The Seismic Risk in a Mega-City. Emergency planning in Istanbul

The most scientific and objective way to assess the size of an earthquake is use the Moment Magnitude Scale (MMS or MW). Developed in the 1970s to succeed the famous Richter Magnitude Scale which was created by the homonymous seismologist (Charles Richter) in 1935, it measures the size of a “hearth shock” in terms of energy released.
Scales such as the Richter trace the energy released by ground motion on a seismograph at a distance of 100 km from the earthquake’s epicenter. Indeed, the value release is equal to the base-10 logarithmic of the maximum trace of the seismographs and it is expressed in micrometer (10⁻³ mm). For instance, an earthquake that produces a maximum trace amplitude of 1 mm would indicate magnitude 3.0 earthquake (log₁₀ 1,000 = 3).

As the Richter scale is logarithmic, each whole number represents a ground-shacking ten times greater than the number before on the scale, so an M7 earthquake causes a quake 1,000 times greater than an earthquake of M_L3. Furthermore, each whole number on the Richter scale represents 31.6 times greater energy released and so the difference between two earthquake with 2 whole point of M_L in difference, it would means a ratio of energy released equal to 1/1,000.

Today one of the most often-used scales is the Local Moment (M_L) scale, which is a corrected version of the Richter. However, as Smith (2001) explains, the “large earthquakes with a great rupture length are not well represented by the M_L scale [because] it does not represent the total energy release in the event”.
Therefore, with an earthquake of M_L=6.5 or greater, the Moment Magnitude (M_W or only M) will give a more accurate reading. Smith (2001) points out that this scale is better because it “is based on the surface area of the fault displaced, its average length of movement and the rigidity of the ground material, rather on a seismograph trace”.

Another way, apparently less scientific and objective than the Richter Scale, but equally famous and used as well, is the Modified Mercalli Scale (MM)⁹, a modern and update version of the original Mercalli (which was in turn an update from the Rossi-Fiorel scale). The MM quantifies the effects of an earthquake on the Earth's surface on nature, humans, and infrastructure. Thus, it does not measure the energy released but the earthquake’s intensity, and so it is directly linked to the hazard impact.

The reason it appears less scientific is that it uses personal reports and observations to measure the intensity, whereas the magnitude is measured by instruments. However, according to Smith (2001), the Mercalli Scale “is probably no less “scientific” than the Richter Scale because seismologists can disagree about the exact rating on the magnitude scale”. Moreover, a principal advantage of this scale is that it can be used to assess earthquakes that happened in the past, before instruments such as the seismographs were invented.