However the intensity is variable over the area affected by the earthquake, with high intensities near the epicentre and lower values further away. These are allocated a value depending on the effects of the shaking according to the Modified Mercalli Intensity Scale. In an example, Magnitude can be likened to the power of radio or television waves sent out from a broadcasting station.
Intensity is how well you receive the signal, which can depend on your distance from the energy source, the local conditions, and the pathway the signal has to take to reach you. Privacy Statement Disclaimer and Copyright. Staff Search. Earthquakes Earthquakes at a Plate Boundary. New Zealands Largest Fault.
Earthquakes and Faults. New Zealand Earthquakes. Major Faults in New Zealand. The distance to the epicenter must also be taken into account because the greater the distance from the earthquake, the smaller the waves get. The effect of distance is factored out of the calculation. There is no upper limit defined for the Richter scale, but after a century of seismograph measurements, it appears that rocks in the earth release their stress before building up enough energy to reach magnitude The Richter scale was found to not transfer very well from the San Andreas fault zone, a transform plate boundary, to the much more powerful earthquakes that occur at convergent plate boundaries, particularly subduction zone earthquakes.
Therefore, the Richter scale has been replaced by the moment magnitude scale, symbolized as M w. The moment magnitude scale is broadly similar to the Richter scale, but it takes more factors into account, including the total area of the fault that moves during the earthquake, and how much it moves.
This produces a magnitude number that is a better indicator of the total amount of energy released by the earthquake. Because the moment magnitude scale has replaced the Richter scale, we will assume from here on that we are referring to moment magnitude, not Richter magnitude, when we speak of earthquake magnitude.
The magnitude scale portrays energy logarithmically to approximately base For example, a magnitude 6. A magnitude 7. A magnitude 9.
Earthquake intensity is very different from earthquake magnitude. Earthquake intensity is a ranking based on the observed effects of an earthquake in each particular place. Therefore, each earthquake produces a range of intensity values, ranging from highest in the epicenter area to zero at a distance from the epicenter.
The most commonly used earthquake intensity scale is the Modified Mercalli earthquake intensity scale. The table below shows approximately how many earthquakes occur each year in each magnitude range and what the intensity might be at the epicenter for each magnitude range. The following table gives intensities that are typically observed at locations near the epicenter of earthquakes of different magnitudes.
Improve this page Learn More. Skip to main content. Module 8: Earthquakes. Search for:. Reading: Magnitude versus Intensity Magnitude and Intensity measure different characteristics of earthquakes. Calculating Earthquake Magnitude The magnitude of an earthquake is a number that allows earthquakes to be compared with each other in terms of their relative power.
Emergency Management. Survey Manual. Earthquake magnitude, energy release, and shaking intensity are all related measurements of an earthquake that are often confused with one another. Their dependencies and relationships can be complicated, and even one of these concepts alone can be confusing. The time, location, and magnitude of an earthquake can be determined from the data recorded by seismometer. Seismometers record the vibrations from earthquakes that travel through the Earth.
Each seismometer records the shaking of the ground directly beneath it. Sensitive instruments, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. Modern systems precisely amplify and record ground motion typically at periods of between 0.
Magnitude is the size of the earthquake. An earthquake has a single magnitude. The shaking that it causes has many values that vary from place to place based on distance, type of surface material, and other factors. See the Intensity section below for more details on shaking intensity measurements.
Magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude as measured on a seismogram.
When initially developed, all magnitude scales based on measurements of the recorded waveform amplitudes were thought to be equivalent. But for very large earthquakes, some magnitudes underestimate true earthquake size, and some underestimate the size.
Thus, we now use measurements that describe the physical effects of an earthquake rather than measurements based only on the amplitude of a waveform recording. More on that later. The Richter Scale M L is what most people have heard about, but in practice it is not commonly used anymore, except for small earthquakes recorded locally, for which ML and short-period surface wave magnitude Mblg are the only magnitudes that can be measured.
For all other earthquakes, the moment magnitude Mw scale is a more accurate measure of the earthquake size. Although similar seismographs had existed since the 's, it was only in that Charles F.
Richter, a seismologist at the California Institute of Technology, introduced the concept of earthquake magnitude. His original definition held only for California earthquakes occurring within km of a particular type of seismograph the Woods-Anderson torsion instrument.
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