Tectonophysics 319 (2000) 275 300 www.elsevier.com/locate/tecto The geometry of the Wadati Benioff zone and lithospheric kinematics in the Hellenic arc B.C. Papazachos a,*, V.G. Karakostas a, C.B. Papazachos b, E.M. Scordilis a a Geophysical Laboratory, University of Thessaloniki, P.O. Box 352-1, GR-54006 Thessaloniki, Greece b Institute of Engineering Seismology and Earthquake Engineering (ITSAK), P.O. Box 53, GR-55102 Finikas, Thessaloniki, Greece Abstract Accurate locations of 961 shallow and intermediate depth earthquakes, which occurred between 1956 and 1995 in the Hellenic arc (34 39 N, 19 28 E), are used to define the plate boundaries in this area. Reliable fault plane solutions for 77 shallow and intermediate depth earthquakes are also used in order to define the interaction between the different plates in the arc. An ocean continent type of interaction occurs on a curved surface, which is defined by the shallow branch (20 100 km) of the Wadati Benioff zone. The intersection of this zone with the earth s surface is a curve which follows the convex (outer) side of the sedimentary arc (western Peloponnese west of Cythera south coast of Crete east coast of Rhodes) and dips at low angle (~30 ) to the Aegean sea. Coupling between the subducted oceanic crust and the overriding of the Aegean lithospheric plate takes place on this surface. The deep branch (100 180 km) of the Wadati Benioff zone is dipping freely (without coupling) at a high angle (~45 ) beneath the south Aegean trough and the volcanic arc. The high shallow seismicity (h 20 km) which is observed in the southwestern convex (outer) side of the arc (Ionian sea) is attributed to the fast southwestward motion of the Aegean plate. Location of strong deep earthquakes (h>100 km) in the fore-arc area of the southwestern part of the Hellenic arc (west of Cythera) indicates that oceanic crust is destroyed in this part of the Hellenic trench due to roll-back of the descending lithospheric slab. 2000 Elsevier Science B.V. All rights reserved. Keywords: subduction; Wadati Benioff zone; dip-parallel tension 1. Introduction vergence and subduction, like the Hellenic arc, is the accurate determination of the geometry of the Knowledge of the spatial distribution of earththe Wadati Benioff zones, because such zones define quake foci in an area of plate interaction is of boundaries of the subducting slabs. importance because the distribution of earthquake The identification of the Wadati Benioff seismic foci defines the boundaries of the lithospheric zone in the Hellenic arc was first accomplished by plates that interact. If reliable fault plane solutions accurate location of intermediate depth earth- are also available for this area, the patterns of quakes (60 km h 180 km) using the difference lithospheric motion can also be determined. An in arrival time at the seismological station of important problem for areas of lithospheric con- Athens of the P and PcP waves (Papazachos and Comninakis, 1970, 1971). This was the first important step for understanding the plate motion in * Corresponding author. Fax: +30-31-476-085. this area because the existence of this seismic zone E-mail address: costas@itsak.gr (B.C. Papazachos) has been the strongest evidence for subduction of 0040-1951/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0040-1951 ( 99 ) 00299-1
276 B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 Fig. 1. Topographic features of tectonic origin in the Aegean area (Papazachos and Papazachou, 1997). international network of permanent seismological stations which concern locations of earthquakes in the Hellenic arc, as well as data of dense portable networks of stations which have been in operation for several months in this area, are used to define as accurately as possible the spatial distribution of earthquakes in the Hellenic arc and particularly in its fore-arc area. In addition, reliable fault plane solutions of shallow and intermediate depth earthquakes are used to further investigate the lith- ospheric kinematics in this area. Fig. 1 shows a map of the Aegean and surround- ing area. The dominant tectonic feature of the area is the Hellenic arc. The Hellenic trench, the the eastern Mediterranean lithosphere (front part of the African plate) under the Aegean lithosphere (front part of the Eurasian plate). Although several attempts have been made to further investigate the Wadati Benioff zone in southern Aegean by the use of additional data (Comninakis and Papazachos, 1980; Papazachos, 1990), no detailed knowledge exists on the spatial distribution of the shallow and intermediate depth earthquakes beneath the sedimentary part of the Hellenic arc and in its convex (outer) part. This is due to the fact that no seismological stations of the regional network exist in the fore-arc area. For this reason the most reliable data of the
B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 277 sedimentary part of the Hellenic arc ( Hellenides instruments during four local seismicity monitormountain chain) and the volcanic part of the arc ing experiments in the southern Aegean area are also shown in this map. between 1986 and 1995 ( Hatzfeld et al., 1989; Hatzfeld et al., personal communication; Scordilis et al., 1995) were processed here using the 2. Data used HYPO71 computer program (Lee and Lahr, 1975). The configuration of the network of the Three different data sets were used in the present temporary stations, parts of which were occupied work to accurately define the Wadati Benioff zone during the previous experiments, is shown in Fig. 2. of the Hellenic arc (Appendix A). The first one Although not all stations were occupied simulta- consists of earthquakes recorded at local seismodifferent experiments. As a result, the aperture of neously, several stations were common between logical networks which were installed and operated in the southern Aegean area in different time each occupation was large enough to determine periods between 1986 and 1995. The second set the location of shallow and deep events with high includes earthquakes with focal depths h>100 km accuracy. In any case, events with large azimuthal as located by the International Seismological gaps and large errors in hypocentral parameters Center. The third set of data includes strong were not used, as will be described later in detail. earthquakes (M 5.5) for which reliable fault A velocity model of five layers above a half plane solutions have also been determined space (Table 1) was used to locate these earth- (Table 2). quakes. This model was based on the tomographic Data collected with both analog and digital results of a model proposed by Papazachos and Fig. 2. Configuration of the temporary stations installed in the southern Aegean area during several experiments (1986, 1988, 1993 and 1995).
