Earthquakes

What is an earthquake?

• An earthquake is the result of a sudden release of energy in Earth’s crust that creates seismic waves

• Body waves (P and S waves) pass through the main body of Earth

• Surface waves travel across Earth’s surface

(USGS, public domain)

Cause of earthquakes

• Build up of force stored as elastic energy in rocks• Force may be due to tectonic boundary interaction

or stresses within a plate

(JF Vigil 1997 USGA, public domain)

Release of energy in an earthquake

(Hsu.shihhung 2006, Wikimedia Creative Commons)

• Rock is deformed by stress until force is greater than the strength of the rock or friction in the boundary

• The rock breaks or moves – this is an earthquake

Earthquake waves

(Tompkins, 2011)

• Waves are generated at the focus

• The epicentreis the point on Earth’s surface directly above the focus

Locating earthquakes

(Tompkins, 2011)

Measuring earthquakes

• Richter scale measures force released

• Mercalli scale measures effect

Mercalli level II (left) versus VI (right) (E Wardoyo, public domain)