How Seismologists Locate Seismic Events.
So an earthquake has happened but where has it came from? We need to know this information so we can see how many and who could be in any trouble. Also locating where it came from can tell us if any other knock on effects may occur like tsunami’s. Also locating which fault line it has occurred from can help compile records. The way we can detect the location of a earth quake is from its wavefront or expanding wavefront from the earthquakes hypocenter. The wavefront will usually travel and be measured by how fast it travels several kilometers per second.
So here is the trick to locating where the seismic activity has came from. All around the earth especially near to fault lines networks of seismometers have been installed on ground level. When an earthquake appears these seismometers pick up any movement in the earth from the wave fronts. These wavefronts will then reach more and more seismometers as it expands from the hypocenter. As the seismometers pick up the wavefronts the time it is recorded is logged along with other stations to give a general direction to where it has came from. This information can also help not just find the location , but the depth and time of the earthquake.
The wavefronts that are detected are defined into four basic types of seismic wave. Two of the preliminary waves travel through the earths core and the other two travel at the earths surface. The two main types of waves are the Primary and Secondary Primary Waves similar to sound waves these waves compress and dilates the earth core as it travels through. Also known as the P wave it is always the first wave to reach the station. It can travel at less the 5 kilometers per second on the surface but much faster through the core at 13 Kilometers per second.
Secondary Waves this waves vibrates or shakes the earth core or rock sideways but only at half the speed of the primary wave. Also known as the S wave it is second wave to be detected but only at the earth surface.
Its using these waves which will help locate the location of the hypocenter. You need to work out the time difference between the primary and secondary waves at as many of the seismometer stations as possible. Once this has been achieved the distance from each station to the earthquakes centre is calculated from standard travel time tables and time travel curves. Usually this information is gathered and processed by computer programs but if you were to draw circle arcs on a globe calculating the distance from the earthquake to the station using these times as a radius. Once this has been done the arcs will cross at a certain point and this hopefully would give you the location from where the seismic activity cam from.