Focal mechanisms of shallow earthquakes in the Aegean Sea and the surrounding lands determined by waveform modelling: a new database

Journal of Geodynamics 36 (2003) 251–274 www.elsevier.com/locate/jog

Focal mechanisms of shallow earthquakes in the Aegean Sea and the surrounding lands determined by waveform modelling: a new database Anastasia Kiratzi*, Eleni Louvari Department of Geophysics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece

Abstract Source parameters for 40 earthquakes with epicentres in the Aegean Sea and the surrounding lands have been determined using P and SH body waveform inversion of digital waveforms from the Global Seismograph Network (GSN). These results are combined with 145 previous solutions, 41 of which were determined by first motion polarities and 104 by waveform modelling. Thus, a new updated database of source parameters for earthquakes for the period 1953–1999 is available. These fault plane solutions confirm our previous knowledge about the seismotectonics of the Aegean Sea and the surrounding lands. They also provide strong evidence that the western part of the Peloponnese (Gulf of Patras to Gulf of Kyparissia) is mainly deforming by strike-slip faulting, parallel in strike to the Cephalonia Fault and are in favour of the models that suggest a connection of the North Anatolian Fault (NAF) zone with the transform zone of the Ionian Islands. The sedimentary arc (inner part of the Hellenic trench) is deforming by normal faulting with the T-axes trending  E–W. We did not determine any fault plane solutions at the eastern part of the Hellenic arc indicating pure strike-slip motions as expected from the presence of the Pliny and Strabo trenches. However, we did determine solutions that indicate normal faulting combined with a considerable strike-slip component. In the central Aegean Sea the newly determined focal mechanisms support the conclusion that the area, extending from the eastern coasts of Evia up to the coasts of Turkey, is exhibiting a clear strike-slip character. Thus, the effect of the propagating tip of the North Anatolian fault into the Aegean is very clearly pronounced. The focal mechanisms in the back-arc Aegean area confirm the existence of normal faulting with T-axes trending NNE–SSW in western Anatolia and NNW–SSE in Greece. A zone showing NNE–SSW extension that runs parallel to Northern Aegean branch of the NAF zone up to the Corinth Gulf is depicted from the mechanisms of few earthquakes. # 2003 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +30-2310-998486; fax: +30-2310-998528. E-mail address: [email protected] (A. Kiratzi). 0264-3707/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0264-3707(03)00050-4

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A. Kiratzi, E. Louvari / Journal of Geodynamics 36 (2003) 251–274

Fig. 1. Location map of the Aegean Sea and the surrounding lands. The dashed line indicates the boundaries of the Aegean plate and the arrows indicate the motion of the plates relative to Euasia.

1. Introduction The tectonic framework of the broader Aegean area (Fig. 1), part of eastern Mediterranean, is dominated by the collision of the Arabian and African plates with Eurasia (McKenzie, 1972, 1978; Jackson and McKenzie, 1988; Jackson, 1994; Papazachos and Kiratzi, 1996; Papazachos et al., 1998; McClusky et al., 2000). Plate motion models indicate that the Arabian plate is moving northwards relative to Eurasia at a rate of 18–25 mm/year, averaged over 3 Myear. The African plate is also moving northwards relative to Eurasia at a rate of 10 mm/year (DeMets et al., 1990). Its leading edge is being subducted along the Hellenic Trench. As the rate of subduction is higher than the relative motion of Africa relative to Eurasia, it requires a southward movement of the arc relative to Eurasia. A major role in the seismotectonic framework of the Aegean area plays the motion of two other smaller plates: the Anatolian plate and the Aegean plate. The Anatolian block is moving southwestwards and this motion is facilitated by the pre-

A. Kiratzi, E. Louvari / Journal of Geodynamics 36 (2003) 251–274

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sence of the subduction where the Aegean plate can easily override the subducting edge of the African plate. The presence of the North and East Anatolian major strike slip faults further facilitate the westward escape of the Anatolian plate. The existence of the Aegean and the Anatolian plate was originally discussed by McKenzie (1972, 1978) and later by Jackson and McKenzie (1988), Papazachos et al. (1998), Papazachos (1999), and McClusky et al. (2000). Over the years, we have either determined the focal mechanisms of large Aegean earthquakes ourselves or collected those published by others mainly using waveform modelling or first motion polarities for some past events (Louvari, 2000 and references therein). In this paper, we present 40 focal mechanisms determined by body waveform modelling, using digital data from the Global Seismic Network (GSN). We modeled the earthquakes that obey the following criteria: (a) they had never been studied before, (b) they had been studied but not with P and SH waveform inversion, (c) only a Harvard CMT solution was available. These events together with previous mechanisms constitute an updated database of regional focal mechanisms for shallow earthquakes (h <40 Km) in the Aegean Sea and the surrounding lands, (34 N–43 N, 18 E–30 E) which consists of 185 focal mechanisms of earthquakes with M55.0 of the period 1953–1999. From these events, 144 have been determined by waveform modelling either by others or us and 41 by first motion polarities. Information on the focal parameters of the events is given in Tables 1–3. Table 1 lists the new focal mechanisms determined in this study, Table 2 lists the focal mechanisms determined previously by waveform modelling, and Table 3 lists the focal mechanisms, based on first motion polarities.

2. Method of waveform analysis The method described in Na´be˘lek (1984); McCaffrey and Na´be˘lek (1987), and the MT5 version (Zwick et al., 1995) were used to constrain earthquake source parameters using P and SH bodywaveform inversion. Synthetic seismograms are generated for P and SH waveforms at a number of points at the surface of the Earth (seismic stations) from a starting earthquake source model. A least squares inversion minimizes the misfit between the synthetic and the observed seismograms at each station by changing the source model accordingly. The parameters included in the inversion are the strike, dip, and rake of the fault plane, the centroid depth, the shape of the source time function and the scalar seismic moment. The iterative procedure continues until a minimum misfit solution is obtained. This approach is now too routinely applied to justify a more detailed analysis. Digital waveform data for the events studied were obtained from the Global Seismograph Network (GSN). We only consider the waveforms with signal/noise ratio >2 recorded by an adequate number of stations with a good azimuth coverage of the focal sphere. Both long-period (sampling frequency 1 Hz) and broad-band (sampling frequency 20 Hz) data were utilized, in the cases that were both available. In these cases we utilized the long-period data to obtain the parameters of the focal mechanism (strike, dip and rake) and then keeping these parameters fixed in the inversion we utilized the broad band data to invert for the centroid depth and the shape of the source time function. The rich in high frequencies broad-band data allow us to better constrain the source depth and the source time function compared to the long period data that lack high frequency content. An FIR (Finite Impulse Response) band pass filter (Oppenheim and Schafer, 1989) was applied to the data, in order to remove the high frequency noise. The corner frequencies of the filter were

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Table 1 Earthquake focal mechanisms determined by waveform modelling in this study Date (YYMMDD)

