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

Stefano Parolai,Ildiko Nagy,Carla Braitenberg,Giuliana Rossi,Alberto Pastorutti

doi:10.3389/feart.2021.641416

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.

Highlights

Nowadays, space geodetic observations cover large portions of the Earth, with time series even longer than 20 years, i.e., long enough to allow distinguishing and characterization of periodical or transient variations
Once we found the direction in which intrinsic mode functions (IMFs) 8 shows the maximum amplitude, we repeat the analysis by moving from directions 5 and 10◦ to North and South, to be sure to find the direction of maximum amplitude of our signal, with an uncertainty of only 5◦
Since global navigation satellite system (GNSS) and interferometric synthetic aperture radar (InSAR) data are limited to the last 20 years, observations on past episodes of slow slip can come from strainmeters, tiltmeters, and dilatometers that had been continuously recorded in the past

Summary

INTRODUCTION

Space geodetic observations cover large portions of the Earth, with time series even longer than 20 years, i.e., long enough to allow distinguishing and characterization of periodical or transient variations. The strain field has been continuously monitored for over 50 years by the NE-Italy subsurface tilt and strainmeter network (Braitenberg, 1999) It consists of two long baseline horizontal pendulums installed in Grotta Gigante (GG), near Trieste (installed in 1959; continuous data:1964–present), and two tiltmeter stations located in the Bus de la Genziana (BG) and Villanova (VI) caves, active since 2005 and 1977, respectively (Figure 3, Table 1; Chiaruttini and Zadro, 1976; Braitenberg, 1999; Zadro and Braitenberg, 1999; Grillo et al, 2018). The results are in good agreement with the directions found through the transient analysis that affected the FReDNet GNSS data in the time interval 2006–2009 (red– black arrows for the horizontal component, yellow arrows for the vertical one, RossiII) In this case, due to time propagation of the transient signal, the image refers to a particular moment: June 2008. The pore fluid ratio λ is about 0.89 ± 0.01, indicating condition of a supra-hydrostatic state, with pore pressure very close to the lithostatic load

DISCUSSION

Findings

CONCLUSIONS

DATA AVAILABILITY STATEMENT