Introduction to Earth Sciences I


Topic 4

Earthquakes, Seismic Waves and the Earth's Interior

 

Objectives:

Earthquakes are one of the most profound expression of the Earth's dynamics. They provide the most direct evidence that the rigid outer shell of the Earth is in motion. They are also the cause of huge tragedy as seen in earthquake losses in the last two years (see Case Studies below). The objective of this part of the course is to provide some basic information on earthquakes as physical phenomena, what makes them happen and why they happen where they do, and how the propagation of seismic energy generated by earthquakes propagates through the Earth and can be used to infer information about the interior properties of the planet. So I hope you will come away from this course having a basic understanding of -

1. What an earthquake is, why they occur, and why they occur where they do.

2. How the seismic waves generated by earthquakes are used to image the Earth's interior and some things we have learned from these images.

Your text is reasonably good on the basics of this subject (Chapters 14 and 15) but neither of the book's author's are seismologists and you can find better treatments of most of the material at Web sites created by researchers at earthquake research centers. The four that I think will be most useful are:

http://vearthquake.calstatela.edu/ which is from Cal State LA and goes through a number of simple exercises in finding earthquake locations etc that we will do in class or in the lab.

http://scecdc.scec.org/eqcountry.html and http://www.scecdc.scec.org/Module/TableofC.html from the Southern California Earthquake Center, a consortium of universities in Southern California (but including Columbia) who combine to do research on earthquakes and their associated hazards. I have had the most difficulty getting to this site. We will use quite a lot of materials from this site.

We will discuss two new discoveries about the deep earth that seismic analysis has revealed. One is the "super-rotation" of the inner core. A description of that can be found at
http://www.ldeo.columbia.edu/press_releases/song/pr.html
which includes an animation. The scientific paper is published in the journal Nature and would be too technical for our purposes.

We will also study the nature of the so-called D'' Layer which is at the boundary between the outer core and the lower mantle. It exhibits a surprising degree of heterogeneity and structure suggesting that the core/mantle boundary is a very dynamic interface. This was published in Scientific American in May 1993 by Jeanloz and Lay. It's a very accessible article that you should be able to read without much difficulty.

4.1 Some Essentials

4.1.1 Earthquakes Occur On Faults

Three pages at SCEC provide some good definitions.

Earthquakes occur when the Earth breaks. The breakage is called a fault. They are the instantaneous failure of a place on the Earth that is under stress caused by the far field action of the major lithospheric plates that are in constant motion driven by the Earth's heat engine (see Topic 2). This is an important point that the popular press often gets wrong -- earthquakes result from the instantaneous localized release of stresses that are broadly distributed and essentially uniform. They do not result from an instantaneous increase in stresses. In this way they are analogous to the landslide on Bak's sand pile in that the avalanches occur as instantaneous isolated events driven the steady drip of sand from the sand faucet.

Earthquakes usually occur on faults that were known to exist before the earthquake happened, and have had a prior history of earthquake activity.. Many earthquakes occur on the San Andreas Fault, for instance. In a simple way this is a type of prediction. We don't really expect earthquakes to occur in places where there are no faults. Put another way, earthquakes usually occur in places where they have occurred before -- on "active" faults. This is where the sandpile analogy breaks down. To be accurate the sandpile would have to be pre-figured with the scars of previous landslides leading to a tendency for new landslides to occur at the locations of old landslides. The Earth has a memory of where it broke before.

Activity: From SCEDC Section #2 Activity #1 using Seismicity for 1932-1996 chart

Although in detail faults really come in all configurations and accommodate stress relief in many different ways, there is a basic classification of faults into three types -- normal faults, reverse faults and strike-slip faults. Your book has some reasonable diagrams that show the motions associated with each type of fault.

Normal faults relieve tensional stresses and result in two parts of the crust moving apart.

