Relation between electrical resistivity and earthquake generation in the crust of West Anatolia, Turkey

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Tectonophysics 445 (2007) 49 – 65 www.elsevier.com/locate/tecto

Relation between electrical resistivity and earthquake generation in the crust of West Anatolia, Turkey Aysan Gürer ⁎, Murat Bayrak Istanbul University, Engineering Faculty, Department of Geophysical Engineering, Avcılar, 34320 Istanbul, Turkey Received 1 November 2005; accepted 25 June 2007 Available online 21 September 2007

Abstract In this paper, we present a relation between the earthquake occurrence and electric resistivity structures in the crust, in West Anatolia and the Thrace region of Turkey. The relationship between magnetotelluric georesistivity models and crustal earthquakes in West Anatolia, during a period from 1900 to 2000, is investigated. It is found that most of the large crustal earthquakes occurred in and around the areas of the highest electrical resistivity in the upper crust, although rare small magnitude earthquakes are observed in some parts of the conductive lower crust in West Anatolian extensional terrain. The high-resistivity zones may represent rocks that are probably mechanically strong enough to permit sufficient stress to accumulate for earthquakes to occur in western Anatolia and the Thrace region. However, some recent studies state that the generation of a large earthquake is not only a pure mechanical process, but is closely related to fluid existence. We also reviewed recent world-wide researches including results from the Anatolian data for the first time and discussed all general findings in combination. Our findings show that the boundary between the resistive upper crust and the conductive lower crust correlates well with the cutout depth of the seismicity in West Anatolia and Thrace. This boundary is also attributed to the fluid bearing brittle–ductile transition zone in world literature. Fluid migration from the conductive lower crust to the resistive upper crust may contribute the seismicity in resistive zones. Alternatively, the upper–lower crust boundary may act as a stress concentrator and fluids may help to release strain energy in brittle parts of lower crust, by small magnitude earthquakes, whereas they may help in focusing strain in mechanically strong and electrically resistive zones for large earthquakes to occur. © 2007 Elsevier B.V. All rights reserved. Keywords: Conductivity; Seismicity; Magnetotelluric; Fluids; Strain concentration

1. Introduction The world-wide observations of electric resistivity and earthquake hypocentral distribution within the crust can be classified into three groups: earthquakes occurring at the resistive side near the boundary of conductive and resistive zones (Gupta et al., 1996; ⁎ Corresponding author. E-mail addresses: [email protected] (A. Gürer), [email protected] (M. Bayrak). 0040-1951/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2007.06.009

Unsworth et al., 2000; Ogawa et al., 2001, 2002; Bedrosian et al., 2002; Fujinawa et al., 2002; Chen and Chen, 2002; Kasaya and Oshiman, 2004; Uyeshima et al., 2005; Goto et al., 2005; Tank et al., 2005), earthquakes occurring in resistive zones (Rokityansky and Ingerov, 1999; Bayrak and Nalbant, 2001; Bedrosian et al., 2002; Tank et al., 2003; Tank et al., 2003; Ogawa and Honkura, 2004; Sarma et al., 2004; Bayrak et al., 2004; Gürer et al., 2004a,b) and in conductive zones (Waghmare 1997; Ichiki et al., 1999; Bragin et al., 2001; Tank et al., 2003). In this study, we present a

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relation between the earthquake occurrence and resistivity structures in the crust for western Turkey. For this purpose, we used MT models along profiles crossing main geological assemblies and important tectonic zones in west of Turkey (Fig. 1), such as Thrace region, NW Anatolia, West Anatolia, Gediz Graben, and south west Anatolia (Gürer, 1996; Bayrak and Nalbant, 2001; Gürer et al., 2001; Gürer et al., 2002; Bayrak et al., 2004; Elmas and Gürer, 2004; Gürer et al., 2004a,b). We compared these resistivity structures with the earthquake distributions along MT profiles. The common point for all of the regions in western Turkey: hypocentral cross-sections of earthquakes along MT profiles show more dense seismic activity in the resistive upper crust. Recent studies show that the generation of a large earthquake is not a pure mechanical process, but is closely related to physical and chemical properties of materials in the crust and upper mantle, such as magma, fluids, and in situ material heterogeneities (Zhao et al., 2002, 2004). Fluid existence in active fault zones plays an important role in the earthquake rupture process (Zhao et al., 1996). Magnetotelluric (MT) is an effective method of imaging the fault zone fluids (Unsworth et al., 2000; Ogawa et al., 2001, 2002; Bedrosian et al., 2002), and brittle–

ductile transition zones within the crust and mantle (Jones, 1999; Heinson 1999; Vanyan et al., 2001). Because fluids and partial melts dramatically affect the electrical resistivity of crustal material. The observed relation between the seismicity and electrical resistivity structure in Anatolia and its likely causes are compared with several observations in the world (Tables 1 and 2). We also compared crustal resistivity boundaries in our MT models with the knowledge from the other geophysical methods, such as the Curie-point depth (CPD) information based on magnetic data, seismic wave velocities, and gravity data. Most of the information from these methods shows the total crustal thickness and moho depth in Anatolia whereas a few of them implies the boundary between brittle upper crust and viscoelastic lower crust. Curie-point depth is one of the methods of examining thermal structure of the crust, using aeromagnetic data and implying brittle–ductile boundaries. The basal depth of a magnetic source from aeromagnetic data is considered to be the CPD, which is known as the depth at which the dominant magnetic mineral in the crust passes from a ferromagnetic state to a paramagnetic state under the effect of increasing temperature. The CPD varies from region to region, depending on the geology of the region and