278 B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 Table 1 that the focal coordinates of the earthquakes Velocity model used for the event relocation in the south located by the local network data are of high Aegean area accuracy and, therefore, can be used for the P velocity (km/s) Layer thickness (km) purposes of the present study. Despite the great number of earthquakes 6.0 19 recorded by the local networks, the number of 6.6 12 shocks with intermediate focal depths was quite 7.9 19 7.95 50 small. This is probably due to the limited time 8.0 20 span of recording, which is less than one year. 8.05 2 However, it has been observed (Comninakis and Papazachos, 1980) that the ISC data of the earthquakes of intermediate focal depth are accurate if Nolet (1997) for the Aegean area. A velocity ratio they have been estimated by the arrival of 26 or of V /V =1.78 was adopted from the same model more stations. In order to increase the number of p s for our calculations. earthquakes of intermediate focal depth, all these The location of each earthquake was derived shocks (73) which occurred in the studied area after several runs of the HYPO71 program, in between 1964 and 1995, with ISC focal depths order to reject the phases with the bigger errors. h>100 km, were included in the data set. The iterations in each run were not started from Due to the small number of available earththe coordinates of the closest station (as is standard quakes with h>100 km, only one earthquake was for HYPO71) but from the epicenter of the previous found to be located by both the local network location. Moreover, different initial depths data, as well as the ISC using the permanent were tried. The majority of time errors of the network data. This earthquake ( 1986 July 15, phases of the earthquakes located in this way are 15:15) is included twice in the catalog of Appendix less than 1.0 s, and only a small number of phases A, where the ISC location is denoted by an aster- have errors between 1.0 and 2.0 s. Since we did isk. The horizontal distance between the two locations not want our results to be contaminated by mislocations, is 12 km, while the depth gap is only 8 km. only data for events with at least 20 P and Although this is the only earthquake for which a S observations were used. comparison was possible, the small bias between The RMS error of the travel times obtained the determined hypocenters confirms that the previously from the location process was smaller than 1.2 s estimated location errors ( ERH and ERZ) in all cases, with a mean value of 0.43±0.17 s. have realistic values. Moreover, it suggests that no The corresponding epicentral error ( ERH ) varied systematic difference should be expected for deep between 0 and 10 km. Similar errors ( ERZ) were (h>100 km) events between the ISC and local determined for the hypocentral depth of located network determinations, although more data are events. In some cases both ERH and ERZ reached necessary in order to reach a final conclusion. values up to 20 km and in very few cases the Another data source is strong earthquakes, corresponding errors obtained even larger values. because their focal coordinates have been estimated In order to avoid the effect of mislocations, only on the basis of a great number of arrivals. events with both errors (horizontal and depth) less The accuracy of these data is also controlled by than 20 km were used. This resulted in a final data the available macroseismic observations and the set of 811 well-located earthquakes recorded by fault plane solutions. In the present work the data the local networks. The mean epicentral error for of 77 earthquakes with magnitude M 5.5, which this data set was ERH=2.5±2.5 km, while the occurred since 1956 in the studied area and for average depth error was estimated to be which reliable fault plane solutions have been ERZ=2.7±2.4 km. Although these quantities determined by NEIS, Harvard (CMT solutions) have a mathematical sense and do not necessarily or individual studies, were also selected. correspond to the true error estimates, they show Information on the fault plane solutions of these
B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 279 earthquakes (with their corresponding references) (denoted by solid triangles) can be recognized at is given in Table 2. distances between 350 and 400 km in the corre- sponding cross-section (lower part of Fig. 4). Comparison with the upper part of Fig. 4 shows 3. Spatial distribution of the earthquake foci that the epicenters of these earthquakes are located south of Peloponnese and west/southwest of Figs. 3, 4 and 5 show the epicenters of the Cythera up to the Hellenic trench, which at this shallow and intermediate depth earthquakes point reaches a maximum depth of ~5 km. The (upper parts) and the corresponding projections of presence of these deep events at this area will be their foci on vertical planes normal to the arc (low discussed later. parts) for the western, central and eastern section Distribution of earthquake foci in the eastern of the Hellenic arc, respectively. Circles show part of the arc is shown in Fig. 5. Shallow seismi- shallow earthquakes (h 50 km), while shaded and city is also limited to the upper 20 km of the crust. black triangles show intermediate depth earth- The shallow part of the Wadati Benioff zone quakes with focal depths between 50 and 100 km (20 km h<100 km), however, does not show a and between 100 and 180 km, respectively. clear dipping from the fore-arc to the back-arc Symbols of different sizes are used to denote area (from point B to C in Fig. 5) in this part of earthquakes of different magnitudes. The horithe arc there is a gap in the seismicity between the the arc. On the other hand, in this eastern part of zontal line at a depth of 20 km shows the lower boundary of the shallow seismogenic layer, while deeper and the shallower segment of the Wadati Benioff zone, that is, at a depth of 80 100 km, dashed lines show the dipping of the Wadati which is also partly observed in the central part Benioff zone. (Fig. 4) but is not expressed in the western part Fig. 3 indicates that in the western part of the (Fig. 3). It should be noted that this gap cannot arc the thickness of the shallow seismogenic layer be considered as an artifact of the location process covers the upper 20 km of the crust in the backsince the velocity model used ( Table 1) exhibits no arc area (Aegean sea) as well as in the fore-arc change in this depth range (strong velocity gradient part of the area ( Ionian sea). The Wadati Benioff or discontinuity) that would tend to force the zone starts to dip in western Peloponnese (point locations out of this depth range. Moreover, if B in Fig. 3) where the shallower seismogenic layer such a case occurred, it should be present in all becomes thicker. Up to a depth of about 100 km cross-sections (Figs. 3, 4 and 5) and not only in (point C in Fig. 3) this zone dips at a relatively the eastern part of the arc. low angle (~30 ) and after that the dip angle of Based on the accurate data used in the present the zone becomes larger (~45 ) up to its maximum study (Appendix A), isodepths of 20, 100 and focal depth (~180 km). In a large part of the fore- 170 km for the earthquakes in the Hellenic arc arc area (between western Peloponnese and Ionian have been drawn in Fig. 6. In the same figure the islands) no strong earthquakes with h>20 km (sec- epicenters of four complete samples of strong tion AB in Fig. 3) occur. intermediate depth earthquakes occurring during A similar distribution of the earthquake foci is the last two centuries (Papazachos and observed in the central part of the arc ( Fig. 4) Papazachou, 1997) are plotted. It is observed that where dipping of the Wadati Benioff zone starts the isodepths fit quite well with the distribution of ( point B in Fig. 4) in the convex side of the arc, the earthquakes, although these isodepths have under the Hellenic trench. An interesting difference been drawn by the use of a different sample of data. between the distribution of the earthquake foci in this part of the arc and the other parts is that some strong intermediate depth earthquakes with 4. Mechanisms of earthquakes in the Hellenic arc relatively large focal depths (h>100 km) occur in the convex (outer) side of the arc, beneath or very The first seismological evidence for the existence close to the Hellenic trench ( Fig. 1). These events of thrust faults in the Mediterranean lithosphere
