Civil Engineering Reference
In-Depth Information
Epicentral Distance
Site
Epi
c
entre
Fault
Focus
Figure 1.8
D e fi nition of source parameters
depths in the upper 5- 10 km. Some intermediate - and deep-focus earthquakes are located in Romania,
the Aegean Sea and under Spain.
The above discussion highlights one of the diffi culties encountered in characterizing earthquake
parameters, namely the defi nition of the source. From Figure 1.8, it is clear that the source is not a
single point, hence the 'distance from the source' required for engineering seismology applications,
especially in attenuation relationships as discussed in Section 3.3, is ill- defi ned. This has led researchers
to propose treatments for point, line and area sources (Kasahara, 1981). It is therefore important to
exercise caution in using relationships based on source-site measurements, especially for near- fi eld
(with respect to site) and large magnitude events. A demonstration of this is the values of ground
acceleration measured in the Adana-Ceyhan (Turkey) earthquake of 26 June 1998. Two seismological
recording stations, at Ceyhan and Karatas, were located at distances of 32 km and 36 km from the epi-
centre, respectively. Whereas the peak acceleration in Ceyhan was 0.27 g, that at Karatas was 0.03 g.
The observed anomaly may be explained by considering the point of initiation and propagation of the
fault rupture or 'directivity', which is presented in Section 1.3.1, possibly travelling towards Ceyhan
and away from Karatas.
Problem 1.1
Determine the source mechanism of faults with a dip
δ
= 60 ° and rake
λ
= 45 ° . Comment on the
results.
1.1.3 Seismic Waves
Fault ruptures cause brittle fractures of the Earth's crust and dissipate up to 10% of the total plate-
tectonic energy in the form of seismic waves. Earthquake shaking is generated by two types of elastic
seismic waves: body and surface waves. The shaking felt is generally a combination of these waves,
especially at small distances from the source or ' near - fi eld ' .
Body waves travel through the Earth's interior layers. They include longitudinal or primary waves
(also known as 'P-waves') and transverse or secondary waves (also called ' S - waves ' ). P - and S - waves
are also termed 'preliminary tremors' because in most earthquakes they are felt fi rst (Kanai, 1983 ). P -
waves cause alternate push (or compression) and pull (or tension) in the rock as shown in Figure 1.9 .
Thus, as the waves propagate, the medium expands and contracts, while keeping the same form. They
