GEOLOGY/GEOPHYSICS 101 Program 10
EARTHQUAKES 
Are you keeping up with the assignments?
I know a course like this takes a lot of time, and you should be spending maybe as much as two hours outside of class for each of the programs that you watch, and that's very typical of college courses. I know it does take a lot of time to do this, but if you put the time into it, you'll get the benefit out of it, and, also, are you following the study plan and the study guide?
You should be synthesizing information; that is, combining all these different sources of information from the TV program, and from the text, and the study guide, and how about your exam?
How did you do on the exam? Were you satisfied with what you got?
Well, I hope you were, but if you weren't, the second exam basically is starting over. The material in the second exam won't cover the material from the first exam, and it gives you a chance to start all over. If you need to improve your exam scores, try following the study plan and try going back and looking at the key terms and concepts, and don't try to just read the material straight through but actually spend some time reviewing and looking at the pictures and going back over it and so forth.
Most importantly, I hope you are getting a sense of the Earth and how it works. I hope you're doing well, but I also hope that you're starting to see things from a different perspective and noticing features of the Earth as you go outside and look at the real world, things that maybe you didn't see before you started the course.
Well, today's lesson is on earthquakes. It's Lesson 9 in the study guide. I don't have any demos for you today, but when it's appropriate, I'll bring some more demos in and we'll do some close ups and so forth, but as for now, I don't have any demos for you today.
This is kind of a hard topic to show demos because it's about earthquakes. Earthquakes remind us that the Earth is really a restless planet. Every time there's an earthquake, what we're feeling is the result of those deep interior forces driven by the internal heat engine of plate tectonics.
Earthquakes are caused mainly by the movement of the Earth caused by the plate movements.
They're also, of course, caused by manmade explosions, and we saw in an earlier lesson how you can use manmade explosions to study the detailed structure of the upper part of the crust.
The energy of earthquake waves is carried worldwide by seismic waves. As we learned in an earlier lesson, the seismic waves not only travel throughout the entire Earth, but they also carry information with them about the material through which they travel, and much of our knowledge of the interior of the Earth comes from studying these seismic waves.
There are thousands of earthquakes worldwide on a daily basis, but most of them are much too small to be felt, and at great distances even from large earthquakes, very sensitive instruments are needed to detect seismic waves, so "seismology" is the study of earthquakes, and this is a rapidly growing science; in fact, at the beginning of the Twentieth Century, there were only a handful of geologists who considered themselves to be seismologists.
By 1960, a worldwide network of more than 400 seismic stations were established to communicate with each other and share information. The science of seismology has grown from a very early descriptive science in which not much was known about earthquakes to a very important branch of geophysics as we study it today.
But before I actually introduce the lesson to you, I want to remind you of the text assignment.
The study of earthquakes is, although a new science as far as geology goes, there are the many observations of earthquakes and attempts to explain them, which date back more than 2,000 years.
One of the first attempts to get a handle on earthquakes came in 136 A.D. A Chinese scholar, named Chang Hing, built a machine which basically could determine the direction from which the earthquake came.
This consisted of a large urn with marbles around the outside, and when the earthquake shook the urn, one of the marbles fell out, and by looking at which marble fell out, you could determine which direction the earthquake came from. This might be called really more of a "seismoscope" than a "seismograph" because it didn't really give much of a record. It measured the presence of an earthquake but didn't really keep any record of the earthquake or how it happened.
In the early 1700s, a French scientist designed a variation of this device using liquid mercury that overflowed from pipes although there's no indication that this particular seismograph was ever built.
The first what you might call modern seismograph was invented in 1731 when Nicholas Cerillo, an Italian scientist, used a simple pendulum that indicated the amplitude of the ground motion. It didn't give much information about the direction or anything else about it, but it showed how big or how much amplitude there was in the ground motion. He used that early seismograph to make an important determination, and that is that the size of the motion or the amplitude decreased with the square of the distance.
I'll come back and elaborate on that concept a little later. The motivation for a real good seismograph came in 1783. A serious earthquake in Italy, in Calabria, resulted in about 50,000 deaths. The first recording instrument that could record earthquakes was invented shortly thereafter.
It basically consisted of a long paintbrush, which was free to swing back and forth, which painted a trace on a piece of ivory, and this particular seismograph, a bell rang when the amplitude was very large. I want to note here that a similar instrument was used to record the New Madrid earthquake, which occurred around the St. Louis area that picked up in Cincinnati by this seismograph in 1811, 1812.
