Journal of Seismology 6: 287–306, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

287

Analysis and interpretation of the aftershock sequence of the August 17,1999, Izmit (Turkey) earthquake

O. Polat1, H. Eyidogan2, H. Haessler1, A. Cisternas1 & H. Philip3

1ULP IPGS∗, 5 rue Rene Descartes, 67084 Strasbourg Cedex, France; 2ITU† Maden Fakültesi Jeofizik Mühendis-ligi Bölümü, Maslak 80626 Istanbul, Turkey; 3Universite Montpellier II, Laboratoire de Geophysique, Montpellier,France

Received 25 March 2001; accepted in revised form 20 October 2001

Key words: aftershock analysis, Izmit earthquake, Izmit fault, Marmara region, seismotectonic, stress tensor

Abstract

A micro-seismic field experiment has been carried out in the Marmara Sea region. The analysis of the events beforeand after the August 17, 1999 Izmit (Turkey) earthquake has been completed. 1446 events have been well locatedout of a total of 3165 recorded within the period from July 15 to November 2, 1999. 67% of the aftershocks withmagnitude greater than 4 have occurred within the first 6 days after the main-shock. Earthquakes of the Izmitsequence are distributed in the first 15 km of the earth crust, but major events are located in between 5 km and15 km depth. The seismicity pattern defines a rupture plane extending for about 150 km in an E-W direction.The rupture is extremely linear but segmented, and its complexity increases towards the western end manifestingbifurcation. A stress analysis has been carried out both at the western end and all along the aftershock zone. 96selected aftershocks, registered between August 21 and October 22, were chosen in order to compute their focalmechanisms and obtain information about the stress regime after the Izmit earthquake. Strike-slip and normalfaulting mechanisms are dominant. The numerous strike-slip mechanisms are compatible with a dextral motion onan EW oriented vertical fault plane. The best stress tensor solution shows a regime in extension with a well-definedσ 3 axis oriented approximately N35◦.

Introduction

The Marmara region is a transition zone between thestrike-slip regime of the North Anatolian Fault (NAF)Zone to the east, and the extension regime of theAegean Sea to the west. Tectonics in this region ischaracterized by the splitting of the NAFZ into threebranches running more or less in an E-W direction(Figure 1).

Activity of the NAF during the 20th century beganwith the destructive Erzincan earthquake (Mw = 8.2,according to our estimation) in 1939 in northeastTurkey, and it migrated westwards by a series of earth-quakes in 1942, 1943, 1944, 1951 and 1967 (Barka,1996; Stein et al., 1997; Toksöz et al., 1979). Previ-ously, there had been an important rupture along thewestern end of the NAF, across the Gallipoli penin-sula, in 1912 (Ms = 7.4). Thus, before 1999, a seismic

gap remained between the 1967 rupture zone and thebroken 1912 segment. Part of this seismic gap hasbeen filled up by the Izmit (Kocaeli) earthquake onAugust 17, 1999 (Mw = 7.6, Delouis et al., 2002;Harvard CMT), but there still remains a gap betweenthe western end of the Izmit rupture and the Gallipolipeninsula, that represents a high risk for the city ofIstanbul with more than 10 million inhabitants.

After the study of the 1992 Erzincan earthquake(Fuenzalida et al., 1997), we centered our attention onthe Marmara Sea and performed a micro-seismicityfield experiment there in 1995 (Gürbüz et al., 2000),within a Turkish-French cooperation. We followed thiswork in 1999 by deploying a network of 20 shortperiod stations. The installation of this network wascompleted on July 15, 1999 one month before thedestructive Izmit earthquake. The first results of thisexperiment are under publication (Polat et al., 2002).

288

Figure 1. Tectonic features of the Marmara Sea region, together with the USGS DEM 30sec topography (after Barka, 1992). The northernbranches of the North Anatolian Fault are seen as scarps in the relief southeast of Istanbul, and across the deep basins on the northern half ofthe Marmara Sea in the west. It cuts the Gelibolu peninsula and goes into the Saros basin (the political boundary of the city of Istanbul is drawnin gray color). AP: Armutlu peninsula, C: Çinarcik, CD: Çatal Delta, D: Degirmendere, GB: Gemlik Bay, GP: Gelibolu peninsula, HD: HersekDelta, IB: Izmit Bay, PI: Prince Islands, SL: Sapanca Lake, Y: Yalova, 1967: Mudurnu Valley rupture zone (M∼7.1).

This paper presents the final results of the 1999Marmara Sea experiment. We show here the analysisof the aftershocks of the Izmit earthquake, and the res-ults of the stress tensor inversion both for the wholeregion and for the western part of the Izmit bay. Wealso show the distributions of magnitudes and depths,as well as field observations along the surface rupturesof the Izmit fault.

