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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 1982 Dynamic Seismo-Electromagnetic Effects. Dr. Malcolm Johnston Earthquake and Volcanic Hazards U.S. Geological Survey IASPEI Karl Kappler Electromagnetic (EM) signals during earthquakes are expected to result from dynamic stress field radiation and sensor movement driven by ground motion. In the epicentral region, small signals at the time of rupture initiation might also be expected from direct EM radiation from the source. EM signals preceding rupture may also occur from initiation of the fault failure process but these are likely to be smaller than those associated with the main seismic energy release. High-resolution EM data within a kilometer of the San Andreas fault and directly above the end of the rupture from the recent M6 Parkfield, California earthquake on September 28, 2004, allows some quantification of these various processes. During the earthquake, the EM instruments operated continuously at 40 sps as the rupture propagated below the buried sensors. A second EM system installed 115 km to the northwest allows correction for large-scale common-mode noise from the ionosphere and magnetosphere. The resolution of magnetic fields at 1 Hz was about 3 picotesla while that in electric field was about 6 microvolt/km. A search for direct source rupture effects expected at this site up to 3 seconds before the P wave arrival shows no apparent signal above the noise during this time. Magnetic and electric field "seismograms" occurred during the seismic wave arrivals with peak-to-peak (P-P) magnetic signals of about 0.4 nT in the north and east directions and 8 nT in the vertical and P-P electric signals of about 60 microvolts/km in the north and east directions. These appear to track the ground velocity seismograms closely with the peak amplitudes occurring during the larger S wave arrivals. Regarding different contributions to the EM seismograms, preliminary models show that stress changes from the dynamic radiation field could generate local oscillating fields of 0.1 nT. In comparison, expected sensor translation of up to 30 cm/sec in a resistive medium of about 3.0 ohm-m through the Earths geomagnetic field could generate magnetic and electric fields of 0.1 nT and less than 100 microvolts/km, respectively, in the EM seismogram. While we are attempting to predict and remove translation and rotation effects, at this point both crustal stress cycling and sensor translation/rotation are apparently important contributors to these signals. Regarding precursory behavior, these EM data show no indications of unusual noise above the 95% confidence limits of 20 picotesla and 20 microvolts/km, respectively, in the ultra-lowfrequency (ULF) bands (0.01 Hz to 20 Hz) during the week before the earthquake. This conflicts with suggestions that magnetic noise some 50 times larger may have preceded the 1989 ML7.1 Loma Prieta earthquake and electric fields some 1000 times larger may have preceded earthquakes of this magnitude in Greece. On the other hand, static offsets at this site caused by the earthquake corresponded to about 0.2 nT as expected from geodetic and seismological models of the earthquake assuming a total remanent and induced magnetization of 2 A/m in the medium and a stress sensitivity of 0.002/MPa. Keywords: dynamic seismogran, electromagnetic, earthquake
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
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