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active crustal regions using empirical approaches. However, such models
are not appropriate for application in all tectonic regimes and the purpose
of the present section is to briefl y outline some of the key features of
GMPEs that are encountered in varying regimes.
Models for shallow active crustal regions
Models for spectral accelerations in seismically active regions with shallow
crustal seismicity are the most commonly encountered. The main reason for
this is that they are able to occur at small source-to-site distances and their
recurrence intervals are relatively high in comparison to other regions.
Therefore, datasets that are compiled in these regions are relatively rich.
However, with the fi rst ever strong-motion recording having been made in
the 1933 Long Beach, California, earthquake, the period of instrumental
observation is very small in comparison with the recurrence intervals of
moderate-to-large earthquakes. In any given region we therefore have just
a small sample of the population of ground motions that could be experi-
enced. The implication is that datasets of strong ground-motions are usually
very unbalanced for individual regions and comprise recordings from the
earthquake scenarios that just happen to have occurred during the period
of observation. For example, Fig. 2.1 shows the magnitude-distance distribu-
tion of accelerograms (for which fi nite rupture models existed) that were
available to developers within the PEER NGA project (Power
et al
., 2008).
In Fig. 2.1, it is clear that there are scenarios for which no ground-motion
recordings have been made. One can also appreciate that where the cover-
8.0
7. 5
7. 0
6.5
6.0
Taiwan
USA
Turkey
Italy
5.5
5.0
0.1
0.5
1. 0
5.0
10.0
50.0
Rupture distance,
R
RUP
(km)
2.1
Magnitude-distance distribution of a subset of the NGA database.
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