The Dinar Earthquake (M w =6.2; October 1, 1995; Afyon-Turkey) and Earthquake Hazard of the Dinar-Çivril Fault

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1 Pure appl. geophys. 152 (1998) /98/ $ /0 The Dinar Earthquake (M w =6.2; October 1, 1995; Afyon-Turkey) and Earthquake Hazard of the Dinar-Çivril Fault ALI OSMAN ÖNCEL, 1 HAYRETTIN KORAL 2 and ÖMER ALPTEKIN 1 Abstract A moderately strong earthquake (M w =6.2) occurred in the town of Dinar at UT on October 1, 1995, taking the lives of 90 people and damaging about 4500 buildings. Its epicenter is located near the Dinar-Çivril fault and its focal mechanism is linked to a northeast-southwesterly tensional stress field arising from the interaction between the subducting African plate and the overriding Aegean-Anatolian plate in the eastern Mediterranean. Surface cracks of the October 1 earthquake have been observed 10 km continuously along the Dinar-Çivril fault. The cracks have displayed a mode of dip-slip; however, some have also indicated lateral slip. The different modes of slip are generally in agreement with the fault plane solution and are indicators of the complex nature of the rupture process. In investigating the earthquake hazard of the Dinar-Çivril fault and proximity, the maximum likelihood method was used to estimate seismic hazard parameters of b-value, seismicity activity rate m and the expected maximum magnitude M max. The data consisted of the historical data covering the period between and instrumental data between 1900 and This method, allowing use of the mixed earthquake catalogue containing both historical and instrumental earthquake data, yielded values of 0.70, 1.92 and 7.14 for b, m and M max, respectively. The recurrence time estimated for an earthquake of a magnitude of M w =6.2 is 123 years. The non-occurrence probabilities of such an earthquake in 1 and 50 years are 0.21 and 0.04, respectively. Key words: Surface cracks, earthquake hazard parameters, south-western Anatolia. 1. Introduction The town of Dinar which has a population of 35,000 is located in the region of lakes in southwestern Anatolia (Figs. 1 and 2). Dinar has been the site of large historical earthquakes and was recently struck by an earthquake with a magnitude of 6.2 (M w ) at 17:57 UT on October 1, The earthquake s epicentral coordinates were N and E and it caused substantial loss of life and extensive property damage, killing nearly 90 people and damaging about 4500 buildings. It was preceded by over 77 foreshocks of M L 1.5 and noted by its 10-km long surface rupture zone which had been observed along a fault transecting the region. This fault begins east of Dinar and extends toward the town of Çivril 1 I stanbul University, Department of Geophysical Engineering, Avcılar-Istanbul, Turkey. 2 I stanbul University, Department of Geological Engineering, Avcılar-Istanbul, Turkey.

2 92 Ali Osman Öncel et al. Pure appl. geophys., in a northwest-southeasterly orientation. It is called the Dinar-Çivril fault and is considered to be a normal fault (Fig. 2). In this paper, we investigate several seismotectonic features of the Dinar earthquake of October 1, 1995 in an effort to understand its seismotectonic implications for southwestern Turkey. In addition, we evaluate earthquake hazard parameters for the Dinar-Çivril fault and its vicinity, using the available seismological information from historical and instrumental periods. 2. Tectonic Framework Dinar lies at the junction of the Hellenic (Cretan) and the Cyprus Arcs which is the site of a recently activated continental breakup of the Aegean-Anatolian plate (Fig. 1). The breakup is caused by different rates of subduction of the African plate beneath the Cretan and Cyprian arcs, as suggested by ALPTEKIN (1973) and elaborated by BÜYÜKAŞIKOǦLU (1979) and BARKA et al. (1997). The general geological outlook of the Dinar region and its close vicinity is influenced by the plate tectonic configuration of the Arabian and the Aegean-Anatolian plates in the eastern Mediterranean (Fig. 1). The ongoing collision between these plates forces the Anatolian block to move toward the west, causing a tensional stress regime and the formation of horst and graben systems in the Aegean Sea and in southwestern Turkey (MCKENZIE, 1972; ALPTEKIN, 1973; ŞENGÖR, 1980). Despite its internal complexity, GPS measurements in the region support the idea that the Aegean-Anatolian plate may be considered as a counterclockwise rotating single coherent unit moving to the west (ALPTEKIN, 1973) at a rate of cm per year (ORAL et al., 1995). However, this movement is restricted by the presence of the Rhodope block lying in the west and this situation has resulted in an intense internal deformation of the Aegean-Anatolian block (ŞENGÖR, 1980). In proximity to the Dinar region there are two prominent tectonic trends (Fig. 2). One of these is a NE-SW oriented lineament characterized by the Burdur-Fethiye, Acıgöl and Baklan faults. The other is a northwest-southeasterly lineament exemplified by the Dinar-Çivril fault. The region contains additional zones of deformation parallel to those prominent ones (PRICE and SCOTT, 1994). Dinar and its close vicinity possess a rather simple surface geology. The hills to the north and east are comprised of limestone, marl and conglomerates of the Eocene and Oligocene age (Fig. 3). The area to the south and west is covered by Quaternary alluvium, containing sand, gravel and clay. The area between the hills and the low ground is overlain by slope sediments of alluvial fans. The Dinar-Çivril fault occurs along the northeastern boundary of these sediments.

