Title: 3.1 Conductivity Fluctuations in a Fault Zone: A Possible Mechanism for Generating Large Amplitude Magnetic Precursors to Earthquakes? Authors: Gary D. Egbert, (COAS, Oregon State University, Corvallis, OR, 97331, egbert@oce,orst.edu) Thomas J. Shankland (Bayerisches Geoinstitut, Universitaet Bayreuth D95440 Bayreuth, Germany, thomas.shankland@uni-bayreuth.de; on leave from Los Alamos National Laboratory, Los Alamos, NM, 87545) For two weeks prior to the 1989 Loma Prieta earthquake, Fraser-Smith et al. (1990) recorded a dramatic 10-30 fold increase in ultra-low frequency magnetic field variations near the epicenter. Several explanations of this phenomena have been advanced, generally invoking fluid flow in the fault zone. Another possible mechanism is modulation of low frequency crustal electric fields by higher frequency fluctuations of bulk rock conductivity, as seen in laboratory measurements. Using spectra of typical background magnetic and electric field variations from central California, and 2-d calculations for various 2-d fault zone models, we estimate the amplitude of anomalous magnetic fields which would result from small ongoing fluctuations in resistivity. The magnitude of the anomalous signal depends on both the amplitude of the low frequency modulated currents and the amplitude of the conductivity fluctuations. Because the spectrum of natural source EM fluctuations is extremely red, large relative increases in magnetic signals are possible even for small conductivity fluctuations at periods T < 100 s. The calculations show that even small ongoing fluctuations in bulk conductivity could be detected with a properly designed experiment. In principle this mechanism could also reproduce the Fraser-Smith observations, but the resistivity changes that would be required for this appear to be too large to be physically plausible. Title: 3.2 MODELING ELECTROMAGNETIC FIELD OF A VOLCANIC MAGMA CHAMBER (Session 3. Electric and magnetic approaches to seismic and volcanic activity studies, oral presentation) Authors: E. B. Fainberg Institute of Geoelectromagnetic Investigations of Russian Academy of Sciences, 142092, Troitsk, Moscow region, Russia fain@geo.igemi.troitsk.ru B.Sh.Singer Cooperative Research Centre for Australian Mineral Exploration Technologies Macquarie University: NSW 2109, Australia bsinger@laurel.ocs.mq.edu.au A generalized thin layer model is applied to analyze the spatial characteristics of the electromagnetic field generated by magma chambers of volcanoes Etna and Vesuvius. To monitor the volcanic conditions, the natural electromagnetic field as well as the field caused by electrokinetic processes inside the volcanoes need to be simulated for realistic earth models. The model chosen by the authors represents major geoelectric features of the subsurface conductive layer and that of the resistive earth crust in the areas of interest. As usually, geoelectric properties of the subsurface layer are characterized by its integrated conductance. The second heterogeneous layer simulates the resistive basement. It is introduced into the models to study effects of the regional faults. Geoelectric properties of the layer are described by its transverse resistance. The laterally heterogeneous part of the model is of a finite thickness. The heterogeneous double layer rests on the stratified medium. Location with respect to the prominent inhomogeneities of the subsurface layer has a significant effect on the natural electromagnetic field. At the same time, effects of the faults plays a role in forming of electromagnetic field. In the natural electromagnetic field, the faults are mainly energized as a result of the vertical deflection of the horizontal currents by subsurface inhomogeneities. The situation is different for the field originated inside the volcanic chambers. In this case, the poloidal component dominates in the electromagnetic field. The field becomes sensitive to the variations of the transverse resistance. The modeling results can be used to choose the reference and field sites when monitoring volcanic conditions. Title: 3.3 Quantifying of electromagnetic field caused by the variation of stress field on the basis of the piezoelectricity of rock Authors: Tsutomu Ogawa and Hisashi Utada Earthquake Research Institute, the University of Tokyo 1-1-1, Yayoi ,Bunkyo-ku, Tokyo,113-0032, Japan ogawa@eri.u-tokyo.ac.jp It is well known that some crystal rocks show piezoelectricity that