The first seismograph that we could claim is a real modern seismograph was invented in 1844 by James Forbes. He used an inverted pendulum that he could adjust to stiffness, and the reason that you want to adjust to stiffness of the pendulum is because it turns out that earthquake waves come in many different periods and frequencies, so the stiffer the pendulum is, the slower it vibrates, and so by tuning this with adjusting the stiffness, Forbes could pick up earthquakes of various periods and various frequencies.
The problem with this was that Forbes had a pencil at the end of a stick, and the pencil simply wrote on a piece of paper, and it turns out that the friction between the pencil and paper reduced the sensitivity of the seismograph and made it pretty much ineffective.
In 1889 there was an important discovery that changed the face of seismology and actually introduced the modern era of seismology. It was quite an accidental discovery. The discovery was that a particular earthquake, which was felt in Tokyo at one time, was felt one hour later in Germany. This earthquake originated in the Himalayas and traveled through the Earth to Japan faster than it traveled through the Earth to Germany.
The actual data that was recorded was far too compressed to give much information about the earthquake or about the seismic waves, but it did show that energy from earthquakes could travel half way around the world. It also showed how long the motion lasted and showed that the ground movement had a horizontal component; that is, that there was actually a sideways shaking.
So what we might call modern seismology began with this discovery of long distance seismic waves. The original signals were fuzzy. The lines were compressed. We didn't get much information, but it very rapidly evolved into a science where new machines were built that could capture this.
By 1900 there were 16 recorders, which were set up on all the continents, and this type of seismograph was used to measure fault displacement in the 1906 San Francisco Earthquake.
I want to review with you how a seismograph works. It's really a very simple sort of arrangement. A modern seismograph is based on the same principles of the early seismographs, but basically there's three different components.
Much of our information about the world in general comes from waves of different types. For example, we use light waves to get information about our immediate surroundings, but astronomers also use light waves to study the sun, and other planets, and other stars. Sound waves, of course, is another way in which we get lots of information about the world, and it's interesting and, I suppose, convenient that seismic waves have the same sorts of properties as other waves because once we understand how waves behave in general, we can then apply that knowledge to the understanding of seismic wave as well.
Seismic waves result from energy, which is stored in the elastic deformation of rocks. As stresses build up along a fault, the rocks deform elastically, and when they rupture, the earthquake waves travel out in all directions, so the energy then is given off during the earthquake, and this energy travels through the Earth as seismic waves.
Now, any kind of waves require two different things to happen. One is some sort of a disturbing force, which in the case of earthquakes is the displacement or the stresses due to plate tectonics, and, secondly, there must be a restoring force; in other words, in order for a wave to travel, once the rock is displaced out of place, there has to be something to snap it back into place, and it's this action that causes the wave to travel.
When an earthquake happens, waves of different types are produced, very much the way ripples are produced when you throw a stone into water. The surface waves travel along the surface, and you can see those in the ripples traveling outward from where the stone. Sound waves travel through the water, and if you were under the water, or if you were a fish living under the water, and someone threw a stone in the surface, you could hear the sound of the stone as it propagates through the water even though you couldn't see those waves.
Another type of waves, called "sheer waves" don't travel at all through liquids, but they do travel through solid rocks, so not only waves of different types produced, but also waves of different frequencies are produced, and seismologists can learn much by analyzing the different types of waves that are produced.
The time that it takes a wave to travel from the disturbance depends upon the type of rock, and seismologists have a very difficult time in unraveling the travel times and making sense out of these traces, and to be honest with you, not being a seismologist, I look at these things. and it might as well be Greek, but seismologists, when they explain it to me how they do it, it all makes a lot of sense, and it's amazing that you can actually get this much information out of the rocks.
One of the difficulties the seismologists had when they first started studying seismic waves was that the information that you get on the seismic trace on the seismograph is simply a collection of wiggly lines.
In order to make any sense out of this, you have to have some way to know when the actual event occurred because in order to measure the travel time from the earthquake to your station, you don't really get that off the seismograph. All you get is suddenly the pen on the seismograph starts wiggling. Luckily, nuclear tests that were conducted in the western desert in the United States in the 1950s gave a very good way to calibrate instruments because in the nuclear test the exact time of the test was known, the exact location of the test was known, and the amount of energy released was known within fairly narrow limits, so that seismographs could then be calibrated for the arrival times of the wave at a particular station knowing the starting locations.