280 B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 Table 2 Information on the fault plane solutions of strong earthquakes (M 5.5) which occurred in the studied area since 1956 Date Time Latitude Longitude h M Plane A Plane B P axis T axis Reference Azimuth Dip Rake Azimuth Dip Rake Azimuth Dip Azimuth Dip 560709 031140 36.7 25.8 22 7.5 65 40 90 245 50 90 155 85 335 5 a 591115 170840 37.8 20.5 10 6.8 46 37 173 310 86 53 253 38 10 31 b 620828 105956 37.8 22.9 95 6.8 241 51 58 107 49 124 355 3 90 65 c 631216 134753 37.0 21.0 15 5.9 296 16 101 105 74 87 197 29 11 61 b 640717 023427 38.0 23.6 155 6.0 267 36 30 153 74 122 218 24 111 49 c 650331 094731 38.6 22.4 78 6.8 286 17 60 136 73 102 216 29 67 57 d 650405 031255 37.7 22.0 28 6.1 226 57 159 126 74 35 82 35 178 10 c 650409 235702 35.1 24.3 67 6.1 23 56 158 127 74 42 251 17 352 43 d 650427 140906 35.6 23.5 5 5.7 22 27 81 191 65 101 83 71 285 21 e 650706 031842 38.4 22.4 28 6.3 281 34 71 79 58 102 316 74 177 12 f 651128 052605 36.1 27.4 73 6.0 350 30 142 89 84 55 208 30 329 42 d 660509 004253 34.4 26.4 10 5.8 295 40 90 115 50 90 205 5 25 85 c 661029 023925 38.9 21.1 6 6.0 324 40 48 194 61 120 263 11 152 62 f 680328 073959 37.8 20.9 6 5.9 354 34 137 122 67 63 231 20 355 59 g 680530 174026 35.4 27.9 7 5.9 293 25 90 110 76 90 202 20 18 70 c 680704 214751 37.7 23.2 15 5.5 235 40 125 97 58 65 56 66 169 10 h 681205 075211 36.6 26.9 7 6.0 86 50 90 266 40 90 354 85 177 5 c 690114 231206 36.1 29.2 7 6.2 282 25 95 75 87 2 190 18 5 70 c 690416 232106 35.2 27.7 8 5.5 301 30 109 100 60 80 197 16 347 71 c 690612 151331 34.4 25.0 19 6.1 294 29 105 95 61 80 192 17 340 72 c 690708 080913 37.5 20.3 10 5.9 354 18 115 147 74 81 243 30 46 61 b 700408 135028 38.3 22.6 10 6.2 278 20 85 90 70 94 357 75 186 23 i 720504 213957 35.1 23.6 40 6.5 308 18 90 129 72 90 219 27 39 63 j 720913 041320 38.0 22.4 91 6.3 235 76 48 131 47 161 357 21 115 42 k 720917 140715 38.3 20.3 8 6.3 46 66 174 313 84 49 258 37 7 25 b 730105 054918 35.8 21.9 42 5.6 306 30 82 136 60 93 218 15 46 74 i 731104 155213 38.9 20.5 8 5.8 320 45 80 154 46 100 237 1 143 83 f 731129 105744 35.2 23.8 1 6.0 316 10 90 137 80 90 226 35 44 55 i 750404 051618 38.1 22.1 15 5.5 70 75 130 323 42 22 300 45 189 20 l 750922 004456 35.2 26.3 64 5.5 310 50 17 209 75 131 267 19 164 37 m 751231 094544 38.4 21.7 1 5.9 235 40 125 97 58 65 56 66 169 10 l 760511 165945 37.4 20.4 16 6.5 327 12 90 147 78 90 237 35 57 55 b 760612 005918 37.5 20.6 8 5.8 297 20 90 117 70 90 206 25 26 35 g 770818 092741 35.3 23.5 13 5.6 270 12 114 114 79 96 197 44 29 56 m 770911 231919 34.9 23.0 7 6.3 320 30 90 140 60 90 229 16 59 74 n 771128 025910 36.0 27.8 71 5.8 166 38 42 40 68 120 110 17 353 57 k 790515 065923 34.6 24.5 35 5.7 253 17 65 100 75 97 184 29 16 59 l 790615 113417 34.9 24.2 40 5.6 21 23 141 150 75 70 253 28 35 56 l 790723 114155 35.5 26.4 11 5.5 61 35 40 183 70 120 56 56 296 17 o 810224 205337 38.2 23.0 10 6.7 264 42 80 71 49 98 288 83 167 4 p 810225 023554 38.2 23.1 8 6.4 241 44 85 54 46 95 251 86 148 1 p 810304 215807 38.2 23.3 8 6.4 50 45 90 230 45 90 56 90 140 0 p 820817 222220 33.7 22.9 9 6.4 219 34 93 36 57 88 127 11 300 78 q 830117 124130 38.1 20.2 9 7.0 40 45 168 140 82 46 263 25 12 37 r,s 830319 214142 35.0 25.3 67 5.7 43 51 139 162 60 46 289 2 19 60 m 830323 231505 38.2 20.3 7 6.2 29 68 174 123 74 22 254 12 358 19 b,r 830927 235939 36.7 26.9 160 5.6 261 20 122 49 73 80 147 28 305 59 k 840211 080251 38.3 21.9 2 5.6 77 28 121 291 66 74 229 65 10 19 q 840522 135706 35.9 22.6 67 5.5 182 55 29 75 66 142 131 7 34 43 t 840621 104346 35.4 23.3 39 6.2 322 16 114 117 75 83 213 30 18 59 b 850421 084942 35.7 22.2 35 5.6 269 36 71 112 56 103 193 10 60 75 t
B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 281 Table 2 (continued) Date Time Latitude Longitude h M Plane A Plane B P axis T axis Reference Azimuth Dip Rake Azimuth Dip Rake Azimuth Dip Azimuth Dip 850722 213229 34.4 28.4 23 5.7 67 48 34 181 65 133 50 43 301 10 q 850907 102050 37.5 21.2 19 5.6 40 44 147 285 68 51 240 51 348 14 q 850927 163948 34.5 26.6 62 5.5 135 76 13 41 77 166 88 1 358 19 t 860913 172434 37.1 22.2 6 6.0 200 50 81 6 40 100 168 83 285 6 u 870227 233454 38.4 20.4 4 5.9 46 37 155 295 75 55 242 48 359 22 q 870529 184032 37.5 21.5 35 5.5 75 26 121 289 68 76 223 65 8 21 q 870610 145011 37.2 21.4 20 5.5 24 44 180 114 90 46 239 31 349 31 q 880518 051740 38.4 20.5 1 5.8 163 38 95 336 52 86 69 7 225 82 q 881016 123405 37.9 20.9 14 6.0 32 87 166 301 76 3 258 12 166 8 q 890820 183230 37.3 21.2 16 5.9 237 37 130 104 63 64 56 63 175 14 q 890824 021314 37.9 20.2 18 5.7 356 38 131 129 62 63 258 13 355 63 q 900709 112218 34.9 26.6 19 5.5 327 64 82 129 27 106 254 70 51 19 q 910319 120925 34.8 26.3 7 5.8 2 71 122 245 36 33 234 53 116 20 q 920123 042416 38.4 20.5 15 5.5 351 42 97 162 48 84 256 3 21 85 q 920430 114439 35.1 26.6 20 6.1 172 38 106 12 53 78 325 78 93 8 q 921118 211041 38.3 22.5 12 5.7 258 31 81 68 59 95 323 75 161 14 v 921121 050721 35.9 22.5 65 6.3 96 78 139 196 50 16 151 18 48 37 t 930305 065508 37.2 21.5 39 5.8 342 42 120 125 55 66 231 7 341 70 q 930318 154706 38.1 21.8 71 5.5 333 30 125 114 66 72 218 19 353 65 t 930714 123148 38.2 21.8 20 5.5 238 73 163 143 73 18 100 24 190 0 q 940111 072252 35.8 21.8 37 5.5 332 64 147 77 61 30 25 2 293 41 q 940225 023050 38.8 20.6 4 5.6 6 59 176 97 87 31 227 19 326 24 q 940416 230934 37.4 20.6 15 5.7 346 18 134 114 77 78 215 31 9 56 q 940523 064612 35.0 24.9 80 6.1 70 70 137 178 50 26 128 12 26 44 t 950615 001549 38.4 22.2 12 6.4 276 34 73 76 58 101 316 75 174 12 q 951210 032750 34.8 24.1 25 5.5 289 22 75 125 69 96 210 23 45 66 q a Shirokova (1972). b Papadimitriou (1993). c McKenzie (1972). d Papazachos et al. (1992). e Lyon-Caen et al. (1988). f Baker et al. (1997). g Anderson and Jackson (1987). h Ritsema (1974). i McKenzie (1978). j Kiratzi and Langston (1989). k Karakostas (1988). l Papazachos (1975). m Taymaz et al. (1990). n Papazachos et al. (1991). o Ekstrom and England (1989). p Taymaz et al. (1991). q Harvard (CMT solutions). r Scordilis et al. (1985). s Kiratzi and Langston (1991). t NEIS determination. u Papazachos et al. (1988). v Karakaisis et al. (1993)
282 B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 Fig. 3. Epicenters of shallow and intermediate depth earthquakes in the western part of the Hellenic arc (upper part) and crosssection of the earthquake foci along the line ABCD (lower part).
B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 283 Fig. 4. Epicenters of shallow and intermediate depth earthquakes in the central part of the Hellenic arc (upper part) and cross-section of the earthquake foci along the line ABCD (lower part).
284 B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 Fig. 5. Epicenters of shallow and intermediate depth earthquakes in the eastern part of the Hellenic arc (upper part) and cross-section of the earthquake foci along the line BCD (lower part).