is ascribed to crystal anisotropy of rock forming minerals. Maxwell equations containing the contribution of the effect of piezoelectricity of the material show that the temporal variation of the stress field causes the electromagnetic field. We evaluate the electromagnetic field generated by the variation of the stress field on the basis of the piezoelectricity of the material filled in the infinite space in this study. Assuming a slip in the space and the seismic moment, the stress field is derived analytically. Then it is shown that the direction and the variation of observed electromagnetic field depends on the anisotropy of the material, the direction of the slip and the fault plane relative to the observation point, and the radiation pattern of the stress field variation. Implications obtained are: (1) It is possible to observe seismo electromagnetic field when an earthquake occurs, and, (2) If such signals are observed, it gives information on the anisotropy of rocks in the crust. Further studies with more realistic models are needed. Title: 3.4 SEISMICITY IN THE SAYAN-BAIKAL PROVINCE IN RELATION TO SOLAR ACTIVITY AND FLUCTUATIONS OF WATER LEVEL IN LAKE BAIKAL Authors: Popov A.M.*, Shpynev G.B.**, A.V. Mordvinov and Vladimirov V.B.* * Institute of the Earth's Crust, Siberian Division of the Russian Academy of Sciences, Irkutsk, Russia ** Institute of Physics of the Sun and the Earth, Siberian Division of the Russian Academy of Sciences, Irkutsk, Russia Fax: (3952) 46 29 00 E-mail: popov@earth.crust.irk.ru Some precursor anomalies observed during geophysical monitoring for the purpose of earthquake prediction in the Sayan-Baikal province turned to be of non-seismic origin. It has been hypothesised that such factors as solar activity and water level fluctuations in Lake Baikal may act as triggers for seismic processes. To test the idea, annual quantities of all M>1.5 seismic events occurred in the region. The relationship between seismicity and lake level can be explained by changes in the deep fluid regime caused by the Baikal water penetrating to midcrustal depths due to water level fluctuations. The presence of meteoric water in the crust is evidenced by vertical variations of its geoelectric properties recorded by transient-field, dipole and vertical electric soundings held in the South Baikal geophysical test site. As shown by inversion of DSS curves, the long transition times represent depths of 12 to 15 km. These results have been supported by geomagnetic studies as well as by 3D modelling where the equation of equilibrium was solved in combination with the general Hook's law. In the fractured rift displacements result in cyclic annual opening and closing of fractures, water becomes captured and pushed down, that is reflected in variations of crustal electrical conductivity and fluid regime. Results of the continuous seismicity and ELTRAN monitoring held in the Sayan-Biakal province were taken through wavelet analysis, and annual periodicity was revealed that agrees with annual water level fluctuations in Lake Baikal. Session 3, Oral Title: 3.5 Electric and Magnetic Signals Related to the Fluid-Driven Earthquakes in the Eastern Part of Izu Peninsula, Japan. Authors: Y. SASAI and M. Uyeshima (Earthquake Research Institute, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan); Y. Honkura (Tokyo Institute of Technology, Meguro-ku, Tokyo 142, Japan); N. Oshiman (Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto 611, Japan) In the eastern part of Izu Peninsula, we have conducted continuous observations of the total force intensity at 20 stations as well as long-distance measurements of self electric potential (SP) with telephone cables. We observed a remarkable decrease in the total intensity preceding swarm earthquakes and subsequent surface upheaval in October 1996. Moreover, SP increased by about 70 mV in the uplift area. The anomalous change in SP suggests that fluids penetrated into the shallower part of the crust in the focal zone. If the injected fluid causes the temperature rise, it results in the thermal demagnetization with dominant decrease in the total intensity. In 1997, the decrease in the total intensity appeared only in a localized area, where small earthquakes took place sporadically. In the case of October 1996 swarm, the observed magnetic anomaly can be interpreted by a thin horizontally embedded demagnetized ellipsoid. Demagnetization is only 1 % of the average magnetization, i.e. temperature rise up to a few degree C. Warm water is supplied through vertical tensile cracks. Hence the swarm earthquakes in the eastern Izu Peninsula are most probably induced by sporadic increase in the interstitial fluid pressure (fluid-driven), but unlikely by the time variation of regional stresses (stress-driven). Title: 3.6 Electric potential variations associated with yearly lake level variations. Authors: Trique, M. (1) [trique@ldg.bruyeres.cea.fr], Perrier, F.