The Richter scale was developed in by American seismologist Charles Richter as a way of quantifying the magnitude, or strength, of earthquakes. Richter, who was studying earthquakes in California at the time, needed a simple way to precisely express what is qualitatively obvious: some earthquakes are small and others are large. An earthquake is a violent shaking of the ground that is usually caused by sudden motion on a geological fault.
For example, the magnitude 6. Today, earthquakes and fault motion are inextricably linked in the minds of seismologists--so much so that upon hearing that an earthquake has occurred, we immediately ask about the fault that caused it. Richter's focus, in contrast, was on the ground vibration itself, which he could easily monitor using seismometers at the California Institute of Technology Caltech.
To Richter, a high-magnitude earthquake was one with strong ground vibration. Thus, for the Richter scale no direct connection is made to any of the properties of the causative fault. Richter's scale was modeled on the stellar magnitude scale used by astronomers, which quantifies the amount of light emitted by stars their luminosity. A star's luminosity is based on telescopic observations of its brightness that are corrected for the telescope's magnification and for the star's distance from Earth.
But because luminosity varies over many factors of ten Betelgeuse is 50, times more luminous than Alpha Centauri, for example , astronomers calculate a logarithm of the luminosity to produce the stellar magnitude: an easy-to-remember single-digit number. Richter substituted measurements of the amount of ground vibration, as measured by a seismograph, for measurements of luminosity.
Now, instruments are carefully calibrated with respect to each other. Thus, magnitude can be computed from the record of any calibrated seismograph. The Richter Scale has no upper limit. Recently, another scale called the moment magnitude scale MMS has been devised for more precise study of great earthquakes.
Tectonic earthquakes can range in size from magnitudes less than zero, resulting from fault slippage of a few centimetres, to the largest events magnitude greater than 9 , where fault displacements are on the order of many metres. The size of an earthquake is not only a function of the amount of displacement but also the area of the fault plane that ruptures. Hence the larger the rupture area, the larger is the earthquake.
A magnitude 7 earthquake ruptures a fault area of about km2 or about 50 km long and 20 km wide. Also depth is an important factor influencing earthquake severity. We know that earthquakes can originate at various depths within the Earth's solid core.
The deeper the earthquake, the more powerful it is, but it is also far less likely to reach the surface. That's why shallow earthquakes are more common and more dangerous, because the shallower an earthquake, the more damage to surface structures it can cause.
When this energy is released suddenly, for example by shearing movements along faults in the crust of the Earth, an earthquake results. The area of the fault where the sudden rupture takes place is called the focus or hypocenter of the earthquake.
The point on the Earth's surface directly above the focus is called the epicenter of the earthquake. San Fernando, California, Highway interchange heavily damaged by the magnitude 6.
Seismographs record a zig-zag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, location, and magnitude of an earthquake can be determined from the data recorded by seismograph stations. The Richter magnitude scale was developed in by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes.
The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included in the magnitude formula to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquakes.
On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude of 5. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value.
Earthquake-induced liquefaction of the earth-filled dam resulted in a landslide that caused partial collapse [Click on image for a larger view] At first, the Richter Scale could be applied only to the records from instruments of identical manufacture. Now, instruments are carefully calibrated with respect to each other. Thus, magnitude can be computed from the record of any calibrated seismograph. Earthquakes with magnitude of about 2.
Events with magnitudes of about 4. Great earthquakes, such as the Good Friday earthquake in Alaska, have magnitudes of 8. On the average, one earthquake of such size occurs somewhere in the world each year. Although the Richter Scale has no upper limit, the largest known shocks have had magnitudes in the 8. Recently, another scale called the moment magnitude scale has been devised for more precise study of great earthquakes.
The Richter Scale is not used to express damage. An earthquake in a densely populated area which results in many deaths and considerable damage may have the same magnitude as a shock in a remote area that does nothing more than frighten the wildlife. This means it provides a closer approximation to the motion — and potential damage — of a building during an earthquake than PGA.
The structural performance of a building during an earthquake depends on many factors, including magnitude, distance to the fault, soil type, building height, and construction quality. All these factors are built into the Spectral Acceleration calculation. From all of that information we can provide a comprehensive plan, which includes physical and organizational measures, to increase resilience.
Cookies help us improve your website experience. By using our website, you agree to our use of cookies. Industry knowledge. Earthquakes and tsunamis.
0コメント