B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 285 Fig. 6. Isodepths of 20, 100 and 170 km for the earthquakes located in the Wadati Benioff zone of southern Aegean and epicenters of four complete samples of intermediate depth strong earthquakes occurring during the last two centuries. under the Hellenic trench was presented about the arc, respectively. Different symbols are used to three decades ago (Papazachos and Delibasis, denote different fault types. 1969). Since then fault plane solutions for several Normal faults and some strike slip ones are shallow and intermediate depth earthquakes in the shown in the inner part of the shallow crustal Hellenic arc have been published (McKenzie, 1970, seismogenic layer, which is due to the known 1972, 1978; Taymaz et al., 1990; Papazachos et al., expansion of the Aegean and the strike slip faults 1991 among others). in the Cephalonia area which is the contact A considerable number of reliable fault plane between the Aegean and the Adriatic microplates. solutions are now available for strong shallow and Thrust faults dominate in the outer part of this intermediate depth earthquakes in the Hellenic arc layer ( Hellenic trench) due to the convergence ( Table 2). Fig. 7 shows plots of the foci of these between the Aegean and the eastern Mediterranean earthquakes on three vertical planes normal to the lithospheres. In the descending lithospheric slab trend of the western, central and eastern part of strike slip faulting with thrust component occurs
286 B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 Fig. 7. Plot of the foci on the three cross-sections for earthquakes of the Hellenic arc for which reliable fault plane solutions are available. The type of faulting for each earthquake is also indicated.
B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 287 due to the fact that maximum tension trends crust, which is suggested by the data used in the parallel to the dip of the Wadati Benioff zone and present study, has also been observed during the maximum compression is almost horizontal and investigation of the spatial distribution of parallel to the arc ( Kiratzi and Papazachos, 1995; aftershocks of several strong earthquakes which Papazachos, 1996). were generated during the last two decades by It is of interest to observe that the earthquakes normal or strike slip faulting (Papazachos et al., with focal depths between 20 and 50 km which 1979, 1983, 1988; Scordilis et al., 1985). No such occur in the convex (outer) side of the sedimentary investigation of aftershocks of strong main shocks part of the arc are generated by thrust faulting, as generated by thrust faulting in the crust of the it occurs with the shallower earthquakes (0 eastern Mediterranean has been made. Therefore, 20 km), although they form part (the most shal- the result that the shallow seismogenic layer is also low) of the Wadati Benioff zone where the deeper 20 km in the fore-arc part of the Hellenic arc earthquakes (h 60 km) have a different focal indicates that it is a physical property of the crust mechanism. We can, therefore, separate each earth- to generate strong earthquakes only in its shallow quake according to its faulting type and define as part, independent of the type of deformation ( horishallow earthquakes those with h 50 km and as zontal expansion, horizontal shortening, etc.). This intermediate depth earthquakes those with result probably leads to the conclusion that accuh 60 km in the Hellenic arc. rately located foci of earthquakes at depths larger This can be more clearly seen in Fig. 8, where than 20 km indicate a dipping of the crust. the stress axes of the focal mechanisms of Table 2 The Wadati Benioff zone starts at a depth of are presented for the same cross-sections as in 20 km under the convex (outer) side of the sedi- Fig. 7. In this figure the projection of each stress mentary part of the arc (western Peloponnese axis is plotted for each cross-section, hence in west of Cythera south of Crete southeast of several cases the T (tension gray arrows) and Rhodes) and dips towards the back-arc area where P (compression solid arrows) axes are oblique it reaches a depth of 150 km under the volcanic and not normal. If the angle between the stress arc in the southern Aegean. Some earthquakes are axis and the cross-section exceeded 45, then the located even deeper, up to a focal depth of about stress axis is not shown. In all cross-sections we 180 km. This Wadati Benioff zone can be sepaclearly recognize a transition from thrust faulting (horizontal P vertical T ) in the fore-arc area to rated into two branches, one shallow (20 km h normal faulting (horizontal T vertical P) in the 100 km) with a dip angle of about 30 and the back arc area in the shallow part of the crust other deep ( 100 km h 180 km) with a dip angle (20 km). The orientation of the P axis remains of about 45. Of course, the existing scattering of characteristically constant (subhorizontal, dipping the intermediate depth seismicity does not allow towards the fore-arc area) even at larger depths the determination of these dip angles with high along the shallow part of the subduction, up to a accuracy ( less than 5 10 ). On the other hand, depth of 50 km or more in the eastern part of the the relatively small hypocentral errors previously slab. At larger depths (60 150 km), the P axis is described, as well as the systematic increase of the not shown, as it is oriented horizontally and paral- dip angle in all three presented cross-sections lel to the strike of the arc. This section of the (Figs. 3, 4 and 5) suggest that this is a robust subduction is dominated by a strong extension, feature of the subducted slab. which more or less follows the dipping direction The observed slope change for the Hellenic of the slab in each cross-section. subduction is in excellent agreement with indepen- dent results in the area. Detailed tomographic results for the area (Papazachos and Nolet, 1997) 5. Conclusions and discussion along similar cross-sections confirm this change of dipping, although they indicate that it starts at a The confinement of the foci of the shallow shallower depth (~80 km). The same results also earthquakes to the upper 20 km of the Aegean suggest that this slope change for the central part
