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equatorial air masses

13.3.1 How Are Earthquakes Measured?
The tragic consequences of earthquakes can be measured in many ways, like
death tolls or force of ground shaking. Two measures in particular are commonly
used. One is a qualitative measure of the damage inflicted by the earthquake, and
it is referred to as intensity. The second is a quantitative measure of the energy
released by the earthquake, termed magnitude. Both measures provide meaningful data.
13.3.2 Earthquake Intensity
Intensity measurements take into account both the damage incurred due to
the quake and the way that people respond to it. The Modified Mercalli Intensity
Scale (Figure 13.6) is the most widely used scale to measure earthquake intensities. This scale has values that range from Roman numerals I to XII which characterize the damage observed and people’s reactions to it. Data for this scale is
often collected right after an earthquake by having the local population answer
questions about the damage they see and what happened during the quake. This
information can then be pooled to create an intensity map, which creates colored
zones based on the information collected (Figure 13.7). These maps are frequently
used by the insurance industry.
Figure 13.5 | A seismograph and the seismogram it produces.
Author: User “Yamaguchi”
Source: Wikimedia Commons
License: CC BY-SA 3.0
Page | 318
Introductory Geology Earthquakes
Intensity Characteristics
I Shaking not felt under normal circumstances.
II Shaking felt only by those at rest, mostly along upper floors in buildings.
III Weak shaking felt noticeably by people indoors. Many do not recognize this as an earthquake. Vibrations similar to a large vehicle passing by.
IV Light shaking felt indoors by many, outside by few. At night, some were awakened. Dishes, doors, and
windows disturbed; walls cracked. Sensation like heavy truck hitting a building. Cars rock noticeably.
V Moderate shaking felt by most; many awakened. Some dishes and windows broken. Unstable objects
VI Strong shaking felt by all, with many frightened. Heavy furniture may move, and plaster breaks. Damage is slight.
VII Very strong shaking sends all outdoors. Well-designed buildings sustain minimal damage; slight-moderate
damage in ordinary buildings; considerable damage in poorly built structures.
VIII Severe shaking. Well-designed buildings sustain slight damage; considerable damage in ordinary buildings; great damage in poorly built structures.
IX Violent shaking. Well-designed buildings sustain considerable damage; buildings are shifted off foundations,
with some partial collapse. Underground pipes are broken.
X Extreme shaking. Some well-built wooden structures are destroyed; most masonry and frame structures
are destroyed. Landslides considerable.
XI Few structures are left standing. Bridges are destroyed, and large cracks open in the ground.
XII Total damage. Objects thrown upward in the air.
Figure 13.6 | (Above) An abbreviated
table of the Modified Mercalli Intensity
Scale. Intensity for a particular
earthquake is determined by the
maximum damage incurred.
Author: Randa Harris
Source: Original Work
License: CC BY-SA 3.0
Figure 13.7 | (Right) An intensity map
for the San Fernando earthquake in
southern California on 2/9/76. Notice
that near the epicenter (marked by a
star), the intensity was extreme.
Author: USGS
Source: Wikimedia Commons
License: Public Domain
Page | 319
Introductory Geology Earthquakes
13.3.3 Earthquake Magnitude
Another way to classify an earthquake is by the energy released during the event;
this is referred to as the magnitude of the earthquake. While magnitude has been
measured using the Richter scale, as the frequency of earthquake measurements
around the world increased, it was realized that the Richter magnitude scale was
not valid for all earthquakes (it is not accurate for large magnitude earthquakes).
A new scale called the Moment Magnitude Intensity Scale was developed, which
maintains a similar scale to the Richter scale. This scale estimates the total energy
released by an earthquake and can be used to characterize earthquakes of all sizes
throughout the world. The magnitude is based on the seismic moment (estimated
based on ground motions recorded on a seismogram), which is a product of the
distance a fault moved and the force required to move it. This scale works particularly well with larger earthquakes and has been adopted by the United States Geological Survey. Magnitude is based on a logarithmic scale, which means for each
whole number that you increase, the amplitude of the ground motion recorded by
a seismograph increases by 10 and the energy released increases by 101.5, rather
than one (so that a 3 magnitude quake results in ten times the ground shaking as a
2 magnitude quake; a magnitude 4 quake has 102
or 100 times the level of ground
shaking as a 2 magnitude quake (releasing 103
or 1000 times as much energy). For
a rough comparison of magnitude scale to intensity, see Figure 13.8. Why is it necessary to have more than one type of scale? The magnitude scale allows for worldwide characterization of any earthquake event, while the intensity scale does not.
With an intensity scale, a IV in one location could be ranked a II or III in another
location, based off of building construction (ex. poorly constructed buildings will
suffer more damage in the same magnitude earthquake as those built with stronger construction).
13.4 Locating an Earthquake Epicenter
During an earthquake, seismic waves are sent all over the globe. Though they
may weaken with distance, seismographs are sensitive enough to still detect these
waves. In order to determine the location of an earthquake epicenter, seismographs
Magnitude Typical Maximum Modified Mercalli Intensity
1.0 – 2.9 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 and above VIII or above
Figure 13.8 | A comparison of magnitude versus intensity scales for
Author: Randa Harris
Source: Original Work
License: CC BY-SA 3.0

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