A strainmeter array to unravel the Alto Tiberina fault slip behaviour, Central Italy - ICDP STAR Drilling Project

Earthquakes are complex natural phenomena that involve multiple spatio-temporal scales. To understand the physical/chemical processes responsible for the faulting that earthquakes occur on, a multidisciplinary approach is highly recommended. Near Fault Observatories (NFOs) aim at providing high-precision and spatio-temporally dense multidisciplinary near fault data, enabling the generation of innovative scientific products.

The Alto Tiberina Near Fault Observatory (TABOO-NFO) is a permanent monitoring infrastructure around the Alto Tiberina Fault (ATF). The ATF is a 60 km long very low-angle normal fault (mean dip 20°) located along a 3 mm/yr extending sector of the Northern Apennines (Central Italy). The presence of repeating earthquakes on the ATF, as well as a steep gradient in crustal velocities measured by GNSS stations, suggest that portions of the ATF are creeping aseismically. Both laboratory and theoretical studies indicate that any given patch of a fault can creep, nucleate slow earthquakes, and host large earthquakes, as also documented in nature for some earthquakes (e.g., Iquique, Tohoku and Parkfield earthquakes). Nonetheless, how a fault patch switches from one mode of slip to another as well as the interaction between creep, slow and regular earthquakes are still poorly documented by near field observation.

TABOO is a state-of-the-art dense network, managed by the Istituto Nazionale di Geofisica e Vulcanologia (INGV), with mean inter-distance of about 5 km between multidisciplinary sensors, deployed both at surface and within shallow boreholes (<250m). Stations record and transmit in real time via dedicated Wi-Fi technology; then data is stored in standard formats on open access thematic portals and distributed via web services (http://fridge.ingv.it). With STAR, during the Fall of 2021 and Spring of 2022, INGV in collaboration with UNAVCO, drilled six 80-160 m deep boreholes surrounding the creeping portion of the ATF, to deploy Gladwin Tensor strainmeters and short period seismometers. Each “observatory” is also equipped with surface GNSS, meteorological instruments, and additional seismic sensors. The two deepest boreholes host fibre optic cables for temperature and strain. The strainmeter array (STAR) instruments are four-gauge strainmeters, from which we can resolve the horizontal strain matrix and measure deformation on the order of nanostrain, and bridge timescales encompassed neither by GNSS nor by Seismometers. With this new suite of instruments TABOO will enable the collection and calibration of strain records with exquisitely high precision, allowing for a quantitative characterization of ATF creep (~1 mm over <1 km²), enhanced monitoring of microseismicity (below Mc 0.5), and allowing correlation between degassing (CO2, Rn) measurements and subsurface strain.

Such unique near fault data Illuminating the spatiotemporal characteristics of creep on the ATF including possible stress triggering of larger earthquakes by transient creep events, are needed to address key questions of global importance in the seismic hazards and risk assessment community about the physics that allows for both seismic and aseismic slip on a single fault patch.

After presenting the field campaigns, we give an overview of the new data, showing how they enable us to detect new dynamic and static strain features missed by the other in situ instruments.