A seismic sequence from Northern Apennines (Italy) provides new insight on the role of fluids in the active tectonics of accretionary wedges

We analyze a seismic sequence which occurred in 2000 along the Northern Apennines accretionary wedge
(Italy). The sequence developed within the Cretaceous–Triassic limestones of the tectonic wedge, where
methane-rich and oil reservoirs are stored. Ruptures mainly developed on WNW–ESE striking thrusts. The
compressive stress field is consistent with that acting at regional scale in Northern Apennines. Seismic
parameters indicate that fluids are involved in the seismogenic process. The amplitudes of the P and S phases
and data from some stations evidence a P to S conversion within Vp/Vs=2.1 layer. The attenuation properties
of crust show a higher attenuation zone located west of the epicentral cloud. Eight hundred aftershocks
delineate a sub-vertical cloud of events between 7 and 14 km depth. The space–time evolution of the
aftershocks is consistent with a diffusive spreading (diffusivity=1.9 m2/s) along vertically superimposed
thrusts. Diffusion also controls the time evolution of the sequence. Fluid pressure is estimated to be roughly
equal to the vertical, lithostatic stress. The overpressure within reservoirs develops by tectonic compaction
processes. The fluids upraise along sub-vertical fractures related to the shortening of the wedge. The 2000
sequence occurred in an area that separates a thermal and deeper petroleum system from a shallower
biogenic system. The divider of these systems controls the attenuation properties of the crust. The fluid–rock
interaction at seismogenetic depth is related to hydrothermal processes more than to compaction. In
accretionary wedges, seismicity activating superimposed thrusts may drive methane and oil upraising from
the upper crust.