Page 259 - Geothermal Energy Renewable Energy and The Environment

248 Geothermal Energy: Renewable Energy and the Environment

where A is the amplitude of the trace (mm). This relationship was then used by Gutenberg and Richter (1956) to
compute the actual energy release
2
log E Q = 2.9 + 1.9 × M L − 0.024 × M L , (12S.2)
where E Q is the energy release (J). Figure 12S.1 shows how energy release depends on magnitude. Note that for each
increase in magnitude, there is an increase in energy released that is much more than a factor of 10. Modifications of
this relationship continue to be refined as new models of faulting are developed (see Abe 1995 for a summary).
For ground shaking, there is no single measurement associated with a specific earthquake or amount of energy
released since the amount of shaking depends upon the local geology. A person standing on unconsolidated sedi-
ments, such as landfill, for example, would experience much more movement of the ground surface for a given
earthquake than someone nearby standing on a bedrock outcrop. Experience over the years has resulted in the
development of a qualitative scale that describes the amount of damage that would be done and the sensation
that a person would experience for a given amount of shaking. This scale is called the Modified Mercalli Intensity
(MMI) scale. The MMI scale goes from I to XII. Table 12S.1 gives a brief summary of what some of the intensity
values mean.

10 16

10 14 Gutenberg-Richter

10 12

Energy (J) 10 10

10 8
Log-linear
10 6

10 4
10 2
–2 0 2 4 6 8 10
Magnitude

FIGUre 12s.1 The relationship between energy released (E) during an earthquake, in Joules, and the mag-
nitude of the earthquake. The curve labeled “Gutenberg-Richter” is derived from Equation 12S.2.s The line
labeled “Log-linear” shows the E vs magnitude relationship for a different model that assumes a log linear
relationship between energy release and magnitude. The difference between the curves provides an indication
of the uncertain in establishing the absolute amount of energy released during an earthquake.

Table 12s.1
description of modified mercalli Intensity scale

Ground motion
mmI Intensity description Velocity (cm/s)
I Not felt except by very few in especially favorable conditions.
During the day, felt by many who are indoors but only noticed by a few outdoors.
III 1–2
If at night, some people awakened. Vibration like that of a passing light truck.
VI Felt by everyone. Damage slight. Some heavy furniture moved. 5–8
Considerable damage, even in well-built structures. Some buildings collapse.
IX 45–55
Conspicuous ground cracking.
XII Total destruction. >60

Source: Bolt, B., Earthquakes, New York: W.H. Freeman, 1988.