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What are earthquake (Seismic) Waves?
A seismic wave is simply a means of transferring energy from one spot to another within the earth. Although seismologists recognize different types of waves, we are interested in only two types: P (primary) waves, which are similar to sound waves, and S (secondary) waves, which are a kind of shear wave. Within the earth, P waves can travel through solids and liquids, whereas S waves can only travel through solids.
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Types of Seismic Waves
There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves. Body waves can travel through the earth's inner layers, but surface waves can only move along the surface of the planet like ripples on water. Earthquakes radiate seismic energy as both body and surface waves.
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Body Waves
Traveling through the interior of the earth, body waves arrive before the surface waves emitted by an earthquake. These waves are of a higher frequency than surface waves.
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P Waves
The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave, and, consequently, the first to 'arrive' at a seismic station. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air. Have you ever heard a big clap of thunder and heard the windows rattle at the same time? The windows rattle because the sound waves were pushing and pulling on the window glass much like P waves push and pull on rock. Sometimes animals can hear the P waves of an earthquake. Dogs, for instance, commonly begin barking hysterically just before an earthquake 'hits' (or more specifically, before the surface waves arrive). Usually people can only feel the bump and rattle of these waves.
P waves are also known as compressional waves, because of the pushing and pulling they do. Subjected to a P wave, particles move in the same direction that the the wave is moving in, which is the direction that the energy is traveling in, and is sometimes called the 'direction of wave prop
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S Waves
The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium. It is this property of S waves that led seismologists to conclude that the Earth's outer core is a liquid. S waves move rock particles up and down, or side-to-side--perpindicular to the direction that the wave is traveling in (the direction of wave propagation).
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Surface Waves
Travelling only through the crust, surface waves are of a lower frequency than body waves, and are easily distinguished on a seismogram as a result. Though they arrive after body waves, it is surface waves that are almost enitrely responsible for the damage and destruction associated with earthquakes. This damage and the strength of the surface waves are reduced in deeper earthquakes.
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Love Waves
The first kind of surface wave is called a Love wave, named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moves the ground from side-to-side. Confined to the surface of the crust, Love waves produce entirely horizontal motion.
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Rayleigh Waves
The other kind of surface wave is the Rayleigh wave, named for John William Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885. A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.
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*Note Animations by Lawrence W. Braile, Professor, Department of Earth and AtmosphericSciences, Purdue University, West Lafayette, Indiana
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P, Compressional, Primary, Longitudinal
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S, Shear, Secondary, Transverse
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L, Love, Surface waves, Long waves
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R,Rayleigh, Surface waves, Long waves, Ground roll
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VP ~ 5 – 7 km/s in typical Earth’s crust; >~ 8 km/s in Earth’s mantle and core; ~1.5 km/s in water; ~0.3 km/s in air
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VS ~ 3 – 4 km/s in typical Earth’s crust;
>~ 4.5 km/s in Earth’s mantle;
~ 2.5-3.0 km/s in (solid) inner core.
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VL ~ 2.0 - 4.4 km/s in the Earth depending on frequency of the propagating wave, and therefore the depth of penetration of the waves. In general, the Love waves travel slightly faster than the Rayleigh waves
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VR ~ 2.0 - 4.2 km/s in the Earth depending on frequency of the propagating wave, and therefore the depth of penetration of the waves
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Note Delta is anglular distance relative to center of the Earth
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Table of P and S-P versus distance P and S-P travel times as a function of source distance for an earthquake 33 km deep. The Time of the first arriving P phase is given, along with the time difference between the S and P phases. The latter time is known as the S minus P time.