05:31:10 02:54:20 10:17:01 08:02:51 09:12:02 10:20:50 01:41:36 18:36:38 09:00:12 14:50:11 23:06:53 13:30:20 18:32:31 11:22:16 11:29:26 12:09:23 11:43:34 11:44:40 20:12:45 20:06:02 12:31:49 07:22:52 15:45:26 23:09:34 02:05:39 00:34:11 08:47:15 03:27:50 07:59:26 00:00:40 10:07:53 13:39:40 12:22:53 21:10:28 21:38:52 13:07:41 19:21:58 03:30:38 04:00:15 12:27:42

Latitude N

36.35 35.67 40.23 38.37 38.77 37.50 38.40 38.37 37.91 37.17 37.10 37.06 37.26 34.90 37.04 34.80 38.34 35.10 39.81 38.02 38.17 35.83 38.67 37.36 38.83 40.54 40.13 34.76 37.84 36.07 35.51 36.45 34.51 38.40 38.73 37.54 37.29 35.96 37.62 37.13

Longitude E

29.39 21.81 26.80 22.10 25.64 21.20 25.10 25.17 28.53 21.39 28.20 28.01 21.14 26.60 29.51 26.30 21.04 26.60 24.40 26.97 21.77 21.83 27.48 20.63 26.49 23.63 21.67 23.99 26.87 27.46 21.18 22.16 23.93 22.45 25.91 20.53 20.86 21.88 20.75 20.98

Depth (km)

22 17 11 3 9 29 6 14 9 27 7 22 16 9 14 12 22 7 8 6 19 14 14 22 10 12 12 24 9 12 16 32 22 13 10 32 17 13 13 9

(+3/ (+4/ (  3) (+5/ (+2/ (+1/ (+3/ (  4) (  3) (+3/ (+4/ (+2/ (  4) (+3/ (+3/ (  2) (  3) (  1) (+3/ (+3/ (  2) (  3) (  3) (+2/ (  2) (  2) (+1/ (+1/ (+2/ (+1/ (  2) (+2/ (  2) (+3/ (+1/ (  3) (+1/ (  1) (+2/ (  2)

Mw

5) 3) 1) 3) 4) 4)

5) 3) 4) 2) 4)

1) 2)

4)

2) 2) 1) 3) 3) 4) 2) 3) 3)

5.6 5.3 5.4 5.6 5.4 5.3 5.5 5.3 5.7 5.4 5.3 5.3 5.7 5.2 5.4 5.5 5.1 5.8 5.3 6.0 5.4 5.3 5.3 5.5 5.5 5.2 6.4 5.2 5.3 6.1 5.3 6.3 5.2 5.3 5.7 6.5 5.3 5.3 5.1 5.2

Nodal plane 1 Strike

Dip

94 318 70 116 237 24 163 52 325 25 271 271 193 217 270 261 151 214 272 146 229 331 259 124 256 84 242 266 261 232 203 123 309 344 58 354 200 176 19 308

75 48 64 36 89 57 59 77 42 67 57 58 74 56 41 30 51 52 51 76 79 60 35 76 60 66 38 9 53 42 86 72 6 63 83 20 73 81 53 61

Nodal plane 2 Rake 104 124 176 71 161 168 22 152 36 176 103 103 174 21 60 40 105 47 148 13 174 126 120 90 131 103 91 54 119 52 47 84 108 45 175 159 109 117 131 54

Strike

Dip

230 93 162 273 147 121 265 149 83 117 114 115 101 319 53 135 354 337 161 53 138 96 114 304 136 294 63 122 124 6 109 322 111 98 149 104 70 283 144 184

20 52 86 56 71 80 71 63 67 86 35 34 84 73 55 71 41 55 66 77 84 46 60 14 49 27 52 83 46 58 43 19 84 51 85 83 25 28 53 45

P axis Rake 48 58 26 103 1 34 147 15 126 23 71 70 16 144 113 114 72 131 43 166 11 45 71 90 41 63 89 95 57 119 174 108 88 144 7 71 43 19 49 136

T axis

Mo *10e17 Ntm

Azimuth

Plunge

Azimuth

Plunge

173 205 293 144 104 248 127 103 309 249 145 147 56 183 270 207 5 188 120 100 93 36 64 214 113 330 337 207 110 225 326 218 203 305 283 210 85 244 82 63

29 2 15 75 14 15 36 9 53 13 74 73 16 37 70 23 77 57 47 1 12 8 69 31 55 66 83 38 67 64 28 27 39 50 1 35 58 31 0 9

22 300 29 13 10 348 32 8 199 343 10 11 148 85 159 77 252 95 220 9 184 293 190 34 14 184 153 38 11 116 77 24 19 44 14 354 305 114 352 167

58 65 21 10 13 31 8 29 14 19 11 12 7 11 7 57 5 2 9 19 4 58 13 59 6 20 7 52 4 9 34 63 51 7 9 49 26 47 59 58

3.34 1.17 1.62 3.37 1.79 1.30 2.55 1.25 3.87 1.38 1.12 1.13 4.47 0.70 1.40 2.49 0.56 5.64 1.13 10.87 1.83 1.27 1.14 2.15 2.10 0.82 45.89 0.74 1.14 15.27 1.01 31.90 0.69 1.09 4.04 64.56 1.03 1.30 0.51 0.78

A. Kiratzi, E. Louvari / Journal of Geodynamics 36 (2003) 251–274

800502 830714 831010 840211 840506 850907 860325 860329 861011 870610 890427 890428 890820 900709 900718 910319 910626 920430 920723 921106 930714 940111 940128 940416 940524 950504 950513 951210 960402 960720 970727 971013 971105 971105 971114 971118 980110 980429 980501 981006

Origin time (h:m:s)

Table 2 Earthquake focal mechanisms determined by waveform modelling and published in the literature Date Origin (YYMMDD) time (h:m:s)

Latitude Longitude Depth Mw Nodal plane 1 N E (km)

Nodal plane 2

P axis

T axis

Mo References *10e17 Ntm

Strike Dip Rake Strike Dip Rake Azimuth Plunge Azimuth Plunge 17:08:40 16:58:08 13:47:53 14:31:23 17:57:54 14:09:06 20:01:51 03:18:42 02:01:45 00:42:53 02:39:25 17:58:09 07:09:02 16:56:58 07:23:50 22:45:42 00:59:10 21:08:42 13:21:34 01:48:29 15:13:31 08:09:13 21:02:23 13:50:29 10:42:24 13:29:38 06:25:13 12:57:23 05:43:28 21:39:57 14:07:15 15:52:14 10:57:44 05:15:08 04:28:57 16:59:45 01:12:37 09:27:41 23:19:19

37.80 40.80 37.00 40.30 39.30 35.60 37.80 38.40 39.10 34.40 38.90 39.20 39.60 40.67 41.41 39.40 40.10 39.10 39.20 38.50 34.40 37.50 39.20 38.30 38.97 39.00 37.57 37.60 39.07 35.10 38.30 38.89 35.20 40.34 36.18 37.40 37.70 35.30 34.90

