Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 476

10.4.4 Intraslab Earthquakes
There are three intraslab earthquakes: shallow, intermediate and deep.
Unfortunately, there are not sufficient records and data to establish the main
differences between these three cases. Magnitudes are high, the maximum recorded
values being around M 8.0. Due to the depth of source, the area affected by the
ground motion is very large. Consequently, the near-source effects are
insignificant. Generally, the velocities are low and pulses can exist only in
recorded accelerations. The seismic waves passing through an important number of
layers, the number of cycles is very large with long periods, corresponding to T1
spectrum type. The influence of site soil conditions is very high, leading to long
duration of ground motions.
10.5 STRUCTURAL RESPONSE TO INTERPLATE CRUSTAL
EARTHQUAKES: NEAR-SOURCE GROUND MOTIONS
10.5.1 Response of Structures for Near-Source Ground Motions
Although the libraries of ground motion records are becoming richer as far as the
recording station coverage is denser in near-source regions, the sporadic
occurrence of large earthquakes in heavily instrumented regions makes it difficult
to investigate the spatial distribution of the structural response and provide a
complete picture of the distribution of damage occurring in the region. To address
this issue, seismologists are increasingly utilizing large-computer simulations to
compute synthetic ground motions based on source parameters and models of the
geological structures (Aagaard et al, 2001) (see Sections 7.5.4 and 7.5.5). Two
scenarios, a strike-slip fault and a thrust (subduction) fault with magnitude of 6.0
and 5.8, respectively, were used to study the behavior of a 9-story moment
resisting frame (structural period 2.3 sec), for a spatial position of the structure at
each grid point in the analyzed region (Park et al, 2004). The impulse period is
about 5-6 sec, greater than the structure period.
Figure 10.2 shows the story drifts distribution for the strike-slip ground
motions, in case of fault-normal and fault-parallel components. For fault-normal
component the distribution of story drifts shows that the maximum values occur
along the fault, with systematic differences from the epicenter site to the end of the
fault. In the first part, the drift at the top story is greater than at the other stories,
but the drift at lower stories is also significant. Contrary, at the sites near the end of
the fault in the forward directivity zone, the drift at the lower stories is greater than
at the other floors. An important attenuation of story drifts can be noted in fault-
normal direction, showing the importance of forward directivity. For fault-parallel
component, the maximum story drift values occur near the epicenter at 2 km
distance from the fault. The largest drift occurs in the top stories.