VisualizingGeologyFirst Edition

Chapter 5Earthquakes and Earth’s Interior

Copyright © 2008 by John Wiley & Sons, Inc.

Barbara W. MurckBrian J. SkinnerDana Mackenzie

On December 26, 2004 the most powerful earthquake in 40 years – and the third most powerful of the last century.

Earthquakes around this region are quite common, the great difference with this earthquake is that it provoked the largest and deadliest tsunami in history.

The quake began when part of the Indian Plate, which is subducting under the Eurasian Plate, suddenly slipped downward approximately 15 meters. The motion pushed the seafloor as much as 5 meters on the Eurasian Plate.

On the surface killer waves generated by the sudden movement and started to sweep towards Indonesia, Thailand, Sri Lanka, and India. When the waves reached shore they were between 20 to 30 meters tall and swept far inland, obliterating everything in their path.

The resulting devastation caused hundreds of billions of dollars in damages and at least 275,000 deaths.

Scientist cannot prevent earthquakes but the science is getting better in understanding the mechanisms associated with plate motions and preparation for future disasters.

Seismology – The scientific study of earthquakes and seismic waves.

Most earthquakes are caused by the sudden movement of stressed blocks of Earth’s crust along a fault. The friction between huge blocks of rock causes them to size up, bringing the motion along that part of the fault to a temporary stop. While the fault remains locked by friction energy continues to build up as a result of the plate motion, causing rocks adjacent to the jammed section to bend and buckle. Finally the the stress becomes great enough to overcome the friction along the fault. The energy released by the slip of two blocks of rock becomes a violent earthquake.

EVIDENCE OF LATERAL FAULT MOTION – San Andreas Fault - California

EVIDENCE OF VERTICAL FAULT MOTION – Alaska, 1964 – Good Friday Earthquake

Elastic rebound theory – The theory that continuing stress along a fault results in a buildup of elastic energy in the rocks, which is abruptly released when an earthquake occurs.

The elastic rebound theory suggests that rocks, like all solids, are elastic (within limits). This means that they will stretch or bend when subjected to stress, and snap back when stress is removed.

Like a guitar string after is plucked, they continue vibrating. These vibrations are called seismic waves. Like sound waves these waves travel long distances from their place of origin.

Seismic wave – An elastic shock wave that travels outward in all directions from an earthquake’s source.

Earthquake hazards and predictionEach year more than a million earthquakes occur around the world. Fortunately, only few are large enough, or close enough to populated areas. Geologist are working hard to improve their forecasting ability to the point where effective and accurate early warnings can be produced.

Earthquake hazards – Earthquakes can cause a total devastation in a matter of seconds. The most disastrous quake in history occurred in the Shaanxi Province, China, in 1556, killing an estimated 830,000 people.

Path of the Tsunami

Experiment to better understand the force of a tsunami

Earthquake hazards –Primary hazards - Ground motion, with the resulting collapse of buildings, bridges, and other structures. To make matters worse, movement on one part of the of a fault can cause stress along another part, generating other earthquakes, called aftershocks.

Secondary hazards – Landslides, fires, ground liquefaction, and tsunamis are considered secondary hazards caused by an earthquake.

Secondary hazards, landslide – Huascaran, Peru

Secondary hazards, open fissure – Golcuk, Turkey

Secondary hazards, fire – San Francisco, California

Secondary hazards, tsunami – Kalutara, Sri Lanka

Earthquake prediction – Charles Ritcher, inventor of the Ritcher scale for quantifying on the severity of earthquakes, once said, “Only fools, charlatans, and liars predict earthquakes”. Today unfortunately, this is more or less correct. However scientist understand the mechanisms a lot better and may prove Ritcher wrong in the near future.

