The Modified Mercalli Scale as an Instrumental Parameter of Seismic Intensity and its Contrast with other Indicators of Potential Damage

Seismic intensity scales began as qualitative descriptions of the effects produced by earthquakes in terms of structural damage, alterations to geological and geotechnical landscapes, and the subjective human perception of seismic events. In oscillatory motions, acceleration is the kinematic quantity through which humans perceive an earthquake, so perceived intensity is based on the forces induced in the human body due to instantaneous peak accelerations. From 1900 to the present, many researchers in the field of seismology and earthquake engineering have developed various empirical relationships between ground motion parameters, such as PGA, and intensity on the Modified Mercalli Scale. Many others have developed equations that define the energy impact of earthquakes based on amplitudes, duration, and frequency content, with the aim of quantifying potential structural damage, starting from the premise that PGA is not a parameter that captures these three factors, including collapses and material and human losses. The Modified Mercalli Scale has evolved to be implemented as an instrumental indicator of the seismic forces introduced and perceived at ground level and within structures, which allows identifying the meizoseismal area, regions of high structural response, site effects, among other results that contribute to rapid emergency response and seismic hazard management, with the integration of other objective ground motion parameters.