Civil Engineering Reference
In-Depth Information
Such intensity-ground motion relationships are nonetheless important so as to make use of historical
data for which only observed intensity information exists.
Closed-form relationships between PGA and relevant intensity scales have been established in Japan
and in the USA. These are given by Kanai (1983) as follows:
PGA
0.25 10
0.50
I
(3.6.1)
=⋅
JMA
0.91 10
0.31
MM
I
PGA
=⋅
(3.6.2)
in which
I
JMA
and
I
MM
are the values of intensity in the JMA and MM scales (
see
Section 1.2.1 ),
respectively. In the above equations, the values of PGA are expressed in cm/s
2
.
Similarly, Trifunac and Brady (1975) suggested the following relationships for horizontal peak
ground acceleration and velocity:
PGA
0.30
I
(3.7.1)
=⋅
10210
.
MM
PGV
0.25
I
(3.7.2)
=⋅
02310
.
MM
where the values of PGA and PGV are in cm/s
2
and cm/s, respectively. Equations (3.7.1) and (3.7.2)
are applicable for
I
MM
ranging between IV and X. It is instructive to note that for every unit increase
in intensity, the PGA and PGV increase by more than 100% and 80%, respectively.
The use of the above relationships should not be indiscriminate but limited to those cases where
historical observation data, based mainly on intensity values, are available.
A recent and signifi cant addition to the library of strong-motion models is the New Generation
Attenuation (NGA) due to Power
et al.
( 2006 ). This ground -motion model is arguably the most robust
model available and has been shown to apply in many parts of the world. Readers are referred to the
source literature on the NGA ground-motion model (e.g. Boore and Atkinson, 2007 ; Campbell and
Bozorgnia, 2007) where details of its background and use are given.
Problem 3.2
Modifi ed Mercalli intensity
I
MM
of IX was assigned to an area of about 80 km long and 30 km wide
during an earthquake that occurred in the Western United States. Compute the peak ground accel-
eration (PGA) from this earthquake. Compute the value of PGA by using both equations (3.6.2)
and (3.7.1) and comment on the results. Estimate the intensity
I
JMA
according the earthquake in the
Japanese Meteorological Agency (JMA) scale.
3.3.1 Features of Strong-Motion Data for Attenuation Relationships
The strong -motion data set (or catalogue) used for attenuation relationship derivation has to fulfi l
a number of requirements. First, all magnitudes should be uniformly recalculated using consistent
approaches. Second, all distances have to be defi ned uniformly. It is necessary to use the distance from
the closest point on the causative fault to the measuring site, not the epicentral distance. This is particu-
larly important when considering large-magnitude earthquakes at short- to - medium distances.
Calculation of the above-mentioned distance is an involved task that requires deep knowledge of the
local tectonic setting, especially when there is no surface manifestation of the fault. Moreover, the data
set should be well populated and reasonably represent distributions in magnitude, distance and soil
condition; otherwise the ensuing attenuation relationship will exhibit statistical bias. Strong- motion
records in databanks may have errors due to instruments and digitization. Since the short- and long -