(1) [perrier@ldg.bruyeres.cea.fr], Hautot, S. (2) [sophie.hautot@univ-brest.fr] & Tarits, P. [pascal.tarits@univ-brest.fr]. (1) Departement Analyse et Surveillance de l'Environnement, Laboratoire de Detection et Geophysique, B.P.12, F-91680 Bruyeres-le-Chatel, France. (2) UMR Domaines Oceaniques, Place Nicolas Copernic, F-29280 Plouzane, France. Session: 3 Electric and magnetic approaches to seismic and volcanic activity studies. Oral preferred. Abstract: Electric potential variations have been recorded from November 1995 to February 1996 and continuously since October 1996 at 14 measurements points on a one km wide ridge separating two artificial lakes in the French Alps. The levels of the two lakes vary by about sixty meters on a yearly cycle, inducing stress variations and fluid percolation. Unambiguous large potential variations are observed over a year at one point [Perrier et al., Geophysical Research Letters, in press] as well as smaller variations at four other points on both sides of the ridge. Potential variations are linearly correlated with the level of the lakes. The correlation, with an amplitude ranging between 0.5 and 2 mV per meter of water level change, is positive for the points located above the maximum lake level on its banks and negative for these located on the bottom of the lake. Various models are discussed in relation with laboratory data as well as geophysical and geochemical surveys performed on the site. Title: 3.7 Magnetotelluric investigation of deep geothermal reservoir at the Kakkonda field, northern Japan Authors: Toshihiro Uchida, Yasuo Ogawa and Shinichi Takakura Geological Survey of Japan 1-1-3 Higashi, Tsukuba 305-8567, Japan Magnetotelluric surveys have been carried out at the Kakkonda geothermal field, northern Japan, by GSJ and NEDO since 1994. The geothermal power plant has an installed capacity of 80 MWe and utilizes steam and hot water produced from reservoirs at depths of 1000 - 3000 meters. A very young granitic rock, less than 300,000 years old, was found at a depth of approximately 3000m. Based on the temperature greater than 500 degrees C measured at the bottom of a 3,730m deep well and other geochemical data, the granite is estimated to be the heat source of the geothermal system. The purpose of the MT surveys is to obtain detailed resistivity structure of the reservoir and the granite. Measurements have been done on four survey lines. Dominant strike directions of the resistivity structure obtained by the Groom-Bailey decomposition of the GSJ data are NW-SE at a frequency greater than 1Hz and NNE-SSW at less than 0.01 Hz. Preliminary two dimensional inversion models along approximately E-W survey lines indicate low resistivity layer at shallower than 500m. Resisivity of the geothermal reservoirs at depths of 1000 - 2000 m is generally high, and the resistivity decreases at a depth greater than 3,000m in the granite. Title: 3.8 Earthquake Prediction by Electrical Prospecting Authors: Keisuke USHIJIMA*, Hideki MIZUNAGA **and Ikuo Oonaru*** * Engineering Geophysics, Kyushu University, Hakozaki, Fukuoka 812, Japan ** University of California at Berkeley, CA 94720-1760, USA *** OYO Corporation, Shiobaru, Fukuoka 810, Japan An advanced geoelectric technique for direct imaging of fluid-flow in the reservoir has been developed by the geophysical laboratory of Kyushu University. Fluid-flow behaviors in fractures can be visualized as a function of time by the automatic measurements of streaming potentials and electrical resistivity distributions on the ground surface. The system was improved for monitoring streaming potentials caused by seismic signals. Laboratory measurements of streaming potentials of soil samples, computer simulations for various 3D fracture models and field applications for injection and production wells have been conducted for predicting seismic events. It is concluded that the events of electroseismic phenomena is much faster than seismic events caused by