Integrating Distributed Acoustic Sensing and borehole seismometer data for seismic velocity measurements and negative magnitude event location: a case study from the TABOO Near Fault Observatory (Northern Apennines, Italy)

Distributed Acoustic Sensing (DAS) technology makes use of fiber optic cables to sense vibrations, at the Earth’s surface, at unprecedented spatial resolution, less than one meter over distances of kilometres. DAS data have been used for monitoring both the Solid Earth (earthquakes, dyke intrusions and more) and the environment (landslides, snow avalanches, groundwater). Despite its wide application and the numerous, successful case-studies, DAS technology presents two significant limitations: the lower S/N ratio with respect to standard seismometers and the strong "directivity effect" (vibrations must propagate in the axial direction of the fiber optic cable). In this study, we illustrate how the integration of DAS and borehole seismometer data can be used to improve earthquake location and obtain novel information on seismic velocity of the buried rock mass. We analyse the DAS data recorded along a 1km fiber optic cable deployed in a full 3D geometry. The fiber optic cables have been installed in the framework of a surface and borehole very dense seismic array partaining to the Alto Tiberina Near Fault Observatory (TABOO-NFO). The cable geometry covers two horizontal planes, off-set one from the other and at different altitudes, and a vertical borehole  going to 130m depth. The infrastructure has been installed across (from the hangingwal to the footwall) the Gubbio fault, a secondary fault segment antithetic to the main Alto Tiberina master fault bounding at depth a normal fault system. in the Alto Tiberina fault system (Northern Apennines, Italy). The center of the cable array coincides with a shallow borehole (130m deep)  instrumented with two short period seismometers, one at the surface and one at the bottom. The integration of the data from the seismometes and those recorded along such 3D geometry allows for a better recognition and location of very small seismic events occurring on the fault, which are going largely undetected by the local (dense) seismic network. Moreover, data from small size events (Mag > 1) can be used to estimate the P- and S- wave seismic velocity of the geological formation traversed by the borehole (namely, Maiolica fm and Marne a Fucoidi fm), defining precise measurements of such velocities at larger scale-length (10s of meters) with respect to measurements obtained on the same rock in the laboratory.