Reverse (or thrust) faults relieve compressional stresses and result in two parts of the Earth moving together. They look much like a normal fault on which the motion has been reversed. Hence the name. SCEDC Section 1, Page 14 and Page 15

Strike-slip faults relieve lateral stresses and result in two parts of the crust sliding along by one another. Strike-slip faults can be right lateral or left lateral.. The difference is easy to recognize -- imagine yourself standing on one side of the fault, the the other side moves to the right, the fault is right lateral, left lateral if it moves left. The best known fault in the United States, the San Andreas Fault, is a strike-slip fault. SCEDC Section 1, Page 16

It is rare for one type of fault to exist alone. Strike-slip faults often have compressional stresses associated with the crust on either side of the fault, especially if it isn't perfectly straight (and they never are) so normal and reverse faults are commonly found as secondary features associated with the main strike-slip structure. SCEDC Section 1, Activity 13 and map showing sense of slip density

 

4.1.2Faults dont slip all at once

The animations of fault motion shown above are intentional simplifications designed to show the basics of how the different types of fault motions take place. In the real Earth faults do not move all in one simple motion. Typically one segment of fault moves leaving the remainder without any motion. The San Andreas is a huge fault several thousand kilometers in length. When an earthquake occurs on that fault only a very small portion of that fault actually moves.

Generally slip on a fault nucleates at one place and ruptures a segment of the fault surface. The fault block model shows this idea. The earthquake is said to occur at the nucleation point and the area affected by fault motion is called the rupture zone. Large earthquakes have very large rupture zones, perhaps several square kilometers, small ones may effect only a few square meters of the fault. The hypercenter of an earthquake is the place where the earthquake nucleates and the epicenter is the location on the earths surface directly above the hypercenter.

SCEDC Section 1, Page 6 (Epicenter/Hypercenter diagram)

Slip on a fault is typically progressive. The fault ruptures in one place and the area slipping moves along the fault from one place to another. The two examples below illustrate this. They show the evolution of slip on two recent earthquakes in southern California, the Northridge and Landers events. Particularly in the Landers case we see the slip progressively move from south to north along the fault triggering activity on three separate fault segments.

SCEDC Section 1, Activity 3 (Rupture Model Animations including Landers rupture and energy models and Northridge rupture model)

We are starting to learn some surprising things about the evolution of fault slip. It now seems that motion on one fault can trigger activity on other faults that are quite some distance away and that new seismic activity may occur quite some time in the future.

What happens is, in principal quite easy to understand. A place like southern California is a complex deformation zone with many different faults of different type and in different states. Some have recently had earthquakes and some have not had earthquakes for many years. Because an earthquake represents a release of stress in the earth this complex array of faults are in different states of stress some ready to ruptuure. When an earthquake releases stress on one fault or segment the earth motion may transfer the stress to another fault region. The release of stress in one place may increase stress in another place. This newly stressed region may then become prone to new earthquake activity.

4.1.3 Damage Can Occur At Considerable Distance From An Earthquake.

The great San Francisco earthquake of 1907 did not occur on a fault that ran directly through San Francisco. The largest ground motions associated with earthquakes occur on the faults themselves but a great deal of damage is often reported a considerable from the trace of the fault. Sometimes the fault itself can be quite difficult to locate and not an especially dramatic feature.

This distant damage results because the energy that is released at the fault by the earthquake cannot stay localized to the region of the fault itself. The energy propagates away in seismic waves which travel through the Earth and disturb the crust as they pass. These waves are something like the waves one sees in a body of water if a stone is dropped in. The ripples move away from the site of the dropped stone in circles and as they do they reducein size. At some great distance one would never know that the stone had been dropped into the water. The same is true of earthquakes in that as distance increases from the earthquake the disturbance reduces in size. A large earthquake will cause disturbances at distance from the quake that are capable of significant damage, just as a large object thrown into water will cause large waves that travel considerable distances.

The motion associated with an earthquake is very complex and the waves that travel away from the earthquake are similarly much more complex than a simple ripple in a pond. Several different types of waves are generated simultaneously and are associated with different types of ground motion. For instance, to effect a city located north of an earthquake the seismic waves obviously must travel north to reach the city. But when they arrive the motion of the ground will be in both a north-south direction (the direction the waves traveled) and east-west, exactly orthogonal to the direction of travel. And the two types of disturbance don't arrive at the same time. Those making motion in the direction of travel of the wave arrive ahead of those making motion in the orthogonal direction. The first type are called Primary or P-waves, the second are called Secondary or Shear waves designated S-waves. Both are elastic waves and both can travel from one side of the Earth to another from sufficiently large earthquakes. Even small earthquakes send waves to great distance and can be detected by sensitive instruments called seismometers that can pick up very tiny ground motions much too small to be detected by people. These two types of seismic waves will prove to be very useful when we come to use seismic energy to learn about the interior of the Earth.

