Analysis of Source Properties for the Earthquake Sequences in the South-Western Carpathians (Romania)
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ANALYSIS OF SOURCE PROPERTIES FOR THE EARTHQUAKE SEQUENCES IN THE SOUTH-WESTERN CARPATHIANS (ROMANIA) A.O. PLACINTA 1, E. POPESCU 1, F. BORLEANU 1, M. RADULIAN 1, M. POPA 1 1National Institute for Earth Physics, P.O. Box MG-2, Calugareni 12, Magurele, 077125, Romania, [email protected] Abstract. The crustal seismicity in Romania is concentrated in front of the Carpathians Arc bend (Vrancea region) and at the contact between the extra-Carpathian platform regions and Carpathians orogen. The region investigated in this paper is characterizing the contact of the western side of the South Carpathians with the Tisza-Dacia region. In this area, the large amount of transcurrent deformation that took place during the right-lateral movement of the South Carpathians in respect to the stable Moesian unit let to the creation of intra-mountainous pull-apart basins able to accommodate this movement. For two such basins, Haţeg and Caransebe ş-Mehadia, recent good quality seismic data have recorded during three earthquake sequences: (1) a sequence of 14 events occurred on 24-31 March 2011 in the Ha ţeg Depression (maximum magnitude of M w = 3.3), (2) a sequence of 35 events occurred on 8 – 11 September 2013 in the Ha ţeg Depression (maximum magnitude of M w = 4.0) and (3) a sequence of 60 events occurred in the Caransebe ş-Mehadia Depression on 31 October – 15 December 2014 (main shock magnitude of M w = 4.1). We apply empirical Green’s functions deconvolution and spectral ratios techniques to determine the source parameters. Despite the relative small size of the events, high-quality waveforms for pairs of co- located events are available in different measuring sites. The new results, together with previous determinations, provide a useful database to investigate the source scaling properties in correlation with seismotectonics modeling of the study region. Finally, source characteristics (location, seismic moment, source dimension, stress drop, focal mechanism, clustering) are discussed in connection with the seismotectonics features at the scale of the entire Carpathians and adjacent extra-Carpathians contact areas. Key words: source parameters, source scaling, earthquake sequence, intra-mountain depression, South Carpathians. 1. INTRODUCTION The area connecting the Balkans with East Carpathians along the highly bended South Carpathians Mountains underwent large-scale strain deformation during Paleocene-Miocene at the contact with Moesia unit (Getic Depression). Oblique convergence during orogenic subduction and collision is commonly adopted as scenario for the contact between the South Carpathians and Moesia [7]. It implies development of transcurrent zones and creation of pull-apart basins, able to accommodate the large amount of right-lateral movement of the South Carpathians in respect to the stable Moesian unit, driven by the rapid roll-back of a slab attached to the European continent [5, 17, 18]. A few recent earthquake sequences were recorded in the area by the seismic network of the National Institute for Earth Physics (M ăgurele, Romania) in 2 two intramountainous depressions