Delta Time of P S-P Time
Deg M S M S
0.0 0 5.4 0 4.0
0.5 0 10.6 0 7.8
1.0 0 17.7 0 13.5
1.5 0 24.6 0 19.0
2.0 0 31.4 0 24.4
2.5 0 38.3 0 29.9
3.0 0 45.2 0 35.4
3.5 0 52.1 0 40.9
4.0 0 58.9 0 46.4
4.5 1 5.8 0 51.9
5.0 1 12.7 0 57.4
5.5 1 19.6 1 2.8
6.0 1 26.4 1 8.3
6.5 1 33.3 1 13.8
7.0 1 40.2 1 19.2
7.5 1 47.0 1 24.7
8.0 1 53.9 1 30.1
8.5 2 0.7 1 35.6
9.0 2 7.6 1 41.0
9.5 2 14.4 1 46.5
10.0 2 21.3 1 51.9
11.0 2 35.0 2 2.7
12.0 2 48.7 2 13.5
13.0 3 2.3 2 24.3
14.0 3 16.0 2 35.0
15.0 3 29.5 2 45.8
16.0 3 42.5 2 57.1
17.0 3 55.2 3 8.7
18.0 4 7.5 3 20.5
19.0 4 18.8 3 33.4
20.0 4 29.7 3 43.8
21.0 4 40.6 3 52.9
22.0 4 51.3 4 1.8
23.0 5 1.9 4 9.2
24.0 5 11.6 4 15.7
25.0 5 20.7 4 22.6
26.0 5 29.8 4 29.4
27.0 5 38.8 4 36.2
28.0 5 47.7 4 42.9
29.0 5 56.6 4 49.8
30.0 6 5.5 4 56.6
31.0 6 14.3 5 3.4
32.0 6 23.1 5 10.2
33.0 6 31.9 5 17.0
34.0 6 40.5 5 23.8
35.0 6 49.2 5 30.5
36.0 6 57.8 5 37.3
37.0 7 6.3 5 44.0
38.0 7 14.7 5 50.7
39.0 7 23.1 5 57.4
40.0 7 31.4 6 4.0
41.0 7 39.7 6 10.7
42.0 7 47.9 6 17.3
43.0 7 56.0 6 23.9
44.0 8 4.1 6 30.4
45.0 8 12.1 6 37.0
46.0 8 20.0 6 43.5
47.0 8 27.8 6 49.9
48.0 8 35.6 6 56.4
49.0 8 43.3 7 2.8
50.0 8 50.9 7 9.1
51.0 8 58.5 7 15.5
52.0 9 6.0 7 21.8
53.0 9 13.4 7 28.1
54.0 9 20.7 7 34.3
55.0 9 28.0 7 40.5
56.0 9 35.2 7 46.6
57.0 9 42.3 7 52.8
58.0 9 49.4 7 58.8
59.0 9 56.3 8 4.9
60.0 10 3.2 8 10.9
61.0 10 10.1 8 16.9
62.0 10 16.8 8 22.8
63.0 10 23.5 8 28.7
64.0 10 30.1 8 34.5
65.0 10 36.7 8 40.3
66.0 10 43.2 8 46.1
67.0 10 49.5 8 51.8
68.0 10 55.9 8 57.5
69.0 11 2.1 9 3.1
70.0 11 8.3 9 8.7
71.0 11 14.4 9 14.3
72.0 11 20.4 9 19.8
73.0 11 26.4 9 25.2
74.0 11 32.3 9 30.6
75.0 11 38.1 9 36.0
76.0 11 43.8 9 41.3
77.0 11 49.5 9 46.6
78.0 11 55.1 9 51.8
79.0 12 0.6 9 57.0
80.0 12 6.0 10 2.1
81.0 12 11.4 10 7.2
82.0 12 16.7 10 12.3
83.0 12 21.9 10 17.3
84.0 12 27.0 10 22.2
85.0 12 32.0 10 27.1
86.0 12 37.0 10 31.9
87.0 12 41.9 10 36.7
88.0 12 46.7 10 41.4
89.0 12 51.4 10 46.1
90.0 12 56.1 10 50.7
91.0 13 0.7 10 55.2
92.0 13 5.4 10 59.5
93.0 13 10.0 11 3.7
94.0 13 14.5 11 7.9
95.0 13 19.1 11 12.0
96.0 13 23.6 11 16.1
97.0 13 28.1 11 20.1
98.0 13 32.6 11 24.1
99.0 13 37.0 11 28.1
100.0 13 41.5 11 32.0
101.0 13 45.9 11 35.8
102.0 13 50.4 11 39.7
103.0 13 54.8 11 43.6
104.0 13 59.2 11 47.5
105.0 14 3.7 11 51.4
This table is based on the iasp91 model of Kennett and Engdahl (1991) and was generated with the program ARTIM written by R. Buland. Kennett, B. L. N. and E. R. Engdahl (1991). Travel times for global earthquake location and phase identification, Geophys. J. Int., v 105, p 429-465.
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