There are two aspects to the problem of earthquake prediction:

Short term and long-term prediction-

Short-term prediction – Identifies the exact time, magnitude, and location of an earthquake in advance to the actual event, providing authorities to issue and early warning. Short term prediction has not been very successful to date. We measure magnetic properties of the rock, the level of well water could drop, an increment in radon gas in the groundwater, strange animal behavior, glowing auras, unusual radio waves, development of small cracks or fractures, swarms of tiny earthquakes.The most successful prediction was made in China in 1975, where slow tilting of the land, fluctuations of the magnetic field, and numerous foreshocks advice scientists of a large earthquake.

Long time prediction – Involves the prediction of a large earthquake years or even decades in advance of its occurrence.

Paleoseismology – The study of prehistoric earthquakes.

Designing for earthquake safety

Japanese schoolchildren practice for an earthquake

Apartment buildings in Niigata, Japan – after an earthquake caused liquefaction of the ground

A major quake in Kobe, Japan

How does the elastic rebound theory explain the violent tremors that occur during earthquakes?

How do scientist predict earthquakes?

What are some of the primary and secondary hazards associated with earthquakes?

Explain the connection between earthquakes and plate tectonics.

The science of seismology

Seismograph – An instrument that detects and measures vibrations of Earth’s surface.

Seismogram – The record made by a seismograph.

Ancient Chinese seismograph

The most advanced seismographs measure the ground’s motion optically and amplify the signal electronically. Vibrations as tiny as one hundredth-millionth of a centimeter can be detected.

Travel path of seismic body waves

Seismic wavesThe energy released by an earthquake is transmitted to other parts of Earth in the form of seismic waves.

Body wave – A seismic wave that travels through Earth’s interior.

Surface wave – A seismic wave that travels along Earth’s surface.

Focus – The location where rupture commences and an earthquake's energy is first released.

Body waves are divided into two types:

Compressional wave – A seismic body wave consisting of alternating pulses of compression and expansion in the direction of wave travel; P wave or primary wave.

Shear wave – A seismic body wave in which rock is subjected to side-to-side or up-and-down forces perpendicular to the wave’s direction of travel; S wave or secondary wave.

Surface waves – Travel long or near Earth’s surface, like waves along the surface of the ocean. They travel more more slowly than P and S waves, and they pass around the Earth, rather than through it. Thus, surface waves are the last to be detected by a seismograph.

Determining the earthquake epicenter Stations:EurekaElkoLas Vegas

S-P interval 49 seconds

S-P interval 72 seconds

S-P interval 64 seconds

Eureka 49 seconds - 480 kmElko 72 seconds – 700 kmLas Vegas 64 seconds – 620 km

Calculating the Ritcher Magnitude

S wave amplitude 235

Calculating the Ritcher Magnitude

S wave amplitude 60

Calculating the Ritcher Magnitude

S wave amplitude 100

Magnitude 7.1 Ritcher scale

Read, study and complete the following exercise at the following web page:

http://www.sciencecourseware.org/VirtualEarthquake/

Method of triangulation

Measuring earthquakes

Geologist use several different scales to quantify the strength or magnitude of an earthquake, by which we measure the amount of energy released during the quake.

The most familiar of these is the Ritcher magnitude scale.

Measuring earthquakes

Ritcher magnitude scale – A scale of earthquake intensity based on the recorded heights, or amplitudes, of seismic waves recorded on a seismograph.

Charles Ritcher developed his famous magnitude scale in 1935.

Measuring earthquakes

The Ritcher scale is logarithmic, which mean that each unit increases by a tenfold. For example a magnitude 6 earthquake has a wave amplitude ten times larger than a magnitude 5 earthquake.

The Modified Mercalli Intensity Scale is another type of measuring an earthquake based on damage estimates.

Measuring earthquakesMoment magnitude – A measure of earthquake strength that is based on the rupture size, rock properties, and amount of displacement on the fault surface.

All scales measure the same thing, the amount of energy released. In either system, magnitude 9 is catastrophic, whereas magnitude 3 is imperceptible to humans.