SCEDC Section 3, Page 9 (Seismic Waves Identified)

SCEDC Section 3, Activity 2 (Measuring Earthquakes -- Seismic Waves)

SCEDC Section 3, Page 10 (P.S. I Rayleigh Love U)

SCEDC Section 3, Page 11 (P is for Primary Waves)

SCEDC Section 3, Page 12 (Secondary Shear Waves)

 

4.1.4 Earthquakes Have A Distinct Global Distribution

Even a cursory look at a map of the world showing the locations of earthquakes makes it clear that they are anything but uniformly distributed. They occur in distinct bands. In fact, most of the Earth is relatively free of earthquakes. A map showing the locations of the deepest earthquakes we see that their locations are even more restricted. Deep earthquakes are essentially absent in the US, for instance, but common along the west coast of South America. If instead of looking at a plan view, we were to make a section through the Earth at one of the locations where deep earthquakes occur we see that they form a very distinct band penetrating deep into the Earth.

Go to Pre-Assembled Event Information page.

These zones of intense earthquake activity mark the major lithospheric plate boundaries -- those places where the plates are being created, destroyed and are sliding past one another. The latter two types of boundary create the most intense activity. The most destructive earthquakes to have taken place on Earth occurred at subducting plate boundaries where the lithosphere is being thrust deep down into the mantle. Where one plate is diving beneath the other tremendous forces are at play. One plate literally rubs against the other, sometimes sticking and building stresses released in earthquakes, sometimes sliding. In a subduction zone dipping at a relatively shallow angle a large area of the two plates is in contact and when such a large area slips instantaneously there is the potential for a very large earthquake. The larger the rupture area the larger the earthquake will be. So if a very large section of a strike-slip fault gives way, a very large earthquake is also possible.

The type of faulting associated with the three different plate boundaries is consistent with the overall motion of the plates -- normal faulting dominates at divergent boundaries where plates are being created, reverse faulting at subduction systems and strike-slip faulting at transform boundaries. It is possible to analyze the seismic waves from an earthquake recorded at a number of locations around the world to determine the location, depth, magnitude and focal mechanism (the type of motion that gave rise to the earthquake). http://vearthquake.calstatela.edu/ is a good source for how to do earthquake location and magnitude determination. That is, we do not need to be on top of an earthquake to know where it occurred and why. Today, there is a world wide network of identical seismometers operated to monitor earthquakes and quickly determine their size and location. An earthquake larger than about magnitude 4 anywhere in the world will be detected by this network and quickly located. Smaller earthquakes are recorded on local parts of the network. The accuracy of earthquake location depends on the number of seismometers that recorded the event and how well they are distributed with respect to the quake. Generally, the more seismometers that record the event the better it will be located but the best locations are obtained if the seismometers actually surround the earthquake.

4.1.5 Locating the Earthquake

The Virtual Earthquake: http://vcourseware5.calstatela.edu/VirtualEarthquake/VQuakeExecute.html

4.1.6 It Is Not Hard To Tell An Earthquake From A Nuclear Explosion

The worldwide network of seismometers was set up in large part in response to the need to establish a mechanism to verify compliance with the Comprehensive Nuclear Test Ban Treaty. But how can we distinguish a nuclear explosion from an earthquake? In fact it is not terribly difficult.

Think about the mechanism of any earthquake. Regardless of whether it is generated by normal, reverse or strike-slip faulting what is always taking place is the sliding of one part of the crust against another. This produces very distinct seismic energy radiation patterns. Imagine yourself standing in front of a normal fault. The ground motion you felt (as would a seismometer at that location) would be down and toward you as the crustal block slides down the inclined fault. On the other side of the fault the opposite motion would occur and to the side some shearing motion would take place. So the motion you feel depends on where you are with respect to the fault and the seismic energy radiated from the fault is similarly different in different directions away from the fault. It is this information that allows us to determine what sort of fault caused an earthquake.

