Environmental Engineering Reference
In-Depth Information
suggests that the rate of decrease in magnitude is relatively small over a distance from the
fault equal to the vertical distance to the focus, and beyond this point the drop-off
increases rapidly. Many investigators report the lack of markedly greater shaking damage
to structures adjacent to the fault as compared with the damage some distance beyond
(Bonilla, 1970).
Moderate Distances and Beyond
In most geological environments high accelerations are severely reduced at even moder-
ate distances. Wave attenuation from frictional resistance occurs exponentially, and if
shaking is caused by shear waves in the surface of the crust, then even at short distances
the exponential decay becomes very effective.
A relationship between peak bedrock acceleration, magnitude, and distance from the
causative fault for focal depths of 0 to 20 km is given in
Figure 11.28.
Attenuation of maximum acceleration with distance from fault rupture for California
earthquakes, prepared from strong motion records, is given in
Figure 11.29.
Curve I
applies to high-intensity sources, associated with effective modes of dislocation, rock
types, rupture depths, etc., as, for example, San Fernando, 1971. Peak acceleration near a
high-intensity fault would appear to be close to 0.6
g
on average. The expectation ranges
indicate that 90% of the time the peak acceleration would be less than 0.4
g
. Curve II
applies to medium-intensity sources, produced by fault dislocations of a less efficient
kind, such as El Centro, 1940. In either case, for distances over 100 km from the source,
peak accelerations on rock are unlikely to exceed 0.1
g
.
Relationships among predominant periods of maximum acceleration, magnitude, and
distance from the causative fault are given in
Figure 11.30.
Distance from causative fault (mil)
0
25
50
75
0.50
Focal depth = 0 km
Focal depth = 20 km
M
= Richter magnitude
0.45
0.40
0.35
0.30
0.25
0.20
M =8.0
0.15
7.5
FIGURE 11.28
Relationship between peak bedrock
acceleration, earthquake magnitude, and
distance from the causative fault for focal
depths of 0 and 20 km. (From Leeds, D.J.,
Geology, Seismicity and Environmental Impact
,
Special Publication Association of Engineering
Geology, Los Angeles, California, 1973. With
permission.)
0.10
7.0
5.5
5.0
6.5
6.0
0.05
0.00
0
20
40
60
80
100
120
140
Distance from causative fault (km)