Fig. 1. Main tectonic elements of Turkey with magnetotelluric (MT) profiles. MT profile numbers are shown in circles. Abbreviations in figure are KFZ, Kırklareli Fault Zone; NAFZ, North Anatolian Fault Zone; WAGS, West Anatolian graben System; EFZ, Eskişehir Fault Zone; FBFZ, Fethiye Burdur Fault Zone; IASZ, İzmir Ankara Suture Zone; EAFZ, East Anatolian Fault Zone; DSFZ, Dead sea Fault Zone; KTJ, Karlıova Triple Junction.

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Table 1 Global observations of electrical resistivity and earthquake occurrence Type

Region/fault zone

Seismicity in resistive and conductive NÊ Japan zone boundaries (RCZB), back arc generally in resistive side Northeastern Japan

Seismicity in resistive zones

Earthquake

Reference

Explanations

Intraplate earthquake zone/ micro earthquakes are marked on MT model Miyagi earthquake (M6.5)

Ogawa et al. Seismicity at RCZB (2001) in MT model

Mitsuhata et al.Microearthquakes just (2001) above the highly conductive deep zone in the overlying moderately resistive region. Japan / Itoigawa– Ogawa et al. Hypocenters in Shizuoka Tectonic line (2002) resistive region near the RCZB Central Japan/ In Western Nagano Prefecture Kasaya and Hypocenters in resistive region Mt. Ontake earthquake M6.8 area. Oshiman (2004) around the RCZB Central Japan Mid-Niigata prefecture Uyeshima The main shock at earthquake (M6.8) area. et al. (2005) the sediment cover/ resistive basement border, aftershocks in sedimentary layer. Central Japan/ High seismicity in western Goto et al. High seismicity in Atotsugawa Fault segment. Low seismicity (2005) RCZB at resistive (a gap) in central segment side, less conductive lower crust in active western segment Northeastern High seismicity in central Fujinawa High resistivity in Japan Arc mountain range; no large et al. (2002) high seismicity earthquakes at Central Basin region, a conductive zone in seismically quiet region Unsworth Hypocenters of USA/San Andreas Repeating micro earthquakes et al. (2000) micro earthquakes fault zone, creeping and characteristic M6 events are beneath the fault Park Field segment every 22 years, micro zone conductors and earthquakes marked on in resistive side at MT models RCZB USA/San Andreas, Microseismicity marked Bedrosian High seismicity at creeping central segment on MT models et al. (2002) the border of fault at Holister zone conductor, inside the conductive zone. NW Taiwan/Puli Fault Chi–Chi Mw7.6 earthquake and Chen and Main seismicity in Sanyi Puli seismic zone Chen (2002) resistive layers at RCZB around Sanyi Puli conductivity anomaly. Less activity in this CA. India/Latur Latur earthquake Mw 6.1, a Gupta et al. Main and stable continental region (1996) aftershocks in the resistive zone just above an upper crustal conductor. Ukrainian Carpathians Intensive seismicity at Rokityansky No seismicity over Transcarpathian deep fault and Ingerov the Carpathian forming a RCZB. (1999) conductivity anomaly epicenters are on the resistive block (continued on next page)

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Table 1 (continued) Type

Region/fault zone

Seismicity in resistive zones

Seismicity in conductive zones

Central Japan/ Itoigawa–Shizuoka Tectonic Line Western India/ Koyna Fault

USA/San Andreas, creeping central segment at Holister Central India

Central Tien Shan–South Kazakhstan Northeastern Japan/ northern Miyagi Prefecture

Earthquake

Reference

Explanations

Potential region for M8 class intraplate earthquake

Ogawa and Honkura (2004) Sarma et al. (2004)

Seismicity clusters mainly in the resistive upper crust

Koyna seismic zone/10 December 1967 Koyna earthquake

Microseismicity marked on MT models

Bedrosian et al. (2002)

Jabalpur Earthquake M6.0

Waghmare (1997)

Earthquake epicenters maximum in Hindu Kush area

Bragin et al. (2001)

Seismically active Miyagi Prefecture and micro earthquakes are

Ichiki et al. (1999)

Koyna fault cuts the high resistive segment in MT model and earthquakes occur in this segment around the fault. A high seismicity clustering, in resistive Fransiscan formation. Epicentral region of main and aftershocks correlates well with Jabalpur Paraswada conductivity anomaly Seismogenic zones coincide with subhorizontal areas of elevated conductivity Hypocenters of micro earthquakes lies within conductive layer along the layering, near to the RCZB.