earthquakes. Title: 3.9p Behavior of Electrical conductivity functions before of the earthquake (Tien Shan alpine zone). Author: L. Abramova Institute of Geoelectromagnetic Investigations of United Inst. of Physics of the Earth, 142092 TROITSK, Moscow region, box 30, RUSSIA.E-mail: abramova@geo.igemi.troitsk.ru A series of geomagnetic studies has been undertaken in the Issyk-Kul seismic region, Tien Shan. Three component magnetometers have been operated at four sites. We take for analysis the data (magnetic tensor components) preceding the earthquake that occurred on January 26, 1986. The magnitude of this earthquake was 5 and epicenter was located between the two of the settlements, approximately on the line connecting these settlements. The results showed that we definitely observed the trend of transfer functions in a period preceding the earthquake (during 2 months). Title: 3.10p MAGNETOTELLURIC AND MAGNETOVARIATIONAL ANALYSIS AT COLLEMELUCCIO (CENTRAL ITALY): TIME EVOLUTION OF ELECTROMAGNETIC PARAMETERS IN RELATION TO LOCAL SEISMIC ACTIVITY Authors: D. Di Mauro (1), T. Ernst (2), A. Meloni (1), P.Palangio (1), R. Teisseyre (2) (1) Istituto Nazionale di Geofisica - Rome, Italy (dimauro@ingrm.it) (2) Institute of Geophysics, Warsaw, Poland MagnetoTelluric (MT) and MagnetoVariational (MV) data, continuously collected since Summer 1991 at Collemeluccio (41 43N, 14 22E) in Central Italy, have been analyzed by means of tensor decomposition and robust schemes respectively. In the frame of a joint research project between the Warsaw Academy of Science (Poland) and the National Institute of Geophysics (Italy), fluctuations of some parameters related to the electromagnetic (EM) induction phenomena such as apparent resistivity and strike direction are presented in their time evolution. MV analysis performed on simultaneous data from L'Aquila Geomagnetic Observatory (42 23N, 13 19E and about 100 km away from Collemeluccio) was also carried out in order to find a possible connection with local and regional seismic activity. Moreover, an attempt to widen the geophysical setting of the area is suggested by setting lateral and in depth electrical conductivity heterogeneity for distinct scale lengths. Title: 3.11p Multivariate Processing of Electromagnetic Data from the VAN Earthquake Monitoring Array Authors: Gary D. Egbert, (COAS, Oregon State University, Corvallis, OR, 97331, egbert@oce,orst.edu) N. Bogris, J. Makris, P. Kapiris, & G. Balasis, (Solid Earth Physics Institute, Univ. of Athens, nbogris@atlas.uoa.gr) Electromagnetic (EM) precursors to earthquakes have been reported by a number of researchers over the past several decades, raising hopes that practical EM prediction of damaging earthquakes might be possible. Among the most extensive and promising reports of precursory EM signals are the Seismic Electric Signals (SES) observed in Greece by the VAN group. One major challenge in any investigation of EM seismic precursors is to separate anomalous tectonically caused EM fields from natural magnetotelluric (MT) fields, cultural EM noise, and electrode noise. We present here preliminary results from a collaborative study of signal and noise characteristics in archived digital data from the VAN monitoring network, using multivariate array processing techniques that have been developed for analysis of MT and magnetovariational array data. These methods allow characterization of the spatial structure of local and large scale noise sources, and allow separation of the dipole array signal into MT, local coherent noise, and residual components. A very important feature of this multivariate approach is that residuals in all channels (magnetic as well as electric) can be reliably estimated. Quantification of the relative size of electric and (much smaller) magnetic signals associated with SES will place strong constraints on the geometry and location of sources, and further clarify the nature of the phenomena. Title: 3.12p THE USE OF ELECTROMAGNETIC DATA FOR MEDIUM AND SHORT TERM PREDICTION OF VRANCEA(ROMANIA) EARTHQUAKES. Authors: B.D.Enescu,A.P.Constantin,C.Ionescu and D.Enescu benescu@rcep.dpri.kyoto-u.ac.jp Specific elements concerning the first application in Romania of a new method for detecting Vrancea earthquake precursors as a tool for earthquake prediction are presented. The main sources and aspects of earthquake precursor generation,particularly electomagnetic,seismo-electric and seismo-magnetic signals,are