9th Congress of the Balkan Geophysical Society

5-9 November 2017, Antalya, Turkey

44031

Crustal Compensation Rate In Central Anatolian Region And Comparison Of Seismic Activity

B. Oruç* (Kocaeli University), T. Sönmez (Kocaeli University), E. Ulutaş (Kocaeli University), H.H. Selim (İstanbul Commerce University), M.B. Doğan (Kocaeli University)

Summary The crustal compensation rate can be estimated from the differences gravimetric Moho and isostatic Moho with respect to gravimetric Moho, and allow the establishment of the relationship between isostatic compensation and seismicity in the central Anatolian region. The compensation rates range between 92% and 108%. What is surprising about the most of events are aligned in the zones that encompass seismic activity with the rates between 99% and 101% that are almost fully compensated. It is understood that the pattern of seismicity is also dependence of rheological structure of crustal blocks. The areas where compansation rate less than 100% and greater than 100% correspond to undercompensation and overcompensation, respectively. The results in undercompensation provide aseismic zones where preseismic locking is high and a major role for accommodating the stress evaluation of crustal blocks, and may then be related to preseismic activity. The crustal thickening obtained from gravity inversion in the eastern part of the central Anatolian region suggests overcompensated crust and the buoyant forces that result from subsidence, controlled by the density contrast between crust and lithospheric mantle. The overcompensation areas are almost aseismic, and indicates isostatic readjustment.

9th Congress of the Balkan Geophysical Society

5-9 November 2017, Antalya, Turkey

Introduction

Central Anatolian region locates in a large area between North Anatolian fault zone, East Anatolian fault zone, the Cyprus and Hellenic arcs. Central Anatolian region is mainly dominated by strike-slip tectonics. Major neotectonic elements in the region are Central Anatolian Fault Zone (CAFZ), İnönü-Eskişehir Fault Zone (İEFZ), Ezinepazarı fault (EF), eastern edge of outer and inner Isparta Angle (OIA, IIA), Tuz Lake Fault (TLF). We have used the EGM2008 model that includes spherical harmonic coefficients up to degree and order 2190 (Pavlis et al., 2008) in order to study crustal structure from these newly data sets of the region. Firstly, radially averaged amplitude spectrum has been applied the grid data of EGM2008 Bouguer anomaly map of the central Anatolian region. An average depth (38 km) of Moho interface has been estimated from the slope of spectrum. After processing a band-pass filtering from the critical wavenumbers from spectra, regional anomalies coming from Moho have been separated. Thus, once the filtering shallower effects, regional anomalies are inverted to estimate the Moho undulation. In addition, we examine the deviations between gravimetric Moho and isostatic Moho from flexural isostasy model in order to map the crustal compensation rates, also compared to seismic activities.

Figure 1 a) Main tectonic features of Anatolian plate (Koçyiğit, 2003) and location of the study area. b) EG2008 Bouguer anomalies derived from the Earth Gravitational Model released by the National Geospatial-Intelligence Agency (NGA) (Pavlis et al., 2008).

Moho Undulations using Parker-Oldenburg Gravity Inversion Algorithm and Isostasic Analysis

Parker (1973) proposed a method based on the Fourier transform of the gravity anomaly and the sum of Fourier transforms of a deeper interface causing regional anomalies. According this relationship, an inversion algorithm, proposed by Oldenburg (1974), in which the undulation of a deeper interface generating regional anomalies is applied to estimate the depths of the interface from an average depth and density contrast between the layers separated by interface. The iterative process terminate after completing a certain iteration number or when the difference between two successive approximations of the depths of interface is lower than previously model. Figure 1a shows regional gravity anomalies obtained by applying band-pass filter with cut-off wavenumbers between 0.03 km-1 (210 km) and 0.06 km-1 (104 km) derived from the spectral analysis to the EGM2008 Bouguer anomalies.We also show gravity anomaly map with superimposing major fault zones in the central Anatolian region. It is clear

9th Congress of the Balkan Geophysical Society

5-9 November 2017, Antalya, Turkey

that tendency regional anomalies are well correlated the NW-SE and NE-SW trending fault zones (Figure 1a). Figure 1b shows the Moho undulation, estimated from the Parker-Oldenburg gravity inversion (gravimetric Moho). In the gravimetric Moho, Moho depths vary in the range of 38-43.5 km, and becomes deeper from central part to east gently. In the central part and the northwest portion of the study area, we estimate the presence of Moho uplift around the ~38 km below the north-central section of İEFZ. In general, gravimetric Moho also shows a linear characteristic of NW-SE and NE-SW trend which approximately corresponds to the fault zones and faults. Especially, we observe that gravimetric Moho depths have steep gradient variations belts towards higher surface elevation area (Figure 1b). From Figure 1a and Figure 2c, one can conclude that the Bouguer anomalies in central Anatolian are negatively correlated with the topographic relief, as expected from isostasic mountain concept. Accordingly, negative anomalies cover the higher elevation regions in the eastern part and southwestern portion of the study area. Negative anomalies can thus be due to a thickened crust with crustal roots of lower density, compared to the lithospheric mantle. Positive anomalies mostly correspond to a low topographic relief and can be interpreted in terms of crustal thinning. As a result, we also examine isostatic Moho undulation based on the theory of Vening-Meinesz regional isostasy (Meinesz, 1940; Moritz, 1990; Abd-El Motaal, 1993). Watts (2001) defines that the lithosphere is a thin elastic plate overlying an inviscid fluid that flexes elastically in response to loads (Figure 2). Compensation is thus achieved on a geological scale by the deflection of the elastic lithospheric plate due to loads. In the 2D case when the plate is infinitely long lithospheric flexure (w) or plate deflection under uniform load (L) is calculated by the following equation (Jeffreys, 1976):