Table 2 Electrical resistivity and earthquake occurrence in Turkey Type

Region/fault zone

Earthquake

Reference

Explanations

Seismicity in resistive and conductive zone boundaries RCZB Seismicity in resistive zones

NW Turkey/North Anatolian Fault Zone (NAFZ), two profiles İzmit profile crossing the epicentral area and Sakarya Profile 30 km east of it

17 August 1999 İzmit Earthquake (M7.4). Seismically active region, many micro earthquakes and after shocks

Tank et al. (2005)

The hypocenters of main shock and after shocks are located on the highly resistive side near RCZB, for both profiles

Seismicity in conductive zones

Tharace Basin/Terzili, Kuzey Osmancık, Kırklareli Fault Zones

A stable area with low seismicity, Small earthquakes (M N 4) around Kırklareli Fault Zone NW Turkey/North Anatolian Fault Zone Seismically active region. (NAFZ)., MT Profile between İznik Lake Many micro earthquakes and İzmit Bay, 40 km west of epicentral and after shocks region of İzmit earthquake West Anatolia, Turkey/West Anatolian Seismically active region, graben system Rapid crustal deformation

Bayrak et al. Hypocentral distribution of the (2004) earthquakes correlates with highly resistive Istranca Massif.

SW Anatolia, Turkey/Fethiye Burdur Fault Zone, Three MT profiles around Fethiye Burdur line

Seismically active region

Gürer et al. (2004a,b)\

NW Turkey/North Anatolian Fault Zone (NAFZ)./MT Profile across the Armutlu Peninsula

Seismically active region. earthquakes swarm activity, presumably triggered by İzmit Earthquake

Tank et al. (2003)

Tank et al. (2003)

After shocks of 17 August 1999 İzmit earthquake occur in the resistive zone.

Bayrak and Nalbant (2001)

Hypocentral distribution of the earthquakes correlates well with the most resistive element, the twin core structure (b2000 W m), of the MT section. The hypocenters of earthquakes are located in the highest resistivity zones or the lowest conductance zones in the upper crust along the MT profiles. The swarm activity is confined in conductive zone. Possible 3-D sea effect because of the geometry of Peninsula

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Table 3 Upper crustal thickness from several geophysical studies in West Anatolia and Thrace Region (location)

Upper crustal thickness

Method

Thrace (Istranca massif)

25–30 km Magnetotelluric ∼ 22–24 km Curie-point depth Thrace (Basin) ∼ 10 km Magnetotelluric ∼ 16–20 km Curie-point depth West Anatolia (NW part, around İzmir ∼ 30 km Magnetotelluric Ankara Suture Zone) 15.5 km Curie-point depth West Anatolia (extensional West Anatolian 8–12 km Magnetotelluric graben system) 6 Curie-point depth 8–11.5 10 km Seismology (Pn and S velocities) SW Anatolia Taurides 18–25 km Magnetotelluric ∼ 20 km (a velocity discontinuity in Seismology (Shear wave Isparta, ISP) velocity) 17° Curie-point depth 24 km

mineralogical content of the rocks. Therefore, one normally expects shallow Curie-point depths in regions that have thinned crust, geothermal potential and young volcanism (Aydın et al., 2005). We mostly used CPD depths to compare variations of the thickness in the brittle upper crust along our MT models. We used seismic wave velocities and gravity information to compare moho depth information obtained from the MT data (Tables 3 and 4). 2. Geology Turkey is one of the most seismically active regions in the world. It is located in ‘Alp–Himalaya Earthquake belt’, and its complex deformation results from the

Reference Bayrak et al. (2004) Aydın et al. (2005) Bayrak et al. (2004) Aydın et al. (2005) Bayrak and Nalbant (2001) Dolmaz et al. (2005a) Bayrak and Nalbant (2001); Gürer et al. (2001) Aydın et al. (2005) Dolmaz et al. (2005a,b) Horasan et al. (2002) (Gürer et al., 2004a,b) Meier et al. (2004) Dolmaz et al. (2005a,b) Aydın et al. (2005)

continental collision between African and Eurasian plates. The major neotectonic elements of the region are the dextral North Anatolian Fault Zone (NAFZ), the Sinistral East Anatolian Fault Zone (EAFZ) and the Aegean– Cyprus Arc which forms a convergent plate boundary between the Afro-Arabian and Anatolian plates (Fig. 1). The geological events in the region such as plate motions, seismic activities, crustal deformations are attributed to these major neotectonic entities (Bozkurt, 2001). The active, right-lateral North Anatolian Fault Zone is the largest strike–slip system in Anatolia. Towards the west of the Marmara region in NW Anatolia, the NAFZ splays into three major strands (Fig. 1). Detailed information about fault geometries a neotectonic of region can be found in Gürer et al. (2003, 2006).