Aftershock analysis of the 1999 Izmit earthquake

A seismic monitoring field experiment was carriedout in the Marmara Sea region between July 15 andNovember 2, 1999. Here, we show the complete ana-lysis of the events before and after the August 17,1999 Izmit earthquake. The phase arrivals from 31.000seismograms have been read for events that were wellrecorded by the 20 short period stations.

We performed a statistical analysis in order to re-tain the best-located events. We selected events with aRMS error smaller or equal to 0.5 sec, and with at least7 P and 3 S phase arrivals. Based on these criteria,1446 events were well located over a total of 3165recorded (Figure 2). Most of the aftershocks follow asuccession of linear trends between Senköy (Çinarcik)and Gölyaka, with concentration of activity NE of the

Armutlu peninsula near Yalova and Çinarcik, east ofIzmit Bay, and Akyazi. Roughly, aftershocks follow atrend of N85◦ from Yalova to Izmit, a trend of N105◦from Izmit to Akyazi crossing Sapanca Lake, and takea NE direction from Akyazi to Gölyaka. The epicenterdistribution is less precise at the eastern end due toweaker station coverage. Three important aftershockclusters have been observed at the west of Hersekdelta: a cluster at the south of the Prince Islands on theNW-SE direction, another one on the E-W direction ofthe NAFZ and a third one between Senköy and Yalova.We also recorded some seismic activity NW of theMarmara Island following an event of magnitude (Md)5.0 on September 20, 1999. There is also a small seis-mic linear cluster activated between Iznik Lake andGemlik bay.

Based on the seismicity analysis, 67% of the af-tershocks with duration magnitudes greater or equalto 4 occurred within the first 6 days following themain-shock (Figure 2 inset). Among these, 55 eventshave magnitudes between 3.5 and 4.0, and 395 earth-quakes have magnitudes between 3.0 and 3.5. Abouthalf (725) of the well located aftershocks have mag-nitudes between 2.5 and 3.0. Finally, 271 events havemagnitudes between 2.0 and 2.5.

289

Figure 2. After-shock distribution of the 1999 Izmit earthquakes (August 17-October 23, 1999) as recorded by a local network. The figureshows the stations that were used for the location. Most of them functioned since July 15, 1999. Aftershocks with magnitudes greater or equalto 4 can be seen in the inset of the figure. Three clusters are seen, together with the epicenter (large star). See Figure 1 for the names of places.

Located events are examined week by week inorder to better understand the time evolution of theaftershocks (Figure 3). We could not observe any re-markable seismic activity before the Izmit main shockexcept for one or two events west of Gölcük andSapanca Lake. Recorded events within the month be-fore the Izmit earthquake have been located mostlywithin the northern half of the Marmara Sea region(Figure 3a). Figure 3b shows the Izmit earthquakewhich is located 40.710◦N–29.957◦E (large star), andits aftershocks during the time interval 17–24 August.The total number of located events during this periodis 331. We observe clusters near Yalova, near Izmitand east of Sapanca Lake. The main elongation ofthe Yalova cluster is to the north, in the direction ofPrince Islands with a concentration to the south ofBüyükada Island. Seismic activity is globally alignedE-W but a cluster east of Sapanca Lake is oriented NE-SW. Figure 3c includes 296 events in the period goingfrom 25 to 31 August. There we observe clearly threecluster zones: Armutlu, Izmit and Akyazi. We didnot record any remarkable aftershock activity withinthe segments Hersek delta-Izmit city and Izmit city-Akyazi. Figure 3d shows 206 aftershocks within theweek of September 1 to 8. Again we observe a gapalong Izmit Bay and the clusters at Yalova, Izmit andeast of Sapanca Lake. Figure 3e includes 168 eventsfrom September 9 to September 16. The same clusters

are present, but seismicity becomes more diffuse. 125events have been located within the week of Septem-ber 17 to 24 (Figure 3f). Seismic activity decreases asexpected but the Yalova cluster is still well defined,the Izmit Bay quietness zone still present, and a Md =5.0 shock generates a new small cluster NW of Mar-mara Island. Figure 3g includes 77 events in the periodSeptember 25 to 30. All of the activity is observed nearthe epicentral area and near Yalova, except for a fewevents at the far east and far west. Figure 3h shows thelocated aftershock distribution from October 1 to 23for about 3 weeks. 231 events have been located withinthis period. The Yalova, Izmit and Akyazi clustersare well defined as well as a small cluster near Mar-mara Island in the west. Finally Figure 3i shows thetotal distribution of activity during the period August17 to October 23. The three main orientations men-tioned above intersect at the epicenter of the mainshock (large star), and near Akyazi. The epicenterscorresponding to the activity before the main shockare included as empty squares for comparison.

