How Do Geologists Use Seismic Waves To Learn About The Earth’S Interior?
- Joe Thomas
Geologists record seismic waves and investigate their path through the Earth. Different types of seismic waves exhibit distinct behaviors. The structure of the planet is revealed by the velocity and courses of the waves. Using data from seismic waves, geologists have discovered that the interior of the Earth is composed of layers.
How do seismic waves enable geologists to investigate Earth’s interior?
Understanding how waves behave as they flow through various materials allows us to understand about the Earth’s strata. Seismic waves indicate that the interior of the Earth is composed of a series of concentric shells, including a thin outer crust, a mantle, a liquid outer core, and a solid inner core.
Earthquakes create seismic waves, which travel differently through various types of material. Primary “p” waves and secondary “s” waves are the two most common forms of seismic body waves. P- waves are capable of penetrating solids and liquids. S- waves travel solely through solids.
We have discovered that the mantle is solid because seismometers can detect both P- and S-waves traveling into it. S-waves that go into the outer core are not detected on the other side, indicating that it is liquid. We are able to determine that the inner core is solid by observing the “phase-shift” of seismic waves as they pass through it.
Some seismic waves travel to and from the inner core as P-waves, but transform into S-waves within the core (Mussett & Khan, 2000). By studying the waves measured by a seismometer, extremely astute geophysicists can identify this phase change. Using a technique known as “seismic tomography,” it is sometimes possible to scan the innards of the earth.
- It is based on the same premise as “computer-aided tomography” or CAT-scans, which are routinely used to inspect the insides of people’s bodies in medicine.
- Seismic tomography is feasible due to the fact that seismic waves move at varying rates through various materials.
- In general, waves move more slowly through soft or somewhat liquid areas of the soil.
Typically, they are hot and sometimes partially molten regions (partially melted to a liquid). We know how quickly seismic waves travel through “normal” portions of the ground, so we can predict when a wave will arrive to a seismometer located at a specific distance from the epicenter of an earthquake.
If a wave comes “late,” we know it went through a warm, porous region of the planet. This graphic illustrates a straightforward illustration of how seismic tomography operates. tomography-1.jpg All three grids depict the same region within the planet. In a, one ray traverses each row. Ray 3 comes late, but it might have slowed down in any of the four little squares, so we cannot determine which square is hot and which is cold.
In b, one ray traverses each column. Ray 6 arrives tardily. The region of overlap between the two slow rays enables us to determine which little square (a small location within the earth) is hot and soft. This is a simple 2-dimensional example, however by measuring seismic waves from several earthquakes at numerous seismometer sites, it is possible to create 3-dimensional pictures.
- This technique has been utilized by geophysicists to photograph “hot zones” under Hawaii and southern Africa.
- I hope this information becomes useful! References: Mussett, A.E., and M.F.
- Han (2000).
- An introduction to geological geophysics with a focus on the planet’s interior.
- Cambridge University Press in New York Answer 3: Sound travels at varying rates through various materials.
Because sound emanates from a source, we can determine where and when an earthquake occurred, and based on the time it takes for the waves to reach different spots on the Earth’s surface, we can determine the path the waves must have taken to get there.
How do geologists utilize seismic waves to estimate the underlying layers of the Earth’s core, mantle, etc., and the matter state of each?
Body Waves – P-waves and S-waves are known as body waves because they pass through the solid body of the Earth. P-waves are capable of penetrating solids, liquids, and gases. S-waves travel solely through solids (Figure 1). Surface waves only travel along Earth’s surface.
- P-waves (primary waves) travel at around 6 to 7 kilometers (4 to 4 miles) per second, making them the first to reach the seismometer. P-waves travel in a compression/expansion manner, compressing and decompressing Earth elements as they go. This results in a change in the material’s volume. When traveling from one layer to the next, P-waves deflect somewhat. Denser or more rigid materials accelerate the velocity of seismic waves. P-waves slow down as they contact the liquid outer core, which is less rigid than the mantle. This causes the P-waves to arrive later and at a greater distance than predicted. As a result, a P-wave shadow zone is produced. Seismographs 104° to 140° from the epicenter of an earthquake do not detect any P-waves.
- S-waves (secondary waves) move approximately half as quickly as P-waves, with a speed of around 3.5 km (2 miles) per second, and they arrive second at seismographs. S-waves travel in an upward and downward manner perpendicular to the direction of wave movement. This alters the form of the Earth’s components as they pass through them. Since only solids resist form change, S-waves can only propagate through solids. S-waves cannot pass through a liquid medium.
