How Do Waves Behave Differently In Earth’S Interior?
- Joe Thomas
Important: – Earth’s interior has boundary alterations. At the boundary, seismic waves are refracted and reflected before returning to the surface. Arrivals of seismic waves at distant sites defined the limits. Seismic shadow zones have taught us a great deal about the earth’s interior. This demonstrates how P waves flow through solids and liquids, but S waves are impeded by the outer liquid core. Animation Freshmen The wave qualities of light are utilized as an example to help us comprehend the behavior of seismic waves.
Animation Freshmen The shadow zone is the region of the earth between 104 and 140 degrees angular distance from an earthquake that does not receive direct P waves. The various phases illustrate how the original P wave evolves as it encounters Earth’s limits. Animation Freshmen The shadow zone is the result of S waves being completely blocked by the liquid core.
Three distinct S-wave phases illustrate how the original S wave is halted (damped) or altered as it encounters Earth’s limits. Animation Freshmen Due to variations in composition, pressure, and temperature inside the Earth’s strata, seismic waves travel along a curved course across its layers.
Animation Freshmen Seismic waves move at various rates through various types of materials. In this two-layer model, two wave fronts hit simultaneously, but the lower layer moves quicker. Animation Freshmen Due to variations in composition, pressure, and temperature inside the Earth’s strata, seismic waves travel along a curved course across its layers.
Animation Freshmen Animated depiction of the competition between the direct seismic wave and the deeper, longer-path significantly refracted seismic wave. Graph documents the times of arrival. Animation Freshmen In this concept of rising velocity with depth, the severely refracted seismic rays accelerate as they travel through five distinct velocity barriers.
- Animation Intermediate In this concept of rising velocity with depth, the severely refracted seismic rays accelerate as they travel through five distinct velocity barriers.
- Animation Intermediate Animation Intermediate Students compare anticipated seismic wave travel periods, derived from a scaled Earth model, with observed seismic data from recent earthquakes, first in small groups and subsequently as a complete class.
This exercise employs models and actual data and stresses the scientific method. Lesson Novice Seismic waves, like other waves, obey the rules of physics. In this project, Physics students have the chance to apply their knowledge of fundamental wave principles (such as reflection, refraction, and energy transmission) as they analyze seismic data to calculate the distance between the surface and bedrock.
Lesson of Intermediate Level To view the distinction between P- and S-wave seismic trajectories as well as their corresponding shadow zones, roll over the buttons. Interactive Freshmen View seismograms of big earthquakes from stations across the world with ease. Plots may be utilized for a variety of purposes, including determining the diameter of the Earth’s outer core as a teaching activity.
Software-Web Application Novice View seismograms of big earthquakes from stations across the world with ease. Plots may be utilized for a variety of purposes, including determining the diameter of the Earth’s outer core as a teaching activity. Software-Web Application Novice Seismic Waves is a browser-based application for visualizing the propagation of earthquake-generated seismic waves through the Earth’s interior and over its surface.
How do various body waves interact with the interior of the Earth?
Internal waves within the earth Earthquakes create two types of waves that go through solid rock: With P or compressional waves, the rock vibrates in the propagation direction. The earthquake’s P waves move the fastest and are the first to arrive. Rock oscillates perpendicular to the propagation direction of S or shear waves.
- In rock, S waves move at approximately 60% the speed of P waves and always arrive after P waves.
- For instance, sound waves are P waves with a sufficiently high frequency to be audible.
- Wiggling or shaking a rope that is secured at one or both ends is an illustration of a S wave.
- Both P and S waves radiate from the epicenter of an earthquake within the ground.
Seismographs frequently record the waves as independent arrivals at great distances from the earthquake. The direct P wave reaches sooner because its path is via denser, faster-moving rocks located deeper inside the ground. The PP (one bounce) and PPP (two bounces) waves move more slowly than the straight P as a result of passing through shallower, slower-moving rocks.
The S waves arrive following the P waves. Surface waves, such as the L wave, are the slowest (and last to arrive on seismograms). L waves are called after the Cambridge mathematician who initially characterized them, A.E.H. Love. Generally, the surface waves are the biggest recorded during an earthquake.
As they move away from the epicenter of an earthquake, body waves in the earth’s interior lose amplitude fast due to their dispersion inside the earth’s volume. However, surface waves propagate more slowly and exclusively on the earth’s surface. The surface confines the energy of surface waves to a smaller volume, hence the wave amplitude required to transport this energy is greater than that of body waves.
Figure 9.3 Using a hefty hammer to strike a huge chunk of rock will generate seismic waves within the rock. Do not attempt this at home! When we strike a rock with a hammer, we also generate a body wave characterized by oscillating vibrations (as opposed to compressions).
This is known as a shear wave (S-wave, where “S” stands for “secondary”), and it is analogous to what occurs when a length of rope is flicked up and down. As seen in Figure 9.4, a wave will form in the rope and travel to its end and return. Compression and shear waves travel extremely rapidly through geological materials.
As seen in Figure 9.5, average P-wave velocities in unconsolidated sediments range between 0.5 km/s and 2.5 km/s, and between 3.0 km/s and 6.5 km/s in solid crustal rocks. Basalt and granite have the highest velocities among the typical lithosphere materials.
- S-wave velocities range between 0.1 km/s and 0.8 km/s in soft sediments and between 1.5 km/s and 3.8 km/s in solid rocks.
- Figure 9.4 A compression wave can be represented by a spring (similar to a Slinky) that is abruptly pushed on one end.
- A shear wave can be represented by a rope that is abruptly flicked.