A very important feature could be observed whenwe considered a statistical picture of the evolution ofseismicity as a function of time. Figure 4 shows thesedata for the period July 15-October 23, 1999. It ispossible to detect a 18 days quiescence period wherethe level of background seismicity lowers sensibly justprior to the main shock. Indeed, the average number

290

Figure 3. Seismicity of the Marmara Sea region before and after the Izmit earthquake (large star) displayed by time windows one week longeach. BI: Büyükada Island, G: Gölcük, IL: Iznik Lake, MI: Marmara Island, see also Figure 1. a) Seismicity before the main shock (emptyrectangles) July 15-August 16. b) 17–24 August. c) 25–31 August. d) 1–8 September. e) 9–16 September. f) 17–24 September. ∼g) 25–30September. h) 1–23 October. i) Cumulated seismicity: a) to h).

291

Figure 4. Histogram of the number of events with time for the time period between 15 July 1999 and 23 October 1999. A quiescence period of18 days is observed just before the main shock. The hyperbola shows the least squares adjusted Omori’s law.

of earthquakes during quiescence is 4.4 times smallerthan that of previous days. Then comes the main shockand the aftershock sequence that decreases accordingto the well known Omori law. Some deviations are dueto the sequel of large aftershocks of magnitude about5 (August 31, September 9, 18, 20, October 5).

Depth distribution along the Izmit aftershock zone

We retained as the ‘best’ solution the one combininglow RMS and the higher possible number of P andS arrivals. In order to avoid bad quality and poorlyconstrained hypocenters we submitted the hypocentraldeterminations to a sorting based on the following cri-teria: RMS less than or equal to 0.5 s, total numberof phases (P+S) taken into account greater or equalto 7, including at least three S-phases, horizontal andvertical errors less than or equal to 2 km and 5 kmrespectively. Based on these criteria we obtained 1316hypocenters over a total of 2405 by using a depth

sweeping procedure (Delouis, 1996) with the Hypo-inverse location program (Klein, 1978). We presentthe corresponding epicenter and depth distributions inFigure 5. The depth of the main shock was fixed at15 km, following the maximum depth of the after-shocks (Delouis at al., in press, give 12 km; KOERIgives 18 km). As we mentioned before, the IzmitBay quietness zone is clearly observed in the Fig-ures 5a and 5b. The hypocentral distribution does notextend east of Akyazi because the poor station cover-age there makes accurate depth determination difficult.The Izmit aftershocks lie above a depth of 15 km, andalmost all of them (90%) are located within a bandzone between 5 km and 15 km depth, as can be seenon the E-W depth-cross section in Figure 5b.

The upper 5 km of the crust shows a low seismicactivity. A similar observation is also reported by Öz-alaybey et al. (2002). This low activity zone can beobserved between 27.5◦E and 30.0◦E. However, mosthypocenters are located closer to the surface from

292

Table 1. Focal mechanism of 96 events along the aftershock zone and west of Izmit bay

