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Thursday, September 27, 2012

Know the Seismic Waves

Earthquake is a jolt in the earth's crust as a companion symptom of tectonic, volcanism activity, or debris that occurs in the earth. At the time of the earthquake, we can feel the vibrations or shocks on where we stand. Earth rocked sideways and upwards. Vibration or shock is seismic waves.

Seismic waves are very low frequency waves that spread through the earth. The emergence of the seismic waves caused by a shift in the Earth's crust segments or can be generated from the explosion at the top of the crust.

The rate of amplitude and direction of propagation of seismic waves depends on the type of seismic waves and the material in its path. Seismograph record the amplitude and frequency of seismic waves and generate information about the earth and the subsurface structure of the earth.
Types of seismic wave. (Picture from: http://www.physics.unc.edu/)
Seismic waves generated naturally as a result of shifting crust (plates) of the earth to the attention of seismologists (seismologists) and physicists to determine the center of the earth quake. While the seismic waves generated artificially as a result of an accidental explosion on the upper crust of the earth useful to geologists and physicists to study the state of the earth of the earth and help search for the sources of fossil fuels.

Seismic waves are divided into two types. First, body wave. Body wave consists of primary wave (commonly abbreviated as P waves) and secondary waves (commonly abbreviated as S waves). Body wave propagates through the earth outwards from the epicenter.

P waves typically arrive at the surface of the earth as a jolt. P waves move faster than S waves, so the P wave is the first wave recorded on seismographs (earthquake recording devices) and became the first wave of people in the area felt the quake. Because it is the first, the wave is called the primary wave.
Seismic wave mapping of the interior of the Earth. (Picture from: http://www.physics.unc.edu/)
P waves (also called compressional or longitudinal waves) travels through solids, liquids (liquid). and gas, and thus going through the whole body of the earth. P waves cause the transmission medium is a liquid, solid, or gas moving back and forth in the direction of the path of deployment. When P waves traveling through rock, the waves move / push tiny rock particles back and forth in the direction of wave propagation. The result is a series of strain and pressure back and forth. This incident is similar to what happens when we pull or a slinky stretch horizontally.

In the earth, the P wave propagates at a rate of about 6 kilometers (3.7 miles) per second at the surface of the rock to about 10.4 kilometers (6.5 miles) per second near the earth's core (approximately 2,900 kilometers below the Earth's surface ). Accretion rate of the P wave to the depths of the earth due to increase hydrostatic pressure, which corresponds to changes in the composition of the rocks. In general, the accretion rate resulting P wave propagates with a curved path.
Seismic waves (Earthquakes) probe the interior of the Earth. (Picture from: http://www.physics.unc.edu/)
The appearance of the P wave was accompanied by a wave of S. S waves are also known as shear or transverse waves. The rate of propagation of the S wave and P wave arrival time resulted in two distinct waves separated when detected by seismographs. Separate arrival times can be used to determine the location of the epicenter.

S waves are transverse shear wave like a wave on the pull rope. Material moves up and down or from side to side. Like the P waves, S waves also spread to the curved trajectory.

S waves can only be spread through a solid medium for liquid and gaseous medium can not sustain shear stress. In the earth, the rate of increase of the S wave about 3.4 kilometers (2.1 miles) per second at the earth's surface up to 7.2 kilometers (4.5 miles) per second near the limit of the earth's core. S waves cause the dots on solid medium moves back and forth perpendicular to the direction of the spread of the wave. When the S waves move, these waves move rock particles outward and push the particles perpendicular to the wave trajectory. It results in the first period roling associated with earthquakes.

The second type of seismic waves are surface waves. Surface waves sometimes resemble waves in the water that moves up and down the earth's surface.
Animation of seismic waves. (Picture from: http://earthquake.usgs.gov/)
This animation shows the seismic waves generated by the 2002 Denali Fault earthquake. P-waves are shown in red; S-waves are shown in blue; Surface waves are shown in yellow. Phase arrival times are indicated on the vertical (Z) seismograms displayed at the top of the animation.

Seismograms are shown for:
COLA: College, Alaska
PFO: Pinyon Flat Observatory, California
SSPA: Standing Stone, Pennsylvania
CTAO: Charter Towers, Australia
DBIC: Dimbokro, Cote d'Ivoire
LBTB: Lobatse, Botswana

This animation is running 100 times faster than the waves actually traveled. Animation of PC program Seismic Waves written by Alan Jones.

Surface wave propagates in the Earth's surface at a rate of approximately 3.5 to 3.9 kilometers per second and slower than body waves. Because the surface wave has a low frequency, long duration of propagation, the width and amplitude of the surface wave is a kind of the most destructive seismic waves. Surface waves move the rock on man-made structures of the building foundation.

Surface wave consists of Rayleigh waves and Love waves. Rayleigh waves are surface waves similar to the rise and deployment of water surface waves. The existence of these waves was predicted long ago by a British physicist Lord Rayleigh in 1885. He was the first to express mathematically the existence of these waves.

Rayleigh waves traveling through the free surface in the form of elastic solids in the earth. The movement of these waves is a combination of longitudinal compression and dilation which then produce an elliptical motion at points on the surface of the earth. In all seismic waves, Rayleigh waves most often (all the time) to spread out and produce a long-duration waves on a seismograph.

Meanwhile, Love waves are surface waves that cause horizontal shifting of the earth surface. The word "Love" is used in the name of the wave is taken from the name of AEH Love. He was a British seismologists who first predicted the existence of these waves and mathematical modeling in 1911.

Love waves are usually spread a little faster than the Rayleigh wave. Love waves spread when a solid medium near the Earth's surface has a vertical elastic properties are diverse. Medium shift entirely by Love waves perpendicular to the direction of the spread of the wave. Energy deployment Love waves produce strong record on seismic stations. *** [GUN GUN GUNAWAN | PIKIRAN RAKYAT 13092012]
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