examined.The prerequisites for using the method in the particular case of Vrancea seismogenic region are described and a a principle diagram of an electromagnetic observatory is given. The devices comprising an electromagnetic observatory and the methodology used in setting up two electromagnetic observatories at Cheia-Muntele Rosu and Vrancioaia,based on the principle diagram,are presented. In the final part of the paper,the results of a preliminary processing of same records in the two observatories are given. Title: 3.13p Possibilities and limits of polarizable and nonpolarizable electrodes,for detecting seismoelectric signals Author: Furnica C. Valentin Institute of Geodynamics of Romanian Academy,19-21,J.L.Calderon Str.,Bucharest, R070201 A ten minute video recording is presented and on an apparently virtual circular structure background (made evident by biodetection),in an bowl with water situated in the center of it, one observes the forming of vortices, visualized by coloring matter. We can discuss aspects regarding temporal and spatial relationships of the phenomenon, detected both in the laboratory and in the field, with the chemical and physical processes which could appear for an electric cell, represented in the measurements made for selecting seismoelectric signals (SES), identical with the recording device of telluric current field. Title: 3.14p Magnetotelluric Studies over seismically active Koyana region of Western Maharastra, India Authors: S.G. Gokarn, Yasuo Ogawa, C.K. Rao Indian Institute of Geomagnetism, Colaba, Mumbai " Geological Survey of Japan, Tsukuba, Japan Magnetotelluric studies were conducted along two E-W profiles over seismically active Koyana region of Western Maharastra passing from Guhaghar-Satara-Nateputa (Profile-I) and second from Guhaghar-Koyana- Karad-Vite (Profile-II). The entire area along the profiles covered by Deccan flood basalts. The impedances were decomposed and the regional strike was estimated. The strike angle obtained from decomposition is North three degrees East which coincides with strike of the Western Ghats in the survey region. The interpretation of the data shows that the thickness of the Deccan trap in both the profiles varies 600 m at the end of the profile to 2ooo m at the center of the profile. A mid crustal conductor was observed at a depth of 30 km with resistivity 800 ohm-m between the stations 24-26. Lithosphere-Asthenosphere boundary was delineated at a depth of 90 km between the stations 21-26. 3.15p Variation in time of the apparent resistivity in the hypocentral zone of the Vrancea earthquakes B.Hobbs (1), D.Zugravescu (2), V.Furnica (2), Angela Dumitrica (2), L.Bordei (2) 1)University of Edinbourgh 2) Intitute of Geodynamics of the Romanian Academy e-mail: angela@sabba.geodin.ro The recordings, as uninterruptedly as possible, of the magneto-telluric field were carried out in the Gresu Station, a component of the Caldarusani-Tulnici Astrogeodynamic Polygon, a station situated in the proximity of the Vrancea seismogenic zone. The Geologger recording system used supplied by the University of Edinburgh, Great Britain, in the framework of the cooperation of the Royal Society of London, and of the Romanian Academy, functioned synchronically by means of a computer time basis. The variation in the time of the natural electromagnetic field was surveyed, respectively the Ex, Ey, Bx, By, Bz, sampled at 20 seconds were recorded, which allowed the storage of information for the frequency range of less than 0.025 Hz. The aim of the processing made by us was to obtain the variation of some of the parameters particularly of the apparent resistivity in the hypocentral area respectively the correlation of these parameters with the variation in time of the tensions responsible for the earthquakes triggering. The establishing of electromagnetic field frequencies carrying the information concerning the variation in time of the apparent resistivity at depths of 80 150 km, where the majority of strong Vrancea earthquakes hypocenters are situated, completed as possible with the establishing of a correspondence between the earthquakes occurence moment and the variation of the resistivity suggests the possibility of setting a network of observatories equipped with band-pass sensors, which might allow collecting in real time of an information of maximum interest for better understanding of the evolution in time of the pre-, co- and post seismic phenomena (seismoelectric signals and information concerning the evolution of apparent resistivity values). 