Date y/m/d Time h:m Lat. ◦N Lon. ◦E Azim. (◦) Dip (◦) Rake (◦) Mag. Ref

990821 23:34 40.67 29.06 88.40 56.11 –110.43 4.0 1

990829 20:16 40.62 29.08 95.55 83.80 –141.24 3.0 2

990830 07:18 40.63 29.12 358.95 65.56 15.82 3.2 3

990830 09:00 40.49 29.17 215.33 53.62 –156.44 3.0 4

990830 15:24 40.71 29.32 138.59 39.73 –56.70 3.3 5

990831 08:33 40.75 29.97 55.40 83.44 162.98 4.5 6

990831 11:06 40.68 29.33 271.14 51.88 –153.36 2.7 7

990831 22:28 40.62 29.09 308.89 56.62 –102.85 4.0 8

990831 23:14 40.62 29.09 155.29 47.29 –52.63 3.3 9

990901 03:23 40.60 29.10 259.99 78.58 –178.24 3.8∗ 10

990901 16:06 40.62 29.15 133.88 69.35 –72.44 3.0 11

990901 23:11 40.75 29.97 339.30 78.66 33.30 3.0 12

990902 16:29 40.72 29.76 58.96 49.76 –76.42 3.4∗ 13

990903 04:18 40.83 28.79 318.47 48.32 –82.20 3.3 14

990903 10:07 40.69 29.21 85.70 68.71 –131.26 3.5 15

990903 11:21 40.61 29.05 115.44 36.64 –67.26 3.1∗ 16

990903 16:45 40.74 29.78 51.82 59.11 163.13 3.0∗ 17

990904 01:01 40.79 30.17 333.41 47.74 –52.56 3.6 18

990904 01:45 40.71 29.40 276.71 86.51 158.11 3.4 19

990904 15:39 40.75 29.96 88.54 86.27 –154.62 3.2 20

990904 20:44 40.77 29.24 277.42 85.38 159.63 3.0 21

990904 23:49 40.70 29.39 111.28 85.01 –115.78 3.2 22

990905 05:29 40.59 28.90 106.47 58.80 –96.25 3.2 23

990905 15:38 40.63 29.12 115.75 67.87 –92.26 3.0 24

990905 22:45 40.45 29.25 214.90 70.62 –120.39 3.0 25

990906 06:33 40.73 29.78 223.73 86.13 –131.47 4.1 26

990906 06:38 40.73 29.79 338.13 78.13 32.70 3.0 27

990906 14:08 40.69 29.40 102.64 62.02 –103.12 3.0 28

990906 15:57 40.60 29.07 135.25 37.31 –56.00 3.4 29

990906 20:38 40.62 29.03 54.42 86.44 160.06 3.3 30

990907 23:25 40.74 29.92 184.60 31.98 43.67 2.7 31

990908 15:35 40.62 29.11 137.41 29.75 –39.55 3.2 32

990908 23:43 40.71 29.53 327.29 72.98 23.66 3.1 33

990909 01:02 40.72 29.53 321.25 70.65 14.20 3.3 34

990909 01:32 40.70 29.16 339.13 63.63 –0.71 4.2 35

990909 19:18 40.72 30.02 337.76 76.66 30.27 3.4 36

990910 11:47 40.61 29.15 117.81 48.75 –78.76 3.1 37

990911 07:26 40.71 29.36 192.57 37.71 –3.87 3.2 38

990912 14:38 40.71 29.42 192.59 37.72 49.43 3.0∗ 39

990912 16:20 40.73 29.70 222.78 79.57 –135.48 3.1 40

990912 21:37 40.71 30.00 195.58 48.86 –72.34 2.9 41

990913 01:27 40.61 29.06 326.76 56.76 –44.90 3.1 42

990917 19:50 40.77 30.09 347.65 70.82 20.52 3.6 43

990918 00:48 40.62 29.16 129.04 70.38 –77.60 4.1 44

990918 02:07 40.64 29.08 341.51 51.22 –31.75 3.1 45

990920 15:04 40.64 29.09 67.20 86.13 –177.92 3.0 46

990920 20:36 40.67 27.50 285.11 86.43 146.34 2.3 47

990920 21:28 40.66 27.51 244.03 64.57 –166.91 5.0 48

990920 22:16 40.65 27.51 47.39 59.60 37.76 3.2 49

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Table 1. Continued

Date y/m/d Time h:m Lat. ◦N Lon. ◦E Azim. (◦) Dip (◦) Rake (◦) Mag. Ref

990920 23:40 40.66 27.52 299.93 74.12 164.97 3.4 50

990921 01:09 40.64 27.49 198.56 68.17 –66.64 3.5 51

990922 01:04 40.64 29.06 161.32 47.54 –49.87 3.1 52

990922 01:49 40.65 29.10 126.55 52.20 –74.01 3.0 53

990922 17:44 40.64 29.18 82.75 63.85 –127.41 3.1 54

990922 23:02 40.56 27.77 65.02 88.48 177.53 3.2 55

990923 03:20 40.75 29.80 107.14 82.56 –119.99 3.1 56

990923 05:05 40.62 29.15 87.37 63.21 –121.39 3.0 57

990923 05:16 40.63 29.05 316.41 61.31 –65.93 3.0 58

990923 06:24 40.75 29.85 249.46 85.76 –178.00 3.3 59

990923 20:24 40.63 29.17 137.37 30.29 –41.08 3.4 60

990924 00:01 40.46 29.16 226.26 67.04 –155.37 3.2 61

990924 03:26 40.62 29.16 258.39 79.03 –178.00 3.1 62

990924 13:44 40.77 30.24 335.79 66.31 4.63 3.2 63

990924 18:28 40.67 27.46 39.66 27.89 –25.09 3.3 64

990924 20:10 40.75 29.25 336.34 38.39 –61.94 2.8 65

990925 09:49 40.62 29.09 143.41 40.58 –54.45 2.9 66

990925 17:05 40.61 29.07 321.82 63.43 –31.27 3.1 67

990927 21:44 40.41 29.06 51.83 59.06 –111.32 3.1 68

990927 23:48 40.37 28.11 297.27 85.29 120.04 3.3 69

990929 00:09 40.79 29.78 201.91 39.33 173.59 3.3 70

990929 11:01 40.70 29.42 189.67 82.61 –50.67 3.1 71

