Saturday, October 29, 2011

How the strength of earthquakes is determined

How the strength of earthquakes is determined

Earthquakes occur when the tectonic plates that make up the earth’s crust interact with one another. Some earthquakes are caused by the plates moving passed one another and others are caused by one plate sliding under the other. The size of an earthquake depends on the amount of force that has built up at the point of contact. Sometimes the plates moving causes only a slight tremor, whereas in other instances the movement can cause cataclysmic destruction.

Earthquakes usually occur in places where two plates meet, called faults. Earthquakes are mostly generated deep within the earth's crust, when the pressure between two plates is too great for them to be held in place. The underground rocks then snap, sending shock waves out in all directions. These are called seismic waves. The underground origin of an earthquake is called the focus. The point at which an earthquake originates on the surface is called the epicentre.

Earthquakes are commonly measured by their magnitude and intensity.Magnitude is a measure of the total energy released during an earthquake. It is determined from a seismograph, which plots the ground motion produced by seismic waves.

The Japanese "shindo" scale for measuring earthquakes is more commonly used in Japan than the Richter scale to describe earthquakes. Shindo refers to the intensity of an earthquake at a given location, i.e. what people actually feel at a given location, while the Richter scale measures the magnitude of an earthquake, i.e. the energy an earthquake releases at the epicenter. The shindo scale ranges from shindo one, a slight earthquake felt only by people who are not moving, to shindo seven, a severe earthquake. Shindo two to four are still minor earthquakes that do not cause damage, while objects start to fall at shindo five, and heavier damage occurs at shindo six and seven.

Magnitude is a measure of the amount of energy released during an earthquake. The magnitudes measured using the Richter scale. The logarithms of the wave heights on seismograms measured in microns (1/1,000,000th of a meter, or 1/1000th of a millimeter) because some earthquakes made very small waves whereas others produced large waves..A wave one millimeter (1000 microns) high on a seismogram would have a magnitude of 3 because 1000 is ten raised to the third power. In contrast, a wave ten millimeters high would have a magnitude of 4.

A seismograph as a kind of sensitive pendulum that records the shaking of the Earth. The output of a seismograph is known as a seismogram. In the early days, seismograms were produced using ink pens on paper or beams of light on photographic paper, but now it's most often done digitally using computers. John Milne was the English seismologist and geologist who invented the first modern seismograph and promoted the building of seismological stations. The horizontal pendulum seismograph was improved after World War II with the Press-Ewing seismograph, developed in the United States for recording long-period waves. It is widely used throughout the world today. The Press-Ewing seismograph uses a Milne pendulum, but the pivot supporting the pendulum is replaced by an elastic wire to avoid friction.

Today, state of the art seismic systems transmit data from the seismograph via telephone line and satellite directly to a central digital computer. A preliminary location, depth-of-focus, and magnitude can now be obtained within minutes of the onset of an earthquake. The only limiting factor is how long the seismic waves take to travel from the epicenter to the stations - usually less than 10 minutes.

The seismograph that Dr. Richter used amplified movements by a factor of 3000, so the waves on the seismograms were much bigger than those that actually occurred in the Earth. Seismologists today do not use the Richter scale as a universal tool for measuring earthquakes, because it does not accurately measure the energy emitted in jolts as big as the one that hit Japan.

Instead Seismologists have since developed a new measurement of earthquake size, called moment magnitude. Moment is a physical quantity more closely related to the total energy released in the earthquake than the Richter magnitude. It can be estimated by geologists examining the geometry of a fault in the field or by seismologists analyzing a seismograph. Moment magnitude has many advantages over other magnitude scales. First, all earthquakes can be compared on the same scale. (Richter magnitude is only precise for earthquakes of a certain size and distance from a seismometer.) Second, because it can be determined either instrumentally or from geology, it can be used to measure old earthquakes and compare them to instrumentally recorded earthquakes. Third, by estimating how large a section of fault will likely move in the future, the magnitude of that earthquake can be calculated with confidence.

To measure all the energy produced by a colossal earthquake, seismologists sometimes have to wait days or weeks to analyze the vibrations of the entire Earth. Using seismic data for an earthquake from a variety of sensors, researchers can infer what they call a “moment tensor,” a three-dimensional plot of both a fault’s orientation and the direction in which it slipped, as well as the distance the fault slipped. This is then used to calculate the total energy released by the earthquake, which the moment magnitude scale’s numbers represent. The moment magnitude scale is calibrated so that it roughly matches the Richter scale’s numbers up to 7.0 or so. But unlike the Richter scale, the moment magnitude scale does not suffer from the saturation problem, and can account for the energy released by unexpectedly large earthquakes.

