IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS008 Oral Presentation 1983
MT 3D modeling of seismogenic zone Duzce in NW Turkey. Rokityansky
I.I., Savchenko T.S., Tank S.B.
Mr. Tymur Savchenko
In 1999 two catastrophic earthquakes: Izmit and Duzce occurred in the western part of the North
Anatolian Fault Zone. In 2005 joint Turkish-Ukrainian team made MT observations in 17 sites in the
Duzce seismogenic zone. Results of the processing, MT tensor analysis and 2D inversion were presented
at 18th EM Workshop at el Vendrel, Spain, September 2006 (Kaya et al., 2006). Results of 2D inversion
of E- and B-polarization MT data yield very good conducting crustal layer with integral conductance
several thousands Simenses under northern part of NS profile. Such conductor must create strong
magnetovariational anomaly at periods several hundreds – few thousands seconds. But induction
vectors do not support the existence of such anomaly. This contradiction of MTS and MVP data in the
framework of 2D approach leads us to supposition that MT field behavior is essentially 3D at long
periods. This idea also supported by parameter Skew at long periods. At short periods T < 0.1-1 s,
parameter Skew is small, additional impedances are much smaller than principal ones and data can be
considered as 1D. In this approach interpretation was made for every site and conductivity distribution
of uppermost sedimentary layer along profile was received (Rokityansky et al., 2007). Having rather
complex data along a profile we do not hope to get reliable results of 3D inversion. So, we preferred the
direct problem 3D modeling, selecting 3D model to fit the results of short period data 1D inversion and
manipulating parameters of upper layer outside of profile by qualitative consideration of the shunting
effect. Shunting effect is a reduction of electric field (and MTS RHO curve) in surface conductor because
of electric charges accumulation at the conductor edges. The effect is well studied for B-polarization in
2D and this result can be qualitatively applied for 3D situation. 6 northern sites of the profile are located
in sedimentary basin with specific resistivity 2 Ohmm and integral conductance around 300 S. The
northern MTS curve ρxy rises much steeper than eastern curve ρyx and their difference attain 1.8
orders at long period 500-1000 s. It can be interpreted as small shunting effect for ρxy and much
stronger one for ρyx. The sedimentary basin in the area is almost isometric (30 km from N to S, 40 km
from E to W). We supposed that eastern and western borders of the basin are high resistive (we
accepted it in the starting model be 2000 Ohmm, but in the final model it was reduced to 600 Ohmm)
while northern and southern borders do not prevent free current flow in N-S direction. We have no
information to the North from our profile and supposed that surface layer has resistivity 5 Ohmm that
sufficient for minimization of shunting effect. In the southern direction we have observations and know
that near surface resistivity rise up to 1000 Ohmm at the distance 5 km from sediments under sites 1-6.
It means that currents can flow in the southern direction only at some depth. We introduced vertically
submerging to the depth 10 km conductor under site 6, which exactly coincide with North Anatolian
Fault Zone active during Duzce earthquake 12.11.1999, and continued to the South as horizontal layer
at the depth 10 km with conductance 100 S. Using code (W. Siripunvaraporn, G. Egbert, M. Uyeshima,
2006) we calculated 7 3D models with slightly varying parameters to improve the fitting with really
observed impedance tensor. The fitting was satisfactory and we make the conclusion that North
Anatolian Fault zone under site 6 together with southward deep conductor provides outflow of NS
currents from sediments under sites 1-5 in the southern direction. The system of vertical and horizontal
conductors can be replaced by fault steeply submerging in south direction.
Keywords: 3dmodeling, seismogeniczones, mts