Reducing the subjectivity in Modified Mercalli intensity analysis: drawing isoseismals
LEQ: How are Earthquakes Measured? Lesson terms: intensity, Modified Mercalli Scale, magnitude,...
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Transcript of LEQ: How are Earthquakes Measured? Lesson terms: intensity, Modified Mercalli Scale, magnitude,...
LEQ: How are Earthquakes Measured?
Lesson terms:
intensity, Modified Mercalli Scale, magnitude, Richter Scale, seismograph, seismogram, Moment Magnitude Scale
Measuring Earthquakes
There are at least 20 different types of measures.We are studying 3 of them:
The Modified Mercalli scaleThe Richter scaleThe Moment Magnitude scale
Earthquake Intensity The Modified Mercalli Scale
• Used before instruments were invented to detect seismic waves.
• Rates the intensity of an earthquake based upon people’s observations of the damage done by the ground shaking during an earthquake.
• Ranges from I to XII • Uses common terms such as "noticeable by
people" "damage to buildings" chimneys collapse" "fissures open in the ground”.
The Modified Mercalli Scale
Developed in the twentieth century to rate earthquakes according to their intensityThe intensity of an earthquake is the strength of ground motion in a given placeIs not a precise measurementBut, the 12 steps explain the damage given to people, land surface, and buildingsThe same earthquake could have different Mercalli ratings because of the different amount of damage in different spots
•The Mercalli scale uses Roman numerals to rank earthquakes by how much damage they cause
Earthquake Strength The Richter Scale
• Uses instruments to detect seismic waves. • The magnitude of the earthquake’s strength
can be determined from the information recorded by the instrument.
• Range from 0 to 9.• Each higher number of magnitude is 10x
stronger than the number below it.
Seismographs• A seismograph records the
vibrations from earthquakes. Mechanical versions work by
way of a large mass, freely suspended. A seismometer is
an electrical version of the instrument.
• In the example on the left, a rotating drum records a red
line on a sheet of paper. If the earth moves (in this case from
left to right) the whole machine will vibrate too.
• However, the large mass tends to stay still, so the drum shakes
beneath the pen, recording a squiggle!
Seismogram: the record of the Earthquake
The record of an earthquake, a seismogram, as recorded by a seismograph, will be a plot of vibrations versus time. On the seismogram, time is marked at regular intervals, so that we can determine the time of arrival of the first P-wave and the time of arrival of the first S-wave.
Seismogram
The Seismogram
Seismologists use the information recorded on the seismogram to measure the magnitude of the
earthquake recorded and the distance to the epicenter from the seismograph.
The Richter ScaleThe Richter scale is a rating of the size of seismic waves as measured by a particular type of mechanical seismograph Developed in the 1930’sAll over the world, geologists used this for about 50 yearsElectric seismographs eventually replaced the mechanical ones used in this scaleProvides accurate measurements for small, nearby earthquakesDoes not work for big, far ones
The Moment Magnitude ScaleGeologists use this
scale todayIt’s a rating system that estimates the total energy released by an earthquakeCan be used for any kind of earthquakes, near or farSome news reports may mention the Richter scale, but the magnitude number they quote is almost always the moment magnitude for that earthquake
Comparing the Scales
Click on the link below to view a chart that effectively compares the 3 scales.
Earthquake Measurement Scales Comparison Chart