Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 474

Type 2 M< 5.5
Type 1 M> 5.5
Figure 10.1 Elastic response spectra types (after Gioncu and Mazzolani, 2002)
10.4.2 Interplate Crustal Earthquakes
There are two interplate types: subduction and strike-slip earthquakes.
Subduction crustal earthquakes . These earthquakes are produced by faults
situated in the Earth crust and characterized by the highest magnitudes recorded in
the world: 1960 Chile M 9.5, 1964 Alaska M 9.2, 1957 Aleutian Islands M 9.1
and 2004 Sumatra M 9.0. Therefore, in function of recurrence studies, this
earthquake type can be classified as a very strong one. The main effects are
concentrated in the so-called near-source zones, assumed to be within a distance of
about 20-30 km from the epicenters. Being very difficult to establish the exact
distance between site and source, all areas where subductions exist must be
considered potentially situated in near-source zones. Within this zone, the ground
motions are significantly influenced by the rupture mechanism. The main effect is
the velocity pulse, where the maximum values of velocity are very difficult to
record due to the focusing effects and only by chance a recording station is located
in that zone. Such a chance happened during the Chi-Chi (Taiwan) earthquake in
an isolated station; the recorded velocity exceeded 350 cm/sec, without the
certitude that there were not other sites with larger velocities. The fact that during
the Northridge earthquake the recorded velocities did not exceed 200 cm/sec, does