Ritcher magnitude 6 – Damage on surface: small objects broken, sleepers awake (Mercalli Intensity VII) – Energy released: about the same as one atomic bomb – Parkfield, CA 2004

Ritcher magnitude 7 – Damage on surface: some walls fall, general panic (Mercalli Intensity IX) – Energy released: about the same as 32 atomic bombs – Kobe, Japan 1995

Ritcher magnitude 8 – Damage on surface: wide destruction, thousands dead (Mercalli Intensity XI) – Energy released: about the same as 1000 atomic bombs – San Francisco, CA 1906

How does a seismograph detect earthquakes?

What are the major types of seismic waves?

What is the difference between the epicenter and the focus of an earthquake?

How do P and S reveal both the strength and location of an earthquake?

Describe the Ritcher and moment magnitude scales

Studying Earth’s Interior

Studying Earth’s Interior

When scientists cannot study something by direct sampling, a second method comes to the forefront: indirect study or remote sensing.The seismic waves from an earthquake are much like X-rays, in the sense that they enter Earth near the surface, travel all the way through it and emerge on the other side.

How geologists look into Earth’s interior – seismic methods

Before 1906, scientist’s understanding of seismic waves was limited. But that year, British geologist Richard Dixon Oldham first identified the difference between P waves and S waves. Based on this study he suggested that the earth has a liquid core.

Three distinct things can happen to seismic waves when they meet such a boundary, called seismic discontinuity:

1. The waves can be refracted, or bent, as they pass from one material into another.

2. They can be reflected, which means that all or part of the wave energy bounces back.

3. They can be absorbed, which means that all or part of the wave energy is blocked.

Seismic discontinuity – A boundary inside Earth where the velocities of seismic waves change abruptly.

Refraction – the bending of a wave as it passes from one material into another material, trough which it travels at a different speed.

Reflection – The bouncing back of a wave from an interface between two different materials.

Absorption – All the energy is blocked.

Seismic tomography – CAT scanning allow scientist to make up three dimensional layers from two dimensional data.

How geologists look into Earth’s interior – other methods.

Direct observation: drilling and xenoliths. Deepest mine - South Africa 3.6 km Deepest hole ever drilled – Kola Peninsula, Russia 12 km

Xenoliths – Magma carries fragments from unmelted surrounding rock.

Diamonds Messengers from the Deep – They form at depths of 100 to 300 km

Uncut diamondFrom South Africa

How geologists look into Earth’s interior – other methods.

Indirect observation: methods from physics, astronomy, and chemistry

Magnetism –Mass of or planet – Density of Earth is

5.5g/cm3 – Surface rocks 2.8 g/cm3

Meteorites -

Earth’s magnetic field

Charged particles from the Sun entering Earth’s atmosphere along magnetic field lines

Why do seismic waves undergo refraction as they pass through the Earth?

Explain why seismic data point to the existence of a liquid core

Identify several ways in which scientists can study Earth’s interior indirectly or remotely

What kind of geological features are revealed as seismic discontinuities?

Why can’t geologist drill a hole down to the mantle?

How do scientists obtain mineral samples that come from the mantle?

What does the Earth’s magnetic field tell us about its interior?

Describe three pieces of evidence that indicate that Earth has a molten iron-rich core.

A multi-layered planet

Crust, mantle, and core

Crust – The outermost compositional layer of the solid Earth; part of the lithosphere

Composition of the crust is 95% igneous or metamorphic rocks.

The boundary that separates the crust from the mantle is called the Mohorovicic discontinuity. Mantle rocks being denser transmit P waves much more quickly.

Mantle – The middle compositional layer of Earth, between the core and the crust. The mantle consists mainly of iron and magnesium silicate minerals.

Asthenosphere – Layer of weak ductile rock in the mantle that is close to melting but not actually molten.

Lithosphere – Earth’s rocky, outermost layer, comprising the crust and the uppermost part of the mantle.

Mesosphere – The boundary to the core and mantle.

It is important to remember that the mantle is mostly solid rock, except for small pockets of melt in the asthenosphere. We know that mantle has to be solid because P and S waves travel through it. Nevertheless, pressures and temperatures deep within the Earth are so high that even solid rock can flow, in very, very small convection currents as described before.

