Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 253

parallel ones. Contrary, the ground motions from the 1999 Kocaeli (Turkey)
earthquake, recorded at Yarimca, show the near-source effects, which are
characteristic for strike-slip earthquakes: ground velocities with almost the same
values in all directions.
The largest dynamic ground velocity is coincident in time with the ground
displacement and occurs over a time interval of several seconds. It is, therefore,
necessary to treat the dynamic components of the seismic load as coincident loads
(Somerville, 2002). Comparing the ground motions, resulting during these two
different faulting mechanisms, the effects of rupture directivity are similar, with
the exception of the fling slip displacements, which are not present in the case of
thrust earthquakes. This indicates that, for the engineering purposes, it is not
necessary to distinguish between different fault types in characterizing the near-
source rupture directivity effects (Somerville et al, 1996). The key ground motion
parameters for near-source earthquakes are: amplification of horizontal and vertical
components, peak ground velocity, pulse period and number of significant pulses
(Bray and Rodriguez-Marek, 2000).
Horizontalandverticalcomponentsamplification. The increase of the horizontal
accelerations with the site distance from the fault, the period larger than 0.6 sec
(for the earthquake magnitude M 7.0) are shown in Figure 7.30 (Somerville, 1997).
One can observe that the influence of near-source increase with the ground motion
period and it is reduced at the distance greater than 30 km. For large magnitudes,
large periods and near source, important increasing can be noticed, over 50%.
Figure 7.30 Amplification factors of ground motions due to near-source effect
(after Somerville, 1997)