3.16p Variation in space of apparent resistivity on profiles situated in the Vrancea seismogenic region B.Hobbs (1), D.Zugravescu (2), V.Furnica (2), Angela Dumitrica (2), L.Bordei (2) (1) University of Edinbourgh (2) Intitute of Geodynamics of the Romanian Academy e-mail: angela@sabba.geodin.ro Deep structure determination by the magnetotelluric method in the Vrancea Seismic region constitutes a difficult problem because of the field conditions of collecting primary information. Magnetotelluric soundings used by us were carried out in the Gresu, Vintileasa, Caldarusani basic stations and in several connection stations situated on the Gresu-Vintileasca and Vintileasca-Caldarusani profiles of particular geodynamic interest, belonging to the Caldarusani-Tulnici Astrogeodynamic Polygon. The three Geologger recording systems used, put at our disposal by the University of Edinburgh, Great Britain, functioned synchronically by means of computer set time basis. The natural electromagnetic field variation in time was surveyed by recording, as uninterruptedly, as possible, the Ex, Ey, Bx, By, Bz components sampled at 20 seconds, allowed the storage of information for the frequency range of less than 0.025 Hz. The aim of the processing was to establish the resistivity variation in the depth, at the sounding points, and to interpolate the values obtained on the studied profiles, respectively the establishing of the resistivity sections allowing to sketch the position of the tectonic blocks forming the crust in this zone of maximum geodynamic interest. Title: 3.17p On a Long-Term Self-Potential Change in a Seismically Active Area in Central Japan Authors: Yoshimori Honkura[1], Masaki Matsushima[1] and Naoto Oshiman[2] [1] Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan yhonkura@geo.titech.ac.jp, mmatsush@geo.titech.ac.jp [2] Disaster Prevention Research Institute, University of Kyoto, Uji, Kyoto 611, Japan g53032@sakura.kudpc.kyoto-u.ac.jp Earthquake swarm activities have often occurred in the northeastern part of the Izu Peninsula, central Japan. Aiming at detecting possible changes in the electric and magnetic fields associated with the activities, we have been carrying out continuous measurements of the magnetic field and the self-potential at some sites near the swarm area. In this paper, we discuss on some characteristic changes in the self-potential data. Self-potential changes arising from oceanic tides can be measured at a site located close to the seashore, whereas such changes do not appear at a site located a few kilometers inland. The self-potential data have been contaminated by rainfall and hence it has been difficult to detect long-term changes. During the period from October to November in 1997, however, a clear gradual self-potential change could be observed. Representation in terms of the electric field change shows that the direction of the changing field is similar to that of the electric field variations due to oceanic tides. The observed long-term potential change therefore suggests a long-term change in the oceanic current system, although the possibility of crustal activity origin cannot be denied at present. Title: 3.18p Intercorelation Earthquakes Epicentre Arrangement with Deep Crust Electrical Conductivity Authors: Ingerov A.I.(1), Vlasov Y.T.(1), Rokytyansky I.I.(2) (1) GRL Slavuta Co.,Ltd,33,K.Marx Ave, Dnipropetrovsk,320044,Ukraine, Fax (380562) 476777 (2) Institute of Geophysics,POB-338/y,Kiev-146,Ukraine (380044) 4743251 root@slavuta.dnepropetrovsk.ua This work is based on 8000 Magnetotelluric soundings (MTS) made gradually during last 20 years on the Ukrainian territory. A common feature of the geoelectrical structure is a regular spatial distribution of vertical or inclined conducting zones coinciding with faults of different orders. These zones display a tendency to be of one of four discrete azimuths: 315 degree (the Ryansnopol anomaly) and 0 degree and 17 degree (the Kirovograd anomaly) are the most conductive, and 90 degree as the less conductive. In the limit of the Krimea the conducting structure thith azimuth 77 degree. Conducting zones of each of 5 azimuth are characterized by MTS different from that giving possibility for zones identification by MTS data. The large conducting structures tend to follow through the certain intervals multiple 50 or 70 km. The remarcable fact is