characterizing the contact between the South Carpathians and Moesia: Ha ţeg Depression and Caransebe ş-Mehadia Depression (Fig. 1). The two depressions are located at the exterior of the Carpathians, in a region where the largest amount of transcurrent deformation took place and significant amounts of dextral strike-slip deformations were supposed to accommodate the clockwise rotation and N-to-E-ward movement of the Carpathians units in respect to Moesia. The goal of the paper is to determine the source parameters of these sequences and their scaling and to interpret the results in the context of the main geotectonical features of the region. The sequences were recorded in March 2011 and September 2013 in the Ha ţeg Depression and in October 2014 in the Caransebe ş-Mehadia Depression. The seismicity as recorded in catalogues in the intra-mountain depressions of the East Carpathians is scanty and sporadic. The sequences recently recorded in the Ha ţeg and Caransebe ş-Mehadia depressions are remarkable from this point of view as amount and quality of waveform data. Fig. 1. Tectonic sketch of the South Carpathians area and associated seismicity (from Romplus catalogue, [11] – updated). Ha ţeg and Caransebe ş-Mehadia depressions are marked by ellipses. 2. REGIONAL SEISMICITY AND TECTONICS The seismic activity in the study region is generally poor: 69 events with magnitude M w ≥ 3 have been recorded since 1900 to the present day (1 June 2015) according to the Romplus catalogue ([11], updated version on www.infp.ro ) (Fig. 2). Seismicity concentrates in four clusters related to: (1) Moldova Nouă fault to the west, (2) Cerna fault, (3) Ha ţeg Depression to the north and (4) the contact between the Getic Depression and the Carpathian orogeny to the east (Fig. 2). Cluster (2) is mentioned as Mehadia-Or şova seismo-active zone by [12]. Cluster (3) is assigned by the same authors as a distinct seismo-active zone. The largest 3 earthquakes were recorded in Moldova Nou ă region (a multiple sequence that took place on more than 6-month interval in 1879-1880, with two shocks of M w 5.3 and eleven shocks with M w above 4), Mehadia-Or şova region (an event of M w 5.6 recorded in 1991) and Tg. Jiu region (an event of Mw 5.2 recorded in 1943). 46.0 Deva Hateg Legende 45.5 epicentres for events with 3<M<4 Caransebes epicentres for events with 4<M<5 0 epicenters for events with M>5 cities latitude ( latitude N) Tg-Jiu 45.0 Mehadia B. Herculane M. Noua Orsova 44.5 21.5 22.0 22.5 23.0 23.5 0 longitude ( E) Fig. 2. Seismicity in the study region (M w ≥ 3) as recorded in the Romplus catalogue ([11] – updated). After 2010 four earthquake sequences took place in the Caransebe ş- Mehadia Depression, located north from cluster (2) and in (3) and (4) zones. The sequence in (4) zone was investigated in detail by [15]. Our attention will be focused on Ha ţeg and Caransebe ş-Mehadia zones. In the Ha ţeg zone two sequences are analysed, one occurred in 24-31 March 2011 (14 events) and the other occurred 8-11 September 2013 (31 events). One earthquake produced rather isolated from the sequence of September 2013, on 31 October 2013, was included in our study taking into account the remarkable proximity of its hypocentre to the sequence hypocentres. The location