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Authors: Marzorati, S.* 
Massa, M.* 
Cattaneo, M.* 
Monachesi, G.* 
Frapiccini, M.* 
Title: Very detailed seismic pattern and migration inferred from the April 2010 Pietralunga (northern Italian Apennines) micro-earthquake sequence
Issue Date: 2013
Series/Report no.: /610 (2014)
DOI: 10.1016/j.tecto.2013.10.014
Keywords: cross-correlation
Alto Tiberina Fault
Subject Classification04. Solid Earth::04.06. Seismology::04.06.01. Earthquake faults: properties and evolution 
04. Solid Earth::04.06. Seismology::04.06.06. Surveys, measurements, and monitoring 
04. Solid Earth::04.06. Seismology::04.06.10. Instruments and techniques 
04. Solid Earth::04.07. Tectonophysics::04.07.07. Tectonics 
Abstract: We propose a very detailed picture of the seismicity occurring in the proximity of the Alto Tiberina Low Angle Normal Fault (ATF, Northern Italian Apennines) by presenting the pattern and evolution of a seismic sequence that occurred on the hanging wall of the ATF in the first months of 2010 and that was characterized by about 1000 events with ML ranging from -0.7 to 3.8. In order to capture the rupture kinematics of the investigated area, a cross-correlation technique was at first applied to calculate very accurate time shifts among the events of the sequence and then to relocate them. Considering the many factors that can affect the accuracy of a routine event location, the whole sequence was relocated with the double-difference method, including both absolute travel-time measurements and cross-correlation differential travel-times. The new locations confirm that seismic activity is mainly arranged along a NW-SE oriented structure, ranging in depth from 4 to 6 km and dipping towards North East with an angle of about 65°. A further analysis of waveforms similarity was performed at a reference station by merging the capability of the cross-correlation technique and the bridging algorithm. The analysis allows us to group events into several earthquake families (from now on multiplets), 11 of which include at least 10 events with a cross-correlation value higher than 0.9. The detected mutiplets allow us to emphasize the spatial and temporal migration of the sequence occurred along a 307°N strike direction with an averaged propagation velocity of about 0.4 km/day. The normal focal mechanisms obtained from the events with ML≥2 validate the supposed extensional tectonic regime of the investigated area. The main nodal planes, characterized by strikes ranging in 312°±12 and dips about -90°, are consistent with the spatial evolution of the aftershocks.
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