990929 17:36 40.72 29.76 309.69 70.54 –76.57 2.9 72

990930 07:55 40.71 29.88 12.10 44.74 –13.93 3.5 73

990930 18:39 40.63 29.19 107.93 32.85 –63.39 2.7 74

991001 06:10 40.61 29.15 115.88 34.32 –63.01 3.3 75

991001 08:12 40.72 29.88 317.24 53.20 –80.09 3.4 76

991001 17:04 40.69 29.35 85.48 85.75 –157.32 3.1 77

991002 23:18 40.35 28.43 227.95 89.46 –143.05 2.9 78

991003 16:19 40.77 29.84 54.60 57.36 –113.21 3.0 79

991003 22:35 40.70 29.35 218.96 61.51 –149.18 3.1 80

991005 04:10 40.76 29.81 51.79 56.96 –29.08 3.7 81

991005 06:42 40.76 29.22 90.70 34.05 –68.60 3.1 82

991005 12:26 40.63 29.17 191.04 36.54 42.26 3.1 83

991006 14:16 40.43 28.70 256.59 58.63 –164.14 3.2 84

991006 18:35 40.63 29.08 271.29 70.77 –171.58 3.2 85

991006 23:22 40.63 29.08 328.70 50.29 –56.77 2.9 86

991007 11:13 41.06 29.34 192.54 37.59 101.24 3.0 87

991007 12:41 40.63 29.08 174.75 56.71 –50.13 3.2 88

991008 09:20 40.73 29.04 265.64 48.59 –155.30 3.3 89

991008 17:43 40.76 30.03 255.46 89.46 173.52 3.4 90

991009 05:23 40.77 29.23 51.92 58.71 –51.00 2.8 91

991020 23:08 40.80 29.08 192.60 37.74 –87.96 4.4 92

991020 23:25 40.80 29.04 123.35 25.03 –32.73 3.0 93

991021 07:33 40.37 28.11 51.78 59.06 21.12 3.7 94

991021 08:20 40.35 28.58 183.04 32.92 17.44 3.1 95

991022 06:43 40.37 28.11 92.09 78.44 –137.36 3.4 96

∗ Magnitude is taken from KOERI.

294

Figure 5. a) Epicenters. b) Hypocentral distribution of the Izmit aftershocks (the depth of the main shock is fixed at 15 km, at the lower depthof aftershocks). The boundaries of the three parallel depth-cross sections (A-A’, B-B’, C-C’) are seen on Figure 5a.

Izmit bay to Akyazi. Near this city, earthquakes lieabove a depth of 10 km. The largest clusters of after-shocks are located between Yalova and south of PrinceIslands. The deepest earthquake of the entire sequencehas been determined here with a depth a 23 km onAugust 21, 1999 (Md = 2.6). Further west, a smallhypocentral cluster is located from 6 to 14 km depth,below the NW margin of Marmara Island, except forone or two events, near the surface (Figure 5b).

In order to illustrate the spatial distribution of thehypocenter locations region by region, we show threeparallel-cross sections (A-A’, B-B’, C-C’) across the

Izmit aftershock zone (Figure 5a and 6). The firstsection (A-A’) is between the longitudes 28.95◦E and29.32◦E and it contains 808 hypocenters. The eventsare at a depth of about 15 km south of Prince Islands.Further south, we observe depths between 5 and 15 kmto the SW of Darica peninsula. Seismicity increasessharply near Yalova and Cinarcik. The cross section B-B’ shows 135 depths between the longitudes 29.32◦Eand 29.85◦E. We observe seismic activity only to thewest of the Hersek delta along the Izmit bay. Althoughmost of the events are located from 5 km down to15 km in depth, few hypocenters are within the first

295

Figure 6. Depth distribution of the Izmit aftershocks on three NS cross-sections (A-A’, B-B’, C-C’). The number of events in each section is808, 135 and 176 respectively.

296

Figure 7. a) Map of 96 focal mechanisms obtained for the period August 21-October 22, 1999 all along the aftershock zone. The mechanismsare shown in an equal area projection on the lower hemisphere. Most of the mechanisms show a dominant strike slip or normal componentexcept for four clear reverse faults (N◦ 31, 39, 83, 87). b) Stress tensor parameters, shape factor R (equal to 1.6 ± 0.1) and likelihood value(94%). Maximum values are normalized to 1. The score indicates the normalized number of polarities consistent with the stress tensor. Theerror ellipses correspond to one standard deviation.

5 km. Finally, further east, the C-C’ cross sectionshows 176 events between the longitudes 29.85◦E and30.45◦E. Hypocenters start near the surface north ofIzmit bay, then depth increases up to 12 km towardsGölcük. The depth of the events decreases from theeast of Sapanca Lake to Akyazi. All depths in the C-C’section are within first 10 km of the earth crust.

