A local-magnitude scale for Mt. Vesuvius from synthetic Wood-Anderson seismograms

The Local-Magnitude scale actually in use at Vesuvius Observatory is based on the measure of seismogram coda duration, and calibrated with data from Irpinia aftershocks. A recent study on local seismic attenuation at Mt. Vesuvius reveals coda shapes highly different from those from Irpinia aftershocks, and a very low quality factor, if compared to the average Q of the region, indicating the necessity of the revision of the Magnitude scale, in order to better compare the seismic energy associated to the local seismicity of Mt. Vesuvius to that of other active volcanoes. Being the seismic attenuation parameters known in the area, we could correct the seismic amplitudes for the path effect to obtain precise estimates of the amplitude level of the displacement spectrum. Hence we estimated the Moment-Magnitude, M, for a set of well recorded micro-earthquakes. To use the Richter formula M-L =log(10) A(max) -log A(0)(R) we estimated the log Amplitude-Distance correction curve, - log A(0)(R), numerically synthesizing an S-wave-packet and letting it propagate in a earth medium with the same attenuation properties of those measured at Mt. Vesuvius. Then we synthesized the Wood-Anderson equivalent seismogram for the same data set and used the distance correction in order to calculate the Wood-Anderson Magnitude. This Magnitude scale was normalized in order to fit the Richter formula valid for Southern California at a distance Delta of 10 km, and results to be M-WA =log A + 1.34log(R) -1.10. The comparison of the Wood-Anderson scale with the Duration-Magnitude scale in routine use at Vesuvius Observatory indicates that care must be taken when the estimate of the Duration-Magnitude is carried out for small earthquakes recorded at a site characterized by a high level of seismic noise.