Abstract
We update the last present-day stress map for Italy relatively to the area of 2016 Amatrice seismic sequence (central Italy) taking into account a large number of earthquakes occurred from August 24 to October 3, 2016. In particular in this paper, we discuss the new stress data from crustal earthquake focal mechanisms selecting those with Magnitude ≥ 4.0; at the same time, we revise the borehole data, analyze the stratigraphic profiles and the relative sonic logs in 4 deep wells located close to the Amatrice sequence along the Apennine belt and toward east along the Adriatic foredeep. From these data we consider the P-wave velocity trend with depth and estimate rock density following an empirical relationship. Then we calculate the overburden stress magnitude for each well. The new present-day stress indicators confirm the presence of prevalent normal faulting regime and better define the local stress field in the area, highlighting a slight rotation from NE-SW to ENE-WSW of extension. The analysis evidences that the lithostatic gradient gradually changes from ~26 MPa/km in the belt to less than 23 MPa/km along the Adriatic foredeep. Finally, at a depth of 5 km we estimate the vertical stress magnitude varying from 130 MPa to 114 moving from the Apennine belt to the Adriatic foredeep. Although the wells are very close each other they show different P wave velocities from the belt to the foredeep with values ~7km/s and ~4 km/s at 5 km depth, respectively.
Highlights
This paper combines present-day stress data from 2016 Amatrice seismic sequence and some information retrieved from the analysis of some deep wells in two different geologic context: the Apennine belt, where the earthquakes occurred, and the contiguous eastward Apennine foredeep.Along the entire central Apennine belt (Fig. 1) the main active process is the ~NE–SW extension, well defined by several normal faulting focal plane solutions and fault data
We have taken into account the available Quick Regional Centroid Moment Tensors [http://autorcmt.bo.ingv.it/quicks.html, Pondrelli et al, 2006] for homogeneity with the other focal mechanism data already published in the previous stress maps
As suggested by the inversion of several earthquake focal mechanisms, we are confident that in the Apennine belt the three principal stresses are perpendicular to each other with one being in a vertical orientation [e.g. Frepoli and Amato, 1997], we have evaluated the magnitude of Sv from the cumulated weight of overburden in the four selected wells
Summary
This paper combines present-day stress data from 2016 Amatrice seismic sequence (until October 3rd, 2016) and some information retrieved from the analysis of some deep wells in two different geologic context: the Apennine belt, where the earthquakes occurred, and the contiguous eastward Apennine foredeep. NE-SW compression is limited to the external areas, along the northern Apennine front (from the Po Plain up to ~42.5° latitude) and toward the central Adriatic Sea, with a localized well-depicted N-S compression [Montone and Mariucci 2016]. Very close to the Amatrice seismic sequence, two deep wells were analyzed [Mariucci et al, 2010] in order to define the minimum horizontal stress orientation (Shmin) through the breakout analysis (Fig. 2). The estimated rock density in that sector of the Apennine belt ranged from 2.65 to 2.33 g/cm (Montone and Mariucci 2015) in good agreement with the result found in central Italy (Umbria - Marche Apennines) by other authors [Federico and Pauselli 1998; Pauselli et al 2010]
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