Learning from Earthquakes - Earthquake Engineering for Structural Design - page 81

important to note the difference between the characteristics of parallel and
normal ground motions. This directivity was very evident during the Loma
Prieta, Northridge, Kobe, Kocaeli and Chi-Chi earthquakes.
- For the near-field sites (an area within a distance of 15-20 km from the
epicenter) the earthquake characteristics are very different in comparison
with the ones recorded at the far-field sites. The three directions recorded
of accelerations, velocities and displacements obtained at Northridge,
Kobe and Chi-Chi prove this very important aspect.
- For surface sources in the near-field sites, even moderate earthquakes in
magnitude can produce high levels of peak accelerations and velocities.
The 1999 moderate Athens earthquake produced high accelerations.
- Recurrence periods, characteristic for a given place, are very different for
interplate and intraplate earthquakes, because the former move along a
very well determined fault, while the latter are characteristic for an area,
where the epicenter positions are undetermined and in continuously
changing. For interplate earthquakes it is possible to use statistical
methodologies, which, in contrast, are useless for the intraplate
earthquakes due to the absence of sufficient data on the same site.
Examining all these aspects, it results that there are three main earthquake
types: crustal interplate and intraplate earthquakes, and deep intraslab
earthquakes. Both earthquake hazard and seismic hazard analyses must be
carried out on the bases of this classification of earthquake types.
Figure 3.8 Surface rupture effects during 1906 San Francisco earthquake
(USGS, nd)