Chapter+4+Earthquakes+-+The+Basics - Chapter 4 Earthquakes...

1
Chapter 4 Earthquakes: The Basics
1.0 Introduction
An accounting of the worst natural disasters in historical time places earthquakes and
their consequences well within the 10 deadliest of processes. For example, in 1201 a
devastating earthquake roared through Syria and Egypt killing approximately 1.1 million
people. The population density in 1201 was rather dramatically less than now, so this was
certainly an awesome catastrophe. The worst earthquake in China – which experiences
many earthquakes – seems to have been in 1556 when about 830,000 were killed in
Shansi Province. Of course, the deadly tsunami of December 2004 – with a dead +
missing and presumed dead toll of about 250,000 – was instigated by an enormous
earthquake. The earthquake in Haiti in 2010 was truly a disaster; had the country not been
so poor, had infrastructure not been so weakly constructed, and had the government been
even mildly adequate, it might have remained a disaster. As things worked out, it turned
into a regional catastrophe with perhaps 316,000 dead (
without a census, it’s hard to know the
real figure
).
So we’d better have a pretty good basic knowledge of earthquakes and the processes that
produce them. In this chapter we’ll study those basic processes, how earthquakes are
located and their intensity measured, the terminology used, and how earthquakes are
predicted ( which is ‘poorly’ ). For those who’ve already had this background, this will be
a quick review chapter.
2.0 Seismic Energy Source
We normally accept the definition of an earthquake to be the ground shaking that
accompanies sudden movement on a fault; true, it may also be the shaking produced by
motion of magma
underground, a fast-moving
landslide, or even an
underground nuclear bomb
explosion – but those are
very rare. If an earthquake
produces ground motion,
then energy must be
consumed in the process. So,
where does the energy come
from?
Figure 1 shows pretty
convincingly that
earthquakes
( dots show
locations )
are associated with
Fig. 1: Earthquake foci around the world.
Modified Figure 2-16b in
Hyndman and Hyndman (2011), Natural Hazards and Disasters, 3rd edition.

2
plate boundaries. Following our plate tectonics discussions, we can accept that the
boundaries of plates are active regions, and most of that action takes the form of:
x normal fault s
x reverse and thrust faults
(rocks on one side of the
fault are pushed over top
of the rocks on the other
side at a low angle)
x strike-slip or transform
faults (Fig. 2).
Just because there is a break or fracture through a rock doesn’t mean that the blocks on
either side move readily ( although if they do, we call the process ‘ creep ’ ). Normally,
there is enough friction ( or resistance to motion ) from the rocks on either side of the
break to allow a great deal of stress energy to build (
Press your open hands tightly together and
try and move one over the other; you will have to use a fair bit of energy to make the movement – and
when you do, the hands move with a ‘jump’
). In nature, when the resistance across the break or