Stress tensor and focal mechanisms inversion

In order to investigate the stress field and the character-istics of faulting along the Izmit rupture zone, we usedan algorithm which performs the simultaneous inver-sion of the orientation and shape factor R of the stresstensor and of individual focal mechanisms for a pop-

ulation of earthquakes (Rivera and Cisternas, 1990).The shape factor R defined by R = (σz-σx ) / ( σy -σx),where σz is the principal stress closest to the vertical,and σx , σy are the other two principal stresses, withthe condition that σy> σx .

State of stress along the Izmit aftershock zone

A selected number of 96 events, recorded between Au-gust 21 and October 22, 1999, are used to determinethe focal mechanism solutions and the present state ofstress after the Izmit earthquake. All of the selectedevents have at least 10 first motion polarities and areinverted jointly. Figure 7a shows the epicenter distri-bution together with their focal mechanism solutions.A list of focal parameter is given in Table 1 and in the

297

Figure 7. Continued.

Appendix. As expected, strike-slip and normal fault-ing mechanisms are dominant except for four cases ofreverse faulting (N◦ 31,39,83,87).

Figure 7b presents the best stress tensor solutions,showing the shape factor R equal to 1.6 ± 0.1. The σ 3axis is almost horizontal and oriented approximatelyN35◦, but the σ 1 and σ 2 axes are inclined about 45◦,and it is difficult to say which one is closer to the ver-tical. Thus, the well defined orientation of σ 3 impliesextension in a N35◦E direction, but the stress regimeis in between extension (σ 1 is closer to the vertical)and strike-slip (σ 2 is closer to the vertical).

State of stress at the west of the Izmit aftershocks

We also carried out a specific inversion to infer thestress regime at the western end of the Izmit aftershockzone. For this, we used the same algorithm for eventslocated in between longitudes 28.30◦E and 29.30◦E,with a selected number of 50 earthquakes located nearÇinarcik and Yalova during the period from August

21 to October 20, 1999. The same selection criteria asabove have been used for this study.

Figure 8a presents the results of the mechanismsolutions with their epicentral distributions. The focalparameter list is given in Table 1. Two clear reversefault solutions (Nb 39,83) are still present here, butthe most dominant mechanisms are in normal fault-ing. Figure 8b shows the best stress tensor solutions,with a shape factor R equal to 1.8 ± 0.3 which rep-resents an extension regime. The σ 3 axis is orientedapproximately N35◦ as before. Although σ 1 is closerto the vertical (extension regime), it is again obliqueand it is not possible to decide between extension andstrike-slip regimes.

Seismotectonic analysis

A field study has been performed along the Izmit faultin order to relate surface ruptures to the aftershockdistributions. The Izmit earthquake generated a re-markably linear set of surface ruptures about 150 km

298

Figure 8. a) Map of 50 focal mechanisms obtained for the period August 21-October 20, 1999 at the western part of the Izmit Bay. Most of theevents are located near Çinarcik and Hersek delta. b) Stress tensor parameters for the region (see Figure 7b for definitions), shape factor R (1.8± 0.3) and likelihood (96%).

long, with a dominant dextral strike-slip character(Barka et al., 2000; Delouis et al., 2002; Gülen etal., 2002; Polat et al., 2002). These ruptures could beobserved mainly east of Izmit up to Gölyaka, but alsoin the Degirmendere-Gölcük region and in the militaryairport at Çatal delta (Figure 9, Figure 8a). The mainbranches of the Izmit fault rupture observed in landare: 1. Degirmendere-Tiktik, 2. Tiktik-Sapanca Lake,3. Sapanca Lake-Akyazi, 4. Akyazi-Gölyaka.

Seventy-six reliable measurements (Table 2) weremade on the Izmit fault segments from Yalova toGölyaka (Figure 10). The maximum horizontal offsetof ∼5 m was measured in the Navy base of Gölcükand near Arifiye (east of Sapanca Lake) with a ruptureazimuth of N98◦ and N82◦ respectively. The offset ofthe Izmit fault rupture gradually decreases further east,as seen in Karadere east of Akyazi, and further eastin Degirmentepe east of Gölyaka, with a maximumvalue of 1m20. The observed azimuths of the ruptureare N74◦ in Karadere and N66◦ in Degirmentepe.

Numerous examples of man-made structures likeroads, houses, alignement of trees, walls, fences,channels, plowed fields, etc., were shifted acrossthe fault and permitted quantitative measurements allalong the Izmit surface rupture. The measured valuesare close to those obtained by Barka et al. (2000),though they present a more complete data set. Eventhough most of the observations confirmed the strike-slip character of the fault (Table 2), there is at least oneplace with an important branch in normal faulting. Thewall of the Stadium of Gölcük was a good indicatorto determine the vertical offset of 1m76. Horizontaloffset is 80 cm and the azimuth of the surface ruptureis N131◦. We considered this to be either a secondaryfault or the limit of a landslide (Polat et al., 2002).

Discussion and conclusions

The seismic monitoring of the Marmara Sea regionwith a local network provided information about the

299

Figure 8. Continued.