Exercise 9.1 How Quickly Will Seismic Waves Arrive? Imagine if a powerful earthquake strikes Vancouver Island’s Strathcona Park (west of Courtenay). How long will it take, assuming the crustal average P-wave velocity is 5 km/s, for the first seismic waves (P-waves) to reach the following locations (distances from the epicentre are indicated)?
|Nanaimo (120 km)|
|Surrey (200 km)|
|Kamloops (390 km)|
In general, mantle rock is denser and stronger than crustal rock, and both P- and S-waves move through the mantle at a higher rate than via the crust. Moreover, seismic-wave velocities are proportional to the degree of rock compression, which increases considerably with depth.
The phase condition of rock has an impact on seismic waves. If there is any degree of melting in the rock, they are delayed. If the substance is fully liquid, P-waves are significantly delayed and S-waves cease to exist. Figure 9.5 P-wave (red) and S-wave (blue) velocity distributions in sediments and solid crustal rocks.
Figure 9.6a Variations in P-wave and S-wave velocity with depth in the Earth. Figure 9.6b Variations in P- and S-wave velocities in the upper mantle and crust (This is an enlarged view of the curves in Figure 9.6a’s upper 600 kilometers) Since the late 1800s, accurate seismometers have been used for earthquake research, and systematic utilization of seismic data to comprehend Earth’s interior began in the early 1900s.
- Over the last several decades, the rate of change of seismic waves with depth in the Earth (as seen in Figure 9.6) has been estimated by analyzing seismic signals from big earthquakes at seismic stations across the world.
- Small discrepancies in signal arrival times at various places have been interpreted to indicate that: The mantle has larger velocities than the crust.
Generally, velocity increases with pressure and, thus, depth. Velocity decreases between 100 km and 250 km deep (called the “low-velocity zone”; equivalent to the asthenosphere). At 660 km deep, velocities accelerate considerably (because of a mineralogical transition).
In the area right above the core-mantle barrier, velocity decreases (the D” layer or “ultra-low-velocity zone”). S-waves cannot traverse the outer portion of the core. At the border between the outer liquid core and inner solid core, P-wave velocities increase considerably. In the early 1900s, Croatian seismologist Andrija Mohorovii (pronounced Mohoro-vi-chich) discovered that at certain distances from an earthquake, two distinct sets of seismic waves arrived to a seismic station within a few seconds of one another.
He reasoned that the waves that descended into the mantle, traveled through the mantle, and were then twisted upward back into the crust reached the seismic station first because, while having a longer distance to travel, they traveled more quickly through mantle rock (as shown in Figure 9.7).
Mohorovii discontinuity is the name given to the barrier between the crust and mantle (or Moho). Its depth extends from 60 to 80 kilometers beneath significant mountain ranges, 30 to 50 kilometers beneath the majority of the continental crust, and 5 to 10 kilometers beneath the marine crust. Figure 9.7 Seismic waves arising from an earthquake are depicted (red star).
Some waves travel through the crust to the seismic station (at approximately 6 km/s), whilst others travel down into the mantle (at approximately 8 km/s) and are bent upward toward the surface, arriving at the station before the waves that traveled solely through the crust.
- Figure 9.8 summarizes our present understanding of the patterns of seismic wave propagation across the Earth.
- As a result of the steady increase in density (and, thus, rock strength) with depth, all waves are refracted (toward the lower density material) as they pass through homogeneous portions of the Earth and so tend to curve outwards toward the surface.
At barriers within Earth, such as the Moho, the core-mantle boundary (CMB), and the outer-core/inner-core boundary, waves are also refracted. S-waves cannot pass through liquids because they are halted by the CMB, and there is an S-wave shadow on the opposite side of the Earth from a seismic source.
- The angular distance between the seismic source and the shadow zone is 103° on each side, resulting in a total angular distance of 154°.
- This information may be used to infer the depth of the CMB.
- P-waves are able to pass through liquids, therefore they can traverse the liquid portion of the core.
- Due of the refraction that occurs at the CMB, waves that go through the core are deflected away from the surface, creating a 103° to 150° P-wave shadow zone on each side.
This information may be utilized to determine the distinctions between the core’s inner and outside portions. Figure 9.8 illustrates patterns of seismic wave propagation inside the mantle and core of the Earth. S-waves cannot get through the liquid outer core, hence they cast a shadow on the other side of the planet.
- P-waves indeed go into the core, but there are also P-wave shadow zones because the waves that reach the core are refracted.9.2 Liquid Cores of Other Planets We know that other planets must have (or must have had) liquid cores like ours, and we might determine their size using seismic data.
- On display are the S-wave shadow zones of planets A and B.
Using the same technique as for Earth (on the left), draw the cores of these two more planets. Using data from several seismometers and hundreds of earthquakes, a two- or three-dimensional representation of the seismic characteristics of a portion of the mantle may be created.
This method is known as seismic tomography, and an example of the resulting image is displayed in Figure 9.9. Figure 9.9 is a P-wave tomographic profile of the region extending from southeast of Tonga to Fiji in the southern Pacific Ocean. Yellow and red indicate rock with modest seismic velocities, whereas blue represents rock with comparatively high seismic velocities.
Open circles are employed in the research as earthquakes. The Pacific Plate subducts under Tonga, as seen in Figure 9.9, as a 100 km thick slab of cold (blue) oceanic crust that has been driven into the surrounding heated mantle. The cold rock is more stiff than the surrounding heated mantle rock, hence its seismic velocity is somewhat higher.
Which waves are capable of penetrating the inner core?
P waves go through the core. Which seismic waves remain on the surface of the planet?