By a property called dispersion, surface waves of different wave lengths travel at different velocities. It is the surface waves which cause most property damage because surface waves produce more ground movement and travel more slowly, so they take longer to pass. Earthquake locations - An earthquake starts at the earthquake focus which is the fault zone along which the earthquake slips. Often the earthquake is located by epicenter or the position on the surface of the earth right above the earthquake focus. Earthquakes are located using a record of the earth vibration called a seismogram .
Several stations are necessary to pin-point the location of an earthquake.
This is done by drawing a circle on the globe with the radius equal to the distance between the epicenter and the seismic station. Having done the same for three stations, the point of intersection of the three circles is the actual epicenter of the earthquake. Earthquake strength - One method for indicating how large an earthquake was is to measure the earthquake's effect on people and buildings. Earthquake intensity is determined on a scale of Roman numerals between I and XII according to the convention called the modified Mercalli scale . This method has drawbacks because buildings are constructed to different standards in different parts of the world.