What is an earthquake: faults are the main cause. Not the only, but what we study most: movement of one geological materials past another, releases energy = earthquake, foreshocks and aftershocks. Small earthquakes preceding a major earthquake: fault rupture and propagation. Faults do not slip all at once. Initial slip begins at hypocenter and propagates along the fault surface. Slippage adds strain to adjacent sections triggering more slippage. Fault slip is the amount of displacement on the fault surface. Igneous = higher energy felt: hammer hits table. Sedimentary = lower energy: hammer hits mattress. Allow earth to move under static pendulum. Work when inertia of an object keeps the mass in place while earth and supports under the mass move. Movement of earth in relation the stationary mass is recorded. Records that reveal the behaviour of seismic waves are called seismograms: primary waves (p) Shear motion at right angles to their direction of travel.