Okoro Daniel O.
Basically, the Earth crust has been divided into three (3), which are;
- The Core
- The Mantle
- The Crust / Lithosphere
So what is Seismology?
Seismology is coined from two Greek words, Seismos (Meaning: Earth Quake) and Logy (Meaning: Study /To Study), can simply be defined as the study of Earth Quake.
It can also be defined as the study of movement of waves into the various part of the Earth.
What is Earth Quake?
Earth Quake is the sudden release of energy into the Earth’s lithosphere (or crust) that cause SEISMIC WAVES.
SEISMIC WAVES are elastic waves / wave energy that are released or introduced into the Earth.
They are often produced naturally when EARTH QUAKE occurs. The point where the seismic wave meet is called the FOCUS.
When the earth quake originates from the FOCUS, they are either reflected or refracted.
SEISMIC REFLECTION: Is the return of some of the energy of seismic wave to the earth surface after the waves bounce back on the rock boundary.
SEISMIC REFRACTION: Is the bonding of seismic waves as they pass from one material to another.
This is similar to the way when light wave travel through GLASS PRISM.
SEISMOMETER: Is used to study seismic waves.
SEISMOGRAPH: Is the recording device that produce the permanent record of the earth motion that is detected.
SEISMOGRAM: Is the paper or printed record of seismic events.
TYPES OF SEIMIC WAVES
Seismic waves are basically divided into two (2);
1. Body Waves (P-waves)
2. Surface waves (S-waves)
Body Waves
A body wave also known as P-Waves is a seismic wave that moves through the interior of the earth, as opposed to surface waves that travel near the earth’s surface. P and S waves are body waves. Each type of wave shakes the ground in different ways.
Of the body waves, the primary, or P, wave has the higher speed of propagation and so reaches a seismic recording station faster than the secondary, or S, wave. P waves, also called compressional or longitudinal waves, give the transmitting medium—whether liquid, solid, or gas—a back-and-forth motion in the direction of the path of propagation, thus stretching or compressing the medium as the wave passes any one point in a manner similar to that of sound waves in air. In the Earth, P waves travel at speeds from about 6 km (3.7 miles) per second in surface rock to about 10.4 km (6.5 miles) per second near the Earth’s core some 2,900 km (1,800 miles) below the surface. As the waves enter the core, the velocity drops to about 8 km (5 miles) per second. It increases to about 11 km (6.8 miles) per second near the centre of the Earth. The speed increase with depth results from increased hydrostatic pressure as well as from changes in rock composition; in general, the increase causes P waves to travel in curved paths that are concave upward.
Surface waves
A surface wave is a seismic wave that is trapped near the surface of the earth. S waves, also called shear or transverse waves, cause points of solid media to move back and forth perpendicular to the direction of propagation; as the wave passes, the medium is sheared first in one direction and then in another. In the Earth the speed of S waves increases from about 3.4 km (2.1 miles) per second at the surface to 7.2 km (4.5 miles) per second near the boundary of the core, which, being liquid, cannot transmit them; indeed, their observed absence is a compelling argument for the liquid nature of the outer core. Like P waves, S waves travel in curved paths that are concave upward.