288 B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 Fig. 8. Stress field along the same cross-sections presented in Fig. 7. Notice that the extension is parallel to the dipping of the Wadati Benioff zone in all cross-sections (depth >50 60 km).
B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 289 of the subduction is quite sharp, as also seen in to the back-arc part of the arc and overriding of Fig. 4. Similar changes in the slope of the subduct- the Aegean lithosphere from northeast to southing slab at depths of 60 100 km have been west contribute to this coupling. Subduction preobserved in other subduction areas, such as the vails at the low part of this branch (55 100 km) Japan island arc ( Wesnousky et al., 1982) or the since the maximum tension trends parallel to the Aleutian subduction zone ( Ekstrom and Engdahl, dip of the zone at these depths ( Kiratzi and 1989). Moreover, theoretical modeling (e.g. Tao Papazachos, 1995). This dip-parallel extension is and O Connell, 1992) also favors the shape of also present at larger depths (100 150 km), as seen kinked slab profiles, especially in oceanic conti- in Fig. 8. nental subduction systems, such the Mediterranean This stress pattern is very consistent with the subduction beneath the Aegean. stress pattern seen in other subduction areas. The shallow branch of the Wadati Benioff zone Subduction parallel tension has been well docuis not well developed, particularly in the eastern mented for similar depth ranges (100 200 km) in part where its dipping from the fore-arc to the other subduction areas, e.g. Philippines (Cardwell back-arc part of the arc is doubtful. This gap in et al., 1980) and used in theoretical modeling (e.g. the slab observed in its eastern section is further Tao and O Connell, 1992). For the Central supported by recent regional tomographic results Aleutians, Ekstrom and Engdahl (1989) find that ( Papazachos and Nolet, 1997), which clearly show the thrust faults with subhorizontal P axes (also a discontinuity in the slab between 75 and 95 km, dipping towards the fore-arc side) remain practialmost exactly at the same place where no intermecally constant along the main thrust zone at depths diate depth earthquakes are detected. This between 15 and 50 km, similar to what is found in agreement between the seismicity distribution and the Hellenic arc. According to the same authors, tomography suggests that this slab tear-up is a down-dip tension dominates at large depths robust feature of the eastern section of the Hellenic (>150 km), although they confirmed the presence arc. Small earthquakes with h>20 km occur even of down-dip compression at depths between 70 in the area between the shallow seismogenic layer and 120 km (Frohlich et al., 1982). ( 0 20 km) of the southern Aegean and this branch The orientation of the compression parallel to of the Wadati Benioff zone. Strong earthquakes the strike of the arc (and the subduction) seen in in the shallow part (20 55 km) of this branch of the Wadati Benioff zone are also caused by horiin the previously mentioned or other subduction the Hellenic arc at depths >55 km is not identified zontal compression (thrust faulting) which trends in a northeast southwest direction while deeper areas. For example, detailed study of the seismicity strong earthquakes (55 100 km) are caused by of the subducted Nazca plate (Cahill and Isacks, tension trending parallel to the dip of the Wadati 1992) confirms that the T axis for intermediate Benioff zone (strike slip faulting with a thrust and deep earthquakes (up to 350 km) has also a component). more or less down-dip orientation along All these observations indicate that this shallow ~3000 km of subduction (10 35 S). However, the branch (20 km h 100 km) of the Wadati P axis was found to be oriented almost vertically Benioff zone defines the area of coupling between or normal to the subducted plate, similarly to the the Aegean and the Mediterranean lithospheres. deeper (>150 km) Aleutian events. The origin of This is strongly supported by the fact that all big the in-plane horizontal compression at depths intermediate depth earthquakes with magnitudes >55 km in the Hellenic arc is not clear, and further up to 8.2 (see Fig. 6) occur in this branch. It is modeling is necessary in order to explain such also supported by the existence of the aseismic special behavior. layer (80 100 km), at least in the eastern arc, The shallow earthquakes ( 0 55 km) in the outer which is probably an area of low rigidity on which (convex) side of the sedimentary arc are mainly lithospheric plates slip easily. It seems that both due to the overriding of Mediterranean by the subduction of oceanic lithosphere (front part of Aegean lithospheric plate which moves fast south- the Mediterranean lithosphere) from the fore-arc westwards, as shown by geodetic data (Smith et al.,
290 B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 1994; Kastens et al., 1996). Such southwestward overriding of the Aegean plate explains the fact Appendix A that no clear dipping of the shallow branch of the Wadati Benioff zone is observed in the concave Table 3 (inner) part of the eastern section of the arc. It Focal parameters of the earthquakes used in the present study. also explains the observation that no earthquakes The earthquake of the 15th July 1986, 15:15 is included twice with h>20 km occur in a large area of the southand ISC (ISC solution denoted by 1) for comparison of the solutions obtained by the local network westernmost part of the arc (sections AB in Figs. 3 and 4) where shallow seismicity is very high, while this is not valid for the eastern part of the arc (no Date Origin time Latitude Longitude h M AB seismic section exists in Fig. 5). The Aegean 560709 03:11:40 36.70 25.80 22 7.5 591115 17:08:40 37.80 20.50 10 6.8 plate, due to its fast southwestward motion, has 610523 02:45:20 36.70 28.50 70 6.4 surpassed the subducted lithospheric slab and 620828 10:59:56 37.80 22.90 95 6.8 reached the Ionian sea where the boundary 640717 02:34:27 38.00 23.60 155 6.0 between