How Are Earthquake Magnitudes Measured?


How Are Earthquake Magnitudes Measured?

The Richter Scale

Figure 1 - Charles Richter studying a seismogram.
The magnitude of most earthquakes is measured on the *Richter scale*,
invented by Charles F. Richter in 1934. The Richter magnitude is calculated
from the amplitude of the largest seismic wave recorded for the earthquake,
no matter what type of wave was the strongest.

The Richter magnitudes are based on a logarithmic scale (base 10). What
this means is that for each whole number you go up on the Richter scale,
the amplitude of the ground motion recorded by a seismograph goes up ten
times. Using this scale, a magnitude 5 earthquake would result in ten times
the level of ground shaking as a magnitude 4 earthquake (and 32 times as
much energy would be released). To give you an idea how these numbers can
add up, think of it in terms of the energy released by explosives: a
magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of
TNT. A magnitude 8 earthquake releases as much energy as detonating *6
million tons of TNT*. Pretty impressive, huh? Fortunately, most of the
earthquakes that occur each year are magnitude 2.5 or less, too small to be
felt by most people.

The Richter magnitude scale can be used to desribe earthquakes so small
that they are expressed in negative numbers. The scale also has no upper
limit, so it can describe earthquakes of unimaginable and (so far)
unexperienced intensity, such as magnitude 10.0 and beyond.

Although Richter originally proposed this way of measuring an earthquake's
"size," he only used a certain type of seismograph and measured shallow
earthquakes in Southern California. Scientists have now made other
"magnitude" scales, all calibrated to


how are earthquakes measured

Earthquake - Wikipedia, the free encyclopedia


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Global earthquake epicenters, 1963–1998
Global plate tectonic movement

Part of a series on
· · Foreshock
· Aftershock

· · Blind thrust
· Doublet

· · Interplate
· Intraplate

· · Megathrust
· Remotely triggered

· · Slow
· Submarine
· Supershear

· · Tsunami
· Earthquake swarm

· · Fault movement
· Volcanism

· Induced seismicity

· · Epicenter
· Hypocenter

· · Shadow zone
· Seismic wave

· · P-wave
· S-wave

· · Seismic scales
· Seismometer

· Earthquake duration magnitude

· Coordinating Committee for
Earthquake Prediction
· Earthquake-sensitive person

Other topics
· Shear wave splitting
· Adams–Williamson equation
· Flinn-Engdahl regions
· Earthquake engineering
· · Seismite
· Seismology

· v
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· e

An *earthquake* (also known as a *quake*, *tremor* or *temblor*) is the
result of a sudden release of energy in the Earth's crust that creates
seismic waves. The *seismicity*, *seismism* or *seismic activity* of an
area refers to the frequency, type and size of earthquakes experienced over
a period of time.

Earthquakes are measured using observations from seismometers. The moment
magnitude is the most common scale on which earthquakes larger than
approximately 5 are reported for the entire globe. The more numerous
earthquakes smaller than magnitude 5 reported by national seismological
observatories are measured mostly on the local magnitude scale, also
referred to as the Richter scale. These two scales are numerically similar
over their range of validity. Magnitude 3 or lower earthquakes are mostly
almost imperceptible or weak and magnitude 7 and over potentially cause
serious damage over larger areas, depending on their depth. The largest


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