20.50 29.10 21.00 28.20 23.80 23.50 29.30 22.40 21.74 26.40 21.10 24.60 21.29 30.69 20.44 24.90 27.50 28.50 28.40 28.50 25.00 20.30 29.50 22.60 29.92 29.80 29.70 29.60 29.67 23.60 20.30 20.44 23.80 26.14 30.77 20.40 28.89 23.50 23.00

12.0 15.0 6.0 14.0 7.0 13.0 2.0 10.0 11.0 16.0 15.0 10.0 10.0 12.0 9.0 10.0 5.0 8.0 8.0 3.3 19.0 10.0 7.7 9.0 8.0 9.0 12.0 12.0 6.0 40.0 8.0 23.0 18.0 15.0 25.0 16.0 4.0 38.0 16.0

6.8 5.9 5.5 6.3 6.0 5.4 5.9 6.1 6.2 5.5 5.5 6.2 5.9 7.2 6.2 6.8 5.7 5.9 6.1 6.7 6.0 5.8 7.1 5.9 5.6 6.1 5.8 5.5 5.9 6.2 6.2 5.8 5.7 6.1 5.4 6.3 6.1 5.5 5.8

46 304 291 100 135 191 91 281 263 132 324 313 153 275 190 216 60 112 90 300 163 346 304 265 280 104 230 235 298 309 45 324 283 68 109 335 276 265 295

37 56 7 40 85 65 74 34 40 46 40 43 42 88 43 81 40 34 40 41 50 13 41 23 31 43 35 65 55 18 68 50 38 55 80 14 69 13 40

173 82 74 90 15 79 103 71 95 110 48 56 85 178 88 173 68 90 104 97 44 108 97 81 100 90 105 89 77 89 174 81 97 145 70 106 131 62 95

310 110 127 280 44 346 311 78 90 284 194 90 326 185 7 307 268 292 288 129 41 148 133 75 112 284 68 53 96 130 313 158 94 316 353 139 164 114 108

86 35 83 50 75 27 21 58 50 47 61 56 48 88 47 83 53 56 51 49 58 78 49 67 60 47 56 25 37 72 84 41 52 62 22 76 45 79 50

53 102 92 90 175 113 52 103 86 70 119 118 95 2 92 9 108 90 79 84 131 86 84 94 84 90 80 92 108 90 22 101 84 41 152 86 30 96 86

253 241 215 190 269 122 342 315 31 28 263 305 190 140 242 81 346 202 248 83 104 241 87 338 38 194 10 147 249 220 267 60 188 279 215 232 142 199 201

38 77 38 85 7 68 59 74 84 1 12 66 85 3 88 1 7 79 79 84 5 33 84 68 74 88 76 70 76 27 20 5 7 48 32 31 49 34 5

11 28 39 10 0 273 192 177 177 120 151 199 59 230 98 172 231 22 10 215 7 53 219 168 198 14 151 324 19 40 1 183 336 13 357 44 35 32 347

31 11 52 5 14 19 28 12 5 76 62 7 3 0 2 11 74 11 6 4 56 57 4 22 15 2 11 20 9 63 11 82 82 4 51 59 14 56 84

171.80 9.60 1.90 41.00 14.70 1.80 8.53 16.70 24.60 2.20 2.30 24.30 9.90 750.00 22.00 224.00 5.00 9.80 17.00 129.00 11.00 7.30 505.00 9.10 2.70 19.39 6.00 2.50 9.50 26.20 21.60 6.10 4.20 20.00 1.82 35.70 14.80 2.00 5.30

Papadimitriou, 1993 Taymaz et al., 1991 Baker et al., 1997 Taymaz et al., 1991 Taymaz et al., 1991 Lyon-Caen et al., 1988 Yilmazturk and Burton, 1999 Baker et al., 1997 Baker et al., 1997 Taymaz et al., 1990 Baker et al., 1997 Taymaz et al., 1991 Baker et al., 1997 Taymaz et al., 1991 Baker et al., 1997 Kiratzi et al., 1991 Taymaz et al., 1991 Eyidogan and Jackson, 1985 Eyidogan and Jackson, 1985 Braunmiller and Nabelek, 1996 Taymaz et al., 1990 Baker et al., 1997 Braunmiller and Nabelek, 1996 Liotier, 1989 Eyidogan and Jackson, 1985 Eyidogan and Jackson, 1985 Taymaz and Price, 1992 Taymaz and Price, 1992 Eyidogan and Jackson, 1985 Kiratzi and Langston, 1989 Papadimitriou, 1993 Baker et al., 1997 Papadimitriou, 1993 Taymaz et al., 1991 Yilmazturk and Burton, 1999 Papadimitriou, 1993 Yilmazturk and Burton, 1999 Taymaz et al., 1990 Papadimitriou, 1993

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591115 630918 631216 641006 650309 650427 650613 650706 660205 660509 661029 670304 670501 670722 671130 680219 690303 690323 690325 690328 690612 690708 700328 700408 700416 700419 710512 710512 710525 720504 720917 731104 731129 750327 750430 760511 760819 770818 770911

256

Table 2 (continued) Date Origin (YYMMDD) time (h:m:s)

Latitude Longitude Depth Mw Nodal plane 1 N E (km)

Nodal plane 2

P axis

T axis

Mo References *10e17 Ntm

Strike Dip Rake Strike Dip Rake Azimuth Plunge Azimuth Plunge 23:34:12 20:03:24 02:10:21 06:19:41 14:43:08 06:59:23 17:23:20 11:44:46 11:34:17 11:41:54 20:02:57 02:11:57 19:39:03 20:53:37 02:35:54 21:58:07 15:16:18 17:20:23 14:10:51 17:39:15 19:27:25 22:22:20 23:41:21 12:41:31 00:02:14 15:27:01 23:51:05 04:17:32 23:13:48 12:01:27 15:43:52 12:45:00 10:43:46 23:35:59 18:14:13 16:39:48 21:57:15 17:24:35 23:34:54 01:02:47

40.70 40.73 41.90 41.93 42.22 34.60 42.15 38.80 34.90 35.50 43.29 39.28 39.28 38.16 38.10 38.18 39.31 37.81 39.20 38.90 39.80 33.70 40.90 38.10 38.20 38.18 38.29 38.09 38.44 40.32 40.05 42.93 35.40 40.67 39.30 34.50 41.70 37.01 38.42 41.16

23.30 23.25 19.03 18.99 18.69 24.50 18.71 26.60 24.20 26.40 20.84 23.11 23.01 22.88 23.05 23.23 20.74 20.06 25.20 24.90 24.40 22.90 19.60 20.20 20.30 20.39 20.26 20.29 20.33 27.22 24.70 17.73 23.30 19.22 22.80 26.60 19.30 22.18 20.36 19.68

