Earthquake Waves. Origin and Characteristics

Transcription

1 Earthquake Waves Origin and Characteristics Unless otherwise noted the artwork and photographs in this slide show are original and by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States.

2 As we will explore off and on over the next two semesters, the outer layer of the Earth is broken up into thin plates that are forced to move around on its surface, probably because of motion in an underlying thicker mantle layer. At the plate boundary between two of these plates they are generally moving in different directions. Plates moving toward each other are said to have a convergent margin between them. Plates moving away from each other have a divergent margin between. Plates slipping sideways past each other have a transform margin between. Almost (but not quite) all major earthquakes are started at a plate margin, the force responsible being the force of the two plates motions.

3 ELASTIC REBOUND Seismic (earthquake) waves are created by a phenomenon called elastic rebound. Elastic refers to a type of material behavior in which the shape or volume of a material is changed because of some force, but in which the change is reversed when the force is removed. Elastic like you find in waistbands is called that because that s how it behaves. It stretches out big enough to go over your hips when you pull on it, then shortens to fit your waist when you let it go. It gets even smaller if you let it go while it is not around your waist, and that snugness is what keeps it in place unless the elastic is shot. If you have ever broken a wooden baseball or softball bat while hitting a ball then you have felt another consequence of elastic rebound. When you hit a ball and the bat does not break the force of the impact does two things: it flattens the ball a little on the side you hit, and it bends the bat back ever so slightly. This stores a little of the energy of the ball s momentum, and of your swing, in the bat and the ball. Some of the energy also goes into the air as a sound wave, creating the famous crack of the bat; (or in an aluminum bat, its ping.) Once the force of your swing has reversed the direction of the ball s path, and the ball begins to lose contact with the bat, all of the stored energy is imparted to the ball. In becoming un-flattened and regaining it s original shape the ball pushes itself away from the bat, and in straightening and regaining its original straight shape the bat pushes the ball away from itself. The ball goes as far as it does because it is given energy from three sources: your swing, the elastic rebound of the ball, and that of the bat. When you break the bat there is also elastic rebound, but all the elastic energy of the ball and bat, plus some of the energy of your swing, makes the bat vibrate like crazy. The two results are that you are likely to get thrown out because you haven t hit the ball very far (too much energy wasted in the bat) and you will have an unpleasant feeling in your hands. The vibrations can feel like a mild electric shock!

4 Now imagine watching a time-lapse movie of a transform margin over a long interval of time. As the arrows in the first map indicate the north (top) plate is being forced eastward (right) and the south (bottom) plate is being forced westward (right). As the forces build the rocks begin to deform elastically bending as shown in the second map. Any material, including rock, can sand a certain amount of elastic deformation, but eventually it reaches its elastic limit and breaks, as shown in the third map. Incidentally, this is a fault. Immediately after breaking, the rock rebounds to its original, unbent state and jiggles, just like the two parts of a broken bat jiggle. As you see in the fourth map, the energy of that jiggling eventually damps out along the fault, but (being energy) it does not simply disappear. Instead, it moves away from the fault by elastically deforming the surrounding rocks in ways we ll see next, radiating in all directions. (This is the way sound moves as well.) THIS IS HOW EARTHQUAKE WAVES ORIGINATE!

5 Wave Types

6 Some waves move away from the fault along the Earth s surface. These we call surface waves and geologists (and others as well) are interested in them because these cause all the damage cause by an earthquake. There are several types, but we will only talk about two, and that only very briefly, and only after we ve seen the waves that move internally. Other waves move within (and throughout) the Earth. These we call body waves. There are two types and there are some interesting differences in the way they propagate through the planet. They do no damage, but we can learn a great deal about the inside of the Earth by studying how they behave. This is where most of our remaining time talking about earthquakes will be spent. Even though seismologists identify several types of waves, the basic motions involve elastic deformation of the rock they move through in only two basic ways.

7 Body Waves

8 All waves move energy over long distances, but the rock itself is generally not changed by their passage, escept for slight changes occurring as each wave passes. Because no mass is actually moved we usually say that the wave energy propagates. The leading wave creates a new wave in front of itself as it cycles. One type of body wave is called a p-wave. The p is most often treated as an abbreviation for primary for reasons we ll get into soon. It can also be thought of as meaning pressure. In a p-wave, each tiny particle each molecule or grain or crystal in a rock pushes the one ahead of itself forward (in the direction of propagation) and then retreats, pulling the bit ahead of itself back. As long as the waves are passing the rock particles jiggle back and forth. At any one moment alternating parts of the rock are being squeezed together (compressed) and stretched apart (dilated).