Table 4 Total crustal thickness (moho depth) from several geophysical studies in West Anatolia and Thrace Region (location)

Crustal thickness (moho depth)

Method

Reference

Thrace (Istranca massif)

35–45 km 34 km 33 km ∼ 45–50 km 36 km 30–40 km

Magnetotelluric Refraction and gravity data combination Seismic wave velocity Magnetotelluric Refraction and gravity data combination Magnetotelluric

33 km ∼ 30 km 28–30 km

Seismology (Pn and S velocities) Seismic data Seismic (Common conversion point crustal image shows Several short positive bands at 28–30 km) Seismic wave velocity Magnetotelluric Shear wave velocity

Bayrak et al. (2004) Makris (1985) Gürbüz et al. (1992) Bayrak and Nalbant (2001) Makris (1985) Bayrak and Nalbant (2001); Gürer et al. (2001) Horasan et al. (2002) Saunders et al. (1998) Zhu et al. (2006)

West Anatolia (NW part, around İzmir Ankara Suture Zone) West Anatolia (extensional West Anatolian graben system)

SW Anatolia Taurides

30.1 ± 2.7 KUL 30–45 km ∼ 40 km in Isparta (ISP) 42.2 ± 2.9 (ISP)

Shear wave velocity

Zhu et al. (2006) Gürer et al. (2004a,b) Meier et al. (2004) Zhu et al. (2006)

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Fig. 2. (a) Final MT georesistivity model obtained by 2-D inversion modeling of the MT data, along Profile 1, (After Bayrak et al., 2004). Dots show hypocentral distribution of the earthquakes occurring along the profile. Earthquakes occur in and around the resistive IM: Istranca Massif. TB: Trakya Basin, KFZ: Kırklareli Fault Zone, KOFZ: Kuzey Osmancık Fault zone and TFZ: Terzili Fault zone in the basin. h (m) shows topographic elevation of MT sites. Triangles with letters (TKO) over the section show MT sites and their names. (b) The conductance to 60 km.

Northern strand constitutes the most active section of the NAFZ and destructive earthquakes occur on this strand such as 17 August 1999 İzmit Earthquake (Ms = 7.4). The southern strand of NAFZ bounds the southern margin of Sea of Marmara. The deep structure of NW Anatolia, across two strands of NAFZ, has been investigated by several MT profiles (Gürer, 1996; Tank et al., 2003; Çağlar and İşseven 2004; Tank et al., 2005). NAFZ meets the sinistral East Anatolian Fault Zone (EAFZ) at Karlıova Triple Junction (Fig. 1). The left lateral slip along the EAFZ contributes to the westward

extrusion of Anatolia. Anatolian block or “wedge” moves westward relative to the Eurasian plate in the north and to the Arabian plate in the southeast, along the North and East Anatolian Fault Zones. This westward extrusion of Anatolian block is thought to be the cause of crustal extension and thinning in West Anatolia by several authors (Şengör et al., 1985). The thinning of the West Anatolian crust is also imaged by several MT profiles in West Anatolia (Bayrak and Nalbant, 2001; Gürer et al., 2001; Çağlar, 2001; Ulugergerli et al., 2007).

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Fig. 3. (a) Final MT georesistivity model obtained by 2-D inversion modeling of the MT data, along Profile 2 in West Anatolia, (After Bayrak and Nalbant, 2001). İASZ: İzmir Ankara Suture Zone, BG: Bigadiç Graben, GG: Gördes Graben, DG: Demirci Graben and CMM: Core of Menderes Massif. (b) The conductance to 80 km. h (m) shows topographic elevation of MT sites. Triangles with numbers over the section show MT sites. The numbers with unit of mW m− 2 denote heat flow values at some sites along the MT profile. (c) Hypocentral distribution of the earthquakes occurring along the Profile 2. Earthquakes are mainly confined in the resistive upper crust. The hypocenters of two older earthquakes in c, marked by their occurrence dates (1942 and 1965), are probably very poorly determined.

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Western Anatolia is one of the most seismically active and rapidly deforming regions in the world (Taymaz et al., 1991; McClusky et al., 2000). The region is currently extending at a rate of 30–40 mm/year (Oral et al., 1995) in N–S direction. As a result of this continental extension, the crust was thinned progressively since the Miocene time and 8 major E–W and 20 NE–SW grabens developed (Yılmaz et al., 2000). Another important tectonic feature in West Anatolia is İzmir Ankara Suture Zone (IASZ) which is regarded as the remnant of an oceanic realm between Sakarya and Tauride continents. Western Anatolia extended terrain is bounded by the Fethiye Burdur Fault Zone (FBFZ) and the south Aegean trench system of the Aegean arc to the south (Barka and Reilinger, 1997). The Aegean arc (in Fig. 1) is an active plate boundary, which accommodates the convergence between the African plate to the south and the Anatolian plate to the north. Aegean arc system plays an important role in the geodynamical evolution of the region. The Eastern part of Aegean arc acts rather as a transform fault (Le Pichon et al., 1979). Several trenches with a strike–slip character have been distinguished such as Pliny and Strabo Trenches (Jongsma, 1977). On the land side, in southwest Anatolia, the Fethiye Burdur Fault Zone (FBFZ) is one of the most striking tectonic elements. FBFZ lies in continuity with the Pliny Fault Zone on the land side (southwest Turkey). Along FBFZ, the normal faulting is distributed with a strongly pronounced lateral strike–slip component (Taymaz and Price, 1992; Temiz et al., 1997). The MT studies, in southwest Anatolia, revealed images of the geoelectrical structure and its relations to the main structural entities in the land side (Gürer et al., 2004a,b). The main MT profiles that are subject of this paper are shown with prominent active tectonic elements of Turkey in Fig. 1. 3. MT data and modeling The MT measurements discussed here can be classified into two groups according to the used measurement systems. The first set of MT data along West Anatolian profile (in Fig. 1, Profile 2) were obtained with a Phoenix Geophysics Ltd. tensor system. The recording period range was 0.003125-2000 s for all

