EARTHQUAKES

2009 Q 4 (a) 10 marks

(i) Name one place where earthquakes occur. Answer: along the San Andreas fault in California.

(ii) Examine the diagram showing the shockwaves of an earthquake and name the areas labelled A and B. Answer: A = The focus, B = The epicentre

(iii) Briefly describe the cause of earthquakes. Convection currents within the mantle cause tectonic plates to move. As the plates slide past each other at transform plate boundaries and collide with one another at convergent plate boundaries they can become snagged. Once this happens, pressure begins to build up along the fault line as the convection currents continue underneath the crust. Eventually the built up pressure will become so great that there will be a sudden jolting movement whereby the plates become unsnagged. This release of pressure sends tremors outwards from the focus through the crust. Shaking of the ground is likely to be strongest at the epicentre, which is the point at the surface directly above the focus. The strongest recorded earthquake happened in Chile in 1960. It measured 9.5 on the Richter Scale and was caused by the subduction of the Nazca Plate underneath the South American Plate.