3 Vol. 152, 1998 The Dinar Earthquake Seismicity Seismicity of western and southwestern Anatolia and the surrounding region comprises deep-seated, mostly off-shore earthquakes and shallow continental earthquakes (Fig. 2). The deep-seated earthquakes occur in the subducting African plate of the Mediterranean Sea and along the Burdur-Fethiye fault zone. Shallow earthquakes, however, appear to be concentrated near crustal faults on the Aegean- Figure 1 Tectonic setting and seismicity for the eastern Mediterranean and Turkey. Compiled from BARKA and HANCOCK, 1984; PHILIP et al., 1989; LYBERIS et al., 1992; ŞAROĞLU et al., 1992; ÖNCEL et al., Seismicity covers the period between 1900 and Arrows indicate the direction of plate motions. The inset indicates the area in Figure 2.

4 94 Ali Osman Öncel et al. Pure appl. geophys., Figure 2 Seismicity of the Dinar and the surrounding region of Afyon-Isparta-Burdur provinces for the time period between 50 A. C. and 1995 (M 3) shown on simplified geology map taken after PRICE and SCOTT (1994) and EYIDOGAN and BARKA (1996). Solid lines between A A and B B indicate orientations used for investigating the depth distributions of earthquakes shown in Figure 5. The inset by solid line indicates the region of Dinar affected by the earthquake. The area inside the larger parallelogram bounded by dotted lines was investigated for earthquake hazard parameters. Circles represent seismicity related to the Dinar earthquake. Plus signs represent earthquakes with magnitudes greater than 4.5 monitored between 1900 and

5 Vol. 152, 1998 The Dinar Earthquake 95 Anatolian plate. The nature of the earthquakes is indicative of the general characteristics of the tectonic framework. The town of Dinar and the surrounding areas have a long earthquake history dating to about 1200 B.C. Appointed by King Meandros as the capital of his kingdom in the VIIIth century B.C., this town has survived many strong earthquakes in the Byzantium, Selçuk and Ottoman times. Historical seismicity data recorded in Dinar spanning two millennia (88 B.C A.C.) suggest that at least 18 earthquakes with intensities V and above in Modified Mercalli scale have affected the nearby region of Afyon-Isparta-Burdur and Dinar. Prominently, the 1875 earthquake resulted in a death toll of 1300 in a region between Çivril and Dinar provinces (Fig. 2). A 20-km long rupture zone was observed (PINAR and LAHN, 1952; AMBRASEYS, 1975). The instrumental seismicity data show 212 earthquakes in the area with magnitudes varying from 4.0 to 6.9. Among these earthquakes, on October 3, 1914 (M s =7.0; I 0 =IX) an earthquake was strongly felt in the town of Dinar. The 1914 earthquake caused a surface rupture along the southeast coast of the Burdur Lake and destroyed about houses and took about 4000 lives (SOYSAL et al., 1980). The most recent destructive earthquake in the region occurred on May 12, 1971 Figure 3 Detailed geological map of the Dinar region. Geology is modified from the 1/ Dinar-Afyon sheet prepared by State Water Works of Turkey. Surface cracks taken from DEMIRTAŞ et al. (1996) and KORAL et al. (1997).

6 96 Ali Osman Öncel et al. Pure appl. geophys., Figure 4 Temporal distribution of fore- and aftershocks of the Dinar earthquake. (M s =6.1; I 0 =IX), destroying over 1400 houses and killing 57 people. The 1971 earthquake had a normal faulting mechanism (Fig. 2) and was related to the Burdur fault (TAYMAZ and PRICE, 1992). On October 1, 1995, the town of Dinar was struck by another earthquake (M w =6.2), causing extensive damage to buildings and a death toll of 90 with over 200 injured (ÖNCEL et al., 1996). The earthquake was strongly felt and caused moderate damage in nearby villages such as Yakaköy, Kızıllı and Yapaǧlı. Figure 3 denotes by dotted line the boundary of the area in which extensive damage was observed. Seismicity related to the Dinar earthquake was recorded by 6 local and 30 permanent on line, dial up and radio-link stations (KOERI RECONNAISSANCE REPORT, 1995). The earthquake possessed unique features. Foreshock activity related to the main event intensified within six days prior to the earthquake (Fig. 4). During that time, 77 foreshocks with magnitudes ranging from 1.5 to 5.4 occurred. Following the earthquake, 671 aftershocks with magnitudes ranging from 2.9 to 5.1 occurred within the first twelve days. This data was used to examine the depth distribution of pre- and aftershocks in sections both normal and parallel to the strike of the fault (Fig. 5). Seismicity in the normal profile appears to be localized. The seismicity along the profile parallel to the Dinar-Çivril fault is scattered.

