic Vibrations - Amount of structural damage is dependent upon: 1) earthquake magnitude, 2) duration of the vibrations, 3) nature of the material upon which the structure is built, 4) design of the structure

liquefaction - The process during which soils and unconsolidated sediments containing abundant water are turned into a fluid-like mass that is not capable of supporting buildings.

2. Tsunami - A 'seismic sea wave' which forms when an undersea earthquake transfers seismic energy to the above water. The seismic energy travels through the deep water as a wave with little effect. When the wave of energy energy approaches shallow water, the height of the wave is increased, sometimes as high as 100 feet. (Tsunami's may also be generated by major submarine landslides, or exploding volcanic islands.) (Fig. 8.22)  

3. Landslides, Ground Subsidence - Both caused by seismic vibrations and/or liquefaction.

4. Fire - Typically caused by rupture of natural gas and electrical lines. Fighting such fires may be complicated by rupture of water lines to fire hydrants.


VI. Seismic Waves - What They Tell Us About The Earth's Interior
(Fig. 8.29, 8.30)

Important seismic discontinuities with the Earth's interior:

1. Mohorovicic Discontinuity (The 'Moho') - The boundary between the crust and mantle. Characterized by an increase in seismic wave velocity as waves pass from the crust to the underlying mantle. Lies at a depth of 5 to 40 km. Is deeper beneath continental crust than beneath oceanic crust. The increase in seismic wave velocity occurs as body waves travel from lower density felsic rocks of the continental crust and mafic rocks of the oceanic crust into higher density ultramafic rocks of the mantle. (Remember: the velocity of a seismic wave is directly proportional to the density of the material through which it travels.)

2. The Low Velocity Zone - The boundary between the lithosphere and the asthenosphere. Characterized by an abrupt decrease in seismic wave velocities, due to presence of small amounts of molten rock. The lithosphere is the cool outer region of the earth, approximately 100 km thick, and includes the crust and the upper parts of the mantle. The asthenosphere is that part of the upper mantle which lies below the lithosphere. It contains rock which exhibits plastic behavior, and extends from depths of approximately 100 km to approximately 660 km. The base of the asthenosphere marks the base of the upper mantle.

3. 660 km discontinuity - An abrupt increase in seismic wave velocities occurs as high internal Earth pressures force minerals to transform into higher density atomic arrangements. The 660 km discontinuity marks the base of the asthenosphere and the boundary between the upper mantle and the lower mantle.

4. Lower Mantle-Outer Core Boundary - Occurs at approximately 2900 km. Due to the liquid nature of the outer core, P-Wave velocities abruptly decrease as the waves move into material of much lower density. S-waves are not transmitted through the liquid outer core.

5. Outer Core - Inner Core Boundary - Occurs at approximately 5100 km. Due to the solid nature of the inner core, P-wave velocities abruptly increase as the waves move into material of higher density.

P-wave and S-wave paths bend gradually as they pass through the Earth due to increasing pressure, and abrupt changes in composition and/or density of the material through which they travel. S-waves do not travel through the liquid outer core, and therefore are only observed within an arc of 105o of the epicenter. Regions of the Earth's surface where S-waves are not observed fall within the S-wave shadow zone. P-waves are bent sharply at the core-mantle boundary, so there is a zone where no P-waves are detected extending from 105o to 140o from the epicenter. This is known as the P-wave shadow zone.

VII. Importance of Meteorites and the Earth's Magnetic Field 

Many meteorites are composed of primarily iron and nickel. Scientists believe that meteorites formed at approximately the same time the solar system formed. Because the earth formed  by accretion of solid bodies similar to iron - nickel meteorites, it is believed that the metallic core is composed of iron and nickel.

The Earth's magnetic field is electromagnetic in origin. It is similar to an electromagnet created by wrapping a wire around a nail, and then connecting the ends of the wire to the positive and negative ends of a battery. The battery causes electrons to flow through the wire, creating an electromagnetic field. The metals in the earth's core are also good conductors of electricity. The liquid metallic outer core flows around the solid metallic inner core, generating the Earth's electromagnetic field. The existence of the Earth's electromagnetic field supports the theory that the outer core consists of metallic liquid, and the inner core consists of metallic solid.


Study Questions - Earthquakes and Earth's Interior