8.0 7.0 9.0 12.0 8.0 35.0 6.0 8.0 40.0 11.0 10.0 10.0 15.0 8.6 6.7 7.0 7.0 15.0 6.0 6.0 7.0 39.0 17.0 11.0 9.1 12.0 7.0 18.0 13.0 10.0 9.0 16.0 39.0 17.0 11.0 38.0 8.0 8.0 13.0 30.0

5.8 6.3 5.4 6.9 5.7 5.7 5.9 5.8 5.6 5.6 5.8 6.6 5.6 6.6 6.3 6.2 5.4 5.6 6.8 6.3 6.5 6.3 5.4 6.8 5.7 5.5 6.1 5.4 5.5 5.4 6.7 5.4 6.0 5.2 5.6 5.8 5.6 5.8 5.7 5.7

265 257 306 316 339 253 322 262 150 61 21 58 79 268 246 230 350 15 47 216 233 230 323 39 41 41 31 62 36 245 50 350 110 282 77 125 345 196 26 323

40 41 28 14 10 17 35 41 75 35 79 41 31 39 44 45 25 76 77 79 62 45 27 45 49 82 69 70 86 68 76 30 72 17 50 77 15 51 61 25

82 96 30 90 113 65 90 108 70 40 156 128 123 76 84 90 106 180 167 175 173 109 92 175 171 177 174 172 167 155 177 148 83 20 105 9 119 90 168 69

75 85 189 136 136 99 142 105 25 186 116 284 296 70 58 50 152 105 314 307 140 24 141 133 137 311 123 155 127 345 141 108 312 173 280 33 135 16 122 166

50 49 76 76 81 75 55 51 25 68 66 59 64 52 46 45 66 89 77 85 84 48 63 86 83 87 84 82 77 67 87 75 19 84 42 81 77 39 80 67

96 85 115 90 86 97 90 75 142 118 12 62 72 101 96 90 82 14 13 11 28 72 89 45 41 8 21 20 4 24 14 64 111 106 73 167 83 90 30 100

305 36 260 226 229 183 232 71 256 58 70 244 238 294 256 0 248 239 271 81 93 127 232 257 262 266 255 287 82 295 275 218 206 249 287 79 231 106 251 249

83 84 27 31 36 29 10 77 27 57 9 64 66 79 86 90 21 9 19 4 24 2 18 27 22 8 10 8 6 1 8 25 27 37 78 3 32 84 13 21

170 171 128 46 41 19 52 184 34 297 336 354 13 168 152 140 48 331 181 172 190 223 49 6 8 356 349 20 351 205 6 347 9 100 178 349 35 286 348 93

5 4 52 59 54 60 80 5 55 18 25 10 17 7 1 0 68 11 0 11 15 77 72 33 33 4 19 20 12 33 12 53 62 48 4 16 57 6 28 67

6.20 35.00 1.73 248.30 4.10 3.90 10.20 6.70 3.50 3.20 6.47 86.70 3.08 105.00 35.00 27.00 1.58 3.02 224.00 38.20 73.20 30.00 1.70 208.00 3.73 2.22 19.20 1.53 1.97 1.34 121.00 1.45 12.00 0.75 3.00 5.50 3.22 6.50 4.11 4.99

Baker et al., 1997 Braunmiller and Nabelek, 1996 Louvari et al., 2001 Baker et al., 1997 Baker et al., 1997 Taymaz et al., 1990 Baker et al., 1997 Taymaz et al., 1991 Taymaz et al., 1990 CMT Harvard solution Louvari et al., 2001 CMT Harvard solution CMT Harvard solution Braunmiller and Nabelek, 1996 Braunmiller and Nabelek, 1996 Taymaz et al., 1991 Louvari et al., 2001 Louvari et al., 1999 Kiratzi et al., 1991 Taymaz et al., 1991 Taymaz et al., 1991 Taymaz et al., 1990 Baker et al., 1997 Papadimitriou, 1993 Louvari et al., 1999 Louvari et al., 1999 Papadimitriou, 1993 Louvari et al., 1999 Louvari et al., 1999 Papadimitriou, 1988 Kiratzi et al., 1991 Louvari et al., 2001 Taymaz et al., 1990 Louvari et al., 2001 Taymaz et al., 1991 Taymaz et al., 1990 Louvari et al., 2001 Baker et al., 1997 Louvari et al., 1999 Louvari et al., 2001

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780523 780620 790409 790415 790415 790515 790524 790614 790615 790723 800518 800709 800710 810224 810225 810304 810310 810628 811219 811227 820118 820817 821116 830117 830119 830131 830323 830324 830514 830705 830806 840513 840621 850116 850430 850927 851121 860913 870227 880109

Table 2 (continued) Latitude Longitude Depth Mw Nodal plane 1 N E (km)

Nodal plane 2

P axis

T axis

Mo References *10e17 Ntm

Strike Dip Rake Strike Dip Rake Azimuth Plunge Azimuth Plunge 880518 881016 890824 900616 901221 920123 920320 921118 930326 930613 940225 950615 951001 960201 960726 960905 960917 970516 990817 990817 990831 990907 990913 991111 991112

05:17:42 12:34:06 02:13:14 02:16:20 06:57:43 04:24:19 05:37:26 21:10:43 11:58:18 23:26:41 02:30:50 00:15:49 15:57:16 17:57:59 18:55:50 20:44:14 13:45:25 07:00:52 00:01:38 03:14:01 08:10:51 11:56:51 11:55:29 14:41:24 16:57:21

38.36 37.89 37.94 39.16 40.91 38.40 36.65 38.34 37.66 39.28 38.76 38.36 38.06 37.77 40.08 42.48 42.90 40.93 40.76 40.60 40.75 38.08 40.80 40.81 40.79

20.42 20.89 20.14 20.54 22.36 20.57 24.51 22.44 21.30 20.49 20.56 22.23 30.13 20.05 20.64 17.95 17.65 20.47 29.97 30.63 29.92 23.58 30.03 30.20 31.21

23.0 29.0 16.0 7.0 16.0 9.0 15.0 7.4 15.0 9.0 9.0 7.2 4.0 20.0 7.0 12.0 7.0 7.0 10.0 1.0 15.0 10.0 16.0 13.0 12.0

5.3 5.9 5.2 5.5 6.1 5.6 5.2 5.7 5.4 5.3 5.5 6.3 6.3 5.5 5.1 5.7 5.4 5.3 7.4 5.3 5.1 5.9 5.7 5.6 7.1

45 32 36 352 54 345 293 270 122 325 22 277 136 173 34 302 285 352 267 192 82 115 268 297 262

70 87 46 33 47 19 45 30 60 30 58 33 43 55 36 40 14 47 85 34 78 57 49 55 53

163 166 142 105 103 68 90 81 5 106 168 76 87 71 93 71 79 135 175 82 141 80 180 179 177