Core – Earth’s innermost compositional layer, where the magnetic field is generated and much geothermal energy resides. The outer core must be liquid (S waves stop) and because of the great pressure and in spite of high temperatures the inner core, must be solid.

What is the nature between the crust and the mantle?

Why and how the inner core differs from the outer core?

What are the major layers of the Earth?

Define the composition of the crust, mantle, and core

Loch Ness

Loch Ness

Loch Ness

What reason can you suggest for this stream’s strange behavior?

Along which type of plate boundary do you think that you’ll have the largest earthquakes?

What is the elastic rebound theory?

List primary earthquake hazards

List secondary earthquake hazards

How many types of seismic waves?

Name the types of wave motion

On the seismogram below label the following: arrival of P and S waves, S-P interval, arrival of surface waves, background noise

What is the difference between the Ritcher Scale, Moment magnitude scale, and the Mercalli scale.

What happens to the waves as they reach a discontinuity?

What is the composition of the mantle and the core?

What is the asthenosphere?

If we seismographs at three different stations A, B, and C. A and B had an S-P interval of 3 seconds and C had an interval of 11 seconds. To which station(s) is the earthquake near to?

Which station(s) received the first P waves?

A magnitude 8 earthquake is how many times greater than a magnitude 7 earthquake?

Label the following : mantle, lithosphere, oceanic crust, asthenosphere, outer core, moho, inner core, continental crust, mesosphere

The scientific study of earthquakes and seismic waves is known as _______.

A) seismograph B) seismogramC) seismology D) vulcanologyE) Tectonics

The _______ scale is a logarithmic scale that measures earthquake intensity.

A) BartonB) WegenerC) MercalliD) RichterE) none of the above

Fragments of unmelted rocks that are sometimes incorporated in magma are known as _______.

Which type of waves would cause more damage?

_______ are the first waves to leave the focus after an earthquake.

The _______ is the point on the surface directly above the point of an earthquake.

Diamonds are incorporated in rocks called _______ that come deep from within the Earth.

The _______ is the part of the Earth's interior where rocks start to melt.

An instrument that measures and detects vibrations in the Earth is known as a _______.

In the figure below, what is the approximate time of the arrival of the P-waves?

A) 1 minuteB) 2 minutes, 15 secondsC) 3 minutesD) 4 minutesE) 5 minutes, 30 seconds

In the figure below, what is the approximate time of the arrival of the S-waves?

A) 1 minuteB) 2 minutes, 15 secondsC) 3 minutesD) 4 minutesE) 5 minutes, 30 seconds

In the figure below, what is the approximate time of the arrival of the surface waves?

A) 1 minuteB) 2 minutes, 15 secondsC) 3 minutesD) 4 minutesE) 5 minutes, 30 seconds

According to the figure below, what is the approximate S-P travel time?

A) 1 minute, 45 secondsB) 2 minutes, 15 secondsC) 3 minutes, 15 secondsD) 0 minutes, 45 secondsE) cannot be determined

The _______ is the Earth's rocky, outermost layer.

The Earth's density as a whole is approximately 2.8 g/cm3.

Secondary hazards sometimes cause more damage than the earthquake itself. (T/F)

A large, destructive wave sometimes caused by am earthquake is called a _______.

According to the figure below, _______ is the location of the earthquake.

A wave could be:

1)

2)

3)

The theory that stress is continually built up along a fault and released when earthquake occurs is known as _______.

The method of using data from three seismic stations to locate an earthquake is known as _______.

_______ are fragments of unmelted rock that are sometimes incorporated in magma.

The average density of the Earth's crust is _______.

A _______ wave is a wave that travels along the earth's surface.

The area inside the Earth where rocks start to turn plastic is known as the _______.

The Richter magnitude scale is a _______ scale.

Swarms of tiny earthquakes that may occur before large earthquakes are known as _______.

Describe the method of triangulation.

Which type of wave cannot travel through a liquid.