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Seismic Waves Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 4, 2011

Surface Waves

Surface waves are also divided into two types, Love waves and Rayleigh waves . Each of the two surface waves travels literally along the surface of the Earth and produces a distinct type of motion.

Love waves produce motion that is perpendicular to the direction of wave travel in a horizontal orientation only . This is the type of horizontal shearing that wipes out building foundations.

Rayleigh waves produce a rolling motion like the waves on the sea. An object like a building on the surface will experience both an up-and-down, bobbing motion transverse to and a back-and-forth motion parallel to, the Rayleigh wave direction of travel. The two movements combine to create a rolling, elliptical action that is extremely difficult for buildings not specially designed for earthquakes to withstand.

Love and Rayleigh surface waves are responsible for a lot of damage that takes place during earthquakes. They are not as basic as P and S waves, but create a lot of problems anyway.

Seismic Moment

Seismologists have developed a standard magnitude scale that is independent of the instrument type. It is called the moment magnitude scale and comes from calculations of seismic moment .

Seismic moment is figured out through the physics of torque (twisting). A torque is a force that changes the angular momentum of a system. It is defined as the force times the distance from the center of rotation. Earthquakes are caused by internal torques, which are caused by stress interactions of tectonic plates and faults. Seismologists calculate moment magnitude with the following factors:

Moment = rock rigidity × fault area × slip distance

Magnitude And Intensity

Magnitude and intensity measure different earthquake characteristics. Magnitude measures the energy released at the source of the earthquake and is determined by seismograph measurements. The magnitude is the same no matter where you are, or how strong or weak the shaking at various locations.

The magnitude of an earthquake is a measured value of an earthquake’s size and energy.

In Richter magnitude, the amount of movement (amplitude) caused by seismic waves determines its magnitude.

Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined from effects on people, human structures, and the natural environment.

The intensity of an earthquake is a measure of the earth’s shaking at a certain location.

Table 12-2 gives intensities that are often seen near the epicenter of different earthquakes’ magnitudes.

Table 12-2 The amount of damage from earthquakes varies with intensity.

Magnitude

Intensity

Effects

1.0–3.0

I

Rarely felt

3.0–3.9

II–III

II – only felt by those lying still and in tall buildings III – felt by people indoors and in tall buildings, but not recognized as an earthquake; cars may rock a bit; vibrations like truck going by; time estimated.

4.0–4.9

IV–V

IV – felt indoors by many, outdoors by few during the day; some awakened at night; dishes, windows, and doors jarred; walls crack and pop; sounds like heavy truck ramming building; cars rocked a lot V – felt by almost everyone; many wake up; dishes and windows broken; objects tipped over

5.0–5.9

VI–VII

VI – felt by everyone, many alarmed; heavy furniture moved; some fallen plaster; overall damage slight VII – damage minor in buildings of good design and construction; slight to medium in well-built structures; lots of damage in poorly built or designed structures; some chimneys broken

6.0–6.9

VIII–IX

VIII – damage minor in specially designed structures; lots of damage and some collapse in common buildings; huge damage in poorly built structures, chimneys, factory stacks, columns, monuments, and walls fall; heavy furniture flipped IX – damage great in specially designed structures; well-designed frame structures jerked sideways; great damage in substantial buildings, with partial collapse; buildings moved off foundations

7.0 and higher

X and higher

X – some well-built wooden buildings destroyed; most masonry, frame, and foundation destroyed; rails bent XI – few masonry structures left standing; bridges destroyed; rails bent completely XII – damage complete; lines of sight and level are distorted; objects airborne

 

Using Richter’s open ended scale allows more accurate reports of earthquake intensity. It doesn’t depend on people’s feelings and observations like earlier scales did.

Today with the Internet and more and more households submitting reports, it is possible for intensity maps to be created within hours of a large earthquake. These intensity maps help seismologists figure out how subsurface matter relates to what is felt in an earthquake and the damage to buildings and homes.

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