Environmental Engineering Reference
In-Depth Information
the magnitude range of interest. The data are taken from seismicity maps such as
Figure
11.10.
Statistically, the computed number of events per year for a given magnitude is usu-
ally presented as a return time once in so many years. The general relationships vary from
region to region with geologic conditions, stress level, and perhaps magnitude. The vari-
ation in stress level is important in that both short-term and secular changes in earthquake
frequency are thereby permitted (Bollinger, 1976).
In the
southeastern United States
(Bollinger, 1976), from Figure 11.10,
log
N
3.01
0.59
I
o
(for
V
≤
I
o
≤
VIII)
(11.14)
The frequency of occurrence for events of various intensities expressed by Equation 11.14 is
given in
Table 11.6.
Several interpretations are possible from these data: the region is overdue
for the occurrence of a damaging shock (
I
o
VII or VIII), there is a change toward a lower level
of activity, or the maximum intensity in some of the historical data has been overestimated.
For the
Ramapo Fault
in New Jersey (period 1937-1977) (from Aggarwal and Sykes, 1978),
log
N
1.70
±
0.13
0.73
M
(11.15)
Equation 11.15 gives a recurrence of shocks of
M
7.0 of once every 97 years.
Early Warning Indicators
General
There is no certain way of predicting where or when an earthquake may occur, although
a number of tentative methods are under long-term study, most of which are related to
subtle geologic changes with time.
Geologic changes occurring with time include:
Fault displacement, tilting, or warping of the surface
●
Stress increase in fault zone or in surface rocks
●
Fluctuation of gravitational or magnetic fields above normal levels (Before the
Hollister, California event of November 1974, of
M
●
5.2, the magnetic field rose
above the normal level.)
Change in arrival times of transient P waves (dilatancy theory)
●
Change in radon emissions from soils and subsurface waters
●
Animal reactions
are considered significant by the Chinese. It appears that domestic ani-
mals can sense microseisms, and shortly before an earthquake they become highly nerv-
ous. The Chinese have even evacuated cities in recent years on the basis of animal
reactions preceding an event.
TABLE 11.6
Frequency of Earthquake Occurrence in the Southeastern United States
a
I
o
Return Period (years)
Years Since Last
Number Expected per
Occurrence
100 Years
V
0.9
0
115
VI
3.4
1
30
VII
13.0
48
8
VIII
51.0
63
2
(IX, X)
(200, 780)
b
(90)
(0.5, 0.1)
a
From Bollinger, G. A.,
Proc. ASCE, Numer. Methods Geomech.,
2, 917-937, 1976. With permission.
b
Extrapolated values in parentheses.