141 301 154 154 253 188 113 80 29 127 118 80 312 24 218 146 116 228 177 2 343 277 359 206 170

74 76 64 58 45 72 45 60 86 61 80 58 47 39 54 53 76 59 85 56 52 34 89 89 88

21 3 51 80 76 97 90 95 150 81 32 99 93 115 88 105 93 53 5 96 15 105 41 35 37

272 257 271 251 249 272 117 337 79 224 246 324 179 276 138 225 204 192 132 253 310 54 126 156 119

3 12 10 13 80 27 90 74 18 16 15 75 87 8 81 7 31 58 7 78 35 76 27 25 27

4 166 15 37 153 109 203 174 341 16 345 177 44 34 306 107 30 292 222 96 207 198 232 257 222

26 8 53 75 1 62 0 15 24 72 30 13 2 72 9 76 59 7 0 11 17 12 28 23 24

1.12 7.47 0.75 2.49 17.00 2.90 0.68 4.10 1.61 1.11 2.15 33.80 31.00 2.56 0.61 4.38 1.65 1.15 1311.00 1.11 0.47 9.22 4.90 2.64 470.80

Louvari et al., 1999 CMT Harvard solution Louvari et al., 1999 Louvari et al., 2001 Baker et al., 1997 Louvari et al., 1999 CMT Harvard solution Hatzfeld et al., 1996 CMT Harvard solution Louvari et al., 2001 Louvari et al., 1999 Bernard et al., 1997 Wright et al., 1999 Louvari et al., 1999 Louvari et al., 2001 Louvari et al., 2001 Louvari et al., 2001 Louvari et al., 2001 Kiratzi and Louvari, 2001 Aktar and Orgulu, 2000 Kiratzi and Louvari, 2001 Louvari and Kiratzi, 2001 Kiratzi and Louvari, 2001 Kiratzi and Louvari, 2001 Kiratzi and Louvari, 2001

A. Kiratzi, E. Louvari / Journal of Geodynamics 36 (2003) 251–274

Date Origin (YYMMDD) time (h:m:s)

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Table 3 Earthquake focal mechanisms determined by first motion polarities Date Origin Latitude Longitude Depth Mw Nodal plane 1 Nodal plane 2 P axis T axis References (YYMMDD) time (h:m:s) N E (Km) Strike Dip Rake Strike Dip Rake Azimuth Plunge Azimuth Plunge 19:06:16 9:23:00 13:02:36 7:07:00 20:31:00 3:11:00 19:10:13 2:25:42 6:33:00 0:26:00 16:00:00 3:12:00 0:08:00 1:31:00 7:39:00 17:40:00 21:47:00 7:52:00 23:12:00 22:12:00 3:49:00 23:21:00 20:20:32 1:02:00 23:11:00 9:01:00 2:01:00 23:18:45 10:10:26 14:05:00 5:49:00 5:16:00 9:45:45 0:59:00 10:31:00 2:10:18 2:35:52 1:18:14 9:27:02 6:17:08 10:06:09

40.02 38.30 39.30 37.66 39.86 36.70 36.40 36.50 40.66 37.00 40.30 37.70 40.20 40.70 37.80 35.40 37.70 36.60 36.10 40.50 38.50 35.20 39.10 39.80 39.10 39.08 41.10 34.90 37.60 39.30 35.80 38.10 38.49 37.50 40.71 39.25 39.19 38.69 36.25 39.30 39.05

27.40 20.80 22.20 27.19 30.49 25.80 28.60 28.60 30.89 28.50 24.80 22.00 24.80 30.40 20.90 27.90 23.20 26.90 29.20 19.90 26.40 27.70 28.50 20.60 29.60 28.60 19.80 26.30 29.70 29.46 21.90 22.10 21.70 20.60 23.26 23.01 22.73 22.38 28.28 23.77 25.22

10.0 10.0 10.0 10.0 10.0 15.0 1.0 1.0 10.0 1.0 33.0 28.0 33.0 16.0 6.0 27.0 15.0 7.0 10.0 18.0 16.0 8.0 8.0 27.0 10.0 30.0 19.0 2.0 2.0 18.0 40.0 15.0 12.0 8.0 12.0 1.0 18.0 8.3 29.0 10.0 10.0

7.1 6.9 6.9 6.8 6.2 7.2 6.8 7.2 7.2 6.2 5.5 6.1 5.6 5.6 5.9 6.0 5.5 6.0 6.3 5.8 5.9 5.5 5.4 5.8 5.5 5.6 5.4 5.4 5.1 5.6 5.6 5.5 5.7 5.8 5.3 5.7 6.1 5.1 5.1 5.0 5.0

59 163 158 55 140 65 83 58 87 65 310 226 132 301 120 314 235 57 100 164 116 104 78 92 277 77 332 144 214 101 136 68 236 115 283 80 84 242 38 39 245

76 34 62 51 56 40 63 85 78 76 89 58 32 50 71 25 40 46 74 40 60 80 39 83 60 50 19 70 89 35 60 75 39 70 56 43 40 52 55 88 78

174 101 97 133 51 90 28 19 179 70 1 161 90 110 64 119 125 72 82 97 90 85 114 61 78 96 90 86 90 87 94 127 125 90 60 78 90 100 170 170 170

150 330 353 291 265 245 339 326 177 189 220 126 312 151 356 103 97 212 307 335 296 311 288 350 74 266 152 336 28 277 308 319 98 295 57 244 264 78 134 309 153

84 57 29 55 50 50 65 71 89 24 89 74 58 44 32 68 58 47 18 50 30 11 55 30 32 40 71 20 1 55 30 40 59 20 44 48 50 39 82 80 80

14 83 77 50 133 90 150 175 12 144 179 33 90 67 142 77 65 108 116 84 90 117 72 166 110 83 90 101 96 92 83 24 65 90 127 101 90 77 35 2 12

284 65 51 260 108 155 31 191 311 359 265 82 222 146 229 203 56 47 196 69 26 198 247 206 216 310 242 237 124 178 223 299 55 205 248 89 174 109 261 264 109

6 12 72 58 58 85 1 10 8 55 0 35 77 74 22 22 66 77 28 5 75 35 73 32 72 83 26 25 46 80 15 47 66 25 65 82 85 80 18 8 16

15 217 253 354 204 335 300 284 43 139 175 179 42 45 356 352 169 314 358 208 206 8 5 333 358 171 62 47 304 9 57 185 170 25 352 342 354 339 2 174 199

14 77 17 2 3 5 38 17 9 28 1 10 13 3 56 65 10 1 60 83 15 55 8 44 14 5 64 65 44 10 75 21 11 65 7 3 5 7 30 6 1