Okay, as a result of early seismic studies, the major layers of the Earth across mantle and core were known as early as the 1920s.
By the 1960s we have the establishment of seismograph networks and the detection of what we might call the fine structure of earthquake waves.
By the 1980s we saw this information being stored in computers and analyzed by computers to get a lot more information.
In fact, the amount of analysis that a modern computer can do is probably more than the entire team of seismologists could have done up until the time the computer was invented. The video today gives a pretty good explanation of the nature of seismic waves but be sure to review Chapter 2, pages 26 to 31, the section on earthquakes and the Earth's interior.
Now, the video also uses really good animations, nice slow motion animations of the various types of wave motion, the compressional motion and the sheer motion, and if you have a chance if you're recording this, go back and look at those over and over again until you have a sense of how the motion actually occurs and how the wave passes through, so I'll summarize this briefly, but I'll let the video do most of the work as far as explaining this to you.
There are basically two types of seismic waves.
The one thing to note here is that the wave's speed depends upon the density and the elasticity of the material. The more dense the material, the faster the waves travel. We also note that the waves are bent and partially reflected when they pass a boundary. The boundaries that were first observed are those that represent the moho or the division between the crust and the mantle and the divisions between the mantle and the upper crust. So it's abrupt changes that mark these major boundaries.
Okay, what we need to understand here, not so much the details of the waves, but to understand that whenever an earthquake happens, waves of all three different kinds are produced.
The surface waves radiate from the epicenter of the earthquake when the body waves reach the surface; in other words, if this is the surface of the Earth, the earthquake happens down here some place. The waves radiate out in all directions, but some of them reach the surface. When they reach the surface, they run then along the surface as surface waves.
The first waves to arrive the surface are the "P" waves; that's why they're called "primary" waves.
The second are the "sheer" waves; that's why they're called "S" waves for "second,"
and the third are the surface waves or the "L" waves. They're called "L" because they're large waves.
Okay, there are a couple of more terms that I need to clarify before we watch the video. I used the word "epicenter" a minute ago. We need to distinguish between the focus of an earthquake and the epicenter. These are really simple terms to understand.
The "focus" is the source of the earthquake waves; in other words, that is the location where the breakage actually occurs, and it usually occurs at some depth in the Earth. Here the waves are generated by rupturing of Earth materials. The fracturing usually begins at a particular point and may spread outward along the fault from that point. Fixing the patterns and sizes of how the focus actually appears is one of the problems that geologists haven't solved, but it seems that the focus and the waves originate within a fairly limited region, more like 50 kilometers rather than five or 500. Again, we don't really have a good sense on that.
The depth of a typical focus in anywhere from right at the surface down to about 700 kilometers although most of them are shallow, and we remember that the continental margins have the focuses of the earthquake clustered along a plane, which dips toward the continent. We called it the benioff zone before.
In contrast to the focus is the epicenter. The "epicenter" is the point on the surface that's vertically above the focus. This is actually where the surface waves originate from, and it's the closest point on the surface of the focus, and it's the point where the waves first arrive at the surface. It's also the place where most of the damage is done in severe earthquakes.
Okay, to locate earthquakes, seismologists use a travel time graph. They simply look at the difference in arrival time between "P" waves and "S" waves, and knowing very little else about the materials that the waves have passed through, it allows them to calculate the distance from the earthquake to that particular station.
In order to find out the location of the earthquake, you need three different stations. The video will show how this is done with triangulation, so I won't elaborate on them, but I will note that each seismograph station can only measure distance and not direction. Computer analysis of lots of different earthquake records can also use the same techniques to find the depth of the focus.
The last thing I want to talk about before the video is the measurement of the size of an earthquake. To specify the size of an earthquake has always been a problem. Descriptions of earthquakes are very crude and highly subjective and, let's face it, most of the people who are observing an earthquake, are not in their most rational behavior at the time.
Most people are frightened by earthquakes, especially large earthquakes, and there are very few people who are cool enough and have the sense enough to sit down with a pencil and a piece of paper and write down their impressions at the time an earthquake is happening, so trying to describe in some way the intensity of an earthquake or the size is a very difficult thing.
A scale was developed 1883, which used 10 degrees of intensity. The problem with these intensity scales is that they're ambiguous, and they're cultural dependent.