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stations. The other group of the MT data (along Thrace, and SWAnatolian profiles; Profiles 1, 2, 3, 4, 5 in Fig. 1) was measured using Geotronics MT equipment with frequency range of 0.001–25 Hz. For each site five components (Hx, Hy, Hz, Ex, Ey) of electromagnetic fields were recorded. The horizontal electric and magnetic field components are measured in orthogonal directions with a cross shaped configuration. Two-dimensional inversions of the MT data set (from Thrace region, West Anatolia and southwest Anatolia) were undertaken using the 2-D Mackie code (Mackie et al., 1997; Rodi and Mackie, 2001) in this study. This seeks a model that fits the observed MT data and is also spatially smooth. The initial model was a 100 Ω m half space for all of the profiles except for the profile crossing the Thrace basin. For Thrace basin due to the thick sedimentary fill it is chosen as 30 Ω m. We assumed a 15% error floor for apparent resistivity and equivalent error floor for phase for all of the MT models. We compared our MT models with the knowledge from the other geophysical methods, such as the Curiepoint depth information based on magnetic data, seismic wave velocities, and gravity data. 4. Georesistivity structures along MT profiles 4.1. Thrace region (Profile1) Our first MT profile (Profile 1) crosses the Tertiary Thrace basin and the adjacent Istranca Massif that are located in northwestern Turkey (Fig. 1). Kırklareli Fault Zone (KFZ) forms the boundary between the Istranca Massif and the Thrace Basin. Electromagnetic images of the Thrace basin, crust and lithospheric upper mantle of Thrace region of Turkey from MT data was obtained using MT data that were acquired at nine stations on an ∼ 80 km traverse (Bayrak et al., 2004). We inverted the data using both H and E polarizations. The final resistivity model (Fig. 2a) has an RMS misfit of 1.48. The main features found are: In the northernmost part of the profile, a wide (∼ 21 km) zone of Istranca Massif with very high resistivity (N 2000 Ω m) is imaged at a depth of 2.5 km extending 25–35 km. Aydın et al. (2005) calculated Curie-point depth as 22–24 km at Istranca massif implying the bottom of the magnetized bodies in the crust. Below the Thrace basin, an undulated zone of conductive lower crust with low

Fig. 4. (a) Final MT georesistivity model obtained by 2-D inversion modeling of the MT data, along Profile 3 in southwest Anatolia, (after Gürer et al., 2004a). CB: Çameli Basin, BF: Basin Faults forming Çameli Basin, FBFZ: Fethiye Burdur Fault Zone, WZ: A weak zone at the continuation of Strabo transform in Mediterranean. (b) The conductance to 80 km. (c) Hypocentral distribution of the earthquakes occurring along the Profile 3. Earthquake clustering is in the lowest conductance region.

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resistivity (b 75 Ω m) is imaged with thickness of ∼ 10 km. CPD map by Aydın et al. (2005) shows 16 km for the Thrace basin implying that crust is thinning from the Istranca massif to the Depth of lithospheric upper mantle (∼ 250 Ω m) varies between 35 and 45 km, beneath the Istranca Massif in the northernmost part of the profile. It takes the lowest value of 17 km at the central part of the Thrace Basin. This may indicate a mantle uplift. Makris (1985) gives the moho depth as 34 km based on refraction and gravity data and Gürbüz et al. (1992) show that it is 33 km using seismic wave velocities at the Istranca massif. Aydın et al. (2005) gives ∼ 16 km Curie-point depth in the Thrace Basin and ∼ 22–24 km at Istranca massif. These findings are quite harmonious with our MT model results. The MT model and the Curie depth point map also show that the crustal thickness decreases from the Istranca massif towards the Thrace Basin. The Intra-Pontide subduction zone is subject of the debate as to its location and existence in Thrace basin. We imaged a slab subducting to the north, beneath the Istranca massif in Thrace Basin for the first time. Georesistivity model also indicated that conductive electrical asthenosphere is not present in northwestern Turkey in contrast to western Turkey (Bayrak et al., 2004). 4.2. West Anatolia (Profile 2) The crustal thinning and lithospheric structures in the West Anatolia have been investigated by Bayrak and Nalbant (2001) and Çağlar (2001) with approximately SE–NW trending, MT profiles across the prominent elements in the region such as E–W trending West Anatolian Graben System (WAGS) and İzmir Ankara Suture Zone (İASZ). Gürer et al. (2001, 2002) investigated the crustal resistivity structure and possible relations of E–W Gediz graben with the neighbor NE– SW grabens, which may indicate their relative ages. Bayrak and Nalbant (2001) inverted the H polarization data along a MT Profile 2 Fig. 1), across the major geological structures of western Anatolia. The spread of the profile was about 175.7 km. The initial model for the inversion was 100 Ω m half space and the final resistivity model (in Fig. 3a) has an RMS misfit of 0.85. Bayrak and Nalbant (2001) imaged upper (N200 Ω m) and lower crust (b 75 Ω m) with varying thickness in West Anatolia. The depth of the lithosphere and astenosphere boundary is around 30 km at the SE and NW ends of the profile and 45–50 km in the middle. Bayrak and Nalbant (2001) observed an interesting twin core structure with a very high resistivity (N2000 Ω m)