Now think about an explosion. Unless the explosive agent is distributed in a line or other pattern we would expect a fairly uniform distribution of energy in all directions from the explosion. There is no reason why any direction away from the explosion would receive more energy than any other. No fault-generated earthquake can produce that sort of radiation pattern. Also. because there is no real sliding motion associated with an explosion very little shearing occurs and hence very little S-wave energy is present. The ratio of S-wave to P-wave energy from an explosion is therefore unusually low and quite unlike a fault-generated earthquake.

 

4.2 Earthquakes Disasters: Two Case Studies

Living in New York there seems to be little need to worry about earthquakes. We hear about earthquakes but they always occur somewhere else, either in a distant part of the world, or on the opposite of the US. Few people living in Manhattan would rate earthquake risk very highly among the hazards to life of living in the city. One is much more likely to be run down by a taxi than to be injured in an earthquake. So why worry?

For the most part there is probably little need for Manhattanites to be too concerned about earthquake hazard. Earthquake have occurred in the northeast and some have been very destructive. Destructive earthquakes are sure to occur again in the area and when they do the potential for property damage is very real and the possibility of loss of life is also real. It is extremely unlikely, however, that loss of life in a Manhattan earthquake will rival that which occurred in two very recent earthquakes that we will use in this class as examples of how different types of earthquakes in different setting can cause loss of property and life. The first is the earthquake and tsunami that took over 2000 lives in a small village on the north coast of New Guinea in July 1998. The second occurred in August 1999 in a densely populated part of Turkey and took more than 12,000 lives, and the count is still increasing. The causes of these earthquakes are very different.

 

4.2.1 Sissano Lagoon, July 17, 1998

The USGS Web site provides an excellent description of this earthquake event. The lives that were lost in this earthquake were not caused by the earthquake itself, but by a submarine landslide that the earthquake caused. This is a very dramatic example of a very common phenomenon ­ earthquakes don't usually kill people. It is extremely rare that people are swallowed up in gaping chasms caused by earthquakes (Superman movies notwithstanding) or crushed by the movement of the Earth itself. Usually people die in the ruins of buildings that collapse on them or by being struck by falling objects. There is an old adage that says .... earthquakes don't kill people; buildings do. In Sissano Lagoon the earthquake of July 17 didn't harm anyone. It was felt by many people, but no damage was done and no-one was injured. What killed more than 2000 people was a tsunami (Tidal wave) that resulted from the earthquake.

Off Sissano Lagoon the seafloor slopes very steeply and rapidly attains depths of 4000 meters (www.walrus.wr.usgs.gov/tsunami/PNG.html). Sediments that collect on the steep slope are unstable and can slide down the slope in a phenomenon that is the submarine equivalent of a landslide or avalanche (a term restricted to the description of snow slides). Witnesses have described how air is pushed in from of landslides creating strong winds. When a submarine slide, called a slump, moves down slope a large amount of water is displaced. Because the surface of the ocean is free to deform the displaced water tends to cause the sea surface to bulge upward and a wave of risen water moves out in all directions from the area of the slump. Tsunami waves created in this manner can travel at frightening speeds of as much as 1000 miles per hour. Traveling away from the coast they cause no discernible effect but traveling toward the coast they are natural hazards of immense destructive force. These waves break as they enter shallow water just as normal ocean waves do at a beach. The villagers of Sissano Lagoon, gathered in the evening for their meal, would have seen a wall of breaking water as much as 50 feet high coming directly at them so fast that escape was impossible. Four villages were completely wiped out and others badly damaged. While the loss of life of 2000 seems gruesome enough, the coastline here is very sparsely populated. A similar event off Miami, Seattle or New York would kill tens of thousands. Is this impossible? The answer is "no".

The offshore slope of the seafloor adjacent to many coastlines is often quite steep. Sediments are thick and unstable. Almost all continental slopes exhibit some real potential for unstable behavior.

