Why are three seismogram readings needed to accurately pinpoint the epicenter of an earthquake?

Answer 1

To locate an earthquake, you need the data from at least three seismometer stations.

The process is known as triangulation and is described in more detail below.

The seismometer records the P and S-wave arrival times. P-waves travel faster through the earth than S-waves and so they arrive at the seismometer station before the S-waves and are recorded by the seismometer first.

The difference in arrival time between the two types of seismic wave can be used to calculate the distance of the earthquake's epicentre from the seismometer (the equation to do this can be seen at the bottom of this answer).

This can then be plotted on a map, by drawing a circle with a radius equal to the distance to the epicentre around the seismometer station. This is then repeated for the other two seismometer stations and the point where the three circles intersect is the location of the earthquakes epicentre.

The above procedure is commonly automated using computers and numerical techniques so that a large number of differing seismic episodes can be processed efficiently.

It should be noted that this is an imperfect process as a number of assumptions must be made about the material through which the seismic waves travel in order to estimate their speed.

DE = DeltaT x (VP - VS) / (VS x VP)

Where:

DE = Distance to epicentre (km)

DeltaT = Difference between P and S-wave arrival time (s)

VP = P-wave velocity (km/s)

VS = S-wave velocity (km/s)

Answer 2

A seismograph is an instrument used to measure the vibrations produced by the movement of waves through the Earth. These waves are produced by the release of energy during an earthquake. Since different directions of motions are produced by earthquakes, there are different types of seismographs. Some of the seismographs record horizontal motions while others record vertical motions.

Answer 3

A seismograph is an instrument used to measure the vibrations produced by the movement of waves through the Earth. These waves are produced by the release of energy during an earthquake. Since different directions of motions are produced by earthquakes, there are different types of seismographs. Some of the seismographs record horizontal motions while others record vertical motions.

Answer 4

Based on the difference in arrival times of P and S waves as recorded on a seismometer you can estimate the distance of the seismometer from the earthquake epicentre, however you will have no idea of the direction.

Seismologists (or more correctly automated computer programs written by seismologists) plot the distance to the earthquakes epicentre by drawing a circle round the seismometer loaction on a map with a radius equal to the epicentral distance.

If this is done for two stations located in differing places the circles will intersect in two places. This narrows down the location of the epicentre to either of those two points. If you add in the data from a third station, all three of the circles should intersect at a single common point marking the epicentre of the earthquake. As such the minimum number of seismometer stations required to locate the epicentre of an aerthquake is 3.

Answer 5

because each seismograph measures a particular type of wave and also because one seismograph can only determine how far away the epicenter is, not in which direction