Exploring Inside the Earth. What Seismic Waves Tell Us About Earth s Interior

Transcription

1 Exploring Inside the Earth What Seismic Waves Tell Us About Earth s Interior

2 Sir Isaac Newton In Ancient times, the center of the Earth was pictured as a mysterious underworld of fiery furnaces and volcanic explosions. With the growth of a more mechanical outlook after the work of Isaac Newton, early geophysicists could make more realistic inferences about the unknown interior from the properties of surface rocks. In particular, Newton s theory of gravity had a critical effect on speculations about the inside of the Earth because it provided a way to measure the Earth s density.

3 In the 20 th century, geophysicists have achieved a far more complete and detailed picture of the Earth s interior, and they have done so almost entirely through the use of a single tool: the analysis of earthquake waves. By examining waves from all over the world, they have been able to locate boundaries and define the composition of the interior But how can we see inside the Earth using earthquake waves? The first step is to look at the earthquake recordings themselves.

4 If one looks at a typical recording of a rather large earthquake, at first, it will seem to be no more than a set of squiggly lines. Closer inspection shows the offset ticks one minute apart, to mark the passage of time, and the typical P waves, S waves, and surface waves. But what are all of the squiggles that follow the P, S and surface waves?

5 Our typical train of seismic waves is typically followed numerous seismic waves that have refracted and reflected through and off boundaries within the interior. Ocean waves reflect and refract as they interact with a shoreline When water waves encounter a boundary such as a steep shoreline, they are reflected back from the boundary; an outgoing train of waves develops, which may be seen passing through the incoming train.

6 When ocean waves roll obliquely onto a shallow beach, the waves in the shallower depths travel more slowly and lag behind those in deeper water. As a consequence, the waves are bent toward the shallower water. The lines of the waves thus turn more and more parallel to the beach before they break. Refraction is the term used to describe the change in the direction of the wave front due to shifts in speed.

7 Seismic waves may also be reflected or refracted at a boundary, but unlike other waves, seismic waves show a unique behavior when incident upon a reflecting surface within the Earth. For example, a P wave hitting a boundary surface at an angle breaks up into both a reflected P wave and a refracted P wave. As well however, it generates a reflected S wave and a refracted S wave. The reason is that at the point of incidence the rock at the boundary is being not only compressed but sheared. Refracted S Refracted P Rock discontinuity (or boundary) Reflected P Reflected S Incident P

8 The Crust-Mantle Boundary The first clear seismological evidence on the separate nature and thickness of the crust came in the early 1900 s when it was inferred from earthquake records that there is a marked structural change at a distance of about km below the surface of the continents. This famous work was performed by the Croatian geophysicist Andrija Mohorovicic.

9 While analyzing P and S waves on recordings close to the epicenter of a Croatian earthquake, he noticed that some waves seemed to arrive later than expected for waves running along the surface of the Earth. To explain the delay, he decided that downward heading P and S waves had been refracted along and up from a boundary at about 54 km depth. Surface Direct P and S waves Crust-Mantle Boundary Refracted P and S waves Subsequent studies showed that what has come to be called the Mohorovicic discontinuity, or Moho for short, is worldwide. This boundary separates the crust from the mantle below.

10 Discovery of the Earth s Liquid Core In 1906 R. D. Oldham examined seismograms recorded around the world from a single earthquake. He noted that at a distance of about 130 degrees, P waves began to be delayed by a minute or so. Furthermore, S waves could only be followed to about 120 degrees; beyond this distance the arrival was delayed by 10 minutes or so. To explain these delays, Oldham theorized that the waves had penetrated a central core that transmitted waves at a slower speed.

11 Beno Gutenberg In 1914, Beno Gutenberg aided by much more extensive observations, determined the first rather precise estimate for the depth of the core at 2900 km. Today, the boundary between the mantle and core is called the Gutenberg discontinuity in honor of Beno Gutenberg. By the 1930 s it became clear that the delay in the P and S waves was due to the liquid state of the core. P waves were being slowed by the cores liquid state, and S waves were actually being blocked by its liquid nature. Any recorded S waves were actually reflections from elsewhere within the Earth s interior.

12 The Discovery of the Earth s Inner Core In 1936 a Danish seismologist, Inge Lehman, published evidence that there is an inner core about the size of the Moon within the outer core. In her examination of recordings, Lehman found waves that could not be explained by contemporary models of the Earth s interior. She believed that the arrival times of these waves could be explained if the waves had reflected off a small inner core. Today, we recognize this boundary as the Lehman discontinuity.

13 Because P and S waves may take any of a multitude of possible paths through the Earth s interior, seismologists need a systematic notation to identify the various paths. The different waves are classified depending on their type and the encounters they have made with different major boundaries within the Earth. P = P wave in the mantle S = S wave in the mantle K = P wave in the outer core I = P wave in the inner core J = S wave in the inner core c = wave reflected at mantle/core boundary i = wave reflected at inner/outer core boundary

14 In this scheme, the notation PKIKP, for example, is interpreted as a P wave starting in the mantle, refracted into the outer core as a P wave (K leg), and then refracted through the inner core as another P wave (I leg). Finally, the PKIKP wave is refracted back out of the inner core into the outer core as a P wave (second K leg); in its final leg, PKIKP travels as a P wave through the mantle to the surface. Earthquake waves also multiply when reflected at the outer surface of the Earth. Reflected P waves with two legs are denoted PP, with three legs PPP, and so on. In the same way, we have SS, SSS, and so on for surface reflections of S waves.

15 Can you determine the paths of the following waves? PKPPKP (or P P for short) SKS (remember S waves can t travel thru liquid!) PSP PKiKP PKJKP ScP PKKKKKKKP (or P7KP for short) PKKKKP (or P4KP for short)