On the Richter scale, each whole-number step represents an approximate thirty-two fold increase in released energy. For example:
6.0 is equal to 32 times the energy of a 5.0
1,000 times the energy of a 4.0 and
32,000 times the energy released by a 3.0

Two tenths rule:
Every two-tenths of a unit represents double the energy released at the focus.
5.0 to a 5.2 is twice as big
5.4 is four times as large as a 5.0
5.6 is eight times as large as a 5.0

Scientists also guage earthquakes by intensity, which is the degree of damage from an earthquake at a particular location. The intensity scale, the Modified Mercalli Scale, is divided into 12 degrees, each identified by a Roman numeral. Modern seismographic systems precisely amplify and record ground motion (typically at periods of between 0.1 and 100 seconds) as a function of time. This amplification and recording as a function of time is the source of instrumental amplitude and arrival-time data on near and distant earthquakes.

Based on their magnitude, quakes are assigned to a class, according to the U.S. Geological Survey. An increase in one number, say from 5.5 to 6.5, means that a quake's magnitude is 10 times as great. The classes are as follows:
Great: Magnitude is greater than or equal to 8.0. A magnitude-8.0 earthquake is capable of tremendous damage.
Major: Magnitude in the rage of 7.0 to 7.9. A magnitude-7.0 earthquake is a major earthquake that is capable of widespread, heavy damage.
Strong: Magnitude in the rage of 6.0 to 6.9. A magnitude-6.0 quake can cause severe damage.
Moderate: Magnitude in the rage of 5.0 to 5.9. A magnitude-5.0 quake can cause considerable damage.
Light: Magnitude in the rage of 4.0 to 4.9. A magnitude-4.0 quake is capable of moderate damage.
Minor: Magnitude in the rage of 3.0 to 3.9.

Sumatra 2004
China 2006

Haiti 2010
New Zealand 2011



The largest earthquake ever recorded on Earth was a magnitude 9.5 that occurred in Chile in 1960, followed in size by the 1964 Good Friday earthquake in Alaska (magnitude 9.2), a magnitude 9.1 earthquake in Alaska during 1957, and a magnitude 9.0 earthquake in Russia during 1952. Two large earthquakes, one a magnitude 9.0 and one a magnitude 8.2, occurred on Dec. 26, 2004 and March 28, 2005, respectively, along the same fault zone off the coast of Sumatra, Indonesia.

A longer fault can produce a bigger earthquake that lasts a longer time.MagnitudeDateLocationRupture Length
(seconds)9.1 December 26, 2004 Sumatra, Indonesia 1200 500
7.9 January 9, 1857 Fort Tejon, CA 360 130
7.9 May 12, 2008 Sichuan, China 300 120
7.8 April 18, 1906 San Francisco, CA 400 110
7.3 June 28, 1992 Landers, CA 70 24
7.3 August 17, 1959 Hebgen Lake, MT 44 12
7.0 October 17, 1989 Loma Prieta, CA 40 7
7.0 October 28, 1983 Borah Peak, ID 34 9
6.8 February 28, 2001 Nisqually, WA 20 6
6.7 January 17, 1994 Northridge, CA 14 7
6.4 March 10, 1933 Long Beach, CA 15 5
5.9 October 1, 1987 Whittier Narrows, CA 6 3
5.4 July 29, 2008 Chino Hills, CA 5 1

The effects of an earthquake will be dependant upon where you live. we have seen over the years that a smaller earthquake, such as a 4.3 can be felt by those who live close to the rivers, in the floodplain, and not felt by anyone else. This is due to the geological makeup of the ground, nearest the river more sand and more shallow water tables. When earthquake vibrations pass through soil which has a high liquid water content, the soil loses the properties of a solid and takes on those of a semi-liquid, like quicksand or pudding, this process is called liquefaction. The foundations of heavy buildings suddenly lose the support of the soil, and they may topple, or settle deeper into the Earth.

Within the past 15 years building codes have become more strict. Those buildings built within this period will have a better chance of riding out an earthquake, however, this does not mean that it will not sustain any damage.

CBS News

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