Seismic waves are waves of energy moving through Earth's layers, and are the result of earthquakes, volcanic eruptions, magma movements, large landslides and large man-made explosions that produce low-frequency acoustic energy. Many other natural and anthropogenic sources create low-amplitude waves commonly referred to as ambient vibrations. Seismic waves are studied by geophysicists called seismologists. The seismic wave field is recorded by a seismometer, a hydrophone (in water), or an accelerometer.
The speed of wave propagation depends on the density and elasticity of the medium. Speed ​​tends to increase with depth and range from about 2 to 8 km/s in the Earth's crust, up to 13 km/sec in the mantle.
Earthquakes create different types of waves at different speeds; when reaching the seismic observatory, their different travel time helps the scientists to find the source of the hypocenter. In geophysics, refracting or reflecting seismic waves are used for research into the Earth's interior structures, and artificial vibrations are often produced to investigate superficial subsurface structures.
Video Seismic wave
Jenis
Among the many types of seismic waves, one can make a wide difference between body waves, traveling through the Earth, and surface waves, moving on the surface of the Earth.
Other modes of wave propagation exist than those described in this article; although of relatively small importance to Earth-borne waves, they are important in cases of asteroseismology.
- Waves of the body are exploring the interior of the Earth.
- Surface waves move across the surface. Surface waves decay more slowly with the distance from body waves, which travel in three dimensions.
- Movement of surface wave particles is larger than the body wave, so surface waves tend to cause more damage.
Body wave
Body waves traveling through the Earth's interior along the path are controlled by material properties in terms of density and modulus (stiffness). Density and modulus, in turn, vary according to the temperature, composition, and phase of the material. This effect resembles the refraction of light waves. Two types of particle motions produce two types of body waves: Main and wave Secondary .
The main wave
The primary wave (P wave) is a compression wave that is elongated. P wave is a pressure wave that moves faster than other waves through the earth to arrive at the first seismograph station, hence the name "Main". These waves can travel through all types of materials, including liquids, and can travel nearly 1.7 times faster than the S wave. In the air, they take the form of sound waves, so they travel at the speed of sound. The typical speed is 330 m/sec in the air, 1450 m/s in water and about 5000 m/s in granite.
Secondary wave
The secondary wave (S-wave) is the shear wave that transverse in nature. After an earthquake, the S-wave arrives at the seismographic station after a faster moving P wave and replaces the ground perpendicular to the propagation direction. Depending on the direction of propagation, the waves can take on different surface characteristics; for example, in case the S wave is horizontally polarized, the ground moves alternately to one side and then the other side. The S-wave can travel only through solids, since liquids (liquids and gases) do not support shear stress. The S-wave is slower than the P-wave, and the speed is usually about 60% of the P-wave in each given material.
Surface waves
Surface seismic waves move along the Earth's surface. They can be classified as mechanical surface waveforms. They are called surface waves, because they diminish as they get farther from the surface. They travel more slowly than seismic body waves (P and S). In large earthquakes, surface waves can have an amplitudes of several centimeters.
Rayleigh Wave
Rayleigh waves, also called ground rolls, are surface waves that move as ripples with motions similar to waves on the surface of the water (note, however, that the movement of related particles in shallow depths is retrograde, and that the restoring forces in Rayleigh and other seismic waves are elastic, not gravity like a wave of water). The existence of this wave was predicted by John William Strutt, Lord Rayleigh, in 1885. They were slower than body waves, roughly 90% of the S wave velocity for a typical homogeneous elastic medium. In a layered medium (such as crust and upper mantle) Rayleigh wave velocity depends on frequency and wavelength. See also the sheep wave.
Wave of love
Love waves are horizontally polarized shear waves (SH waves), only in the presence of a semi-infinite medium coated by a top layer of finite thickness. They are named after A.E.H. Love, a British mathematician who created the mathematical model of waves in 1911. They usually travel a bit faster than Rayleigh waves, about 90% of S wave velocity, and have the greatest amplitude.
Stoneley Wave
Stoneley waves are a type of boundary wave (or interface wave) that spreads along solid fluid boundaries or, under certain conditions, also along solid-solid borders. The Stoneley wave amplitude has a maximum value on the boundary between two related media and decomposes exponentially toward each depth. These waves can be generated along the liquid borehole wall, becoming an important source of coherent noise in the VSP and making low frequency components of the source in sonic logging. The equation for the Stoneley wave was first given by Dr. Robert Stoneley (1894-1976), Emeritus Professor of Seismology, Cambridge.