at a depth from 5 km to 25 km under the İzmir Ankara Suture Zone. The boundary between the resistive upper crust and the conductive lower crust varies from 8– 12 km in the extensional West Anatolian graben system to 30 km around the İzmir Ankara Suture Zone (IASZ). Curie-point depth is calculated as 6–11.5 km at West Anatolian graben system (Dolmaz et al., 2005a,b; Aydın et al., 2005) and as 15.5 km around İASZ (Dolmaz et al., 2005a,b) indicating a thinning in the crust from IASZ to the West Anatolian graben system. Horasan et al. (2002) gives a low velocity zone at 10 km depth in West Anatolia using the 21 April 2000 Denizli (Honaz) and the 9 July 1998 İzmir (Doğanbey) earthquakes in the graben province. These results are quite compatible with that of the MT model in Fig. 3a. The Moho depth around IASZ is given as 36 km in the map based on gravity and refraction data by Makris (1985) where as it is 33 given km by Horasan et al. (2002) 30 km by Saunders et al. (1998) and Zhu et al. (2006) for West Anatolian graben system. 4.3. SW Anatolia (Profiles 3, 4, 5) SW Anatolia is located at the junction of the eastern region that marks a seismically active part of the broad transition zone between the African and Eurasian plates. Magnetotelluric data in H and E polarizations along three MT profiles (Profile 3, Profile 4 and Profile 5), across the Lycian nappes and the Beydağları relative autochton and Fethiye Burdur Fault Zone (FBFZ) in SW Anatolia, was inverted (Gürer et al., 2004a,b). The initial models were 100 Ω m half spaces for three of profiles. Final resistivity models (in Figs. 4a, 5a, 6a) have RMS misfit of 1.48, 1 and 1.5 for profiles, respectively. Resulting models along three profiles reveal the conductive lower crust (b75 Ω m and the resistive N 350 Ω m) upper crust. The depth to the upper/ lower crust boundary varies from 10 to 25 km in the region. The resistive upper crust is interrupted by more conductive vertical zones. Some of these zones along three profiles coincide with prominent geological structures of the region such as, the Fethiye Burdur Fault Zone (FBFZ), Strabo Fault Zone and Basin forming normal faults of Çameli Basin (Figs. 4a, 5a, 6a). In SW Anatolia, at the station ISP in Isparta, a shear wave velocity discontinuity is obtained at mid crustal depths, at 20 km, by Meier et al. (2004). In the northwestern end of our MT model in Fig. 4, where it is closest to ISP, we observed the lower crustal thickness as 23 km. Curie depth point estimate for SW Anatolia is also compatible with the MT model results as 17–24 km by Dolmaz et al. (2005a,b) and Aydın et al. (2005).

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Fig. 5. (a) Final MT georesistivity model obtained by 2-D inversion modeling of the MT data, along Profile 4 in southwest Anatolia, (after Gürer et al., 2004b). FBFZ: Fethiye Burdur Fault Zone, thick arrow denotes the major fault of this fault zone. (b) The conductance to 80 km. (c) Hypocentral distribution of the earthquakes occurring along the Profile 4. Seismicity is mainly in the resistive upper crust and resistive side of the upper mantle.

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Moho depth in the region is estimated by several researches, based on different geophysical methods, as 40–42 km (Meier et al., 2004) for the seismic station ISP. This depth is compatible with the depth determined from our MT model, in Fig. 4, as 40 km.

