At least 5 aftershocks rattle DC area – US news – Life – msnbc.com

 

 

 

 

Modified Mercalli Intensity scale

The lower degrees of the Modified Mercalli Intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage.

The small table is a rough guide to the degrees of the Modified Mercalli Intensity scale.[1][2] The colors and descriptive names shown here differ from those used on certain shake maps in other articles.

The large table gives Modified Mercalli scale intensities that are typically observed at locations near the epicenter of the earthquake.[1]

The correlation between magnitude and intensity is far from total, depending upon several factors including depth of the earthquake, terrain, and population density. For example, on 19 May 2011 an earthquake of magnitude 0.7 in Southern California, USA 4 km deep was classified as of intensity III by the United States Geological Survey (USGS), while a 4.5 magnitude quake in Salta, Argentina 164 km deep was of intensity I.[3]

Moment Magnitude Typical Maximum
Modified Mercalli Intensity
1.0 – 3.0 I
3.0 – 3.9 II – III
4.0 – 4.9 IV – V
5.0 – 5.9 VI – VII
6.0 – 6.9 VII – IX
7.0+ VIII or higher

I. Instrumental Generally not felt by people unless in favorable conditions.
II. Weak Felt only by a few people at best, especially on the upper floors of buildings. Delicately suspended objects may swing.
III. Slight Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.
IV. Moderate Felt indoors by many people, outdoors by few people during the day. At night, some awaken. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rock noticeably. Dishes and windows rattle alarmingly.
V. Rather Strong Felt outside by most, may not be felt by some outside in non-favorable conditions. Dishes and windows may break and large bells will ring. Vibrations like large train passing close to house.
VI. Strong Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight.
VII. Very Strong Difficult to stand; furniture broken; damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Noticed by people driving motor cars.
VIII. Destructive Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture moved.
IX. Violent General panic; damage considerable in specially designed structures, well designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X. Intense Some well built wooden structures destroyed; most masonry and frame structures destroyed with foundation. Rails bent.
XI. Extreme Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.
XII. Cataclysmic Total destruction – Everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock move position. Landscape altered, or leveled by several meters. In some cases, even the routes of rivers are changed.

[edit]Correlation with physical quantities

The Mercalli scale is not defined in terms of more rigorous, objectively quantifiable measurements such as shake amplitude, shake frequency, peak velocity, or peak acceleration. Human perceived shakings and building damages are best correlated with peak acceleration for lower-intensity events, and with peak velocity for higher-intensity events.[4]

[edit]Comparison to the moment magnitude scale

The effects of any one earthquake can vary greatly from place to place, so there may be many Mercalli intensity values measured for the same earthquake. These values can be best displayed using a contoured map. Each earthquake, on the other hand, has only one magnitude.

At least 5 aftershocks rattle DC area – US news – Life – msnbc.com.

http://earthquake.usgs.gov/earthquakes/states/maryland/history.php

http://www.mgs.md.gov/esic/fs/fs13.html

http://earthquake.usgs.gov/earthquakes/states/maryland/hazards.php

http://en.wikipedia.org/wiki/Earthquake

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One Response to At least 5 aftershocks rattle DC area – US news – Life – msnbc.com

  1. Moment magnitude scale
    From Wikipedia, the free encyclopedia
    The moment magnitude scale (abbreviated as MMS; denoted as MW) is used by seismologists to measure the size of earthquakes in terms of the energy released.[1] The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped.[2] The scale was developed in the 1970s to succeed the 1930s-era Richter magnitude scale (ML). Even though the formulae are different, the new scale retains the familiar continuum of magnitude values defined by the older one. The MMS is now the scale used to estimate magnitudes for all modern large earthquakes by the United States Geological Survey.[3]
    Contents [hide]
    1 Definition
    2 Comparative energy released by two earthquakes
    3 Radiated seismic energy
    4 Nuclear explosions
    5 Comparison with Richter scale
    6 See also
    7 Notes
    8 References
    9 External links
    [edit]Definition

    The symbol for the moment magnitude scale is Mw, with the subscript w meaning mechanical work accomplished. The moment magnitude Mw is a dimensionless number defined by

    where M0 is the magnitude of the seismic moment in dyne centimeters (10−7 N·m).[1] The constant values in the equation are chosen to achieve consistency with the magnitude values produced by earlier scales, the Local Magnitude and the Surface Wave magnitude, both referred to as the “Richter” scale by reporters.
    [edit]Comparative energy released by two earthquakes

    As with the Richter scale, an increase of one step on this logarithmic scale corresponds to a 101.5 ≈ 32 times increase in the amount of energy released, and an increase of two steps corresponds to a 103 = 1000 times increase in energy.
    The following formula, obtained by solving the previous equation for M0, allows one to assess the proportional difference fΔE in energy release between earthquakes of two different moment magnitudes, say m1 and m2:

    [edit]Radiated seismic energy

    Potential energy is stored in the crust in the form of built-up stress. During an earthquake, this stored energy is transformed and results in
    cracks and deformation in rocks
    heat,
    radiated seismic energy Es.
    The seismic moment M0 is a measure of the total amount of energy that is transformed during an earthquake. Only a small fraction of the seismic moment M0 is converted into radiated seismic energy Es, which is what seismographs register. Using the estimate

    Choy and Boatwright defined in 1995 the energy magnitude [4]

    [edit]Nuclear explosions

    The energy released by nuclear weapons is traditionally expressed in terms of the energy stored in a kiloton or megaton of the conventional explosive trinitrotoluene (TNT).
    A rule of thumb equivalence from seismology used in the study of nuclear proliferation asserts that a one kiloton nuclear explosion creates a seismic signal with a magnitude of approximately 4.0.[5] This in turn leads to the equation [6]

    where mTNT is the mass of the explosive TNT that is quoted for comparison (relative to megatons Mt).
    Such comparison figures are not very meaningful. As with earthquakes, during an underground explosion of a nuclear weapon, only a small fraction of the total amount of energy transformed ends up being radiated as seismic waves. Therefore, a seismic efficiency has to be chosen for a bomb that is quoted as a comparison. Using the conventional specific energy of TNT (4.184 MJ/kg), the above formula implies the assumption that about 0.5% of the bomb’s energy is converted into radiated seismic energy Es.[7] For real underground nuclear tests, the actual seismic efficiency achieved varies significantly and depends on the site and design parameters of the test.
    [edit]Comparison with Richter scale

    Main article: Richter magnitude scale
    In 1935, Charles Richter and Beno Gutenberg developed the local magnitude (ML) scale (popularly known as the Richter scale) with the goal of quantifying medium-sized earthquakes (between magnitude 3.0 and 7.0) in Southern California. This scale was based on the ground motion measured by a particular type of seismometer at a distance of 100 kilometres (62 mi) from the earthquake’s epicenter.[3] Because of this, there is an upper limit on the highest measurable magnitude, and all large earthquakes will tend to have a local magnitude of around 7. The magnitude becomes unreliable for measurements taken at a distance of more than about 600 kilometres (370 mi) from the epicenter.
    The moment magnitude (Mw) scale was introduced in 1979 by Caltech seismologists Thomas C. Hanks and Hiroo Kanamori to address these shortcomings while maintaining consistency. Thus, for medium-sized earthquakes, the moment magnitude values should be similar to Richter values. That is, a magnitude 5.0 earthquake will be about a 5.0 on both scales. This scale was based on the physical properties of the earthquake, specifically the seismic moment (M0). Unlike other scales, the moment magnitude scale does not saturate at the upper end; there is no upper limit to the possible measurable magnitudes. However, this has the side-effect that the scales diverge for smaller earthquakes.[1]
    The concept of seismic moment was introduced in 1966,[8] but it took 13 years before the (Mw) scale was designed. The reason for the delay was that the necessary spectra of seismic signals had to be derived by hand at first, which required personal attention to every event. Faster computers than those available in the 1960s were necessary and seismologists had to develop methods to process earthquake signals automatically. In the mid 1970s Dziewonski[9] started the Harvard Global Centroid Moment Tensor Catalog.[10] After this advance, it was possible to introduce (Mw) and estimate it for large numbers of earthquakes.
    Moment magnitude is now the most common measure for medium to large earthquake magnitudes,[11] but breaks down for smaller quakes. For example, the United States Geological Survey does not use this scale for earthquakes with a magnitude of less than 3.5, which is the great majority of quakes. For these smaller quakes, other magnitude scales are used. All magnitudes are calibrated to the ML scale of Richter and Gutenberg.
    Magnitude scales differ from earthquake intensity, which is the perceptible shaking, and local damage experienced during a quake. The shaking intensity at a given spot depends on many factors, such as soil types, soil sublayers, depth, type of displacement, and range from the epicenter (not counting the complications of building engineering and architectural factors). Rather, magnitude scales are used to estimate with one number the size of the quake.
    The following table compares magnitudes towards the upper end of the Richter Scale for major Californian earthquakes.[1]
    Date Seismic moment (dyne-cm) Richter scale ML Moment magnitude Mw
    1933-03-11 2 6.3 6.2
    1940-05-19 30 6.4 7.0
    1941-07-01 0.9 5.9 6.0
    1942-10-21 9 6.5 6.6
    1946-03-15 1 6.3 6.0
    1947-04-10 7 6.2 6.5
    1948-12-04 1 6.5 6.0
    1952-07-21 200 7.2 7.5
    1954-03-19 4 6.2 6.4
    [edit]

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