Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 283

- Attenuation of ground motions is different; the full areas of intraplate
earthquakes have been quite large, even if the magnitude is moderate.
- Higher contrast of shaking on soft soils versus hard rock, showing the high
influence of site soil type. Especially the liquefaction of non-cohesive soils seems
to be very important for this earthquake type.
- Intraplate earthquakes with relatively small magnitude can produce large peak
accelerations and large ground velocities only in the near-source region (see 1396
Basel, Switzerland, and 1811-1812 New Madrid, and 1994 Northridge, USA,
earthquakes). This is due to the reduced depth, approximately 80% being less than
10 km deep.
- For source-to-site distance less 10 km, the predicted levels of ground motion are
very strong, depending on the focal depth of the earthquake (see Figure 7.18).
Considering the threshold of 0.2g for damaging motions, it can be observed that
only those motions with focal depths less than 10 km from the surface would be
expected to generate potential damage on limited surface around the epicenter.
- In the near-source zone, the intraplate earthquakes also depend on the source
type and the rupture propagation. There are observation evidences that thrust faults
produce higher ground motion than normal faults for comparable hypocentral
depths and equivalent magnitudes. In the same conditions, the magnitude of thrust
faults can be more than three times the one corresponding to normal faults (Duan
and Oglesby, 2005).
Due to these characteristics, only the near-source of intraplate earthquakes can
be very damaging. Unfortunately, the lack of intraplate records at short distance
from the epicenter encouraged the use of interplate earthquake records, even if it is
known that there are important differences between these two near-source
earthquake types. This is a high priority of professionals to create a network of
seismic stations also in stable continental zone, where some earthquakes have been
experienced, especially along the existing rifts which are already known.
7.7 GROUND MOTIONS PECULIARITIES OF INTRASLAB SOURCES
Earthquakes delineate the subducting lithosphere down to nearly 700 km depth
below the Earth's crust. The classification of intraslab faults is presented in Section
6.4. Unfortunately, there is not information about the earthquakes produced by
these faults at the same level as for the other earthquake types.
For shallow intraslab earthquakes (depth until 70 km), the events are produced
by shear dislocations as a primary mechanism, which is therefore not very different
from the one of subduction earthquakes. Therefore, the same rules as for these
earthquakes can be used, with the only exception for the magnitude, which could
be larger.
The characteristics of earthquakes at depth below 70 km are the subject
controversial discussions, by provoking the interest of seismologists. For
intermediate intraslab earthquakes (70 km to 300 km), the events include both
mechanical and thermal processes (the transformation and dehydration of the crust
components) (Kirby et al, 1991), but the last are moderate. For these earthquakes,
the interests manifested by seismologists are more concentrated in the