5. Resistivity and earthquake distribution In this study, we compared two dimensional georesistivity structures and the distribution of earthquakes occurring between 1900 and 2000, within the 40 km wide zone along our MT profiles, from several regions of western Turkey, such as Thrace region, northwest Anatolia, West Anatolia and southwest Anatolia. We used the software by Jones et al. (2003) for projecting earthquake hypocenters obtained from KOERI data base (from Kandilli observatory) to the lines along our MT profiles. We compared and discussed our results with the other observations of crustal resistivity in seismically active regions, from the world (Table 1) and from Turkey (Table 2). Thrace region is a relatively stable area with low seismicity rates. Small earthquakes (Ms ≤ 4) around the Kırklareli Fault Zone (KFZ), along Profile 1, define the limited seismic activity in the region. The highly resistive Istranca Massif, overlying a conductive subducting slab, shows a very good correlation with the hypocentral distribution of earthquakes occurring within the 40 km wide zone along our MT profile (Fig. 2a, b). Fig. 2b shows the conductance to 60 km along profile. In the figure it is obvious that the decreasing thickness of the upper crust cause the increasing the conductance to about 1000 S. In northern part of the georesistivity model, the conductance takes lowest value, around 100 S and it coincides with the earthquake occurrence zone in the crust. Briefly, the seismicity in Thrace region is consistent with resistive zones in the crust and the lowest conductance throughout the entire depth of the MT model. In West Anatolia, along Profile 2, hypocentral cross section of the earthquakes along SE–NW trending MT profile (Bayrak and Nalbant, 2001), shows a dense seismic activity in and around the most resistive twin core structure in the resistive upper crust (Fig. 3a). The boundary between the resistive upper crust and conductive lower crust has an undulated character in the region. Fig. 3b shows the conductance to 80 km. Hypocenters of the earthquakes are mostly located in the resistive upper crust, at the low conductance region, Fig. 6. (a) Final MT georesistivity model obtained by 2-D inversion modeling of the MT data, along Profile 5 in southwest Anatolia, (after Gürer et al., 2004a). The resistive zone extending to 80 km depth, under the southern most end of the model, may be caused by a local site effect. FBFZ: Fethiye Burdur Fault Zone. (b) The conductance to 80 km. (c) Hypocentral distribution of the earthquakes occurring along the Profile 5. Seismicity is confined in resistive upper crust. The hypocenter of the deepest earthquake in c is probably poorly determined.

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except a few earthquakes in conductive lower crust (Fig. 3c). West Anatolia is an extensional region and a conductive electrical asthenosphere is observed in the region (Fig. 3a). There is almost no seismic activity beneath the conductive lower crust in mantle depths. The main cutout boundary of the seismicity is the upper/ lower crust boundary (Fig. 3c). However, a secondary seismicity in some localizations of the conductive lower crust, is observed along West Anatolia MT profile in Fig. 3c. This seismicity consists of rare and relatively low magnitude earthquakes. The hypocenters of two older earthquakes in Fig. 3c, marked by their occurrence dates (1942 and 1965), are probably very poorly determined as noted by Bayrak and Nalbant (2001). Earthquake occurrence areas in the conductive lower crust may indicate preservation of brittle rheology in some local parts of the lower crust in West Anatolia, as concluded by Simpson (1999) for world-wide observations. In SW Anatolia, we also compared the hypocentral distribution of the earthquakes with the georesistivity models (in Figs. 4a, 5a, 6a) along three MT profiles (Profiles 3, 4, 5). Along the Profile 5, an earthquake clustering is observed in and around a resistive core in the upper crust where the conductive lower crust is not present beneath it (Fig. 4c). In other words, the earthquake clustering area coincides with the lowest conductance part (Fig. 4b) of the georesistivity model (Fig. 4a). Deep extension of this earthquake clustering within the resistive zone is located in between two lower crustal conductors. Outside this clustering area, hypocenters of the earthquakes are mainly located in the resistive upper crust and rare earthquake occurrence is observed in the conductive lower crust. Along the other two profiles in SW Anatolia, earthquake hypocenters are also confined in the resistive upper crust underlined by lower crustal conductors (in Figs. 5 and 6). The resistivity of upper mantle is not homogeneous along the Profile 4. It varies from 80 Ω m in the northwest to 250 Ω m in the southeast of the profile. There is an interesting earthquake occurrence region in the resistive side of the upper mantle along the Profile 6. This is an interesting observation of an earthquake occurrence area under the conductive lower crust in the resistive zone. 6. Discussion Electrical resistivity has a good connection with existence of pore fluids. The higher fluid contends in the rocks produces the low resistivity. Selected observations of electrical resistivity and seismicity from several regions of the world and Turkey are summarized in Tables 1 and 2, respectively. There appears to be a good

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correlation in the world literature between the seismicity and the boundaries of resistive and conductive zones (Table 1). In general, earthquakes appear to be occurring in the resistive side of the resistive–conductive block boundaries or in resistive zones in the crust. These observations are mostly explained by migration of fluids from the permeable conductive zones to the less permeable resistive zones (Gupta et al., 1996; Ichiki et al.; 1999; Ogawa et al., 2001; Ogawa et al., 2002; Fujinawa et al, 2002; Bedrosian et al., 2002; Chen and Chen, 2002; Kasaya and Oshiman, 2004; Ogawa and Honkura, 2004; Uyeshima et al., 2005; Goto et al., 2005). An alternative explanation of earthquake occurrence at the conductive–resistive block boundaries can be the local stress concentration near the structural boundary (Ogawa et al., 2001). However, the role of fluids in earthquake generation using the P and S wave velocity structures, is also reported by several researchers (Zhao et al., 1996; Zhao et al., 2002; Kayal et al., 2002; Mishra and Zhao 2003; Zhao et al., 2004). Areas with large coseismic slip and high aftershock activity appear to be associated with high P wave velocity, high Poisson's ratio and high electrical conductivity which may represent existence of fault zone fluids (Zhao et al., 2004). For this reason, fluids may also have a role in stress concentration and distribution at the structural boundaries for earthquake generation. The comparison of resistivity model and hypocentral distribution of earthquakes in western Turkey and Thrace (Table 2) indicates that hypocenters are generally located in and around the resistive bodies in the upper crust. In other words, the seismicity clusters mainly in the resistive crust that is underlain by the lower crustal conductors. Very few hypocenters are located within the conductive lower crust. A similar result has been reported by Ogawa and Honkura (2004). Bonner et al. (2003) are also showed that only a small percentage of crustal earthquakes occur below the cutout depth, which is at or near the brittle–ductile transition in the crust, which is mainly determined by temperature conditions. In the world-wide reports from MT studies, this transition zone is mostly attributed to the resistive upper-conductive lower crust boundary and the depth to the top of this boundary and the conductance of the upper crustal layer are also known as a well resolved parameters of the resistivity models based on MT data (Li et al., 2003). Our results showed that the top of the lower crustal conductors mostly correlate well with the cutout depth of seismicity in western Turkey, where the average value of heat flow (110.8 mW m− 2) is higher than world average. Electrical resistivity is strongly affected by pore fluid