 

4.2.2 Turkey, August 1999

At perhaps the worst possible time of day, 03:02 a.m., on August 17th, a magnitude 7.4 earthquake struck near the town of Izmit in Turkey. At that time of day almost everyone is in bed asleep and unable to take any actions to evade the catastrophe. Loss of life in earthquakes is strongly dependent on the time of day at which the earthquake strikes. People die in houses and other buildings. The more people who are out of their houses during an earthquake, or at least awake enough to get out of their houses or find a safer place within, the lower the death toll. The fact that structures, not the Earth, kill people was bought home graphically and sadly in Izmit.

The New York Times ran headlines on August 18th through the 21st on the quake together with an article in the Week in Review Section on Sunday the 22nd, and an article in Science Times on Tuesday, the 24th. Watching the changing emphasis of these pieces it becomes increasingly apparent that the social cost of this event was due to many factors under the control of humans. In a 7.4 magnitude earthquake people will die and property loss will be high. The state of our science and the understanding of earthquakes does not presently allow any useful prediction skill - we don't know when an earthquake will occur or how large it might be, but we do have a refined knowledge of where earthquakes will occur. The Izmit earthquake took place on the Anatolian Fault, a structure of known historic earthquake activity. Seismologists knew that large earthquakes would occur in this region - not when or how large exactly - but that they certainly would occur. Knowing that, we can take some preventative actions. Primarily we can build earthquake resistant houses and buildings. The second day of reporting in the Times, the headline featured the key issue, "Recrimination and Rescue in Turkey; Toll hits 4,000. Shoddy Housing Vulnerable". What is always apparent in an earthquake is that the damage seems capricious. Some structures stand almost unscathed, others completely collapse. Why ?

In general there are two reasons:

1) Poor construction - although people live in areas that have experienced repeated earthquakes over centuries, and their description appears in the historic records of those people, construction practices for structures that house people put those individuals in danger. In general, the knowledge of engineering practices that allow buildings to resist earthquake shaking is quite new. So, many older structures in earthquake prone areas are highly vulnerable to damage by shaking and loss of life occurs from building collapse. Today it is possible to make most structures earthquake resistant, even large sky scrapers such as the _____________________ in Malaysia. Part of the secret is to allow the building to experience a controlled yielding to the shaking motion. Buildings have internal members that absorb the motion. Think about how a car would be shaken if there were no shock absorbers at the wheels. In a car the shock absorbers take up a large amount of the motion that would be transmitted to the car due to bumps in the road. Building design now includes shock absorbing elements in their frame structure which act the same way. Smaller, domestic structures can be improved in many ways also. One simple measure is to ensure that the main frame of the house is firmly attached to the foundation. There have been many instances where houses have literally shaken off their foundations. In the Kobe earthquake one of the series of issues was that many traditional Japanese style houses have fairly sparse, light frame construction with heavy tiled roofs. This works if the structure is static, but if shaken the roof often collapses the light frame beneath, completely squashing the underlying structure.

In the final analysis, however, there are few structures that can withstand a direct hit from a huge earthquake. The job of the earthquake engineer is to design structures that minimize damage - not eliminate it. In the Izmit earthquake it appears that human greed was the major cause of loss of life. Although earthquake building codes existed many new buildings were built in ways that did not conform to code using poor construction materials. Corrupt local officials have been blamed, but the outcome of inquiries is not yet known.

2) Poor underlying ground conditions - No mater how strongly a building is constructed it will suffer major damage if it is not built on solid ground. In many graphic news photos of earthquake damage we see many in which a building appears to have toppled over almost intact. In most instances this occurs because the ground beneath the structure has failed. Most often the ground is not solid rock but a weak soil and the foundations are set into this weak material. The shaking associated with an earthquake causes the weak material to become even weaker - in fact, it can lead to a liquification of the soil in places. Where the foundation sits on the liquefied soil it will sink causing the building to topple over.

As an aside, the leaning tower of Pisa is leaning because it is built on very weak material and it rains a lot in the Pisa area. The water table is quite high so the weak soil is often saturated. It is in a near-liquid state. The soggy soil is somewhat weaker on the Southern side so the tower, which has sunk as a whole several meters into the ground, has simply sunk more on one side than the other, causing the lean.