Let me read for you the description of a sixth degree earthquake. "There was a general awakening of those asleep, a general ringing of bells, oscillation of chandeliers, stopping of clocks, and some startled persons leave their dwellings."
Now, this is all fine and good, but if there are no chandeliers, and if there are no bells to ring, and if what if the people are not startled, what if they're used to earthquakes. What if they live in a region where there are lots of earthquakes, and an earthquake that may startle some people, for example, in the middle of the United States, where there aren't many earthquakes, may not even phase someone who lives in earthquake prone areas like Japan.
A more modern scale was developed in 1902 called the "Mercales" Scale and modified in 1931, and it's still in use today. You can see this Table 2 on page 153 in the textbook.
It has a 12 point scale, the lowest intensity can't be felt at all; the highest intensity is total damage. Keep in mind that intensity, as I've been describing, is a measure of the effect of an Earthquake at a particular place, and it depends not only on the structures involved and the perceptions of people, but also depends upon the rock type through which the earthquake wave was traveling.
What was needed was some way to put the earthquake size on to a more objective scale. This was done by Richter. Richter recognized that since the amplitude of earthquake waves diminishes with the square of the distance, that there's a relationship between the total amount of energy in an earthquake, and the distance and the amplitude of an earthquake in a particular place, so Richter devised a scale in which he used numbers where each successive number represents a factor of ten and the amplitude of the earthquake.
In other words, if an earthquake is reported by a particular station, and it has an amplitude of one unit; in other words, there's a small wiggle, then the earthquake of the next magnitude would have an amplitude ten times that; in other words, ten times the wiggle. On this scale, a magnitude 4 earthquake is noticeable by people; a magnitude of 7 or above represents a major earthquake. Just to note the Loma Pietro earthquake, the so called World Series Earthquake that hit northern California in 1989 had a magnitude of 7.1.
Okay, the other thing to note here is that there's a relationship between the energy of an earthquake and the amplitude. The more energy there is, the more amplitude.
But it turns out that on the Richter scales the amount of energy between one earthquake and the next is about a factor of 30, so that, for example, a magnitude 5 earthquake releases about the same amount of energy as a small atomic bomb like the one used during World War II.
A magnitude 6 earthquake, one number higher, releases about 30 times more energy than magnitude 5, but a magnitude 8 earthquake releases about a million times more energy than a magnitude 4, so even though they're only separated by four numbers, a magnitude 8 earthquake is really much more energetic and much more destructive or potentially destructive than a magnitude 4.
The largest earthquake ever recorded was about an 8.6, and there have been four of them in history. Well, the Richter scale is theoretically open ended, but most seismologists believe that the strength of rocks is such that we'll never see more than about a magnitude 9.
One last thing to note here. There are wide variations in the total amount of energy from one earthquake to the next. In fact, there are many small earthquakes in a given year of relatively low energy but infrequent large earthquakes. In fact, only one or two major earthquakes is a given year.
Well, today's video shows some of these aspects that I've talked about, but a good portion of the video, the last half or so, shows active research on the San Andreas Fault in a place called Parkfield, where there have been fairly periodic earthquakes occurring every 20 years or so.
You should pay special attention to this section to see how seismologists supply various methods to study fault movements, so with all that in mind, let's watch the video. Music Major funding for "Earth Review" was provided by the Annenberg CPB Project.
Historian Will Durand once observed that civilization exists by geological consent subject to change without notice. Durand may not have had earthquakes specifically in mind when he made that statement, but it is difficult to imagine a more cataclysmic event than a major earthquake.
Ancient mythology held that tremors occurred due to the movements of giant animals beneath the Earth. While science has since proved such notions incorrect, these myths did contain at least one element of truth, the cause for the abruptly shifting land does reside deep below our feet, but instead of the land sitting on a mythological beast, we ride atop a gigantic heat transporting machine that keeps deep rock flowing and churning.
Radioactive decay generates the heat energy, which ultimately powers movements of Earth's rocky tectonic plates. The friction between the rough edges of these slowly moving segments of the Earth's lithosphere results in a series of jerky starts and stops, which is the direct cause of most major earthquakes.
The same heat engine that produces earthquakes is the driving force raising the world's great mountain ranges. If this uplift did not take place, the relentless force of erosion would reduce the Earth's landscape to a single flat plane.