7 Vol. 152, 1998 The Dinar Earthquake Surface Cracks The earthquake of October 1, 1995 occurred near the NW-SE trending Dinar- Çivril fault which could be traced between the towns of Dinar and Çivril in the province of Afyon. A surface rupture zone was observed along this fault between the town of Dinar and the village of Yapaǧılı. It acquired three different orientations. The rupture zone near Dinar is roughly N-S oriented. To the west, it has a Figure 5 Depth distributions of earthquakes. A-A indicates a profile normal to the Dinar-Çivril fault line. B B indicates a profile parallel to the fault line. All earthquakes in the area defined by perpendicular lines to profiles are projected on a vertical plane.

8 98 Ali Osman Öncel et al. Pure appl. geophys., Photo 1 View of the surface break near the town of Dinar, facing northwest. Note two different orientations of the crack geometry. north-northwesterly orientation and continues through the village of Kızıllı extending to the village of Yapaǧılı. Thereafter it has a west-northwesterly orientation (Fig. 3). The rupture zone generally follows the boundary between the consolidated sediments of Eocene and Oligocene and unconsolidated alluvial and slope deposits (Fig. 3). The surface rupture indicated in Figure 3 was first mapped for the preliminary report prepared by the Seismology Section of the Earthquake Research Division-General Directorate of Disaster Affairs (DEMIRTAŞ et al., 1996). The same rupture pattern was still visible in the field months after the main shock. The surface rupture observed along a 10-km long section of the 55-km long Keçiborlu-Dinar-Çivril fault was noted to be composed of small cracks as short as

9 Vol. 152, 1998 The Dinar Earthquake 99 Photo 2 a) Example of lateral slip, indicated by the en echelon geometry. Photo was taken to the east of the village of Yakaköy, facing north; b) Example of dip-slip observed near the Dinar-Çivril road, amounting to 35 cm.

10 100 Ali Osman Öncel et al. Pure appl. geophys., Figure 6 An illustration of the properties of the earthquake catalogue (KIJKO and SELLOVOLL, 1987, 1990). This approach allows combination of historical and instrumental catalogues with variable threshold magnitudes. 1 m long and as long as several tens of meters (KORAL et al., 1997 )(Photo 1). Some of these cracks have been arranged in en echelon patterns and have not fallen in a straight line (Photo 2a). Others have displayed linear or anastomasing geometry. Those having orientation close to north-south have been rather linear and displayed dip-slip displacement less than a few centimeters. Those having orientation close to east-west have displayed anastomasing pattern with average dip-slip displacement of cm (Photo 2b). The dip-slip movement was consistently toward the south or southwest. Surface cracks have been connected to each other in a pattern that produces an overall view of a north-northwest/south-southeasterly trend, similar to that of the Dinar-Çivril fault (Fig. 3). 5. Seismic Risk Both historical and instrumental earthquake catalogues were utilized to estimate parameters for seismic hazard assessment. A maximum likelihood method described by KIJKO and SELLOVOLL (1988, 1990) was followed. This method allows mixed earthquake catalogues to be used efficiently. Only the main shocks were used in computations to avoid overestimation of hazard parameters (ÖNCEL and ALPTEKIN, 1996). The main shock catalogue was compiled by eliminating aftershocks from the NEIC catalogue for the time period between 1916 and 1995, and Ambraseys and Finkel s catalogue (AMBRASEYS and FINKEL, 1987) for the time period between 1899 and Window method was employed for deleting aftershocks according to KEILIS-BOROK et al. (1980). The historical part of the catalogue was compiled for the period between 50 A.C. and 1899 from SOYSAL et