The Izmit event provides an example of how, although earthquakes cannot be predicted with precision some aspects of the way they behave can give clues to where they might occur next. (http://www.sciencenews.org/sn_arc99/8_28_99/fob2.htm). The Izmit event occurred on the North Anatolian Fault; a feature that has experienced significant historic activity. In fact, there is a distinct pattern to the seismicity. Major earthquakes have been marching east to west over the last century. Each earthquake ruptures a certain length of the fault and in doing so releases stress in that region. What appears to be happening is that each stress release event also increases the stress on another part of the fault. On the North Anatolian fault this progressive release/loading phenomenon is moving west implying that the area that has been loaded is now to the west of Izmit. Istanbul, an ancient city with many old structures, lies to the west of Izmit. It is generally thought that the Izmit earthquake has made Istanbul more vulnerable to earthquakes in the future. Just when, where and how strong the quake will be is not known.

Earthquakes marched west along the North Anatolian fault this century (top).

These shocks created stress, in red, in the region of the August 17th, 1999 quake (bottom).

 

4.3 An Earthquake Or A Clandestine Nuclear Test

 

4.4 Earthquakes And The Interior Of The Earth

The analysis of the seismic waves sent out by earthquakes provides ius wiuth some of the greatest insights into the interior structure of the Earth. They provide perhaps the most direct way to view the Earths interior since they can be used to create an image of the interior.

This is because the interior of the Earth is composed of layers and structures that propagate seismic energy at different speeds. We learnt above that an earthquake gives rise to many types of seismic waves, the P-wave and S-wave being the first two to arrive at any site from an earthquake. The interior layers of the Earth propagate these waves at different speeds, generally increasing downward in the earth. At the top of the mantle the p-wave velocity is about 8 km/sec and this increases downward to about ____ km/sec. The S-wave speeds vary from ___ to ___ km/sec.

Most important for deriving an understanding of interior properties, S-waves do not propagate in liquids. Remember that the S-wave is a shear wave that results fron a motion in the earth that literally tries to shear the rock. Liquids cannot be sheared, of course, so S-waves cannot pass through them. So S-wave velocities increase in the mantle until the core is reached then they go abruptly to zero in the liquid outer core.

Summary of the average seismic-wave velocity and density profiles through the Earth according to the PREM model. The velocities of compressional (VP) and shear (VS) waves are given on the left, density on the right, and pressure as a function of depth on the top scale (see Table 2). From :Encyclopeadia Britannica view this link for more context.

 

The seismic waves bend (refract) as they pass through different layers and bounce off the interfaces between layers of very different material just as sound echoes off the walls of a canyon. They refract around the liquid outer core giving rise to shadow zones on the opposite side of the earth from the location of the earthquake.


From: Geophysical Properties of the Earth

By recording and analyzing the many seismic waves that are recorded at many locations throughout the Earth we can build up a picture of the structures beneath in much the same way as is done in medical imaging. The technique is called seismic tomography and is borrowed directly from the CAT scan techniques of medicine.
A detailed description of Seismic Tomography: http://www.geof.ruu.nl/~bijwaard/abstracts/vakidioot/vak_uk.html

Tomography using S-waves or P-waves or a combination of the two has in recent years been used to create remarkable images of the interior structure. What is being images are changes in seismic velocity which are associated with temperature variations in the Earth. Particularly for S-waves propagation speed is very sensitive to temperature with faster regions being associated with colder material, warmer with slower velocities. Thus, we are able to image the decent of a slab at a subduction zone because that material is relatively cold compared to its surroundings. Similarly regions of hot upwelling mantle appear as distinct regions of slower velocity material. One thing we have learnt most recently is that some slabs seem to penetrate much deeper into the mantle than most scientists had thought possible. In fact, they appear, in places, to penetrate to the core itself!

As well of these general pictures of mantle structure emerging from tomographic analysis special properties of seismic waves can be used to reveal features of the deep structure in surprising detail. Here are two examples.

 

4.4.1 Inner Core Rotation

Lamont news site: http://www.ldeo.columbia.edu/press_releases/song/pr.html

 

4.4.2 Structure of Core-Mantel Boundary

Text coming soon
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