9 Here is a well-known object that transmits energy in the form of p-waves. In the left hand photo Ms. Standridge is just beginning the pull phase of the game and in the right she is just finishing it up. The zones of compression and dilation within this experimental device are indicated by the arrows.

10

11 The other type of body wave is called as s-wave. The s is most often treated as an abbreviation for secondary for reasons we ll get into. It can also be thought of as meaning shear. In an s-wave, each tiny particle pushes the one ahead of itself up and down, perpendicular to the direction of propagation. As long as the waves are passing the rock particles jiggle back and forth. At any one moment alternating parts of the rock are moving upward and downward. Actually there is a 3-dimensional motion in most cases, but that s difficult to draw and this is a close enough approximation for you to get the idea.

12 Jiggling the end of a rope tied at the other end shows how this works. Alternating parts of the rope move upward and downward as the wave moves the energy along the rope. The arrows show the direction of most recent motion at different places on the rope in each photograph. The lower photograph shows the rope with more vigorous jiggling. Because each wave carries more energy the wavelength is greater. In the top picture there were two full waves (one you can t see); in the lower, only one full wave is in the same length of rope. Thanks to Deborah Standridge for the energy input to the Slinky and rope.

13 Surface Waves

14 There are surface waves that move like p-waves others that move like s-wavesw, but the two that we ll examine, the two that cause the most damage, both move more-or-less like s-waves. Rayleigh waves are rather like the waves that move across the ocean s surface. As the energy passes the ground surface rises and rocks back and forth a little. This causes structures on the surface to sway back ad forth parallel to the direction of propagation. Tall buildings, of course, are affected most noticeably. I had a Spanish professor once who was on the top floor of a tall hotel in Mexico City during an earthquake in the 1960's. He said he knew what was happening when he awoke flying through the air toward the door and then suddenly found himself sliding back across the floor toward the bed. A Rayleigh wave did this to him he didn t really move, for the most part. The building rocked beneath him. Love waves are similar, but the ground motion is horizontal. Structures twist back and forth as a Love wave passes. In both cases a building that is not specifically designed and reinforced to take the stresses will be severely damaged in a moderate to strong earthquake, either by toppling over (Rayleigh wave) or by shearing off its foundation and collapsing.

15 Behaviors That Waves Have In Common (p- and s-waves)

16 1) Earthquake waves, like all energy waves, radiate outward from the source as concentric spheres of WAVEFRONTS. We will be interested in the pathways followed by small "bundles" of the energy as they propagate along WAVEPATHS through the Earth. A wavepath is identified by drawing an arrow at right angles to the concentric wavefronts. (For mathematical purists the paths are perpendicular to a tangent to the wavefront at the point where the wavepath crosses it.) The diagram illustrates the idea. The crests of s-waves or the compressed parts of p-waves are indicated by the circles and move away from the source of energy (the broken place on a fault) in concentric circles, just as the waves do on water into which something has been dropped. This, of course, is a 2-dimensional representation of a 3-dimensional system. If the energy is input near the Earth s surface, as in a pebble dropped into water, the energy doesn t just move across the surface. It also moves downward. The rings you see are where the spherical wavefronts intersect the visible surface. The next slide illustrates the idea.

17 You see this. Surface You can t see this.

18 2) All waves move at different rates in materials of different types. For our purposes we will say that they move faster in denser substances, though this is not strictly true. (Other factors can override the density to alter the rate.) Sound waves are essentially the same as seismic waves and they move faster through water than air. (That s why the noises around a quiet lake sound so eerie. The waves coming through water arrive at your ear earlier than the ones moving through air, so you hear everything twice. The effect is more noticeable with larger distances from the sound a point we will get back to. Thus the wavepath first recorded by a seismograph is NOT the one that follows a straight line path from the epicenter to the seismograph. Rocks are under greater pressure at greater depth and the extra pressure makes them denser. So the wavepath first recorded is one that passed deeper into the earth and curved back upward to the recorder. The straight-line wavefront arrives too, but later. The first arrival is the path that best balances speed and distance to get to the recorder first. That wave is refracted. We will see refraction again in a slightly different way. As we examine what can be inferred about the inside of the Earth from earthquake waves we will be seeing a lot of these curved wavepaths.