11 Vol. 152, 1998 The Dinar Earthquake 101 al. (1980) and the intensities were converted to magnitudes using the formula of M S =0.659 I for Turkey and vicinity (ÖNCEL, 1992). The input data consist of the following files: (a) historical data (four events) encompassing the time interval from 50 to 1899, with a magnitude uncertainty of 0.5. (b) instrumental data during subdivided as follows: Subcatalog 1: , magnitude uncertainty 0.3; Subcatalog 2: , magnitude uncertainty 0.1. The maximum likelihood method was used for computation of earthquake hazard parameters that are seismic activity rate m, the b-value of the Gutenberg- Richter magnitude-frequency relation, and the expected maximum magnitude M max. The instrumental part of the catalogue may be divided into subcatalogues with respect to specified threshold magnitudes. This method considers the artificial seismic gaps in the catalogue which may occur from instrumental or operational problems. Each event is presumed to have a magnitude uncertainty with an upper and a lower magnitude bounds (Fig. 6). The earthquake occurrence is assumed to be a Poisson distribution and can be represented by a doubly truncated Gutenberg-Richter relation. The largest expected magnitude, M max, is computed with respect to the largest observed magnitude in the time period of the catalogue used (KIJKO, 1988). Earthquake hazard parameters computed from the previously described data are as the following: b= ; 2.9 = ; M max = (for x =0.3), where x is the standard error of the maximum observed magnitude. The return periods for earthquakes are given in Figure 7. Recurrence time of an earthquake of the same magnitude as the Dinar earthquake of 1995 (M w =6.2) is 123 years. Discussion and Conclusion Surface cracks of the Dinar earthquake indicate the complex nature of the rupture process. The cracks have distinct shapes of dip-slip and lateral-slip motion (Photos 2a and b). Cracks showing dip-slip motion are predominant, however there are also cracks showing right and left lateral displacement. Lateral displacement is also evident in rotation experienced by damaged buildings (Photo 3). The overall slip pattern indicates a normal faulting event with a small lateral component. This is consistent with the orientation and dip-slip character of the Dinar-Çivril fault, the general tectonic setting of the region and the fault plane solutions suggested by different workers (e.g., EYIDOĞAN and BARKA, 1996; PINAR, 1996) and seismological centers (e.g., NEIC and EMSC). These solutions differ from that suggested by HRW in that there is no apparent strike-slip component.

12 102 Ali Osman Öncel et al. Pure appl. geophys., Figure 7 Mean return times for the main shocks in the Dinar-Çivril fault. Epicenters of fore- and aftershocks are distributed in a weakly elongated northeasterly pattern (Fig. 2). The depth distribution of these shocks in the section normal to the Dinar-Çivril fault is relatively concentrated, but encroaches northeast of the earthquake area bounded by the Dinar-Çivril fault (Fig. 5). The seismic activity has however a broad pattern in the section parallel to the strike of the fault (Fig. 5). The depth distribution patterns, both in the normal and parallel sections, provide support for the view that a northwesterly fault may be the origin of the shock pattern for the earthquake. For a reliable estimation of seismic risk, it is necessary to identify main shocks in the earthquake catalogue (KEILIS-BOROK et al., 1980; ÖNCEL and ALPTEKIN, 1996). Inclusion of aftershocks in the seismic risk analysis may result in overestimation of return times of large earthquakes. For this reason, the window method (KEILIS-BOROK et al., 1980) has been employed to eliminate aftershocks in the seismic risk analysis. Computation of the earthquake hazard parameters b, m and M max for the Dinar-Çivril fault, utilizing the maximum likelihood method on the historical data covering the historical period of and instrumental data between 1900 and 1992, yielded values of 0.70, 1.92, and 7.14, respectively. The

13 Vol. 152, 1998 The Dinar Earthquake 103 Photo 3 View of counterclockwise rotation (shear) experienced by a building. The rotation is most evident in the window frames and the roof. The building is located on a N40 E trending road. Photo faces North. recurrence time estimated for an earthquake of a magnitude of M w =6.2 is 123 years. The non-occurrence probabilities of such an earthquake in 1 and 50 years were computed 0.21 and 0.04, respectively. Acknowledgements We thank Dogan Seber and an anonymous referee for their useful editorial suggestions. We acknowledge the kind support and assistance of Dr. Nalbant during preparation of the paper. REFERENCES ALPTEKIN, Ö. (1973), Focal Mechanisms of Earthquakes in Western Turkey and their Tectonic Implications, Ph.D. Thesis, New Mexico Inst. of Min. and Tech., Socorro, New Mexico, 190 pp. AMBRASEYS, N. N., Studies in Historical Seismicity and Tectonics, Geodynamics Today, Chap. 2 (The Royal Soc., London 1975).

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15 Vol. 152, 1998 The Dinar Earthquake 105 ŞENGÖR, A. M. C.(1980), Principles of Turkey s Neotectonics, Turkish Geological Association Conf. Ser., 2, 40 pp. (in Turkish). SOYSAL, H., SIPAHIOĞLU, S., KOLÇAK, D., and ALTıNOK, Y. (1980), Historical Earthquake Catalogue of Turkey and Vicinity, TÜBI TAK Proje No. TBAG 341. TAYMAZ, T., and PRICE, S. (1992), The 1971 May 12 Burdur Earthquake Sequence, SW Turkey: A Synthesis of Seismological and Geological Obser ations, Geophys. J. Int. 108, (Received December 4, 1996, accepted October 22, 1997).