Without the present range of elevations, rivers would lose their principal sources of water. The land would ultimately erode down to tide level, and most of the diversity in the world's flora and fauna would disappear, and yet, while there are indirect benefits to living in a world with earthquakes, it's the capacity of tremors to violently disrupt human activity that commands our attention.
Earthquakes often occur here on the San Andreas Fault in California. This fault is actually the boundary between two tectonic plates, the North American and Pacific Plates.
In 1857 this segment of the San Andreas caused a massive earthquake. The ground on either side of the fault shifted suddenly and violently, moving as much as four meters. Instruments to measure the strength of the quake didn't exist at the time, but estimates based on eye witness accounts of the damage range as high as 8.2 on the Richter Scale.
Since then, California has experienced several devastating earthquakes; in fact, thousands of earthquakes on hundreds of different faults shake California each year, but earthquakes are not just a California story, they occur throughout the world. In regions that are densely populated and where construction practices are primitive, the destruction and loss of life from a powerful earthquake can be almost incomprehensible.
In 1990, 50,000 people died in Iran. The year before 25,000 people were killed in Armenia, and in China, hundreds of thousands of people were killed in a single earthquake in 1976.
Earthquakes are shockwaves.
They are produced when rocks break and vibrate when two blocks of the Earth's crust slide past one another. This usually occurs along a fault, which is a zone of weakness in the Earth's crust. Movement of the Earth's crust causes stress to accumulate in the rocks of the fault zone.
I can feel the stress accumulating in my hands as I try to slide them past one another. The rocks will eventually break when the accumulated stress exceeds the strength of the rocks in the fault. The movement of the tectonic plates is the principal source of stress in the Earth's crust, stress that accumulates over hundreds or even thousands of years is released in an instant as an earthquake.
Geologists study the mechanics of earthquake behavior in order to develop and understanding of where and when earthquakes are most likely to happen. Most of the energy in an earthquake is expended when blocks of rock move into new positions.
The energy fractures the rock, forming a plane of slip, called a "fault" along which future earthquakes can also occur. Some energy generates frictional heat, and a small part of the energy released creates seismic waves.
These waves can be categorized according to the form they take and the speed at which they travel. Those that travel in such a way that the matter through which the waves are traveling are alternately compressed and dilitated. These are called "P" waves. The reason they're called "P" waves is because these are the first ones, the primary waves to arrive at a seismograph.
"S" waves, which are sometimes called "secondary" waves or "sheer" waves are waves which travel by a wavelike motion where the matter is vibrating up and down at right angles to the direction that the wave energy is being propagated. This causes a sheer stress to develop in rocks through which it travels is why they're called "sheer" waves or "S" waves.
The "S" waves travel somewhat slower than the "P" waves, the difference in speed about 2 kilometers a second. This difference in velocity between "P" and "S" waves is highly significant. It enables seismologists to calculate the precise location where an earthquake has occurred.
If one lies close to the epicenter of an earthquake, the "P" and the "S" waves will not have had enough time really to become separate from one another as they travel through the Earth, and when they arrive all at once, the earthquake is felt as a sharp jolt. If, however, one lies at a great distance from the epicenter, the "P" waves will have far outraced the "S" waves, and so arrive much earlier than the "S" waves do.
The time interval between the arrival of the "P" and the "S" waves thus is a function of the distance to the source, the epicenter. Having a single seismograph record showing the time interval between the arrival of "S" and "P" waves is not adequate for locating the epicenter.
It simply tells one how far away the earthquake occurred. The direction is still missing from this information. In order to determine that direction, one examines seismograph records from other stations in the regions as well. One can pull out a map and easily locate the epicenter by drawing circles around each respective seismic station representing the distance inferred to the source of the earthquake based on the time interval between "S" and "P" waves, and where these circles intersect, bingo, that's your source.
That's where the earthquake energy first reached the Earth's surface.
In the second century the Chinese built a device that could detect the initial ground motion during an earthquake. It consisted of eight metal balls ranged around the circumference of a large sphere. If the Earth shook hard enough, a small pendulum inside the sphere swung back and forth knocking one of the balls off its stand. This indicated the direction that the earthquake vibrations came from.
At the turn of the century a more advanced device was designed, one to make a permanent record of ground motion during an earthquake. This instrument, called a duplex pendulum seismograph recorded the shaking of the ground as a continuous squiggle on a piece of paper. It was this device that recorded the ground motion of the 1906 San Francisco Earthquake. The invention of a device that could accurately measure the strength of an earthquake was a significant scientific achievement.