this plate and the African plate is located 650405 03:12:55 37.70 22.00 28 6.1 today. Therefore, the interaction between the 650409 23:57:02 35.10 24.30 67 6.1 Aegean and the African plate in this western 650427 14:09:06 35.60 23.50 5 5.7 650706 03:18:42 38.40 22.40 28 6.3 boundary is of continent continent type now. 651128 05:26:05 36.10 27.40 73 6.0 The deep branch (100 km h 180 km) of the 660509 00:42:53 34.40 26.40 10 5.8 Wadati Benioff zone in the Hellenic arc is sepa- 680328 07:39:59 37.80 20.90 6 5.9 rated from its shallow part by an aseismic layer 680530 17:40:26 35.40 27.90 7 5.9 (80 100 km), at least in the eastern part where the 680704 21:47:51 37.70 23.20 15 5.5 presence of a slab discontinuity is suggested at 681205 07:52:11 36.60 26.90 7 6.0 690114 23:12:06 36.10 29.20 7 6.2 these depths. This branch of the zone (100 690416 23:21:06 35.20 27.70 8 5.5 180 km) is probably due to the free sinking of an 690612 15:13:31 34.40 25.00 19 6.1 oceanic lithospheric slab, as also indicated by the 690708 08:09:13 37.50 20.30 10 5.9 dip-parallel extension axis presented in Fig. 8. Such 700408 13:50:28 38.30 22.60 10 6.2 a sinking, without the presence of strong coupling, 720504 21:39:57 35.10 23.60 40 6.5 720913 04:13:20 38.00 22.40 91 6.3 explains the fact that no earthquakes with magni- 720917 14:07:15 38.30 20.30 8 6.3 tudes larger than about 7.0 occur in this deep 730105 05:49:18 35.80 21.90 42 5.6 branch of the Wadati Benioff zone. This sinking 731129 10:57:44 35.20 23.80 1 6.0 occurs, mainly, under the southern Aegean sea 750404 05:16:18 38.10 22.10 15 5.5 and particularly under the volcanic arc. 750922 00:44:56 35.20 26.30 64 5.5 There are, however, several well located strong 751231 09:45:44 38.40 21.70 1 5.9 760511 16:59:45 37.40 20.40 16 6.5 intermediate depth earthquakes in the southwest- 760612 00:59:18 37.50 20.60 8 5.8 ern part of the sedimentary arc (in the Ionian sea, 770818 09:27:41 35.30 23.50 13 5.6 west of Cythera) with focal depths larger than 770911 23:19:19 34.90 23.00 7 6.3 100 km (see Fig. 4). This indicates that the oceanic 771128 02:59:10 36.00 27.80 71 5.8 lithosphere of the Eastern Mediterranean is sink- 790515 06:59:23 34.60 24.50 35 5.7 ing in this part of the Ionian sea. This sinking 790615 11:34:17 34.90 24.20 40 5.6 790723 11:41:55 35.50 26.40 11 5.5 of the oceanic lithosphere in this part of the 810224 20:53:37 38.20 23.00 10 6.7 subduction is also indicated by recent tomographic 810225 02:35:54 38.20 23.10 8 6.4 information (Papazachos and Nolet, 1997). These 810304 21:58:07 38.20 23.30 8 6.4 results support the idea that roll-back of the 810320 15:39:08 36.24 22.63 109 4.8 descending lithospheric plate towards the remain- 810508 09:19:30 35.79 27.21 110 4.7 ing scarp of oceanic crust beneath the Ionian sea 811031 08:20:20 37.12 23.86 127 3.9 811116 11:39:45 36.64 26.82 161 4.4 may cause gravitational spreading or gravitational 811228 21:18:10 37.52 23.32 119 3.6 collapse of the expanding Aegean lithosphere 820124 05:37:03 36.61 27.52 146 4.2 ( LePichon and Angelier, 1981; Dewey, 1988). 820418 23:18:00 36.65 27.11 155 4.7
B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 291 Date Origin time Latitude Longitude h M Date Origin time Latitude Longitude h M 820509 22:48:21 35.88 26.32 133 4.3 860609 19:14:18 38.59 21.63 10 1.9 820726 17:17:07 36.88 23.72 106 4.6 860610 00:34:26 37.23 22.88 0 1.9 820817 22:22:20 33.70 22.90 9 6.4 860610 03:35:04 37.86 19.76 26 2.7 820827 01:41:09 36.84 23.61 110 4.2 860610 03:55:06 38.43 21.99 9 2.5 821128 12:12:11 36.43 26.22 140 4.4 860610 08:48:25 37.91 21.88 9 1.3 830117 12:41:30 38.10 20.20 9 7.0 860610 13:15:40 38.19 21.42 11 1.8 830228 17:28:44 36.30 27.72 107 4.5 860610 15:25:04 37.47 22.08 5 1.4 830319 21:41:42 35.00 25.30 67 5.7 860610 18:26:05 37.03 21.23 0 2.0 830323 23:15:05 38.20 20.30 7 6.2 860610 23:20:11 38.01 20.01 0 3.4 830423 08:58:40 36.24 26.43 136 4.3 860611 00:49:10 34.82 22.60 4 3.4 830927 23:59:39 36.70 26.90 160 5.6 860611 03:07:09 35.34 21.48 0 3.6 830927 23:59:39 36.72 26.93 160 5.7 860611 06:55:02 37.90 22.51 11 2.1 831007 04:14:04 37.98 23.27 136 4.7 860611 07:51:41 37.37 20.60 0 3.9 831031 14:04:13 38.15 22.94 120 4.2 860611 08:21:06 37.63 21.68 25 1.5 840211 08:02:51 38.30 21.90 2 5.6 860611 08:25:12 37.63 21.68 26 1.6 840228 08:48:14 36.18 25.64 158 4.9 860611 08:47:47 38.18 22.04 14 0.9 840522 13:57:06 35.90 22.60 67 5.5 860611 10:09:12 36.94 23.27 9 1.9 840620 15:29:34 36.69 27.05 166 4.3 860611 10:47:41 38.33 22.33 7 1.5 840621 10:43:46 35.40 23.30 39 6.2 860611 10:54:19 37.93 22.19 46 1.9 840923 14:19:25 36.52 26.49 155 4.4 860611 13:14:42 37.22 22.03 0 1.1 841010 21:11:18 36.85 23.48 103 4.7 860611 16:51:37 37.56 20.87 7 2.0 841120 15:41:50 35.58 26.52 121 4.6 860611 18:56:59 36.50 22.21 3 2.4 841216 12:08:07 37.10 24.11 138 4.2 860611 19:38:55 36.51 21.25 0 2.9 841216 19:40:48 36.35 26.82 147 4.4 860612 04:26:53 38.38 21.95 12 1.7 850203 16:40:46 37.78 23.81 195 3.8 860612 04:47:42 38.37 21.95 7 1.9 850217 10:45:27 36.61 27.67 128 4.7 860612 06:32:22 35.42 22.94 5 3.3 850225 19:26:08 36.44 26.70 157 4.4 860612 08:31:49 37.43 22.17 15 1.4 850421 08:49:42 35.70 22.20 35 5.6 860612 08:45:03 38.38 22.22 12 1.3 850423 12:46:44 36.28 26.95 137 4.3 860612 09:54:28 37.78 20.39 0 3.0 850714 15:09:53 35.92 26.17 106 4.9 860612 13:00:13 37.31 22.17 16 2.4 850722 21:32:29 34.40 28.40 23 5.7 860612 13:44:47 37.50 22.37 12 1.5 850907 10:20:50 37.50 21.20 19 5.6 860612 18:49:43 37.58 20.99 9 2.0 850914 15:33:54 37.41 24.23 165 3.5 860612 19:04:52 37.64 21.08 0 3.0 850927 16:39:48 34.50 26.60 62 5.5 860613 01:48:21 35.80 21.48 0 3.1 851203 18:12:40 36.64 26.89 156 4.7 860613 10:33:36 38.29 22.04 6 1.6 860221 17:24:44 36.38 26.52 146 4.9 860613 15:21:29 38.03 21.57 18 1.5 860607 18:42:26 37.33 22.13 11 2.4 860613 15:23:36 38.02 21.57 14 2.7 860607 22:50:33 38.33 21.82 4 1.7 860613 16:11:43 37.95 21.59 16 1.7 860608 04:54:51 35.19 21.46 1 5.2 860613 16:24:13 38.12 22.58 12 1.6 860608 05:12:00 35.45 21.76 46 4.3 860613 23:25:35 37.46 21.99 11 1.5 860608 05:58:20 38.51 21.66 8 2.6 860614 00:50:11 38.54 21.65 0 1.9 860608 07:25:58 38.37 21.98 11 1.6 860614 01:34:29 38.48 21.94 10 2.0 860608 07:40:50 38.16 23.05 0 3.0 860614 05:21:49 36.53 21.63 12 1.7 860608 11:49:00 36.40 21.64 0 2.6 860614 08:43:24 37.40 21.96 15 0.9 860608 14:42:32 37.85 21.93 18 1.5 860614 08:47:32 38.55 21.65 13 1.6 860608 19:21:45 37.90 22.50 13 2.2 860614 09:20:27 38.45 21.77 11 2.0 860608 20:07:13 37.93 21.95 22 1.1 860614 09:27:01 38.42 21.75 9 1.4 860608 20:07:45 37.92 21.95 19 1.4 860614 09:35:08 37.60 19.72 3 3.2 860608 22:42:21 37.86 21.90 18 1.3 860614 10:38:20 36.36 22.14 10 2.0 860609 00:43:07 38.08 21.75 22 1.0 860614 14:12:06 37.03 21.77 12 1.7 860609 00:44:54 38.13 22.34 9 1.4 860614 14:30:27 37.68 23.04 0 2.6 860609 08:57:40 36.19 22.11 0 3.2 860614 17:50:23 36.03 21.99 0 4.7 860609 15:18:33 37.77 22.90 64 1.9 860614 18:52:54 36.02 22.02 0 2.4 860609 23:52:23 37.73 19.71 8 2.2 860614 19:05:10 36.02 21.99 0 2.7