)()()()(4

4

xhgxLxgwx

wD Lcm

(1)

where L is the vertical load, g is gravitational acceleration (9.8 m/s2), L is the density of mountain

(2670 kg/m3), m and c are the lithospheric mantle and crustal density, respectively. The parameter D

is the flexural rigidity, controlled by the effective elastic thickness (Te), elastic modulus (E) and poisson ratio (v) and is given by

)1(12 2 eET

D . (2)

Moritz (1990) has given an analytical solution of Eq.(1) with boundary conditions. After calculating deflections (w), the Moho depths with surface elevation at sea level are easily calculated from simplified geometrical structure of the lithospheric flexural model, as shown in Figure 2. In utilizing the isostasic Moho, the density contrast of the crustal root with respect to the underlying lithospheric mantle is set equal to -500 kg/m3 according to standard constants ρc=2800 kg/m3 and ρm=3300 kg/m3. Other parameters that are assumed to be constant for all calculations are the average values for the continental crust of E=1010N/m2, v=0.25, Tc=33 km (Pamukçu and Yurdakul, 2008) and Te=21.29 km (Oruç, 2016). The calculation are performed on 7000 surface elevation data points (70×100). It is clear that the isostatic Moho depths in the central Anatolian region are very close to the gravimetric Moho depths. However, isostatic Moho are characterized by short wavelength variations in addition to the different zones, identified at different depths.

. Figure 2 A simplified lithospheric flexural model, used for modelling isostatic Moho. Tc and Te are compensation depth and effective elastic thickness, respectively.

9th Congress of the Balkan Geophysical Society

5-9 November 2017, Antalya, Turkey

Figure 3 a) Regional gravity map obtained by applying band-pass filter derived from the spectral analysis to the EGM2008 Bouguer anomalies and superimposed fault zones and faults. IIA:Inner Isparta Angle, OIA:Outer Isparta Angle, İEFZ:Inönü Eskişehir Fault Zone, TLF:Tuz Lake Fault Zone, EPF:Ezinepazarı Fault, CAFZ: Central Anatolian Fault Zone. b) Gravimetric Moho, estimated from Parker-Oldenburg inversion algorithm with an average depth of 38 km and density contrast of -500 kg/m3. c)Topography from a global digital elevation model (DEM) with a horizontal grid spacing of 30-arc seconds (Gesch et al., 1999). d) Isostatic Moho based on the regional or flexure model.

Crustal Compensation Rates and Seismicity

Isostatic compensation rate has close relation to the seismic activity. Bott (1971) showed the isostatic compensation could be defined as the percentage of the observed crustal root with respect to the expected isostatic root. Accordingly, isostatic compensation rate is estimated from differences between gravimetric Moho (Figure 3b) and isostatic Moho (Figure 3d) with respect to gravimetric Moho. As shown in Figure 4, crustal compensation is greater than 100% and corresponds to overcompensation when the gravimetric Moho is deeper than the isostatic Moho. In the case of undercompensation, the gravimetric Moho is shallower than the isostatic Moho. Most of the central Anatolian region is said to have compensated since crustal compensation rates are almost 100%. However, the overcompensation areas are within 101 %-108% in the eastern part where the CAFZ passes through the higher plateau areas. The overcompensation rates suggest almost asesismic (postseismic) activity in the region. However, undercompensation rates also correspond to asesismic areas, but may then be related to preseismic activity since some of the events are observed under the condition of isostatic compensation which is almost fulfilled. Thus the areas with undercompensation rates will have to be compensated. It is interesting to note that earthquakes (Figure 4) generally occur at the rates between 99%-101% along the OIA, and northeast part of the central section of İEFZ.  

 Figure 4 Isostatic compensation rate estimated as differences between gravimetric Moho (Figure 3b) and isostaticMoho (Figure 3d) in percent. The epicentr distribution of earthquakes for the period 1975-2016, which has been compiled from the Kandilli Observatory and Earthquake Research Institue (KOERI) databases with moment magnitudes greater than 4.

9th Congress of the Balkan Geophysical Society

5-9 November 2017, Antalya, Turkey

Conclusion  We observe that the deviations between the gravimetric Moho and isostatic Moho in the central Anatolian region. The overcompensation rates are defined when the gravimetric Moho is deeper than the isostatic Moho, hence the surface elevation should rise so that crustal compensation could keep. The reduction of seismic activity is thus expected. The eastern part of the central Anatolian region has the effect with deeper gravimetric Moho, and seismicity is thus very low. The crust is in isostatic equilibrium, when gravimetric Moho is coincident with the isostatic Moho. However, isostatic process can still be in progress, and may be explained by mechanical movements of the crust caused by tectonic forces with complex internal deformation. In that case we expect that the new findings from newly data sets of central Anatolian region are useful and will contribute to rheological studies. Acknowledgements  This study was supported by the Scientific and Technological Research Council of Turkey (TÜBİTAK), Project Number 115Y217. References Bott, M. H. P. (1971). Evolution of young continental margins and formation of shelf basins. Tectonophysics, 11(5), 319-327.

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Oruç, B.(2016). Orta Anadolu Bölgesinin Litosfer Yapısının Bouguer Gravite Verileri ve İzostazik Analizlerle Araştırılması. TÜBİTAK 115Y217 Numaralı Proje Birinci Bilimsel Gelişme Raporu (onaylanmış).

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