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existence in the crust. Consistency of low electrical resistivity under the upper crust and cutout depth of earthquakes may indicate the brittle–ductile boundary and extra fluid distribution at this zone in western Turkey. Wannamaker et al. (2004) also discussed the role of the fluid existence below the domain of seismicity. They concluded that fluids may have a role in curtailing stress build-up either by reducing the depth extends of the brittle regime or by reducing effective normal stresses. In West Anatolia, a limited number of smaller magnitude earthquakes are observed in some parts of the conductive lower crust, indicating some local brittle areas inside it (Figs. 3c an 4c). The world reports of seismicity in conductive zones (Ichiki et al., 1999) and conductive side of resistive–conductive block boundaries (Bedrosian et al., 2002) are mainly related to the micro earthquake activity. These observations may imply that enough strain can accumulate in the resistive side of fault blocks to produce large earthquakes whereas it is released by micro earthquakes in the conductive side, due to the micro cracks and water content in the rock mass reducing effective rock strength. In other words, water existence may form a large conductive block and facilitate creep behavior around a fault zone with micro earthquake activity as reported by Bedrosian et al. (2002). Weak crustal boundaries, such as fault intersections, also act as stress concentrators and cause anomalous stress build-up in their vicinity (Gangopadhyay and Talwani, 2005). As a result of strain release with micro earthquakes in conductive side, stress built-up around the structural boundaries can cause higher stress focusing and accumulation in resistive sides. This may explain the earthquake clustering in resistive sides of the conductive–resistive block boundaries in the fault zones (in Tables 1 and 2). The MT data, subject of this study, was measured with approximately 5 km site interval. The horizontal resistivity boundaries which are consistent with the thickness of the seismogenic layers are quite well defined by this MT site interval. However, new MT measurements around the fault zones with 1–2 km station spacing are needed for detailed comparison of lateral resistivity discontinuities with seismic activity. 7. Conclusions We correlated seismicity and electrical resistivity distribution in West Anatolia and Thrace regions of Turkey, using the resistivity models. The main results and findings of this study can be summarized as follows. 1) Our MT studies in Anatolia show two subzones of crust with varying thickness: the first is the resistive

2)

3)

4)

5)

upper crust while the second is the conductive lower crust. The resistive upper crust is interrupted by more conductive vertical zones which coincide with the surface traces of the major fault zones of Anatolia (such as strike–slip NAFZ, FBFZ and graben forming normal faults). The top of the conductive lower crust correlates well with the cutout depth of regional seismicity. Hypocenters of the earthquakes in western Anatolia and Thrace are confined in and around the highly resistive areas of the resistive upper crust. The cutout depth at upper/lower crust boundary probably forms the brittle–ductile transition zone in the region. In West Anatolia, some rare and relatively small magnitude earthquake occurrences are observed in conductive (b 75 W m) lower crust, in contrast to Thrace region, northwest and southwest Anatolia. This seismicity may indicate preservation of brittle rheology in some local areas of the conductive lower crust. However, considering the high heat flow value (average110.8 mW m− 2) and main cutout depth of the seismicity at the top of the lower crust, highly conductive lower crust generally appears as a brittle– ductile rheology transition zone in West Anatolia. Our observations in West Anatolia and world reports of seismicity in conductive zones mainly show relatively small magnitude or micro earthquake activity. Probably enough strain can not be accumulated in brittle fluid bearing conductive zones to occur large earthquakes because of the weakening of the crust the due to the fluid existence. As a result, resistive crust is probably mechanically strong enough to produce high seismic activity with greater magnitude earthquakes in West Anatolia and Thrace Region. Fluid migration from the conductive lower crust may contribute to generation of large earthquakes by triggering numerous small magnitude earthquakes and concentrating the strain accumulation to resistive rigid zones in the upper crust.

These findings imply that electromagnetic methods can be useful tools to map highly resistive and probably highly rigid areas, to generate the large earthquakes and physical conditions that may contribute to the earthquake generation. Acknowledgments This study was supported by the research fund of the Istanbul University UDP-G/485, 1781/2112200), and by TÜBİTAK (grant 102Y054). We thank O.M.

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