These modern seismographs operate on the same general principles as the early pendulum instruments, but they are much more sensitive. Not only do they record vibrations, but they can be used to measure earthquake strength and duration, and also to determine the location of the earthquake almost immediately.
In 1935 California Geophysicist, Charles Richter, combined the measure of ground motion with the distance from the earthquake's epicenter to yield a value representing the total amount of energy released during the earthquake. He called this the "magnitude of the quake."
The scale that Richter devised runs from magnitude minus 2 to infinity although seismologists believe that rocks can at most only store elastic energy equivalent to magnitude 9 before they snap.
The smallest tremors that can be discerned by humans feeling like the rumbling of a passing train measure about 2.5 on the Richter Scale.
A magnitude of 4 is equivalent to the amount of energy released by 1,000 tons of explosives.
A magnitude just above 8 represents about as much energy as produced by 200 one megaton nuclear bombs. The reason for the huge jumps in energy between one unit and the next on the Richter Scale is that each unit of magnitude increases logarithmically by a factor of just over 30 times more energy released.
For example, a magnitude 5 earthquake releases about one-thirtieth of the energy of a magnitude 6 earthquake, so to compare a magnitude 5 earthquake with a magnitude 7.1 Loma earthquake is a factor of about 900 in terms of the energy released between those two earthquakes.
Small earthquakes really are insignificant in terms of the release of strain on the fault that is the cause of large damaging earthquakes. Small earthquakes really tell you that a fault has enough stress on it to cause bigger earthquakes.
One of the highest magnitude earthquakes ever recorded occurred in 1964 in Alaska. It registered 8.6 on the Richter Scale.
Fortunately, very large earthquakes are rare. Of the thousands of quakes that occur world wide each year, only a hundred or so are strong enough to destroy human life and property, and only one or two produce major geological changes.
The most obvious effect of an earthquake is just that. The earth quakes and trembles. There are many different types of ground vibrations. The rapid vibrations of the primary and secondary waves and the slower rolling motion of what are called surface waves. Near the epicenter where all these vibrations are concentrated at the same location ground motion is similar to the complex sea surface in an ocean storm with waves of all sizes mixed together, but the longer waves travel faster and may not die out for hundreds of kilometers.
The amount of time it takes two successive wave crests to pass a stationery point is called the wave's "period." For longer seismic waves, this period may be several seconds. Other materials including buildings have a period associated with them. In a strong gust of wind a skyscraper will flex and bend. The time it takes for the skyscraper to oscillate back and forth once is called its natural period.
When the period of a seismic wave matches the natural period of a building, the seismic energy is simply added to the oscillation of the building, wave after wave. As a result, the swaying of the building increases dramatically. Other buildings of different heights will not be similarly affected because their natural periods are different. An example of this type of amplification occurs when a child is pushed on a swing. A gentle push in time with the swing's natural period sends the swinger higher and higher.
In the 1985 Mexican Earthquake, seismic surface wave periods matched the natural periods of Mexico City buildings between 10 and 14 stories high. Structures in this size range were seriously damaged.
Given the destructive power of many earthquakes, geologists hope to develop the ability to forecast quakes in time to warn any people who might be in danger, but to do this requires a better understanding of earthquake behavior.
One pastoral stretch of the San Andreas Fault near Parkfield, California has been especially prone to earthquakes. Since 1857 residents of this area have been jolted by moderately strong earthquakes on an average of once every 21 to 22 years. Besides recurring at fairly regular intervals, Parkfield earthquakes also resemble one another in their magnitudes, lengths of fault rupture, and epicenter locations. For this reason, the Parkfield area seems like an ideal setting for an earthquake research project.
Geophysicists are using a sophisticated array of devices to closely monitor the rocks along the fault. The goal is to spot any changes that might signal an impending earthquake.
Evelyn Roloffs is the chief scientist overseeing the Parkfield Earthquake Prediction Experiment. The Parkfield Experiment has three goals.
One of the most fundamental aspects of the Parkfield Experiment focuses on the structure of The San Andreas Fault itself. To learn more about this structure, geophysicists have set up the Vibrocise Project.
