How earthquakes are measured?

Answer 1

Earthquake magnitudes are measured using seismometers. Based on the amplitude of the seismic waves, seismologists can make a preliminary estimate of the moment and Richter magnitudes of the earthquake.

For a more accurate estimate of the energy released, seismologists use the moment magnitude scale (MW).

It is calculated from the seismic moment (M0 - which has the unit of dynes/cm =where 1 dyne/cm = 1x10-7 N/m) which is derived based on the elastic moduli of the crust where the earthquake occurred, the length of the slip surface and the cross sectional area of the slip surface as follows:

M0 = G x AFx DF

Where:

G = Shear modulus of the rock mass

AF = Area of the rupture along the fault

DF = average displacement on AF

In order to make the moment magnitude scale (Mw) consistent with older magnitude scales such as the Local Moment (or "Richter") scale the seismic moment (M0) is converted into a logarithmic scale using the following equation:

Mw = 2/3 x log10 x (M0) - 10.7

This ultimately yields a dimensionless number and as such, Moment Magnitude has no units.

Earthquake intensity is measured using the Modified Mercalli Scale. The scale indicates the perceived damage from an earthquake. These parameters are subjective: they depend on the conditions of buildings and the opinions of witnesses. In order to make the measurements more objective, ground accelerations as measured by seismometers are also used to calculate earthquake intensity.
On the Richter scale.
sizemograph

Answer 2

Earthquakes are measured in magnitude. Magnitude measures how hard the earthquake hit.