Engineering Geology

Chapter 8
An earthquake propagates three types of shock waves. The first pulses that are recorded
on a seismograph are the P or compression waves. The next pulses recorded are the S or
shear waves. S waves usually have larger amplitude than the P waves but the latter travel
almost three times as fast as the former. Both P and S waves travel directly from their
source to the surface. The third type of shocks are known as the L waves. These waves
travel from the focus of the earthquake to the epicentre above, and from there they radiate
over the Earth’s surface. P waves are not as destructive as S or L waves. This is because
they have smaller amplitude, and the force their customary vertical motion creates rarely
exceeds the force of gravity. On the other hand, S waves may develop violent tangential
vibrations strong enough to cause great destruction. The intensity of an earthquake
depends on the amplitude and frequency of wave motion. S waves commonly have a higher
frequency than L waves, nevertheless the latter may be more powerful because of their
larger amplitude.
The hazards due to seismicity include the possibility of a structure being severed by fault
displacement but a much more likely event is damage due to shaking (Fig. 8.4).
The destruction wrought by an earthquake depends on many factors. Of prime importance
are the magnitude of the event, its duration and the response of buildings and other
elements of the infrastructure. In addition, other hazards such as landslides, floods, subsidence,
tsunamis and secondary earthquakes may be triggered by a seismic event (Khazai
and Sitar, 2004).
The severest earthquakes wreak destruction over areas of 2500 km 2 or more, however,
most only affect tens of square kilometres. The strengths of earthquakes may be
expressed in terms of intensity or magnitude. Earthquake intensity scales are a qualitative
expression of the damage caused by an event at a given location. The most widely
accepted intensity scale is the Mercalli scale given in Table 8.1. The magnitude of an earthquake
is an instrumentally measured quantity and is related to the total amount of elastic
energy released when overstrained rocks suddenly rebound and generate shock waves. It
is expressed on a logarithmic scale. An earthquake of magnitude 2 is the smallest likely to
be felt by humans, and earthquakes of magnitude 5 or less are unlikely to cause damage
to well-constructed buildings. The maximum magnitude of earthquakes is limited by the
amount of strain energy that a rock mass can sustain before failure occurs, hence the
largest tremors have had a magnitude of around 8.9. Such an event causes severe
damage over a wide area.
The magnitude of an earthquake event depends on the length of the fault break and the
amount of displacement that occurs. Generally, movement only occurs on a limited length
of a fault during one event. A magnitude 7 earthquake would be produced if a 150 km
long fault underwent a displacement of about 1.0 m. The length of the fault break during
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