At the heart of this effort is a specially equipped truck that shakes the ground, triggering waves of seismic energy. Radiating into the earth, the seismic waves moves at different velocities through different rock types. Analysis of the velocity changes makes it possible to unravel the intricacies of the subsurface geologic structure.
As the waves penetrate the Earth, they are reflected and refracted off the various rock layers, and by measuring first the direct wave, which travels directly from the source to receivers, we can get the velocity of the rocks, and then by looking at the later arriving reflected and refracted scattered waves, we can see possibly where the structure changes and where the layering beneath the Earth is.
With an understanding of the geologic structure of the fault zone, geophysicists set up other experiments to measure movement along the fault.
One device that can detect movement in a very specific localized area is called a "creep meter." Each creep meter consists of a wire stretched across the fault between two fixed points. Whenever the fault slips, the length of wire between the two points changes recording the amount of motion along the fault. Although the creep meter only covers about a ten meter zone, it runs at very high sensitivity and continuously monitors any fault motion at the surface.
To derive the same kinds of information as provided by a creep meter but over a wider area, geophysicists use what's known as an alignment array. This involves setting up markers, then conducting surveys across The San Andreas Fault to determine the local slip rate, the width of the slip zones, and patterns of deformation near the fault trace. One hundred eighty degrees, 11 minutes, 3.0 seconds. Yes to 180 degrees, 24 minutes
The alignment array surveys are also useful in helping scientists determine the best places to install creep meters. The alignment array is similar to the creep meter in that it's measuring fault slip right at the surface trace. However, the creep meter is a continuous monitoring device for the center of that slip area; whereas, the alignment array goes much farther away from the fault to be sure that we're not missing some important slip on auxiliary fractures out to the side, and the alignment array, of course, we only get data when we make surveys; whereas, the creep meter can be run continuously, and we can look at that every day to see how much the center of this zone has actually moved.
Another important component in the Parkfield effort to better understand the mechanics of faulting is the "geolometer." This shoots rays of lazer light at 18 reflectors set up like the spokes of a wheel on the hills around Parkfield.
We put reflectors at different points around the valley, and we take measurements of those, so we keep track of how long it takes that light to go out to that reflector and come back with some sophisticated equipment and computers, and by knowing how fast the light travels and knowing how long it took it to get there and get back, we can calculate a measurement within a fraction of a millimeter on a 9 kilometer line when the conditions are really good.
This network of light is so sensitive that it can detect the slightest bending of rocks along the fault zone. The geolometer system covers a much broader area than either a creep meter or an alignment array survey. They're all designed to look at the same kind of thing with different sensitivities and at different scales, so they all contribute to an understanding of the mechanics of the faulting.
Before the next Parkfield Earthquake, scientists hope to see an acceleration of the slip that occurs on The San Andreas Fault between earthquakes. If that acceleration takes place at the surface, it can be detected directly by a creep meter, but geologists must use other techniques to detect deep movement along the fault that does not appear as slip at the surface. As stress accumulates in the Earth, ground water will respond by rising or falling. This can be measured in water wells that have been installed by members of the Parkfield research team.
A sudden drop in water level, for example, is sometimes observed as the ground swells and cracks in the days preceding an earthquake. Seismologists studying Parkfield are also interested in measuring foreshocks, the seismic activity that precedes an earthquake. To do this requires a detailed network of seismometers, which the Parkfield team has set up. Tiny earthquakes are common occurrences along most active faults, but shortly before a large quake, the frequency and distribution of small shocks may change.
Being the person who looks at the seismicity, I'm expecting to see some changes in the patterns of the earthquake. The earthquakes occur in the fault zone in a fairly complicated manner.
Some places the activity's shallow; some places the activity's deep.
As we get more detailed information about the fault zone, we see certain knots or clusters that are active. What we're seeing, what we're expecting to see is that the larger earthquakes will start at magnitude 2s, 2 and a halfs, 3s, and they'll start migrating down the fault zone towards the area that will fail first in that earthquake. This is the pattern that's been seen in `34 and in `66 and that we're keeping a very close eye on the activity to see if that kind of activity occurs again.
In the case of the Parkfield Experiment, the data obtained in the field are then regularly transmitted by satellite to the U. S. Geological Survey in Menlo Park, California for analysis. Here the data are monitored by scientists for any unusual activity. Depending on the extent of such activity, the Parkfield research team may declare any one of four prearranged alert levels.
