Earthquake Analysis and Measurement

What are Earthquake Waves?

          A seismic wave is that which is propagated by some kind of elastic deformation, or, a change in shape that disappears when the forces are removed.

 Body Waves

          A seismic wave that can travel through the interior of the earth is a Body wave.

P-waves and S-waves are body waves.

 Primary Waves

P waves are compression waves because the rocky material in their path moves back and forth in the same direction as the wave travels alternately compressing and expanding the rock. P waves are the fastest seismic waves; they travel in strong rock at about 6 to 7 km per second.

Secondary Waves

S waves, which shear, or twist, rather than compress the rock they travel through. S waves travel at about 3.5 km per second. S waves cause rocky material to move either side to side or up and down perpendicular to the direction the waves are traveling, thus shearing the rocks.

Both P and S waves help seismologists to locate the focus and epicenter of an earthquake.

Surface Waves

Earthquakes contain surface waves that travel out from the epicenter along the surface of the Earth. Two types of these surface waves occur: Rayleigh waves, named after British physicist Lord Rayleigh, and Love waves, named after British geophysicist A. E. H. Love. Surface waves also cause damage to structures, as they shake the ground underneath the foundations of buildings and other structures.

Rayleigh Waves

On the surface of the Earth, Rayleigh waves cause rock particles to move forward, up, backward, and down in a path that contains the direction of the wave travel. This circular movement is somewhat like a piece of seaweed caught in an ocean wave, rolling in a circular path onto a beach. 

Love Waves

Love wave causes rock to move horizontally, or side to side at right angles to the direction of the traveling wave, with no vertical displacements.

Rayleigh and Love waves always travel slower than P waves and usually travel slower than S waves. 

What is Earthquake Magnitude?

Magnitude is a measure of the strength of an earthquake, or the amount of strain that rocks in Earth's crust release when an earthquake occurs. The Richter scale and the moment magnitude scale are used to measure the magnitude of earthquakes. 

What is Earthquake Intensity?

Earthquake intensity is a measure of the effects of an earthquake in a particular place. Earthquake intensity is the severity of shaking felt due to an Earthquake. It decreases with its distance from epicentre.

Recording an Earthquake

The vibrations produced by earthquakes are detected and recorded by instruments called seismographs. The time of occurrence, the duration of shaking, the locations of the epicenter and focus, and estimates of the energy released can be obtained from data from seismographs set up around the world

         Richter Scale

Richter Scale, method of ranking the strength or size of an earthquake. The Richter scale, also known as the local magnitude scale, was devised in 1935 by the American seismologist Charles F. Richter to rank earthquakes occurring in California. Richter and his associates later modified it to apply to earthquakes anywhere in the world.

The Richter scale ranks earthquakes based on how much the ground shakes 100 km (60 mi) from the earthquake's epicenter, the site on the earth's surface directly above the earthquake's origin. The amount of ground movement is measured by an instrument called a seismograph. Seismographs can detect movements as small as about 0.00001 mm (about 0.000004 in) to movements as large as about 1 m (about 40 in). In order to deal with numbers in such a broad range, the Richter scale is a logarithmic scale—each increase of 1 on the Richter scale represents a tenfold increase in movement. Thus, an earthquake registering 7 on the scale is 10 times as strong as an earthquake registering 6, and the earth moves 10 times as far.

Moment magnitude Scale (Mw)

Today, seismologists prefer to use a different kind of magnitude scale, called the moment magnitude scale, to measure earthquakes. Seismologists calculate moment magnitude by measuring the seismic moment of an earthquake, or the earthquake's strength based on a calculation of the area and the amount of displacement in the slip. The moment magnitude is obtained by multiplying these two measurements. It is more reliable for earthquakes that measure above magnitude 7 on other scales that refer only to part of the seismic waves, whereas the moment magnitude scale measures the total size. The moment magnitude of the 1995 Kobe, Japan, earthquake was a 7.0 moment magnitude; in comparison, the Richter magnitude was 6.8 for that tremor.

Modified Mercalli Scale

Modified Mercalli Scale, scale for measuring the intensity of earthquakes, adapted from the original Mercalli scale. The Mercalli scale was devised in 1902 by Italian seismologist Giuseppe Mercalli. American seismologists Harry O. Wood and Frank Neumann created the Modified Mercalli scale in 1931 to measure the intensity of earthquakes that occur in California. The Modified Mercalli scale, or a scale similar to it, is now used worldwide. The scale has 12 levels of intensity. Each level is defined by a group of observable earthquake effects, such as shaking of the ground and damage to structures such as buildings, roads, and bridges.

The levels are designated by the Roman numerals I to XII. Levels I through VI are used to describe what people see and feel during a small to moderate earthquake. Levels VII through XII are used to describe damage to structures during a moderate to catastrophic earthquake. On average, about one earthquake of level X to XII occurs worldwide every year; 10 to 20 earthquakes of level VII through IX occur each year; and over 500 earthquakes of level I to VI occur every year. Each year over 100,000 earthquakes occur that are not noticed by the human population and therefore are not rated on the Modified Mercalli scale.

Next: Predicting Earthquakes

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