parameters are given in Table 1 and the epicentre distribution for the two sequences is represented in the Fig 3. The locations were obtained using the Joint Hypocentre Determination (JHD) algorithm [19]. Table 1. JHD location parameters for the earthquakes occurred in the Ha ţeg zone. No yyyy/mo/da hh:mm:ss Lat Lon ML Mw h RMS No. GAP (N 0) (E 0) (km) (s) phas. 1 2011/03/24 11:02:24 45.610 22.834 3.5 3.1 3.3* .37 38 60 2 2011/03/25 02:08:27 45.592 22.832 2.4 2.3 5.5 .27 13 124 3 2011/03/25 02:22:17 45.627 22.746 2.4 2.3 10.8* .41 9 124 4 2011/03/25 02:29:20 45.598 22.830 2.5 2.4 6.5* .16 16 121 5 2011/03/25 09:30:53 45.935 22.795 2.5 2.1 5.7 .10 6 142 4 6 2011/03/25 09:48:58 45.584 22.834 2.5 2.4 7.6 .09 10 123 7 2011/03/25 09:51:12 45.660 22.843 2.4 2.3 9.4* .19 8 114 8 2011/03/25 12:01:22 45.594 22.835 3.5 3.2 5.2 .35 34 73 9 2011/03/25 14:51:26 45.608 22.823 2.5 2.4 5.2 .23 11 120 10 2011/03/25 15:12:24 45.602 22.827 4.0 3.3 4.8* .29 37 47 11 2011/03/26 23:37:03 45.594 22.856 2.1 2.2 2.1 .15 8 108 12 2011/03/27 01:13:02 45.639 22.824 2.4 2.3 4.1* .26 14 101 13 2011/03/27 19:38:30 45.607 22.836 2.2 2.2 4.3* .31 13 87 14 2011/03/31 06:29:38 45.621 22.841 2.3 2.3 7.5* .38 13 109 15 2013/09/08 13:00:41 45.615 22.834 4.2 3.6 5.1* .26 57 43 16 2013/09/08 13:07:06 45.557 22.856 1.7 2.3 7.4* .26 11 99 17 2013/09/08 13:07:36 45.611 22.872 1.5 2.2 10.5 .01 6 100 18 2013/09/08 13:09:51 45.628 22.850 0.9 2.0 0.0* .20 7 110 19 2013/09/08 13:14:52 45.616 22.848 1.7 2.3 5.5 .03 8 88 20 2013/09/08 13:15:48 45.610 22.845 1.0 2.2 8.4 .01 6 101 21 2013/09/08 13:22:11 45.612 22.840 4.7 4.0 5.6 .22 83 43 22 2013/09/08 13:27:51 45.608 22.825 1.4 2.2 3.0* .19 8 100 23 2013/09/08 13:36:50 45.659 22.861 0.5 2.2 0.0* .21 6 198 24 2013/09/08 13:41:41 45.582 22.816 1.3 2.2 9.3 .01 5 99 25 2013/09/08 13:47:49 45.593 22.800 0.9 2.2 11.8 .02 5 99 26 2013/09/08 14:12:22 45.588 22.836 1.1 2.2 10.4 .01 5 100 27 2013/09/08 14:21:01 45.612 22.857 2.2 2.4 6.1 .21 23 85 28 2013/09/08 15:01:39 45.617 22.895 1.0 2.2 0.0* .30 7 104 29 2013/09/08 15:14:17 45.616 22.837 1.3 2.2 6.4 .02 6 101 30 2013/09/08 15:48:52 45.546 22.931 0.8 2.2 19.6 .03 5 222 31 2013/09/08 16:19:48 45.611 22.835 1.5 2.3 6.4 .17 14 87 32 2013/09/08 16:51:10 45.614 22.837 1.1 2.2 2.2 .05 9 108 33 2013/09/08 17:20:34 45.608 22.844 2.2 2.4 6.4 .17 25 85 34 2013/09/08 18:04:17 45.628 22.854 1.1 2.3 0.0* .07 7 110 35 2013/09/08 19:51:28 45.610 22.856 1.3 2.3 3.7 .16 14 90 36 2013/09/08 21:09:33 45.596 22.841 0.8 2.2 11.1* .31 11 90 37 2013/09/08 22:04:00 45.632 22.861 0.7 2.2 0.0* .46 9 111 38 2013/09/08 22:31:41 45.607 22.851 1.1 2.3 4.9 .25 15 90 39 2013/09/08 23:46:34 45.604 22.841 1.2 2.3 5.2 .07 12 89 40 2013/09/09 15:30:16 45.612 22.832 2.5 3.6 2.8* .40 41 60 41 2013/09/09 16:27:17 45.635 22.846 1.1 2.4 6.0 .21 18 85 42 2013/09/09 21:14:40 45.617 22.852 0.7 2.2 2.5 .06 11 101 43 2013/09/10 03:02:55 45.647 22.837 0.8 2.3 8.2 .19 12 88 44 2013/09/11 03:38:57 45.425 22.902 2.2 2.2 23.8* .21 8 250 45 2013/10/31 06:10:49 45.600 22.847 2.6 2.9 2.9 .37 44 46 * location with fixed depth marked rows: selected events for source parameters evaluation 5 45.7 Hateg 45.6 45.5 Caransebes 45.4 45.3 latitude 45.2 45.1 45.0 44.9 Mehadia 44.8 22.0 22.1 22.2 22.3 22.4 22.5 22.6 22.7 22.8 22.9 23.0 longitude Fig.