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 I rpinia aftershocks. A recent study on local seismic attenuation at Mt. Vesu vius reveals coda shapes highly different from those from Irpinia aftershoc ks, and a very low quality factor, if compared to the average Q of the regi on, indicating the necessity of the revision of the Magnitude scale, in ord er to better compare the seismic energy associated to the local seismicity of Mt. Vesuvius to that of other active volcanoes. Being the seismic attenu ation 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 =l og(10) A(max) -log A(0)(R) we estimated the log Amplitude-Distance correcti on curve, - log A(0)(R), numerically synthesizing an S-wave-packet and lett ing it propagate in a earth medium with the same attenuation properties of those measured at Mt. Vesuvius. Then we synthesized the Wood-Anderson equiv alent seismogram for the same data set and used the distance correction in order to calculate the Wood-Anderson Magnitude. This Magnitude scale was no rmalized 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 ta ken when the estimate of the Duration-Magnitude is carried out for small ea rthquakes recorded at a site characterized by a high level of seismic noise .