McKenzie, 1972 Anderson and Jackson, 1987 McKenzie, 1972 McKenzie, 1972 McKenzie, 1972 Shirokova, 1972 McKenzie, 1972 McKenzie, 1972 McKenzie, 1972 McKenzie, 1972 McKenzie, 1972 McKenzie, 1972 McKenzie, 1972 McKenzie, 1972 Anderson and Jackson, 1987 McKenzie, 1972 Ritsema, 1974 McKenzie, 1972 McKenzie, 1972 Anderson and Jackson, 1987 McKenzie, 1972 McKenzie, 1972 McKenzie, 1972 Anderson and Jackson, 1987 McKenzie, 1978 McKenzie, 1978 Anderson and Jackson, 1987 McKenzie, 1978 McKenzie, 1978 McKenzie, 1978 McKenzie, 1978 Papazachos, 1975 Papazachos, 1975 Anderson and Jackson, 1987 Soufleris and Stewart, 1981 Papazachos et al., 1983 Papazachos et al., 1983 Burton et al., 1995 Yilmazturk and Burton, 1999 Panagiotou, 2000 Panagiotou, 2000

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530318 530812 540430 550716 560220 560709 570424 570425 570526 590425 640411 650405 651220 670730 680328 680530 680704 681205 690114 690403 690406 690416 690430 691013 700328 700423 700819 710103 710512 720314 730105 750404 751231 760612 780619 800709 800709 830919 871005 970321 970716

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determined by visually comparing the FFT amplitude spectrum of the signal to that of the noise. For most cases the valuable frequency content of the waveforms was approximately lying in the frequency band f1=0.01 Hz and f2=0.2 Hz for the long-period waveforms and f1=0.1 Hz and f2=2 Hz for the broad-band. All waveforms were converted into displacement records before performing the inversion. The data consisted of digital P- and SH-waveforms from stations in the epicentral distance 30–90 to avoid the upper mantle complexities. P and SH waveforms were generated by combining direct (P or S) and reflected (pP and sP, or sS) phases from a point source buried in a half space. Amplitudes were adjusted for geometrical spreading, a simple function of epicentral distance (Langston and Helmberger, 1975), and for attenuation using Futterman’s (1962) operator with t*=1s for P and t*=4s for SH. The source time function was described by the amplitudes of a series of overlapping symmetrical triangularly shaped pulses, the number, and duration of which we selected. In all cases we used the polarities at close stations (
<30 ) to further constrain the orientation of the nodal planes and check the solution. For all the events we assumed as initial source structure a half-space with Vp=6.0 Km s 1, Vs=3.7 Km s 1 and density =2.75 g cm 3 (Panagiotopoulos and Papazachos, 1985; Papazachos and Nolet, 1997). This half-space structure was adopted for modelling the events with centroid depth 47 Km. The events whose centroid depth was in the range of 7–25 Km were modelled using a one-layer model over a half-space with: 1st layer with thickness 7 Km, Vp=6.0 Km s 1, Vs=3.7 Km s 1 and density r=2.75 g cm 3, and for the half-space Vp=6.8 Km s 1, Vs=3.9 Km s 1 and density 2.90 g cm 3 (Panagiotopoulos and Papazachos, 1985; Papazachos and Nolet, 1997). The events whose centroid depth was h> 25 Km were modelled using one-layer over a half -space, as well, with: 1st layer of thickness 25 Km, Vp=6.4 Km s 1, Vs=3.7 Km s 1 and density r=2.85 g cm 3, and for the half-space Vp=7.9 Km s 1, Vs=4.5 Km s 1 and density 3.30 g cm 3 (Panagiotopoulos and Papazachos, 1985; Papazachos and Nolet, 1997). A shallow water layer, above the upper layer, was included in the cases of submarine sources. Receiver structure was assumed to be a homogenous half-space. To test the robustness of the minimum misfit solution, since the covariance matrix associated with this solution usually underestimates the true parameter uncertainties we used the procedure of McCaffrey and Na´be˘lek (1987) and Molnar and Lyon-Caen (1989), to find more realistic uncertainties. Tests of the robustness of the minimum misfit solution were performed primarily for the centroid depth, but in each case, other previously published or Harvard CMT best doublecouple solutions were also examined.

3. Events modelled In the following section we discuss the modelling procedure and illustrate the way in which source-parameter uncertainties were assessed for a few earthquakes along the Hellenic arc whose solutions are of special interest. The dates are reported in YR/M/D convention. 3.1. Events in the western part of the Hellenic arc Fig. 2 shows the focal mechanisms of the earthquakes of western Peloponnese and the Ionian Sea up to the western part of the island of Crete, four of which are discussed in the following,

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starting from north towards south. Note that in this figure besides the focal mechanisms determined by us, which are those with their dates framed with rectangles, other previously published solutions are also included. 3.1.1. Event 930714 M 5.4, Patras This event occurred very close to the city of Patras in northern Peloponnese, and even though of moderate magnitude it caused considerable damage to many constructions. The earthquake was well recorded at teleseismic distances with a good signal to noise ratio at all azimuths. The inversion of 19 P-wave and 23 SH-wave waveforms yielded a minimum misfit solution shown in Fig. 3. It is a strike-slip solution and the NE–SW trending plane is also well constrained by the

Fig. 2. Focal mechanisms along the Western part of the Hellenic trench determined by us (dates of occurrence with frames) or previously published. Note the strike-slip events connected with the Cephalonia–ada fault and the distributed strike-slip faulting along the western coast of Peloponnese.

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use of first motion polarities (insets at Fig. 3). The source depth of 19 Km, places the source at the lower parts of the crust. The Harvard CMT solution (strike 238 , dip 73 , rake 163 , h=20 Km) is very similar to ours, while the first motion polarities solution (strike 331 , dip 73 , rake 35 ) of Karakostas et al. (1993) produces synthetics with mismatch with the observed. 3.1.2. Event 890820 M 5.7, Gulf of Kyparissia This event occurred offshore western Peloponnese. 15 P and 14 SH waveforms were used in the inversion and the minimum misfit solution shown in Fig. 4 indicates a strike-slip fault, at a depth

Fig. 3. In this figure and subsequent four figures the teleseismic waveform modelling is given. Comparison between the observed (solid lines) and the synthetics (dashed lines) P and SH waveforms for the Patras earthquake of 14 July 1993. The event header gives strike, dip, rake, depth (km) and Mo in Nt-m. Solid and open triangles mark P and T axes, respectively. Stations are ordered clockwise by azimuth and the station code is accompanied by a letter corresponding to its position within the focal sphere. The waveform amplitudes are normalized at a distance of 40 and a gain of 6000. Solid bars at either end of the waveform mark the inversion window. The source time function is shown below the Pwave focal sphere, with the waveform time scale below this. Waveform amplitude scales (in microns) are to the left of the focal sphere. The insets show the first motion polarities at stations with epicentral distances smaller than 30 used to further constrain the mechanism.

