The Alto Tiberina fault (ATF) in the Northern Apennines is a low-angle normal fault (mean dip 20°) that is the target of The Alto Tiberina Near Fault Observatory (TABOO), a research infrastructure managed by INGV and part of the European Plate Observing System. We propose to deploy a STrainmeter ARray in shallow boreholes (STAR) to complement and enhance TABOO. Existing seismic data from TABOO reveal microseismicity, at a consistently high rate on the ATF fault plane, including repeating earthquakes (RE). REs together with a steep gradient in crustal velocities measured by GPS and transient surface motion lasting for few months and coinciding with seismic swarms, show that portions of the ATF are creeping aseismically.
Recent studies document that any given patch of a fault can creep, nucleate slow earthquakes, and also host large earthquakes (e.g. Iquique earthquake, Tohoku earthquake). Why a fault patch would switch from one mode of slip to another runs contrary to standard theory. Thus, these observations are forcing a revolution in our way of thinking about how faults accommodate slip. STAR will collect Open Access data needed to address questions about the relationship between creep, slow slip, dynamic earthquake rupture, and tectonic faulting. Understanding the physics allowing for both seismic and aseismic slip on a single fault patch, has implications for seismic hazard and risk assessment globally. STAR will consist of six 80-160m deep vertical boreholes, covering the portion of the ATF that exhibits REs at shallow depth (~4 km), instrumented with strainmeters, downhole seismometers and pressure transducers. Each site will be also equipped with surface GPS and a meteorological instrument allowing correlation between seismicity, degassing (CO2, Rn) measurements and subsurface strain. STAR will provide the international community an opportunity to study creep at local scale and over periods of minutes to months poorly constrained by other geophysical instruments. US National Science Foundation has already contributed with equipment and salary support toward the deployment of STAR (including Gladwin Tensor Borehole Strainmeters, seismometers, GPS instruments, data loggers, pressure transducers, and metpacks). STAR is also supported by existing TABOO power and communications infrastructure, managed by INGV. A number of international collaborators working on the physics of fault slip and earthquakes are also committed to explore STAR data. Here, we seek funds to cover the costs of drilling of the shallow boreholes permitting deployment of the STAR array.