by Lauro Chiaraluce, Richard A. Bennett, Massimiliano Rinaldo Barchi, Marco Bohnhoff

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 multidisciplinary 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 80-160 m 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 met-packs). 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.

• The ATF is an active low-angle normal fault (dip 20°), which bears directly on the decades long debate regarding the activity and mechanics of such misoriented structures. Repeating earthquakes, GPS time series and other data provide strong indications that the ATF fault is creeping at relatively shallow depth.
• To resolve the spatiotemporal characteristics of creep, in particular, whether the creep is occurring in steady state or episodically, whether creep coincides or is adjacent to repeating earthquakes, and how creep relates to seismic swarms requires strainmeters, the only instruments capable of measuring small creep events. The proposed Strain Meter array (STAR) will provide an opportunity to study creep at local scale and over periods of minutes to months poorly constrained by other geophysical instruments. STAR multisensors stations (downhole seismometers, pressure transducers and strainmeters in boreholes plus GPS and meteorological instrument at surface) will also enhance the correlation between multidisciplinary data describing the same process. Considering phenomena intersecting different research fields is a fundamental requirement to better understand the multi-scale, physical/chemical processes responsible for earthquake preparatory phase and faulting. Creep on the ATF can load the earthquake-prone high-angle faults in its hanging wall. STAR will provide critical information to determine if this process happens continuously, episodically, or in a more complex spatiotemporal pattern, including possible stress triggering by transient creep events, an issue of global importance in the seismic hazards community. Thus, in addition to the ATF hazard problems of locally confined (even if crucial) importance, STAR will allow us to address the global question of how aseismic slip relates to seismic slip on a single fault patch. Data from STAR will also guide the interpretation of fault friction experiments from the laboratory.

Creeping, Earthquakes, Europe, Italy, Monitoring, Strainmeters

Latitude: 43.374053, Longitude: 12.352727