Figure 9. Observed (solid lines) and inferred (broken lines) surface ruptures, together with the epicenters located within the period August17 to November 2, 1999. Four clear segments are shown: 1. Degirmendere-Tiktik. 2. Tiktik-Sapanca Lake. 3. Sapanca Lake-Akyazi. 4.Akyazi-Gölyaka. Site names (see also Figures 1 and 3): A: Akyazi, AD: Adapazari, GY: Gölyaka, H: Hendek, IZ: Izmit, K: Kullar, KD:Karadere, KM: Karamürsel, T: Topçular.

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Figure 10. Field measurements along the Izmit surface rupture. The slip is shown at selected places. Site names (see also Figures 1, 3 and 8and Table 2): Ar: Arifiye, Dt: Degirmentepe, G: Gölyaka, IB: Izmit Bay, Kd: Karadere, Tt: Tepetarla, tk: Tiktik.

seismicity before, and after, the 1999 Izmit earth-quake. The main results are discussed below.

The Izmit earthquake generated a remarkable setof surface ruptures over 150 km (see also Barka et al.,2000). Observed deformations mostly correspond to‘en échelon’ tension gashes alternating with pressureridges, thus showing a dominant dextral strike-slipcharacter. Quantitative measurements of the offsetsgive a maximum value of 5 m. These values varyalong the fault, indicating segmentation, even thoughthe segments do not change orientation in an importantway, except for the eastern Gölyaka branch. Some dis-cussion is found in the literature between partisans of asingle fault, and those who propose different branches(Imren et al., 2001; Okay et al., 2000; Gökasan et al.,2001). A multidisciplinary approach shows that theIzmit rupture cannot be considered as a long, linear,single fault.

Clustering in the epicentral distribution (1446 se-lected events over a total of 3165), and the con-centration of aftershocks around the ruptured areasat depth, also contribute to the recognition of seg-ments. Aftershocks lie above a depth of 15 km, andalmost all events (90%) are located within a bandzone between 5 km and 15 km. Most of the importantclusters are detected NE of Armutlu peninsula, nearYalova and Çinarcik to the east of Izmit Bay, and nearAkyazi (Bolu). The clustering at the western end ofthe Izmit Gulf shows a complex structure, suggestingthe presence of at least three branches of the NAF,

and connecting the Izmit rupture to the dormant faultsunder the Marmara Sea.

Four segments have been identified mainly fromsurface ruptures. The first one is located between De-girmendere and Tiktik. The second one is betweenTiktik and West Sapanca Lake. A third one, goesfrom East Sapanca Lake to Akyazi, and the fourthone runs from Akyazi to Gölyaka. A fifth segment canbe deduced from the aftershock clustering, betweenÇinarcik and Hersek.

The cluster located to the south of Prince Islandsmay lead to the activation of the northern part of theMarmara Sea, the last remaining gap between the 1912Gallipoli earthquake and the sequence of ruptures thatbegan at Erzincan in 1939. Three aligned pull-apartbasins, corresponding to segments of the NAF, maybreak over there, independently, or as a whole.

We could not observe any significant seismic activ-ity along the rupture zone of the Izmit earthquakebefore the main shock, with the exception of one ortwo events west of Izmit Bay and Sapanca Lake. Nev-ertheless, the month before the Izmit earthquake ischaracterized by an 18 days quiescence period, justprior to the main shock. This result is well controlledthanks to the density of our network after July 15,1999. The time distribution of the aftershocks fol-lows Omori’s law, except for perturbations due to theactivity following events of magnitude around 5, inparticular the Marmara Island shock of September 20.

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Table 2. Horizontal (h) and vertical (v) offset measurements along the Izmit fault

No. Fault location Slip information Explanation

Lat. (◦) Lon. (◦) Az∗ (◦) H∗ (m) V∗ (m)