These alert levels indicate the probability of a sizable earthquake occurring at Parkfield within the following 72 hours. The seismic activity for the past two days is shown here, and what we have are some earthquakes that represent an increase in the action going on. These earthquakes are located here at the north ... If and when there is a high level alert, the USGS will inform the California office of emergency services, which will then issue a warning to the public.
This warning signifies that the seismologists at the USGS in Menlo Park believe there is, at least, a 30 percent chance a magnitude 6 or higher earthquake will occur in Parkfield during the next 72 hours.
There are no guarantees, of course, that the Parkfield research team will be able to predict the next sizable quake in that region in time to issue a warning, but members of the Parkfield team believe that their project is valuable even if a short term warning is not issued. It's enabling us to record the details of fault slip to a degree that they've never been recorded before, and if there are precursors recorded by instruments that are involved in the alert level scheme, we'll be able to detect them after the fact, and then maybe we can use them to an issue of prediction of the next moderate earthquake in California.
In the meantime, the data recording devices at Parkfield and the scientists at Menlo Park maintain their seismic vigil. Along The San Andreas Fault and on active faults all over the world, blocks of the Earth's crust continuously try to slide past one another. This causes the rocks in the fault zone to bend and deform accumulating tremendous tectonic stress in the process.
Eventually, this massive store of energy will exceed the breaking strength of the rocks triggering an earthquake. There is no doubt that this will occur. The principal questions are when and how strong the resulting earthquake will be.
While earthquake research continues, the hopes of a reliable short term forecasting system are still probably decades away. This task is complicated by the fact that each fault is unique, and successive earthquakes on a given fault can vary considerably.
In the meantime, there are tangible steps that we can take to prepare ourselves for the earthquake that will surely occur. Nature has forced us to live with these sudden releases of energy, but knowledge gives us the power and the chance to survive them.
Music Major Funding for "Earth Review" was provided by the Annenberg CPB Project.
Amazing how people have used these new technologies to study earthquakes, the laser beam and all these various other things. The focus of this show was about the major earthquake belts, and we learned that most of the earthquakes around the world take place in regions of plate boundaries, and we also learned that aseismic ridges, such as the Hawaiian Islands, have little or no major seismic activity, but we do have small earthquakes here in Hawaii, which are associated with volcanic eruptions and island structures.
The only really highly seismic activity is on The Big Island of Hawaii here in the Hawaiian Islands. Mostly there are small earthquakes, and they don't do much damage. There are swarms of small earthquakes called "harmonic tremors" that are associated with magma movement underground and some slumping associated with faulting along rifts and flanks.
There have been two really large earthquakes in The Hawaiian Islands, one in 1868 estimated at about 7 and 1/2 magnitude near South Point, and another one on November 29, 1975 at 7.2 magnitude near Kalapana which causes subsidence of about six feet of the black sand beach, which has now been covered by lava and also killed a couple of campers who were sleeping on the beach further up the coast.
I want to note also that much of the damage that's done during major earthquakes is not so much from the earthquake itself but from after effects, mostly from fire.
Major earthquakes often have the effect of destroying gas lines or breaking gas lines and electric lines, and the combination of gas and electricity, of course, is a dangerous combination. Fire is the cause of most damage from earthquakes in populated areas.
In the 1906 San Francisco Earthquake, fires ranged out of control for several days, partly because water supplies are also damaged during a major earthquake, and there simply was no water available to put the fire out.
Another effect of earthquakes, which often does severe damage, is landslides. Unstable land on hillsides can be shaken lose to damage either the structures already on the hillside or those in the valley below and may dam streams causing flooding.
Okay, one final thing. Some geologic materials become liquids when they're shaken. We don't usually think of rocks as having this property, but volcanic materials and clays of various kinds, if you shake them up, they can actually turn to a liquid and flow under their own weight. You can try this at home with the coffee grounds in your coffee filter. If you take the coffee filter and shake the damp coffee grounds in the filter basket, you'll see the coffee grounds turn into a liquid this way.
Well, that's about it for this time. I'll remind you the next lesson is Lesson 10, "Geologic Time." You should read Chapter 8, pages 175 to 191 and carefully study the text diagrams, especially those which show cross cutting relationships. You might also want to go back to Lesson 3 and reread the pages 20 to 22 on Geologic Time and be sure to read the section on the Birth of the Solar System in Chapter 8; that's Box 8.1, page 185.
Well, that's it for this lesson, and I'll see you next time.