The 2013 Marche offshore earthquakes: new insights into the active tectonic setting of the outer northern Apennines
S. Mazzoli1, C. Macchiavelli2 & A. Ascione1
1Department of Earth Sciences, Environment and Georesources (DiSTAR), University of Naples Federico II, Largo San Marcellino 10, 80138 Naples, Italy
2Scuola di Scienze e Tecnologie, Università di Camerino, Via Gentile III da Varano, 62032 Camerino (MC), Italy
A seismic sequence offshore the Marche region sheds new light on the active tectonic setting of the outer northern Apennines, revealing the existence of a previously unknown transcurrent seismogenic structure. In contrast to the Po Plain area, where vigorous seismicity testifies to active frontal thrusting, the seismotectonic setting of the Adriatic domain is characterized by the occurrence of active crustal strike-slip faults dissecting the thrust belt. As recent studies suggest that thrust faults represent the unique seismogenic sources characterizing the Adriatic sector of the outer northern Apennines, our results pose new challenges to the identification of seismogenic structures in the region.Starting in June 2013, a seismic sequence affected the offshore portion of the central Marche region in Italy. The largest shock in the time span analysed in this study (June–October 2013) occurred on 21 July (at 01:32:24 GMT), with an epicentre immediately offshore Mt. Conero) and a hypocentral depth of 8.4 km according to the preliminary data made available by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). Preliminary evaluations assign magnitudes Ml = 4.9 and Mw = 4.9 (INGV data available at http://cnt.rm.ingv.it/tdmt.html) to this event, whose epicentre is located c. 18 km SE of Ancona, the regional capital of Marche and the main seaport of the central Adriatic region. The main event was preceded by a series of shocks, the largest of which occurred on 13 June (at 05:44:52 GMT) at a hypocentral depth of 6.0 km; the assigned magnitudes are Ml = 3.9 and Mw = 3.8 (http://cnt.rm.ingv.it/tdmt.html). During the seismic sequence that followed the main event, a shock of assigned magnitudes Ml = 4.0 and Mw = 4.1 (http://autorcmt.bo.ingv.it/QRCMT-on-line/QRCMT13-on-line/E1307210307A.html) occurred on 21 July (at 03:07:24 GMT) at a hypocentral depth of 10.1 km. The available focal mechanisms (http://cnt.rm.ingv.it/data_id_old//7229061720/map_dmt_review.pdf,http://cnt.rm.ingv.it/data_id_old//7228517040/map_dmt_review.pdf,http://autorcmt.bo.ingv.it/QRCMT-on-line/QRCMT13-on-line/E1307210307A.html) show similar, dominantly strike-slip fault plane solutions with ENE–WSW left-lateral and NNW–SSE right-lateral nodal planes for all three events (Fig. 1). P- and T-axes are respectively SW–NE and NW–SE oriented, both plunging at a low angle. A similarly plunging P-axis also characterizes the thrust-type fault plane solution obtained for a further shock, of assigned magnitudes Ml = 4.4 and Mw = 4.2 (http://cnt.rm.ingv.it/tdmt.html,http://cnt.rm.ingv.it/data_id_old/7229525640/map_dmt_review.pdf), which occurred on 22 August (at 06:44:50 GMT) at a hypocentral depth of 8 km.