Recurrence of Fault Valve Behavior in a Continental Collision Area: Evidence From Tilt/Strain Measurements in Northern Adria

Abstract

We analyzed the data recorded by the NE-Italy subsurface tilt and strainmeter network evidencing a coherent transient signal in the recordings of four tiltmeter sites in the 1984–1990 period that produced a tilt along the main fractures. Borrowing from classical seismology techniques, we used the uprise times to locate the transient signal source. The propagation velocity is compatible with a fluid diffusion process that starts from a source located close to the hypocenter of the February 10, 1983 Uccea earthquake, MD = 4.2 at the Italy-Slovenia border, at an estimated depth of 10.8 km. Our results add to the previous interpretation of a transient signal recorded by several global navigation satellite system (GNSS) stations in the 2006–2009 period in terms of fluid diffusion below the Bovec basin (Slovenia). That source was located upon continuation to the northwest of the Ravne fault, few kilometers to the northeast from the present one, and about 6 km from the July 12, 2004 Bovec–Krn earthquake, Mw = 5.1, depth ~6.1 km. These observations suggest that the area is subject to fault valve behavior episodes that released fluids trapped at depth to the surrounding region as pore-pressure bulges. The convergence between Alpine and External Dinarides structures in this area puts highly permeable dolomitic limestones in contact with low-permeable fine-grained limestones and flysch formations. Therefore, the conditions for overpressure generation can be created, whereas fault movements, from time to time, in close relation with seismic events, can enable fluid diffusion in the surroundings. We also estimated the possible fluid influx needed to maintain overpressure and possible discharge across both the faults. The study provides insights on pore–fluid pressure variations related to slow slip events from a context different from subduction or transform margins, i.e., in a continental collision area.

Seismic clusters and fluids diffusion: a lesson from the 2018 Molise (Southern Italy) earthquake sequence

“…Various experiments indicated that fluid pressure mainly induces aseismic deformation that triggers seismicity through stress perturbations on fault asperities with appropriate background stresses and frictional instability conditions (De Barros et al, 2020). The alternating bursts of seismicity and quiescence observed in some swarms can be explained by the fault-valve model (Sibson, 1992;Rossi et al, 2021). According to Hainzl (2004), at smaller Dh the intervals between subclusters become longer and the total duration of activity increases.…”

Present-Day Surface Deformation in North-East Italy Using InSAR and GNSS Data

“…Several other studies used geodetic observations, focusing on non-tectonic processes affecting the study area. Rossi et al [57][58][59] described transient deformation signals associated with pore pressure variations in rocks in response to tectonic stress. Stocchi et al [60] estimated the post-glacial isostatic rebound along the Alpine belt as ~1/3 of the measured vertical rates.…”

Surface deformation analysis in Northeast Italy by using PS-InSAR and GNSS data 

“…Other studies focused on the definition of the geometrical and kinematic behaviors of the active faults and their seismogenic potential (e.g., D'Agostino et al, 2005;Bechtold et al, 2009;Cheloni et al, 2014;Moulin et al, 2016;Serpelloni et al, 2016;Anderlini et al, 2020). Rossi et al (2016Rossi et al ( , 2018Rossi et al ( , 2021 have evidenced transient deformations propagating in the region and interpreted them due to pore pressure variations in response to the tectonic stresses.…”