1 40.693 29.372 West of Aerial base, Topçular

2 40.686 29.401 Compressive fissures, Topçular

3 40.722 29.782 82 Degirmendere

4 40.725 29.794 3.00 Wall, Yüzbasilar

5 40.725 29.796 82 5.00 Offset of building, Yüzbasilar

6 40.726 29.796 82 4.30 Offset of wall, Yüzbasilar

7 40.726 29.797 98 Yüzbasilar

8 40.726 29.799 98 5.00 Wall of Navy Base Yüzbasilar

9 40.724 29.836 130 Slump, Gölcük

10 40.722 29.840 131 0.80 1.76 Gölcük Stadium

11 40.721 29.840 144 Normal fault Gölcük

12 40.719 29.844 144 1.50 Ford Auto Plant, Normal fault, Gölcük

13 40.713 29.854 120 2.00 Normal fault Gölcük

14 40.712 29.854 120 2.00 Normal fault Gölcük

15 40.711 29.855 152 2.20 Normal fault Gölcük

16 40.711 29.856 152 1.70 Gölcük

17 40.711 29.857 Gölcük

18 40.709 29.858 119 0.70 1.80 Gölcük

19 40.709 29.858 Gölcük

20 40.709 29.859 157 1.80 Gölcük

21 40.708 29.859 1.76 Gölcük

22 40.707 29.860 80 0.70 Gölcük

23 40.707 29.861 110 0.70 Gölcük

24 40.707 29.862 80 0.50 Gölcük

25 40.708 29.863 Normal fault, Chicken factory, Gölcük

26 40.721 29.939 61 Yuvacik

27 40.722 29.942 142 Yuvacik

28 40.721 29.945 Yuvacik

29 40.721 29.947 98 2.30 Offset of Channel, Yuvacik

30 40.721 29.948 82 2.60 Yuvacik

31 40.721 29.960 83

32 40.721 29.963 85

33 40.723 29.967 Kullar

34 40.723 29.967 Kullar

35 40.721 29.972 Small wall Kullar

36 40.721 29.972 85 2.30 Small wall Kullar

37 40.722 29.987 2.20 Kullar Mosque

38 40.723 30.007 85 1.30 Tiktik village

39 40.723 30.014 82 1.40 Tiktik

40 40.723 30.014 82 1.30 Tiktik

41 40.723 30.016 82 Tiktik

42 40.723 30.020 Tiktik

43 40.721 30.033 85 0.30 Tiktik

44 40.722 30.046 84 3.30 Tiktik

45 40.722 30.053 95 3.30

46 40.721 30.056 95 1.70

47 40.721 30.066 94 2.40 Tepetarla

48 40.720 30.072 94 2.40 Tepetarla

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Table 2. Continued

No. Fault location Slip information Explanation

Lat. (◦) Lon. (◦) Az∗ (◦) H∗ (m) V∗ (m)

49 40.719 30.127 74 2.30 Acisu

50 40.721 30.134 76 3.00

51 40.721 30.135 83 2.40 Religious School

52 40.719 30.135

53 40.721 30.140 83 2.85 0.80

54 40.719 30.148 88 1.20 Karaburun, NW of Sapanca lake

55 40.719 30.152 88 1.20 Karaburun

56 40.712 30.339 2.70

57 40.711 30.343 0.80

58 40.709 30.363 87 3.00 Sapanca

59 40.710 30.370 85 4.30 Sapanca

60 40.709 30.370

61 40.709 30.382 1.90 Toyota Auto Plant

62 40.708 30.403 82 4.80 South of Toyota

63 40.707 30.424 82 5.20 Türk Çaybasi, East of Toyota, Arifiye

64 40.705 30.436

65 40.703 30.451 102 2.50

66 40.703 30.451 95

67 40.702 30.454

68 40.700 30.482 2.50

69 40.695 30.619 Akyazi

70 40.706 30.726 0.80 Camili village, Akyazi

71 40.727 30.826 74 1.20 Karadere, east of Akyazi

72 40.748 30.889 62 1.20 Gölyaka

73 40.749 30.891 62 1.40 Gölyaka

74 40.748 30.892 Gölyaka

75 40.750 30.896 66 Gölyaka

76 40.754 30.909 66 1.20 Degirmentepe village of Gölyaka

∗ Az: Azimuth of the fault rupture from North, H: horizontal slip, V: vertical slip.

The calculated stress tensor obtained from the af-tershocks is characterized by a stable σ 3 axis orientedN215◦. The σ 1 and σ 2 axes are inclined, and not welldefined by the inversion process. Thus, we may con-clude that the stress regime after the Izmit earthquakeis in between extension and strike-slip all along theaftershock zone.

Acknowledgements

We dedicate this work to the memory of Aykut Barka(† 01-02-02), a great seismologist and friend.

We thank to the Ministry of Foreign Affairs ofFrance via the Embassy at Ankara (Turkey) for thefinancial support. This work was also supported bythe CNRS-INSU (France), and the TUBITAK (Tur-key) YDABCAG project number 199Y075. We thank

to Gülsün Saglamer and Naci Görür for providingmany facilities at the Campus of the Istanbul TechnicalUniversity, and to Ahmet Mete Isikara for accessingand using the possibilities of the Kandilli Observatoryand Earthquake Research Institut of Bogazici Univer-sity. The authors wish to thank to Bertrand Delouisand Michel Bouchon for helpful comments on themanuscript.

Appendix

Focal mechanisms of 96 events after the inversion.The 95% confidence ellipse of the pole of the faultplane, and the slip vector are shown. All mechan-isms are represented on the lower hemisphere equal-area projection. Black filled squares are compressionpolarities.

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