Environmental Engineering Reference
In-Depth Information
the interval between the two points in time where the acceleration amplitude is between
the record start to 0.05
g
. The
significant duration
is defined as the time required to build up
from 5 to 95% of the energy in the ground-motion acceleration. A measure of the energy is
the integral of
a
2
d
t
for the total duration of the record, where
a
is the ground motion
acceleration (Erdik and Durukal, 2003). Bolt (1973) provided relationships between brack-
eted duration, source to site distance, and magnitude given in Table 11.5.
∫
11.2.8 Recurrence and Forecasting
General
Prediction Basis
Forecasting the location, magnitude, and time of occurrence of an Earthquake is the role
of the seismologist, and is necessary for seismic design and for the early warning of an
impending event.
A number of factors are considered in forecasting events:
Statistical analysis of historical data (recurrence analysis)
●
Measurements of fault movements, crustal warping, and stress increases
●
Changes in seismic wave velocities (dilatancy theory)
●
Changes in the Earth's magnetic field and other geophysical properties
●
Seismic Risk Analysis
Seismic risk analysis is based on probabilistic and statistical procedures to assess the prob-
able location, magnitude, occurrence, and frequency of earthquake occurrence. Procedures
require evaluation of historical records and of the regional and local geology, particularly
with respect to faults and their activity. The recurrence of events of various magnitudes is
examined, and then the attenuation relationships are evaluated to allow the development
of the probability of ground motion at the site for various magnitudes in terms of the geo-
logic conditions (Donovan and Bornstein, 1978).
In recent years, emphasis has been placed on “probabilistic seismic hazard analysis”
(PSHA) (
Figure 11.50).
The methodology quantifies the hazard at a site from all earthquakes
TABLE 11.5
Bracketed Duration in Seconds
a
Magnitude
Distance (km)
5.5
6.0
6.5
7.0
7.5
8.0
8.5
10
8
12
19
26
31
34
35
25
4
9
15
24
28
30
32
50
2
3
10
22
26
28
29
75
1
1
5
10
14
16
17
100
0
0
1
4
5
6
7
125
0
0
1
2
2
3
3
150
0
0
0
1
2
2
3
175
0
0
0
0
1
2
2
200
0
0
0
0
0
1
2
a
From Bolt, B.A.,
5th World Conference on Earthquake Engineering
, Rome, 1973. With permission.
Note:
1.
Bracketed duration
: The elasped time, for a particular frequency range, between the first and last accel-
eration excursions on an accelerogram record greater than a given amplitude level.
2. Acceleration
0.05
g
, frequency
2 Hz.
