How are the Richter scale and a seismograph used?

Answer 1

A seismograph (modern term seismometer) is an instrument that measures the movement of the ground surface. During an earthquake, seismic waves move or propagate away from the earthquakes epicentre and cause the ground to move

A seismometer measures the acceleration at the grounds surface as seismic waves pass by. They also record the frequency and wavelength of the various waves along with their amplitude.

The Richter scale provides an estimate of the energy released by an earthquake based on the amplitude of the seismic waves detected at a seismometer station and a correction based on the distance of the earthquakes epicentre from the station (as seismic waves lose their energy as they propagate through the earth so if this correction was not made, then different seismometer stations at different distances would give differing Richter magnitudes for the same earthquake).

The Richter scale itself is a logarithmic mathematical formula which is calibrated so that a ten fold increase in amplitude relates to a single hole number increase on the scale (e.g. an earthquake with a Richter magnitude of 5 has seismic waves with a maximum amplitude 10 times larger than those for a magnitude 4). It has a number of practical limitations, in that it is poor at recording earthquakes with magnitudes greater than 7 and at distances greater than 650 km for a seismometer.

The equation for calculating the Richter magnitude (MR) is shown below:

MR = (Log10A) - (Log10A0)

Where

A = maximum zero to peak amplitude of seismic wave (mm) recorded.

A0 = Empirical function derived from the distance from seismometer station to earthquake epicentre

Log10A0 From 0 to 200 km distance:

Log10A0 = 0.15 - 1.6 log(distance in km)

Between 200 and 600 km distance by:

Log10A0 = 3.38 - 3.0 log(distance in km)