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1 Chapter 4 Earthquakes: The Basics 1.0 IntroductionAn accounting of the worst natural disasters in historical time places earthquakes and their consequenceswell within the 10 deadliest of processes. For example, in 1201a 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 1556when about 830,000 were killed in Shansi Province. Of course, the deadly tsunami of December 2004– with a dead + missing and presumed deadtoll of about 250,000– was instigated by an enormous earthquake. The earthquake in Haiti in 2010was 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 perhaps316,000 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 SourceWe 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: xnormal faults xreverse 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) xstrike-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