Abstract
We propose a very detailed picture of seismicity that occurred in the proximity of the Alto Tiberina Fault (ATF, northern Italian Apennines), a low angle normal fault, by presenting the pattern and evolution of a seismic sequence on the hanging wall of the ATF in the first months of 2010 that was characterized by about 1000 events with ML ranging from − 0.7 to 3.8. To capture the rupture kinematics of the investigated area, a cross-correlation technique was initially applied to calculate very accurate time shifts among the events of the sequence, and then to relocate them. The whole sequence was relocated with the double-difference method, which includes both absolute travel-time measurements and cross-correlation differential travel-times. The new locations confirm that the seismic activity was mainly arranged along a NW–SE-oriented structure that ranged in depth from 4 km to 6 km, with dipping towards NE at an angle of ca. 65°. In comparison with geological data, the position of the seismic sequence is compatible with the evaporite Triassic layer. The main nodal planes are consistent with the spatial evolution of the aftershocks and with the tensional state of stress in the area. An analysis of waveform similarity was performed at a reference station by merging the capability of the cross-correlation technique and the bridging algorithm. The detected mutiplets allow us to emphasize the anisotropic spatial and temporal migrations of the seismicity that occurred along a 307° N strike direction with an averaged propagation velocity of ca. 0.4 km/day. We explain the highlighted anisotropic behavior of the migration with the hypothesized presence of overpressured fluids and with physical properties of Triassic evaporites. This suggests the importance of such very detailed relocation of weak and micro-seismicity for improvement of our knowledge of fault system geometry and its evolution.
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