With approximately 500,000 earthquakes happening each year and as many as 100,000 of those being felt, seismic events represent a major hazard for humanity. In the last twenty years, geoscientists have discovered and characterized by means of indirect geophysical and geodetic techniques peculiar seismic phenomena occurring at convergent margins, which are referred to in the scientific community as “episodic tremor and slow slip events” (ETS). Tremors are a persistent low-frequency seismic signal associated with slow slip, a geodetically detected slip larger than the average plate motion. Although ETS do not represent a direct threat to humans, the fact that they are located in proximity of megathrust earthquakes and are believed to mark stress transfer to the megathrust faults makes them of great interest to geoscientists. Therefore, their better characterization can help to shed light on the very hazardous subduction seismicity. Unfortunately, direct observation of the geological record of these phenomena is only possible by analysing rocks exhumed from fossil subduction zones, such that the rock record of deep seismic activity is scanty because subducted rocks commonly re-equilibrate during exhumation en-route to the surface.
The Italian Northern Apennines (Italy) expose deeply subducted rocks exhumed back to the surface that still preserve mineralogical assemblages and structures acquired at great depth in the subduction channel. The studied rocks display evidence of coeval discontinuous and continuous deformation in the form of veins and pervasive foliation, respectively. Veins are composed of quartz and carpholite fibres oriented parallel to the stretching lineation of the host rock and display crack-seal textures. Veins thus formed by incremental growth of the constituent fibres by repeated brittle failure (fracture opening) and sealing of the fracture. These veins display strong analogies with dilational hydroshear veins, a key indicator and well known geological record of ETS occurring at shallow depth within accretionary wedges in subduction channels. By using mineralogical assemblages, thermodynamic modeling and age dating, we constrained the formation of both veins and foliation to pressure and temperature conditions of ~1 GPa and 350 °C at c. 20 Ma. These results suggest depths of 30-40 km and cold geothermal gradients typical of subduction zones. We propose that episodic pulses of aqueous fluids released due to destabilization of hydrated minerals during subduction can be held responsible for the formation of the observed veins.
Concluding, we suggest that these diagnostic veins can be regarded as a powerful fingerprint of deep ETS occurring in subduction zones. Based on the widespread occurrence of these veins, we propose that deep ETS are indeed common at the scale of the entire Apennine orogen. Therefore, the results of this Action require some degree of reinterpretation of the seismotectonic role of several metamorphic units worldwide and, at the same time, assist in doing it.
