In these hours the INGV receives many requests for clarification on the value of the magnitude of the strongest shock of the sequence taking place in central Italy, the one that occurred on 24 August at 03.36.
Here are some clarifications that may help clarify things:
The value of the magnitude is NOT used to compensate for damages caused by earthquakes; for this purpose, the intensity values calculated on the basis of the Mercalli scale (actually the Mercalli-Cancani-Sieberg scale) were used in the past.
Like any physical parameter, the magnitude estimate is affected by uncertainty. In particular, the magnitude value calculated by INGV is 6.0 ± 0.3.
The estimates provided by INGV, the USGS and other international agencies are within the expected variability.
The data used and the parameters of the reference crustal model may differ, contributing to the uncertainty of the estimate. INGV uses a model of crustal velocities calibrated specifically for central Italy and a density of seismic stations greater than that of other international agencies that use global velocity models.
Even INGV, using global models and another data analysis technique (RCMT) obtains a magnitude value of 6.2.
There are different definitions and methods of estimating magnitudes (local magnitude, moment magnitude, surface and volume wave magnitude, duration magnitude, …). These are used according to the type of instrumentation that recorded the seismic waves released by the earthquake and the distance between the stations and the epicenter (local, regional or global scale). For the same seismic event, they should provide identical magnitude estimates, but actually provide slightly different values although usually within the uncertainties of each estimate.