Free Oscillation from Earth
The free oscillation of the Earth is a standing wave, the result of interference between two surface waves moving in the opposite direction. Rayleigh wave interference produces sferoidal oscillation S while the Love wave interruption gives toroidal oscillation T . The mode of oscillation is determined by three numbers, for example, n S l m , where l is the order number corner ( or spherical harmonic level , see Spherical harmonics for more details). The m number is the azimuth sequence number. It may take 2 l value 1 from - l to l . The n number is the radial command number . This means a wave with n zero crossings within a radius. For spherically symmetric Earth, the given periods n and l do not depend on m .
Some examples of spheroidal oscillations are the "breathing" mode 0 S 0 , which involves the expansion and contraction of the entire Earth, and has a period of about 20 minutes; and the "rugby" mode 0 S 2 , which involves two-way alternating expansions, and has a period of about 54 minutes. Mode 0 S 1 does not exist because it will need a change in the center of gravity, which will require external power.
From the fundamental toroidal mode, 0 T 1 represents a change in the Earth's rotational rate; although this happens, is too slow to be useful in seismology. The 0 T 2 mode describes the northern and southern hemispheres relative to each other; this has a period of about 44 minutes.
The first observations of free oscillations from Earth were made during the 1960 massive earthquake in Chile. Currently the period of thousands of modes is known. This data is used to determine some large-scale structures of Earth's interior.
P and S waves in Earth's mantle and core
When an earthquake occurs, seismographs near the epicenter are capable of recording P and S waves, but those located at longer distances no longer detect the first high frequency S waves. Since the shear waves can not pass through the liquid, this phenomenon is the original proof for the observation that is now firmly established that the Earth has a liquid outer core, as demonstrated by Richard Dixon Oldham. Such observations have also been used to argue, with seismic testing, that the Moon has a strong nucleus, although recent geodetic studies show that the core is still liquid.
Maps Seismic wave
Notation
The path traveled between waves of focus and observation points is often described as a ray diagram. This example is shown in the picture above. When reflection is taken into account there are a number of infinite paths that the wave can take. Each path is denoted by a set of letters that describes the passage and phase through the Earth. In general, uppercase indicates transmitted waves and lowercase letters indicate reflected waves. The two exceptions to this seem to be "g" and "n".
As an example:
- ScP is a wave that begins the journey to the center of the Earth as a wave S. After reaching the outer core, the wave reflects as wave P.
- sPKIKP is a wave path that begins its journey to the surface as an S-wave. On the surface it reflects as a P-wave. The P wave then travels through the outer core, the inner core, the outer core, and the mantle.
Use of P and S waves in search of an event
In the case of a local or near earthquake, the difference in arrival time of the P and S waves can be used to determine the distance to the event. In the case of an earthquake occurring at a global distance, three or more geographically diverse observation stations (using public clocks) record the arrival of the P-wave allowing the timing and unique location of the planet for the event. Typically, dozens or even hundreds of P-wave arrivals are used for hypocentric counting. The misfit generated by the hypocenter calculation is known as "the residual". The remaining 0.5 seconds or less is typical for distant events, residuals of 0.1-0.2 s typical for local events, which means that most reported arrivals P correspond to well-calculated hypocenter. Usually the location program will start assuming the event occurred at a depth of about 33 km; then it minimizes the residue by adjusting the depth. Most of the events occur at shallower depths of about 40 km, but some occur as deep as 700 km.
A quick way to determine the distance from the location to the origin of the seismic waves less than 200 km away is to take the difference on the arrival time of the P wave and the S wave in seconds and multiplied by 8 kilometers per second. Modern seismic arrays use more complex earthquake location techniques.
At teleseismic distance, the first P wave arrives to travel deep into the mantle, and may even bias to the outer core of the planet, before traveling back to the Earth's surface where the seismograph station is located. Waves move faster than if they traveled in a straight line from an earthquake. This is due to the increased speed on the planet, and is called the Huygens Principle. The density on the planet increases with depth, which will slow the waves, but the rock modulus increases more, so it means deeper faster. Therefore, longer routes may take less time.
Travel time must be calculated very accurately to calculate the right hypocenter. Because the P wave moves at many kilometers per second, down on the calculation of travel time even half a second can mean errors of many kilometers in terms of distance. In practice, the arrival of P from many stations is used and the error is canceled, so the computed quake center may be quite accurate, about 10-50 km or more worldwide. A dense collection of nearby sensors such as those in California can provide accuracy of about a kilometer, and much greater accuracy is possible when the time is measured directly by cross-correlation of the seismogram waveform.
See also
- Adams-Williamson Equations
- Helioseismology
- Seismology of reflection
References
External links
- EDT: The MATLAB site for seismic wave propagation
Source of the article : Wikipedia
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