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of 22 Km. The nearly N–S nodal plane is very well constrained both by the polarities at teleseismic stations (KEV, BDF, HRV, SCP, GAC and GDH) and by the first motion polarities at the stations closer than 30 , as shown in the insets of Fig. 4. The focal mechanism reported by Harvard for this event (strike 237 , dip 37 , rake 130 , depth 15 Km), is considerably different from ours as it corresponds to a normal fault with a very small strike-slip component, which

Fig. 4. Minimum misfit solution for the event of 20 August 1989. Notation as in Fig. 3. At the bottom synthetics are produced for the CMT Harvard solution.

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produces synthetics with considerable mismatch with the observed as is shown at the bottom of Fig. 4. Fixing the depth at 10 Km also produces synthetics with considerable mismatch with the observed both in the shape and the amplitude. We draw the attention here, to the focal mechanisms of two other moderate-size events for which we inverted the waveforms, the 850907 and the 870610 events (see fig. 2 for their location in the Gulf of Kyparisia). They both indicate strike-slip mechanisms with their foci located in the lower crust, as for the previous events (depths 29 and 27 km, respectively). 3.1.3. Event 971118 M 6.5, Strophades Island This strong (M 6.5) earthquake occurred at sea, SW of Zante Island and NW of the small Strophades Island in the Ionian Sea. Events of similar magnitude occurred in this area on 27 August 1958 (M 6.4) and on 10 April 1962 (M 6.3). No fault plane solutions are available for these events, but the major axes of their isoseismals have a NNW–ESE trend (Papazachos et al., 1997). The waveforms for the 971118 event are rather complicated and clearly indicate a second pulse at 9 s after the direct P arrival, observed at all azimuths (Fig. 5), suggesting source complexity or a deeper centroid depth, or both. We used an adequate number of long-period seismograms to constrain the focal mechanism parameters for this event. We used one source at first and we obtained quite good fit to the P-wave pulses but not as well for the SH-waves. The addition of a second source yielded a much better fit, especially to the SH-waves (Fig. 5). The second source had a time delay of 9 s and it was placed 30 Km ESE (N111 ) in respect to the first source. The second source had a larger scalar moment and a stronger strike-slip component (Fig. 5). The Harvard CMT solution for this event (strike 8 , dip 31 , rake 162 , depth 22.9 km) is not very different from our single source solution. 3.1.4. Event 971013 M 6.3, Gulf of Messinia This strong event (M 6.3), which was well recorded in all azimuths, had epicentre offshore the Gulf of Messinia and was widely felt in southern Peloponnese. The inversion of 25 P-wave and 25 SH-wave waveforms yielded a reverse mechanism with the source at 32 km (Fig. 6). The CMT Harvard solution (strike 119 , dip 70 , rake 90 , h=44.2 km) is similar to ours. The focal mechanism reported by GeoForschungsZentrum (GFZ) in Potsdam is not far from the solutions shown and their depth estimate is 31 km (based on 11 depth phases). 3.2. Events along the eastern part of the Hellenic arc 3.2.1. Event 960720 M 6.1, Rodos Island The epicentre of this event is located between the islands of Karpathos and Rodos (see Fig. 8 for location). Twenty-four P-wave and 24 SH-wave waveforms were used in the inversion and the focal mechanism was further constrained by the first motion polarities at 16 stations from distances less than 30 . We obtained a better fit to the data by employing two sources in the inversion shown in Fig. 7. Both sources imply normal faulting with considerable strike slip component. Both sources are shallow at a depth of 12 and 13 km, respectively (obtained through the inversion of broad band records). It is worth noting that the occurrence of this event supports previous observations of a zone of E–W extension oblique to the strike of the Hellenic trench which in this area is 50 . An aftershock of the 960720 event which occurred

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on 22 July 1996 (M 5.0) has a similar CMT Harvard focal mechanism. Additionally, two more moderate size events that occurred near the east coast of Crete at 900709 and 920430 (Fig. 8) are worth noting because they indicate normal faulting with the T-axes trending in the same E–W direction. 3.3. Focal mechanisms and slip vectors In Fig. 8 we present the focal mechanisms of the updated database (events listed in Tables 1–3). Different colours are used to separate the main types of faulting (green for normal faulting, black for strike-slip and red for thrust and reverse faulting).

Fig. 5. Minimum misfit solution for the event of 18 November 1997. Notation as in Fig. 3. Two sources were used to model this event.

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The collision zone of coastal Albania – NW coastal Greece up to the island of Lefkada has been well defined in the past (i.e. Papazachos et al., 1991; Louvari et al., 2001; and references therein). The strike-slip faulting along the Cephalonia—Lefkada fault zones has been already discussed in previous work (Scordilis et al., 1985; Louvari et al., 1999). The reverse–low angle thrust faulting connected to the Hellenic trench is recognized to start 50 km S of Zante and follows the bathymetric expression of the Hellenic Trench (from Peloponnese to Crete, along off-coast of Crete, east of Karpathos and Rodos up to southern Turkey). Most of the focal mechanisms that we determined (shown in Fig. 2) indicate that western Peloponnese (from Patras up to the Gulf of Kyparissia) is deforming by strike-slip faulting an

Fig. 6. Minimum misfit solution for the event of 13 October 1997. Notation as in Fig. 3.

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Fig. 7. Minimum misfit solution for the event of 20 July 1996 near the island of Rodos. Notation as in Fig. 3. Two sources were used to model this event. The insets show the first motion polarities at stations with epicentral distances smaller than 30 used to further constrain the mechanism.

observation previously made in Papazachos et al. (1998). In addition the entire region that extends from the Gulf of Kyparissia up to the island of Zante and the deformation of Zante itself seems to be connected with strike-slip motions. Of special interest is a zone of normal faulting, evidenced by only a few earthquakes. This zone indicates NNE–SSW extension, along faults that trend N–S or NW–SE. The evidence for this zone is the focal mechanisms of two events at North Aegean Sea (events 651220 and 920723), one at the northern side of the Corinth Gulf (971105), and the other at the Gulf of Patras (910626). These focal mechanisms combined with the mechanism of 670501 in central Greece, indicates extension connected to the N–S faults in a direction that follows the major tectonic structures of

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Fig. 8. Fault plane solutions of shallow earthquakes for the period 1953–1999 with M> 5.5 discussed in the text and presented in Tables 1–3. A lower hemisphere equal area projection is used with the solid quadrants denoting compression and the white ones denoting dilatation.