292 B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 Date Origin time Latitude Longitude h M Date Origin time Latitude Longitude h M 860614 19:18:03 36.07 22.07 0 4.2 860620 00:48:14 36.71 21.32 0 4.2 860614 19:45:08 36.01 22.01 0 2.6 860620 02:18:27 36.78 21.40 0 3.6 860614 20:16:39 38.45 22.27 0 2.8 860620 05:48:15 36.75 21.31 0 4.0 860614 20:52:57 35.97 21.95 0 2.6 860620 07:31:22 36.66 21.42 10 3.3 860614 22:52:05 36.00 22.00 0 3.1 860620 09:47:03 36.75 21.32 0 3.8 860615 00:45:41 36.02 22.06 0 3.4 860621 00:40:04 36.75 21.35 0 3.6 860615 00:48:43 35.87 22.11 18 2.6 860621 20:59:46 36.84 21.49 0 3.4 860615 01:19:29 37.66 23.65 25 2.3 860622 06:34:41 36.76 21.33 0 3.7 860615 03:41:35 37.14 21.85 1 2.0 860622 08:00:43 38.53 21.70 0 4.1 860615 08:58:25 37.60 22.71 54 1.7 860622 19:46:48 37.41 20.70 0 3.4 860615 09:44:21 37.43 22.18 15 1.0 860623 02:55:58 36.96 22.35 47 2.1 860615 09:51:22 35.99 22.00 0 3.1 860623 14:31:19 38.55 21.65 0 3.2 860615 10:20:19 38.43 21.94 7 2.3 860624 08:39:44 36.60 22.78 47 3.3 860615 10:39:09 38.42 21.99 13 1.8 860624 22:04:32 38.34 21.78 9 4.1 860615 11:07:05 38.14 22.68 13 2.0 860625 06:38:05 36.71 21.29 1 3.8 860615 13:53:51 36.03 22.05 0 2.5 860626 10:28:56 37.81 21.13 16 3.1 860615 15:18:38 35.66 22.16 27 3.9 860626 12:32:44 37.60 19.87 4 3.8 860615 19:21:59 38.39 21.81 0 2.6 860626 21:14:13 36.73 21.38 0 4.0 860615 20:46:55 37.71 22.81 62 1.7 860627 08:40:37 36.88 23.21 24 3.8 860615 22:06:33 37.74 22.52 9 1.9 860627 11:33:16 35.74 23.89 22 4.6 860616 01:46:33 37.86 21.15 13 2.6 860628 17:03:25 37.73 21.90 58 2.5 860616 12:55:53 35.97 22.02 0 3.1 860629 19:28:56 37.62 21.98 19 1.7 860616 14:14:16 37.40 21.13 21 2.0 860629 20:57:55 36.87 21.44 6 3.5 860616 16:19:30 37.46 22.08 8 1.8 860629 20:59:56 36.90 21.48 13 2.9 860617 02:52:28 38.14 21.66 19 1.9 860629 22:37:41 36.90 21.46 11 3.1 860617 03:04:18 38.14 21.65 18 2.0 860630 07:33:59 38.32 20.66 0 4.1 860617 06:42:01 37.75 22.03 9 1.7 860630 08:03:47 37.72 21.80 0 3.7 860617 23:37:36 37.60 23.42 75 2.2 860630 09:57:48 37.72 21.81 9 3.6 860617 23:39:40 37.63 21.74 10 1.2 860630 16:32:33 37.65 20.99 0 3.1 860618 06:45:23 38.42 22.02 9 2.2 860630 17:05:46 36.71 21.81 7 3.2 860618 08:51:03 36.60 21.19 0 2.9 860630 21:20:24 36.92 21.62 10 2.3 860618 11:24:28 37.43 22.17 14 1.3 860701 01:40:17 37.21 23.08 49 2.3 860618 11:40:34 36.69 21.15 0 3.1 860701 09:32:00 35.93 24.32 2 4.5 860618 11:55:54 35.88 22.11 0 3.0 860701 11:49:25 36.75 21.39 0 3.6 860618 12:25:19 36.69 21.26 3 2.7 860702 12:06:43 38.24 20.68 0 3.2 860618 14:06:59 37.49 22.35 9 1.3 860703 05:19:52 38.42 22.03 2 2.7 860618 16:54:27 37.43 20.49 0 3.2 860703 10:34:30 35.75 26.28 112 4.1 860618 21:30:40 36.22 21.64 3 3.4 860704 05:55:05 38.35 22.01 4 2.7 860619 01:14:00 37.85 22.94 0 1.7 860704 14:06:15 38.11 22.00 19 3.5 860619 02:07:31 37.57 21.61 17 1.9 860705 09:52:46 37.85 22.59 78 4.2 860619 02:45:45 36.66 21.27 0 3.4 860705 14:09:14 37.89 24.14 168 3.3 860619 05:45:48 37.55 21.80 26 1.8 860705 21:24:17 36.66 21.27 0 3.7 860619 05:52:12 37.54 21.82 25 2.2 860706 02:29:06 36.82 21.93 2 2.3 860619 05:52:39 37.56 21.81 24 2.1 860706 06:24:17 37.48 22.09 8 2.7 860619 05:54:19 37.53 21.86 29 0.8 860706 20:29:48 35.42 22.75 1 1.4 860619 06:36:28 36.69 21.26 0 3.8 860707 17:41:14 37.05 21.84 0 2.9 860619 14:44:24 38.09 20.98 28 2.9 860707 22:08:44 36.99 21.74 9 2.8 860619 15:14:46 36.67 21.30 0 4.7 860708 16:15:07 37.76 21.02 0 4.2 860619 15:21:31 36.77 21.34 0 3.6 860708 17:28:50 38.10 23.01 6 2.7 860619 16:20:22 36.77 21.35 0 3.5 860708 21:54:13 38.19 22.57 8 2.2 860619 16:32:16 36.70 21.31 0 3.7 860709 08:23:25 37.92 21.06 0 3.0 860619 19:10:40 36.76 21.35 0 3.9 860709 12:23:24 36.71 21.38 0 3.2 860619 20:20:29 36.74 21.35 0 4.0 860710 02:33:30 36.76 21.37 0 3.3 860619 22:28:45 36.75 21.34 0 4.1 860710 03:57:54 38.33 22.01 0 2.8
B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 293 Date Origin time Latitude Longitude h M Date Origin time Latitude Longitude h M 860710 06:30:59 37.78 21.12 0 3.4 880712 06:02:21 36.58 24.26 96 3.2 860710 06:57:27 37.78 21.10 5 2.5 880713 19:59:59 34.85 22.24 2 4.0 860710 07:16:33 38.08 22.79 11 2.5 880713 21:29:14 34.88 25.63 16 2.5 860710 07:34:38 37.79 21.12 0 3.3 880714 18:31:08 35.24 23.63 44 3.8 860710 07:47:38 37.76 21.10 7 2.8 880714 21:17:01 36.72 25.72 0 3.0 860710 08:24:57 37.78 21.12 0 3.1 880715 02:32:48 35.19 27.26 0 3.8 860710 18:14:29 38.40 21.93 0 2.9 880715 03:18:57 35.34 27.10 68 3.6 860711 15:43:35 37.47 20.56 1 3.7 880715 08:32:18 34.98 25.75 11 2.4 860711 17:54:39 35.71 23.06 0 3.6 880715 12:39:54 36.69 25.73 6 3.4 860711 20:13:03 37.50 20.57 0 3.5 880715 16:54:56 35.73 22.71 54 4.3 860712 00:14:16 37.21 21.09 0 2.9 880715 17:44:17 37.00 26.81 6 2.8 860712 00:48:12 37.19 21.08 0 2.8 880715 22:55:45 35.69 25.69 8 2.9 860712 06:17:54 37.47 20.54 0 4.2 880716 01:54:34 37.33 22.92 74 4.8 860712 07:47:05 37.47 20.61 1 2.7 880717 06:58:14 34.84 25.37 17 2.6 860713 01:57:52 37.41 22.01 16 2.0 880717 11:52:59 36.14 26.17 10 2.9 860713 07:20:05 37.46 20.63 0 2.9 880717 21:29:12 36.47 28.16 43 4.0 860713 08:41:36 38.09 22.63 73 1.9 880718 01:25:22 35.98 22.93 7 3.6 860713 11:35:27 35.50 22.85 8 3.8 880718 01:44:22 35.29 23.38 59 4.1 860713 18:16:47 37.56 21.72 8 2.5 880718 04:00:54 35.99 23.86 14 3.1 860713 21:22:48 38.41 21.80 11 2.5 880718 10:51:12 34.65 25.24 13 2.7 860714 13:42:33 37.50 22.34 10 2.1 880718 