GEOPHYSICAL RESEARCH LETTERS, VOL. 22, NO. 14, PAGES 1909-1912, JULY 15, 1995 Evidence of active extensionin Quaternary volcanoes of Central Italy from breakout analysisand seismicity P. Morttone, A. Amaro and C. Chiarabba Istituto Nazionale di Geofisica, Roma, Italy G. Buonasorte and A. Fiordelisi ENEL - DPT Vice Direzione Termica e Attivit• Geotermiche, Pisa, Italy Abstract. We presentactive stress directions obtained from the Alban Hills, is relatedto Late Pleistocene N-S right-lateral borehole breakout analysis performed on 15 geothermal wells lo- shear zones. catedin the westerncoastal regions of Central Italy. The study Since the mid 70's, the study region has been extensively area(a 200 km by 50 km NW-elongated areabordering the Ap- drilledfor geothermal research by a joint venture between ENEL ennines) includes several Quaternary high-K alkalinevolcanoes (Italian National Electric Company) and Agip (National Oil active mainly after 0.6 Ma. We analyzed both paperlogs and Company). We analyzed datafromthe Quaternary volcanoes of digital data to detectbreakout directions, the two techniquesLatiumandTuscany, namely Amiata,Vulsini, Sabatini, and A1- yielding similar results. Thebreakout results show a predominant bani volcanoes (Figure1), active between 0.60 and about 0.02 ENE direction of SHmin, with local deviations in oneregion Ma [Fornaseri, 1985]. Data from deep wells (mostly 2-3 km)are (Sabatini volcano) where no seismicity isobserved. The compari- available for breakout analysis in all the volcanoes, excepting the son of breakout data with stress directions inferred from Colli Albani(Figure 1). Seismological dataare available for inversion of microearthquake (M<4) focal mechanisms computed Amiata, Vulsini and Albani (Figure 1), thanks to the presence of in three of the fourvolcanoes suggests thatthe whole area is microseismic networks thatrecorded thousands of earthquakes presently undergoing NE toENE extension. with M<4 [Buonasorte etal., 1987; Arnato etal., 1994]. Contrary to the other volcanoes of similar age and composition, no earth- quakes havebeenrecorded in the Sabatini volcano during many Introduction years of microseismic monitoring. In thisletter, we show horizon- tal directions of stress inferred from breakout orientations; then, The activestress field in Central Italy is poorly known,due to we compare these results with stress directions obtained by in- the lack ofa systematic collection ofstress indicators, such as, version of earthquake focal mechanisms in order to constrain the borehole breakouts and earthquake focal mechanisms. The data stress tensor in the different volcanoes. Where available, we also included inthe World Stress Map (WSM) database for this re- consider Quaternary geological joint and fault slip data found in gion [Zoback, 1992] are limited toa few focal mechanisms of previous works. Inaddition, we collected new data on joint sys- earthquakes that occurred in the Apennines. Only one fault plane terns on pyroclastic formations of the Vulsini Volcanic Complex. solution [Gasparini et al., 1985] is available for the volcanic province of Latium - southern Tuscany (Tuscania earthquake, 6 February 1971, M=4.6). This region experienced important geo- Breakout analysis dynamic processes in Quaternary times,including the build up of a suite of high-K alkaline volcanoes inthe last 0.6 Ma associated Breakout directions from deep wells are, together with earth- with extensional tectonics in the back-arc region ofthe Northern quake focal mechanisms, themost reliable indicators of the Apennines, active since Lower Pliocene. The magmatic activity active stress field [Zoback, 1992]. Breakouts are spalled regions developed along a belt located between the uplifting Apenninic onopposite side of the borehole that produce intervals in the chain and thestrongly subsiding Tyrrhenian area. The post- well with ellipsoidal cross sections [Cox, 1970; Babcock, 1978]. Tortonian extensional tectonics caused the formation of a series Bell and Gough [1983]suggested that thebreakouts are theresult of NW-elongated main basins filled bysedimentary sequences oflocalized shear failure, inresponse toa concentration ofcom- [Locardi etal., 1975] and interrupted by NE-SW trending struc- pressire stress atthe borehole wall,within an anisotropic stress tures asevidenced by surface geology, gravity data and deep field. Breakouts are aligned inthe minimum horizontal princi- wells. During the same period, a widespread magmatic activity pal stress (SHmin)direction. developed in the northern Tyrrhenian areaand migrated tothe We identified borehole breakouts analyzing bothnon- southeast, where volcanism reached itsclimax between 0.6 and computed fieldlogs (paper logs), and digital data. In thetradi- 0.3 Ma [see Barberi etal., 1993]. According toFaccenna etal. tional analysis of paper logs we identified the breakouts [1994], thelast tectonic episode inferred from mesostructural according to thecriteria suggested by Plumb and Hiclonann data atthe southwestern border ofthe Apenninic units, north of [1985] as listed below: 1) the logging tool mustnot rotatein the zoneof elongation. 2) the largercalipercurveshould exceed the bit-size, the smaller one should not be lessthanhole gauge and its trace is parallel Copyright 1995 by the American Geophysical Union. tothe bit-size; 3) breakout elongations coincident (within + 10 ø) with azimuthof Papernumber 95GL01326 holedeviation are discarded, because they maybe caused by 0094-8534/95/95GL-01326503.00 thetool weight or by erosion of the drill strings; 1909