Greece. It is also interesting to mention that this type of faulting (the mechanisms of the events 630918 and 670730 in North Anatolian Fault have the same T-axis orientation) mark the northern border of the Anatolian – Aegean plates as proposed by McClusky et al. (2000). In addition, the events south of the three-leg Chalkidiki peninsula, exhibiting normal faulting with NE–SW trending T axes probably account for the particular shape of this peninsula. Southern Peloponnese has more or less the same shape and indeed at its southern part the focal mechanism of the Kalamata 1986 event (860913) indicates normal faulting with E–W trending T axis. Not to mention that similar mechanisms are observed at the western and eastern part of the island of Crete, defining the shape of the peninsulas. The few available data indicate that the events that

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sparsely occur at the low-seismicity area of the Sea of Crete have normal faulting mechanisms on N–S trending faults. The recent event of 9 June 2001 M 4.2 which occurred east of the island of Kythira has such a mechanism (MEDNET solution). The North Anatolian Fault strands affect a broad area of the Aegean Sea from Evia Island up to Chios Island. The new fault plane solutions clearly indicate that this part of the Aegean Sea is deforming by strike-slip faulting. In addition the results of Panagiotou (2000) and Kiratzi (2002) have shown strike-slip motions crossing the central and southern part of the Evia Island reaching the tip of southern Evoikos Gulf. This was the first time that such evidence was found and the information comes from moderate size earthquakes.

Fig. 9. Comparison of earthquake slip vectors and of velocity vectors obtained from GPS and SLR measurements in respect to Eurasia (data from Oral, 1994; Reilinger et al., 1997; Clarke et al., 1998; Cocard et al., 1999; McClusky et al., 2000). Earthquake slip vectors represent movement of the hanging wall relative to the footwall (from Louvari, 2000).

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Fig. 10. Map showing the orientation of P and T axes in the Aegean Sea and the surrounding lands. Focal mechanisms are also plotted with beach-ball sizes that scale with earthquake magnitude. Note the continuation of the NAF zone through the normal faulting system of central Greece and its connection with the transform zone of Ionian Islands and western Peloponnese. The question marks denote areas, which lack reliable focal mechanisms and where we must focus our studies in the future.

Fig. 9 compares the horizontal projection of earthquake slip vectors from our database and GPS and SLR velocity vectors, in a Eurasia-fixed reference frame. Earthquake slip vectors are plotted only for those events for which we could have a clue of the fault plane. Thus, only the slip vectors of the low-angle thrust faulting events along the Hellenic trench (where the fault plane was assumed as the low-angle plane) and those of the strike-slip faulting in the North Aegean Sea and the Kefallonia fault, both dextral in the sense of motion, are included in the fig. It is seen that the overall velocity field as provided by the GPS measurements has a general motion to the SW and this is reflected in the orientation of the earthquake slip vectors. Fig. 9 thus indicates that the upper crust, at least up to the depth of the seismogenic layer ( 15 Km) deforms in the same pattern. The GPS and SLR velocity vectors indicate that the area north of the North Anatolian Fault and northern Greece move slowly and probably belong to stable Eurasia. On the contrary

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there is a clear increase of the Anatolian plate velocity towards the west, and of course we can also visualize the rapid motion of the Aegean plate towards the Hellenic trench, with velocities that reach a value of 30 to 40 mm/year (McClusky et al., 2000).

4. Conclusions and discussion Fig. 10 is a sketch-map that summarises the main results of this work and leaves a lot of questions, concerning the present day deformation of Greece, open to discussion. The major zones of deformation were drawn with focal mechanisms is the main source of information. Thus, the new updated database and the distribution of mechanisms indicate: (1) Low-angle thrust and reverse faulting along coastal Albania, western Greece, the convex side of the Hellenic arc (SW of Zante up to NE of Rodos island). This type of faulting is attributed to the collision between the Eurasian and the Adriatic plates and to the northward motion of Africa and the subduction along the Hellenic trench. (2) Normal faulting in the back arc Aegean area that covers the Aegean Sea and parts of the adjacent lands (eastern mainland and northern Greece, western Turkey, southern Bulgaria, southern former Yugoslavia). (3) Strike-slip faulting along a belt that marks the boundary of the Aegean plate with the Eurasian plate. This belt starts from the North Anatolian Fault (NAF) in the east, crosses the northern Aegean Sea where it is distributed along a number of sub-branches that extend up to the southern coasts of Evia island, stops abruptly against central Greece before becoming evident again along the Cephalonia–Lefkada transform fault zone and Peloponnese, in the west. The new focal mechanisms indicate extensive strike-slip motions between the Ionian islands and western Peloponnese. The dextral sense of motion of the Cephalonia – Lefkada transform fault is well known. However, the sense of motion of the strike-slip faults of the western Peloponnese could not be evidenced from the aftershock distribution of the major events (i.e. Vartholomio in 1988, Patras in 1993, Pirgos in 1993). We favour a dextral sense of strike slip motion parallel to the strike of the Cephalonia strike-slip fault, that would result in clockwise rotations of the of the NW margin of the Hellenic trench. The new data support the model of McKenzie (1972) and LePichon et al. (1995), which proposes a continuation of the North Anatolian Fault zone in the southwestward direction towards the Gulf of Corinth. The presence of the sinistral strike-slip faults bounding northern Evia (which have not been evidenced so far with the focal mechanism of recent event, except the Skyros 2001 event further north) suggest that these faults play the role of transfer faults transferring the motion of the NAF into the Aegean mainland, which is facilitated by the rotation of small blocks in central Greece as suggested by McKenzie and Jackson (1986). A feature that deserves further attention in the future is the possible continuation of the thrust faulting from the north (Albania coast) south of the island of Lefkada through the island of Cephalonia and Zante. A number of mechanisms indicate NE–SW compression and if one takes into consideration the McKenzie (1972) and Anderson and Jackson (1987) pure thrust solution for the August 1953 event this suggestion if further supported. The area between Peloponnese and Crete is under WNW–ESE extension. This kind of extension is also extending up to southwestern Anatolia. Fig. 10 shows that the dextral shear motion that is transferred from the east through the subbranches of the NAF zone is connected with the dextral motion of Cephalonia and possibly of

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western Peloponnese through the normal faulting system in central Greece and the Gulf of Corinth. This normal faulting system has variable orientation, as deduced from the focal mechanisms and changes from pure E–W (central Greece-central Corinth Gulf) to NNE–SSW in the eastern part of the Gulf. Even though there is no focal mechanism available near the Gulf of Argolis in eastern Peloponnese we may speculate that the deformation in this area is NNE–SSW extension accounting for its shape as well. Another feature that has to be discussed in the future when more data are available is the zone of NNE–SSW extension that starts from the Athos peninsula in Chalkidiki and run in parallel to the Northern Aegean branch of the NAF probably up to the Corinth Gulf. The overall pattern observed in the back-arc Aegean region is the gradual change of NNE-SSW trending T-axes in western Anatolia to NNW–SSE trending T-axes in the normal faulting systems of Greece. This faulting ends against the E–W trending extension that runs parallel to the main mountain belt starting from Albania in the north, a zone that more or less forms the boundary between the extension east of it and the outer compression west of it. The deformation of the Aegean Sea and its surrounding lands has been always an attractive field and more work and data are required to resolve its characteristics, especially by focusing our studies in the areas designated in this paper.

Acknowledgements The comments and suggestions of the two reviewers are gratefully acknowledged. This work was financially supported by OASP (Greece) and General Secretariat of Research and Technology (GSRT—Ministry of Development).

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