19 REFLECTION

20 3. When a wavepath reaches a layer with abruptly different density some (but not all) of the energy reflects off that surface. We call such a surface a density discontinuity. This phenomenon is called reflection. An echo is a reflection of sound waves. Generally, abrupt changes in density coincide with changes in rock type. Sedimentary rocks usually have lower density than igneous rocks. Mudstone has a lower density than sandstone, and so on. Deep within the Earth some discontinuities probably show where one kind of miner is collapsed by pressure into a different structure. Diamond, for example, requires pressures that are far higher than surface pressures, so the diamond structure can only form if the rock is at the proper depth. A source of waves, for example and earthquake, will send wave fronts off in every directions, but any sensor we use to detect the waves will see one and only one point on the wave the one that goes to the disconformity and reflects off at a point half-way between the source and the sensor. In other words the angle of incidence of the recorded wave (I) is the same as the angle of reflection (R). All the other wavepaths will reflect but the sensor will only see this particular one. sensor Half-way point source Crust (density ~2.5) Mantle (density ~3.5) R I

21 REFRACTION

22 Much of the energy of the wave continues across a discontinuity but the wave front that continues in a straight line is not the wave-front of interest for a particular sensor. How this is known is calculated from the time required for the wave to reach the sensor when it reflects from an even deeper discontinuity. A later slide will elaborate a little on this but it is really more complex than we need to consider in any great detail. This change in direction is called refraction. As with reflection, there are wavepaths going in all directions into the lower layer but a sensor will only see one of them. If the layer below a discontinuity has a higher density than the upper layer then the wavepath seen by a sensor will bend downward. This is the more common case because of the increasing pressure with depth. If the layer below a discontinuity has a lower density than the upper layer then the wavepath seen by a sensor will bend upward. density ~2.5 density ~3.5 R I density ~3.5 density ~2.5 R I

23 The angle at which the path is continues is called the angle of refraction. What the angle is is dictated by the difference in density between the two layers. The example shows that if the density changes from 2.5 to 3.5 the angle of refraction is greater than if the difference is Any change in density produces only one angle of refraction. We will return to the importance of this. If the layer below a discontinuity has a higher density than the upper layer then the wavepath seen by a sensor will bend downward. density ~2.5 density ~3.5 R I density ~2.5 density ~3.0 (Angle of refraction from the other diagram)

24 The real reason for this refraction is that the wave moves faster in the denser medium. (This is not perfectly true. There are many other variables that control speed, but for our purposes we can assume this is the case with the waves we ll discuss.) The sensor first records the reflection from the lower discontinuity that follows the path indicated because that is the wavepath that reaches the sensor first. It is the velocity difference that matters. sensor Half-way point source density ~2.5 density ~3.5 density ~2.5

25 Light rays refract as well as the pass across materials of different density, but, unlike seismic waves, moving slower in the denser media. The arrows indicate images of the same fishes as seen through the corner of an aquarium. Many thanks to Dr. Brown s guppies for posing for my picture.)

26 Remember the earlier slide that mentioned first-arriving waves follow curved paths? Remember that wavefronts start off in every direction from the earthquake the sensor detects the fastest path first. We refer to this as a refraction as well because the wavepath that goes a tiny bit deeper with each incremental movement can outrun the one that continues directly toward the sensor. Of course this falls apart once the wave is half-way through. For the second half of its trip it seeks the more direct path, but, again, gets there one tiny step at a time, mirroring its path into the first half of the trip. Thus we say that waves refract continuously as they move deeper because density increases, and then again in the opposite sense as they move shallower. It is the pressure of overlying rock causes the change in density with depth. As we talk about Earth s internal structure you will see many such curved paths and I wanted you to know why we draw them that way.

27 Behaviors That Are Different In p- and s-waves

28 Besides the difference in the ways the material moves as the wave passes (parallel or perpendicular to the direction of propagation) there are two very important differences in the way that p- and s-waves behave: 1) p-waves are faster than s-waves. (This is why they are called primary they arrive at a sensor first. Secondary waves arrive later. 2) P-waves can move through any material solid, liquid, or gas. (Sound waves are just p-waves moving through air.) S-waves, in contrast, can only move through solid materials. In liquids and gasses the atoms and molecules are not attached together as completely and tightly as they are in solids. When one mass of water pushes against the next it can compress it, but when one mass moves sideways past the next most of the energy of that motion is simply lost to friction between the loose molecules. S-waves are damped very quickly once they move into a liquid. They are also slowed down in a plastic material (like silly putty), which behaves in some ways like a liquid. I haven t said much about these two points but they are very important in understanding the internal structure of Earth. Don t let the fact that I covered them in one slide fool you.