Where seismic waves accelerate or decelerate, they refract, altering their travel direction. When seismic waves hit a sharp border between two very different strata, a portion of the seismic wave energy is reflected, rebounding back at the same angle at which it was initially emitted.
- P-waves decelerate near the core-mantle boundary, indicating that the outer core is less rigid than the mantle.
- At the mantle-core border, S-waves vanish, indicating that the outer core is liquid.
Figure 3. The course of an individual P-wave or S-wave is denoted by letters. The form of waves moving through the center is the letter K. This animation illustrates a. As a result of the study of seismic waves and how they travel through the earth’s strata, we have been able to differentiate the following features of the earth’s interior:
- The depth of the crust. This is a measurement of the crust’s thickness based on the sudden acceleration of seismic waves as they enter the mantle. Inferred from the difference in speed between P- and S-waves, the boundary between the crust and mantle is referred to as the Mohorovicic discontinuity, named for the Croatian seismologist who first detected it
- it is commonly referred to as the Moho. We mostly rely on seismic waves to determine the thickness of oceanic and continental crusts.
- The depth of the lithosphere. When passing from the lithosphere into the asthenosphere, seismic waves slow down. This is due to the decreased stiffness and compressibility of the rocks in the sublithosphere layer. Low velocity zone describes the sublithospheric region where seismic waves move more slowly. The region of low velocity likely coincides with the asthenosphere.
- The region separating the upper and lower mesospheres (upper and lower mantle). This manifests as a rise in seismic wave velocity at a depth of 660 kilometers.
- The barrier between the mantle and the center of the planet. S-waves halt abruptly, probably because the outer core is liquid, and the speed of P-waves decreases dramatically when they enter the liquid core, where there is no stiffness to contribute to P-wave speed.
- The main inside. This was originally detected by the refraction of P-waves going through this portion of the core, as a result of a sudden rise in their speed, which was not observed for P-waves traveling just through the outer portion of the core.
- Seismic tomography: imaging slabs and masses in the ground at multiple orientations, not simply in layers. By merging data from several seismometers, three-dimensional representations of regions of the earth with varying seismic wave velocity may be created. Seismic tomography reveals that there are masses of what may be subducted plates that have reached below the asthenosphere into the mesosphere and, in some cases, entered into the lower mesosphere, which is the deepest portion of the mantle. In some locations, subducted plates seem to have accumulated at the base of the upper mesosphere without piercing the lower mesosphere.
Seismic Waves and Determining the Earth’s Structure: A Comparison and Connections Even if the technology to go through all of the Earth’s strata does not exist, scientists may still learn a great lot about Earth’s structure via seismic waves. Seismic waves are vibrations in the ground that convey energy and occur during seismic events such as earthquakes, volcanic eruptions, and even explosions caused by humans.
- There are two distinct types of seismic waves: primary and secondary.
- Primary waves, sometimes called P waves or pressure waves, are longitudinal compression waves that resemble the action of a slinky (SF Fig.7.1 A).
- S waves, or secondary waves, are slower than P waves.
- The motion of secondary waves is equivalent to violently shaking a rope perpendicular to the direction of wave flow (SF Fig.7.1 B).
SF Figure 7.1 C depicts the motion of primary or P waves (on top) and secondary or S waves (on bottom). Seismometers (Figure 7.2) are used by scientists to measure seismic waves. Seismometers monitor the ground’s vibrations in comparison to a stationary device.
Seismometer data, often known as a seismogram, depicts velocity on the y axis and time on the x axis (Fig.7.3). Observe in SF Fig.7.3 that the P wave arrives first due to its higher velocity. SF Table 7.1 demonstrates that P waves have a greater velocity than S waves while going through various types of minerals.
The velocity of seismic waves relies on the qualities of the material through which they travel. For instance, the greater the density of a substance, the quicker a seismic wave travels (SF Table 7.1). P waves are able to move through liquids, solids, and gases, but S waves can only pass through solids.
|Mineral||P wave velocity (m/s)||S wave velocity (m/s)||Density (g/cm 3 )|
Because of SF Figure 7.4 depicts the propagation of waves through the Earth. Note that P waves go through all layers of the ground, however S waves cannot travel through the solid core, resulting in a S wave shadow on the opposite side of the earthquake: Compare-Contrast-Connect: Seismic Waves and Determining the Structure of the Earth
The importance of seismic wave research lies not only in our ability to understand and predict earthquakes and tsunamis, it also reveals information on the Earth’s composition and features in much the same way as it led to the discovery of Mohorovicic’s discontinuity.