SEISMICITY RATE CHANGE BEFORE THE IRPINIA (M=6.9) 1980 EARTHQUAKE

The stability of seismicity rate in central and southern Italy was exa mined in two data sets: from 1975.0 to 1995.0 with M greater than or e qual to 3.4 and from 1987.5 to 1996.0 with M greater than or equal to 2.5. These are the approximate minimum magnitudes of complete reportin g for the respective periods. The first set was used to evaluate the p ossibility that the 23 November 1980 Irpinia (M 6.9) earthquake was pr eceded by precursory seismic quiescence; the second was used to evalua te the conditions under which a current seismic quiescence could be de tected in central or southern Italy, During the years before the Irpin ia earthquake, the seismicity rate in the northern half of the rupture area and north of it was low, Whether this was a case of precursory q uiescence or not is subject to interpretation because the background r ate cannot be established for the years before 1975. If we accept the relatively constant seismicity rate in the Irpinia volume during the d ecade after the mainshock as representative for the background rate, w e have a clear case of precursory quiescence lasting at least 1.3 yr u p to the mainshock. Alternatively, it can be postulated that the seism icity rate during the decade following this shock was elevated regiona lly because of the stress redistribution and that the low rates seen b efore it represent the normal background rate, Even if this reasoning is accepted, the fact remains that a volume including the northern par t of the rupture produced no M greater than or equal to 3.4 earthquake s during 1.3 yr before the Irpinia earthquake but produced 10 earthqua kes during the 4.7 previous yr, Given these facts, we favor the interp retation that the Irpinia 1980 earthquake was preceded by precursory s eismic quiescence. In the entire data set, there are three other cases of quiescence of higher significance without a mainshock following. S ince no other mainshock with M > 6 exists in the data set, no missed e vent exists, We propose that in Italy precursory seismic quiescence ma y precede mainshocks and that it may be detected in the future by the improved catalog of the modern data set beginning in 1987.5. Major mag nitude scale changes give the mistaken appearance that fewer large-mag nitude earthquakes occur in Italy now than in years before 1987, We po stulate that the rate of earthquakes has not changed and that the magn itude scale should be adjusted.