11:52:00 35.95 22.68 0 4.1 860714 17:15:09 38.08 22.04 10 3.2 880718 23:34:15 38.36 22.31 10 4.0 860714 21:05:10 37.99 21.53 19 2.1 880719 04:34:58 36.31 26.71 93 3.8 860714 23:30:59 37.74 22.10 0 2.5 880720 02:09:18 35.82 24.99 32 3.3 860714 23:56:27 37.75 22.11 1 2.7 880720 05:54:22 35.12 27.24 0 3.6 860715 07:31:45 36.65 21.89 0 2.8 880720 06:33:15 36.54 23.84 74 3.0 860715 09:07:50 37.60 22.66 58 2.3 880720 13:00:41 35.31 23.44 42 3.7 860715 11:12:27 38.30 22.13 15 2.9 880721 16:33:57 35.67 26.56 28 3.5 860715 15:15:53 36.66 23.11 124 3.9 880721 20:03:23 35.56 23.63 24 3.5 8607151 15:15:54 36.63 23.26 116 4.1 880722 10:50:33 37.38 22.02 76 3.6 860715 19:04:48 38.39 20.62 5 2.7 880723 00:25:11 35.74 27.05 45 3.7 860715 23:42:42 37.70 22.00 16 1.9 880723 09:19:50 36.88 22.02 8 4.5 860716 04:10:26 37.47 20.56 0 3.5 880724 00:39:16 37.94 22.55 5 3.6 860716 14:09:24 37.66 23.03 9 2.6 880724 06:02:34 36.77 23.83 9 3.5 860716 15:50:14 38.41 21.94 8 2.4 880724 08:28:37 34.82 24.83 0 3.4 860716 20:28:10 37.70 21.63 21 1.8 880724 16:19:13 34.66 24.54 8 3.4 860729 17:40:50 36.69 27.94 109 4.5 880724 17:26:29 35.58 25.34 12 2.6 860811 01:25:23 36.20 26.83 128 4.2 880724 22:20:04 36.87 22.06 4 4.1 860817 04:05:35 36.53 26.96 154 4.2 880725 01:26:50 34.96 24.30 15 3.5 860913 17:24:34 37.10 22.20 6 6.0 880725 09:22:30 35.72 27.13 45 3.0 870120 17:10:43 36.44 26.89 147 4.4 880725 11:11:02 34.88 25.76 0 3.9 870227 23:34:54 38.40 20.40 4 5.9 880725 14:36:02 36.43 26.72 120 3.4 870507 08:56:52 36.63 26.75 153 5.2 880725 14:45:28 35.42 26.16 10 3.1 870529 18:40:32 37.50 21.50 35 5.5 880725 18:09:24 34.87 25.74 15 2.7 870601 02:28:30 36.70 25.46 166 4.4 880725 20:47:36 36.76 23.85 1 3.4 870610 14:50:11 37.20 21.40 20 5.5 880726 02:52:39 34.57 25.21 0 3.3 870611 05:28:57 36.34 26.53 147 4.0 880726 03:27:05 34.91 24.26 8 3.4 870619 18:45:41 36.80 28.20 59 5.5 880726 03:29:54 34.74 24.12 0 3.4 870711 13:55:54 36.64 26.90 161 4.0 880726 17:25:31 36.59 25.57 0 3.2 870831 23:20:49 36.58 27.71 113 4.4 880726 21:56:45 36.11 27.21 1 3.1 880518 05:17:40 38.40 20.50 1 5.8 880726 23:40:14 36.16 27.20 2 3.3 880602 07:34:19 36.28 26.73 130 4.4 880727 01:21:10 36.26 28.07 9 3.5 880710 11:03:26 36.11 29.08 28 4.4 880727 05:00:13 35.39 24.76 45 4.7 880711 20:17:46 36.86 24.08 0 3.0 880727 10:55:06 35.51 25.77 19 3.1
294 B.C. Papazachos et al. / Tectonophysics 319 (2000) 275 300 Date Origin time Latitude Longitude h M Date Origin time Latitude Longitude h M 880727 11:58:03 36.74 23.84 0 3.5 880812 01:28:29 35.35 23.51 4 2.1 880729 01:51:31 36.39 26.55 104 3.4 880812 22:40:40 35.44 27.28 9 4.1 880730 01:30:06 36.57 26.56 138 3.2 880813 16:39:40 36.61 25.53 4 3.4 880730 06:46:43 36.21 28.05 3 3.6 880813 22:29:43 35.60 26.98 12 3.8 880731 21:39:52 36.12 27.38 11 3.1 880814 02:30:54 35.44 23.42 0 3.5 880731 23:48:29 35.10 26.53 0 3.8 880814 03:22:24 34.45 24.94 0 4.2 880801 12:08:14 34.71 24.23 0 3.5 880814 07:27:01 37.27 26.89 8 3.5 880801 15:48:55 34.47 26.16 15 4.0 880814 07:28:04 37.27 26.92 4 4.2 880801 16:46:00 35.72 23.69 16 2.9 880814 13:28:28 35.57 23.72 10 2.7 880802 12:30:21 36.80 26.19 12 3.0 880815 05:37:16 35.36 23.58 9 2.4 880802 15:16:09 34.90 23.59 27 3.7 880815 12:22:06 35.25 22.70 16 3.4 880803 06:33:54 35.16 24.79 103 3.4 880815 20:08:56 35.17 25.06 11 3.4 880803 07:30:06 35.13 26.79 0 3.5 880816 05:49:08 34.69 26.03 61 4.1 880803 08:09:27 35.72 24.68 23 3.1 880816 06:02:11 34.72 26.02 56 4.3 880803 10:12:35 34.84 26.05 50 4.1 880816 16:10:30 35.22 23.46 19 3.8 880803 12:09:59 37.57 22.65 50 3.3 880816 23:39:22 34.47 23.80 6 4.1 880803 19:35:59 36.02 23.27 17 3.8 880817 02:10:38 37.26 26.89 9 4.8 880803 21:18:43 35.74 27.58 2 3.8 880817 11:58:00 36.60 26.82 128 3.7 880804 05:26:10 35.38 23.38 28 3.7 880817 14:07:55 36.93 21.72 0 3.9 880804 06:00:09 36.16 27.25 4 3.0 880817 21:07:21 35.88 23.10 2 2.9 880804 11:07:56 35.81 25.19 6 3.0 880818 09:01:48 36.99 26.79 8 4.2 880804 11:50:09 37.53 26.92 6 3.1 880818 10:13:49 37.00 26.81 12 3.8 880804 14:06:25 36.28 22.92 3 3.5 880818 12:00:45 37.73 23.08 11 2.9 880804 18:43:29 35.23 23.24 27 3.4 880818 13:10:55 35.17 23.45 31 4.2 880804 22:40:16 34.94 24.38 15 3.0 880818 13:25:41 35.19 23.49 29 3.5 880805 00:46:07 35.69 25.19 50 2.9 880818 18:07:15 35.15 23.62 20 3.0 880805 04:02:35 34.81 23.90 29 3.9 880818 22:17:49 34.93 25.43 19 4.2 880805 05:21:07 35.21 23.15 14 3.4 880818 22:25:59 35.08 23.22 0 3.1 880805 08:07:44 36.49 23.08 53 3.2 880819 01:52:01 37.26 26.86 4 3.8 880805 12:53:24 35.63 26.00 27 4.6 880819 06:04:52 35.84 22.91 2 3.6 880805 17:04:57 35.54 26.65 6 3.3 880819 22:00:36 34.70 24.72 8 3.8 880805 20:25:23 36.52 22.86 28 3.6 880820 12:16:30 35.39 23.46 24 2.9 880805 22:37:58 36.10 27.23 0 2.9 880820 14:29:48 35.70 23.43 0 3.0 880806 04:22:00 35.35 27.39 20 3.3 880820 15:30:13 36.42 26.45 103 3.7 880806 06:01:49 34.76 23.87 14 3.0 880820 15:32:32 35.76 25.39 11 3.4 880806 12:35:53 36.17 23.42 15 3.3 880821 10:30:42 36.92 24.26 6 3.2 880806 13:00:01 36.15 23.36 19 3.1 880821 15:23:02 37.00 26.80 8 4.3 880806 13:58:41 35.20 27.18 1 3.9 880821 21:25:24 37.00 26.81 10 3.8 880806 14:49:34 36.75 26.13 13 2.9 880821 23:55:56 34.31 23.93 12 3.4 880807 02:39:56 36.66 23.61 30 3.2 880822 01:47:41 36.06 23.13 13 2.9 880807 11:23:03 35.78 24.98 66 3.4 880822 11:05:19 35.85 23.47 19 3.1 880807 12:07:37 36.29 27.23 2 2.9 880823 04:14:54 37.03 26.85 15 4.6 880808 19:27:44 34.79 23.44 5 3.0 880823 05:40:25 36.92 27.58 4 4.5 880809 07:28:55 35.98 22.50 11 4.0 880823 08:50:19 34.73 24.15 1 3.7 880809 13:09:14 37.62 26.73 13 3.6 880823 10:07:43 35.19 23.87 15 2.8 880809 21:48:50 34.94 27.39 58 4.2 880823 21:51:54 36.83 26.19 12 3.9 880810 00:10:50 36.33 27.21 16 3.3 881016 12:34:05 37.90 20.90 14 6.0 880810 01:06:01 35.14 23.11 11 3.4 881023 17:21:22 36.72 28.35 111 4.7 880810 05:04:14 36.85 22.95 56 3.8 881222 15:30:12 36.74 23.21 126 3.6 880810 14:08:12 35.58 24.69 25 2.9 890427 23:06:52 37.10 28.20 12 5.5 880810 15:36:17 35.34 23.48 0 2.3 890820 18:32:30 37.30 21.20 16 5.9 880810 15:39:23 34.84 23.56 3 2.6 890824 02:13:14 37.90 20.20 18 5.7 880810 21:19:22 35.34 27.47 31 4.2 891014 07:14:41 36.70 25.32 169 4.1 880812 00:48:02 35.83 22.55 0 3.1 891101 13:59:27 36.47 26.99 142 4.6