Environmental Engineering Reference
In-Depth Information
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150
°
−
120
°
−
90
°
60
°
60
°
40
°
40
°
FIGURE 11.8
National Strong Motion Program
recording stations as of April 10, 2002.
(Courtesy National Strong Motion
Program, USGS.)
20
°
20
°
−
150
°
−
120
°
−
90
°
km
0
5001000
emphasis on locations with a history of high-intensity earthquake activity. A list of inter-
national sources of strong motion networks is available from the
National Center for
Earthquake Engineering Research
(NCEER), in Buffalo, New York.
11.2.4
Intensity and Magnitude
Earthquake Strength Measurements
Two different scales are commonly used to provide a measure of Earthquake strength as
related to ground motion forces at the surface: intensity and magnitude.
Intensity is a qualitative value based on the response of people and objects on the
Earth's surface. Given as “felt” reports, values and their geographic distribution
very much reflect population density.
●
Magnitude is a quantitative value computed from seismogram data. Presently, a
number of different forms are recognized by seismologists.
●
Seismic moment
is a parameter suggested in recent years to rate the strength of an
Earthquake. It includes the rigidity of the rock in which the rupture occurs, times the
length of fault face which moves, times the amount of slip. The San Fernando event (1971)
has been computed to have a seismic moment of nearly 1026 erg.
Intensity (I or MM)
Modified Mercalli Scale of Intensity (MM)
Intensity scales were developed as a basis for cataloging the force of an event for compari-
son with others, and the change in force with distance (attenuation) from the epicenter. The
first intensity scale was developed by DeRossi of Italy and Forel of Switzerland in 1883 (in
the literature referred to as RF, followed by a Roman numeral representing the intensity).
The DeRossi-Forel scale was improved by Mercalli in 1902 and modified further in
1931. In 1956, Richter produced the version given in
Table 11.4.
It correlates ground motion
with damage to structures having various degrees of structural quality. (The author has
added the columns for approximate comparative peak ground velocity, acceleration, and
magnitude. The magnitude is the Richter magnitude.)
