Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 278

earthquakes occur along some ancient faults, the main characteristics correspond to
the interplate collision faults.
The catalogue for crustal earthquakes, with magnitude larger than M 4.5,
produced in stable continental regions, contains 1373 events from 495 to 2003
(Schulte and Mooney, 2005, 2007). These earthquakes are classified in the
following categories:
- Interior rifts, if located within 20 km of identified rifts.
- Rifted continental margins, if located within 20 km of these margins.
- Non-rifted crust, if located further than 50 km of any of the above.
If one compares this classification with the one presented in Section 5.3, the
interior rifts correspond to the normal faults produced in zones of crust weakness,
non-rifted crust to reversal-thrust faults, while the rifted continental margins to
faults produced in subducted and collided zones. From the analysis of these data,
one can find that 27% are associated to the rifted crust, 25% to rifted continental
margins, 36% to non-rifted crust and 12% remain uncertain. So, over half of all
events are associated with the two rifted types. From 1373 earthquakes presented
in the database, only 118 earthquakes have magnitude larger than M 6 and 14
exceed magnitude M 7 (all in rifted crust). Examining the map of intraplate
earthquakes, one can observe that they are not evenly distributed and several zones
of concentrated seismicity seem to exist. So, 12 regions are responsible for 74% of
all the events and 98% of the seismic energy is released in these zones. An analysis
of the main intraplate earthquake characteristics is performed by Gangopadhyay
and Talwani (2003) and Stepp (1996).
Situated in crustal zones, the ground motions must have the features of near-
source earthquakes, but with some modified characteristics. Figure 7.55 shows the
case of Banat (Romania) earthquake recorded on 11 December 1991 (Gioncu,
2000). The records refer to the Timisoara site, situated 40 km from the epicenter,
and to the Banloc site which is the place of the epicentre. In the epicentral Banloc
area, one can see that their records have the characteristics of near-source ground
motions (velocity pulse) (Fig. 7.55), but having very short period (0.2 to 0.4 sec),
low accelerations and short duration. The records of the Timisoara site have the
characteristics of intermediate-source area, due to the attenuation effect. The same
characteristics of intraplate earthquakes were observed for other earthquakes. So,
the intraplate ground motions have the features of near-source earthquakes, but
with reduced periods, accelerations and shorter duration.