Cyclic migration of weak earthquakes between Lunigiana earthquake of October 10, 1995 and Reggio Emilia earthquake of October 15, 1996 (Northern Italy)

The cyclic migration of weak earthquakes (M greater than or equal to 2.2) w hich occurred during the year prior to the October 15, 1996 (M = 4.9) Reggi o Emilia earthquake is discussed in this paper. The onset of this migration was associated with the occurrence of the October 10, 1995 (M = 4.8) Lunig iana earthquake about 90 km southwest from the epicenter of the Reggio Emil ia earthquake. At least three series of earthquakes migrating from the epic entral area of the Lunigiana earthquake in the northeast direction were obs erved. The migration of earthquakes of the first series terminated at a dis tance of about 30 km from the epicenter of the Reggio Emilia earthquake. Th e earthquake migration of the other two series halted at about 10 km from t he Reggio Emilia epicenter. The average rate of earthquake migration was ab out 200-300 km/year, while the time of recurrence of the observed cycles va ried from 68 to 178 days. Weak earthquakes migrated along the transversal f ault zones and sometimes jumped from one fault to another. A correlation be tween the migrating earthquakes and tidal variations is analysed. We discus s the hypothesis that the analyzed area is in a state of stress approaching the limit of the long-term durability of crustal rocks and that the observ ed cyclic migration is a result of a combination of a more or less regular evolution of tectonic and tidal variations.