Seismology
The study of earthquakes' causes and effects, from minute pulsations to the most catastrophic natural occurrences inside the earth, is known as seismology.
TABLE OF CONTENTS
Relevance of Seismology
Seismic waves, which are energy waves that pass through the Earth as a result of an earthquake, can reveal a lot about the Earth's internal structure because they move through different materials at various speeds.
P-wave and S Wave
P-wave is also known as
longitudinal waves because the displacement of the medium is in the same direction as, or the opposite direction to, (parallel to) the direction of propagation of the wave; or
compressional waves because they produce compression and rarefaction when travelling through a medium or
pressure waves because they produce increases and decreases in pressure in the medium.
P-waves creates density differences in the material leading to stretching (rarefaction) and squeezing (compression) of the material.
Compare P-wave and P-wave
P- waves are faster and they can travel through both solids and liquids.
S-waves are slower and cannot travel through liquids.For both kinds of waves, the speed at which the wave travels also depends on the properties of the material through which it is traveling.e.g-see photo
Thus, if there is an earthquake somewhere, the first waves that arrive are P-waves. In essence, the gap in P-wave and S-wave arrival gives a first estimate of the distance to the earthquake.
Relevance of P-wave and S-wave:
By observing the arrival of seismic waves at stations all over the world, scientists can gather information about the internal structure of the Earth. For instance, they are aware that the Earth's outer core is liquid because s-waves cannot traverse it.
Seismic waves travel in curved paths through the Earth (because of the increasing pressure, materials are more dense towards the core, travel velocity of seismic waves increases).
Refraction of seismic waves causes them to curve away from a direct path.
Reflection causes them to glance off certain surfaces (e.g. core mantle boundary) when they hit it at too shallow of an angle.
The result of this behavior, in combination with the fact that S-waves cannot travel through liquids is the appearance of seismic shadows, opposite of the actual earthquake site.
When an earthquake occurs there is a “shadow zone” on the opposite side of the earth where no s-waves arrive.
Similarly earth has a solid inner core because some p-waves are reflected off the boundary between the inner core and the outer core.
By measuring the time it takes for seismic waves to travel along many different paths through the earth, they figure out the velocity structure of the earth. Abrupt changes in velocity with depth correspond to boundaries between different layers of the Earth composed of different materials.
What is refraction on Wave s
Refraction is critical refraction, and it can be used to estimate layer velocities.
Seismic Refraction (SR) is a surface geophysics technique that uses seismic wave refraction on rock/soil units and geologic layers to describe subsurface geologic conditions. Snell's Law, a formula used to describe the relationship between seismic wave angles of refraction when passing through a boundary between two different isotropic media, governs a geophysical principle used in the technique (e.g. soil to bedrock).
Laying out a spread of geophones in a straight line with seismic impact source points placed with and off the ends is a common practise in seismic refraction surveys.
What is reflection on waves
When a wave is an incident on any surface, a part of the incident wave is reflected and a part is transmitted into the second medium. If the wave is incident obliquely on the boundary, the transmitted wave can also be termed as a reflected wave.
Here, the incident and the refracted waves obey Snell’s Law of refraction, and the incident and the reflected waves obey the laws of reflection. The reflection of a wave or a pulse can happen from two types of surfaces, it can either be a fixed wall or a ring, as shown in the image below.
Fixed End Reflections
Let us consider the situation where a string is fixed to a rigid wall at its right end. When we allow a pulse to propagate through these strings, the pulse reaches the right end, gets reflected as shown in the figure above. When the pulse arrives at the fixed end, it exerts a force on the wall and according to Newton’s third law, the wall exerts an equal and opposite force on the string. This second force generates a pulse at the support, which travels back along the string in the direction opposite to that of the incident pulse. In a reflection of this kind, there is no displacement at the support as the string is fixed there. The reflected and incident pulses have opposite signs, and they cancel each other at that point. Thus, in the case of a travelling wave, the reflection at a rigid boundary takes place with a phase reversal or with a phase difference of π.
Free End Reflection
When the right end of the string is tied to a ring, which slides up and down without any friction on a rod, we term it as a free end. In this case, when the pulse arrives at the right end, the ring moves up the rod and as it moves, it pulls on the string, stretching the string and producing a reflected pulse with the same sign and amplitude as the incident pulse. Thus, in such a reflection, the incident and reflected pulses reinforce each other, creating the maximum displacement at the end of the string: the maximum displacement of the ring is twice the amplitude of either of the pulses. Thus, the reflection occurs without any additional phase shift. In the case of a travelling wave the reflection at an open boundary the reflection takes place without any phase change.
Summarise the above result, we can say that the reflection of waves at a boundary between two media takes place accordingly. A travelling wave, at a rigid boundary or a closed-end, is reflected with a phase reversal but the reflection at an open boundary takes place without any phase change.
Mathematically, if the incident wave is represented as yi(x, t) = a sin (kx – ωt), then, for reflection at a rigid boundary, the reflected wave is represented by
yr (x, t) = a sin (kx + ωt + π). = – a sin (kx + ωt)
And when the wave gets reflected at an open boundary, the reflected wave is represented by
yr (x, t) = a sin (kx + ωt).
SHADOW Zones
The shadow zone is the area of the earth from angular distances of 104 to 140 degrees that, for a given earthquake, that does not receive any direct P waves. The shadow zone results from S waves (not shown in animation) being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid core.
A seismic shadow zone is an area of the EARTH surface where seismograph cannot detect p-wave direct and/or s-wave from an earthquake. This is due to liquid layers or structures within the Earth's surface. The most recognized shadow zone is due to the core-mantle boundary where P waves are refracted and S waves are stopped at the liquid outer core; however, any liquid boundary or body can create a shadow zone.e.g:-magma reservoirs with a high enough percent melt can create seismic shadow zones.
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