the concentration of the Crimean centres of earthquakes in the field of crossing fault structures various azimuth. Thus the basic role is plyed by breaks with azimuths 17,77,110 and 345.The similar picture is marked and with realization of researches MTS in area Spitac earthquake in Armenia (1988), though it is based on smaller volume of the data. In territory of Ukrainian Carpathians the centers of earthquakes with azimuths 0,17,35,345 degree. The special attention is deserved by fact, that epicentre of crust earthquakes are absent above Carpathians anomaly conductivity, that can testify to the raised viscosity earthly crust in depths of 10-15 km. Title: 3.19p Directional properties of VAN's SES inferred from a resistivity mapping around Ioannina station, Greece Authors: Wataru KANDA(1), Makoto UYESHIMA(2), John MAKRIS(3), Yoshiaki ORIHARA(4), Hideaki HASE(5), Toshiyasu NAGAO(6), and Seiya UYEDA(4) (1) Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University (2) Earthquake Research Institute, the University of Tokyo (3) Solid Earth Physics Institute, the University of Athens (4) International Frontier Program on Earthquake Research, The Institute of Physical and Chemical Research (RIKEN) (5) Faculty of Science, Kyoto University (6) Earthquake Prediction Research Center, Tokai University (1) Yokoyama, Sakurajima, Kagoshima, 891-1419, Japan (2) Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan (3) Zografos, Athens, 157-84, Greece (4) Orido 3-20-1, Shimizu 424-8610, Japan (5) Noguchibaru 3088, Beppu, Oita 874, Japan (6) Orido 3-20-1, Shimizu 424-8610, Japan Email: kanda@svo.dpri.kyoto-u.ac.jp, uyeshima@utada-sun.eri.u-tokyo.ac.jp, sepi@leon.nrcps.ariadne-t.gr, yorihara@riken.go.jp, hhase@iord.u-tokai.ac.jp, nagao@scc.u-tokai.ac.jp, suyeda@st.rim.or.jp In the summer of 1997, we made a bipole-dipole mapping survey around Ioannina VAN station, where detection of the preseismic electric signal (SES) has been repeatedly reported. Since we had found the characteristic directional properties of the electric field in the previous study, the present study was aimed to examine it by investigating the shallow electric structure around the station. The apparent resistivity tensor was derived from two sets of measured voltages at each receiver position. As a rough sketch of the resistivity tensor distribution, we found that the electric field was enhanced along the direction parallel to the trend of the basin at receivers located in the conductive basin, and perpendicular to it at the resistive mountainside.We modeled the measured voltages by the conductance distribution with thin plates. The results showed that the VAN station is located on the resistive portion near the conductance gap. Furthermore, we simulated the apparent resistivity tensor near the VAN station if the current bipoles were located at various positions on the inverted conductance distribution model. Although the directional property similar to that of MT and lightning electric field was not reproduced there, the similar property was found if the source bipoles were put more than about 5 km away from other receiver positions. Title: 3.20p The rezults of physical modelling geo- electromagnetic and geothermic fields correlation. Authors: Kobzova V., Moroz I. Carpathian Branch of Subbotin Institute of Geophysics NASU Ukraine 290601 Lviv Naukova str.3b E-mail va@carp.lviv.ua 3-layer model of a deep Earth's section containing nonstationary heat anomaly in the surface layer is made due to installation "Model". Changes of homogeneous electromagnetic field caused by heat inhomogeneity have been registered. An experimental chart of dependance between magnetic and heat fields has been obtained. The basic result of the experiment consists in the following: technical means, methods and materials for joint physical modelling of geothermic and geoelectromagnetic fields in inhomogeneous layered media simulating real geological situation are developed for the first time. In line with the law of similarity physical modelling results of connection between electromagnetic and geotermic fields may correspond both high-speed and long-duration processes in nature from volcanogenic veruptions to evolutionary processes in seismoactive zones. Title: 3.21p Modeling the Geoelectric Structure by Synthesis of Decomposition Techniques and Mohr Circle Analysis Authors: J. Makris, N.Bogris and K. Eftaxias Solid Earth Physics Institute Panepistimiopolis Zografou, GR-157 84 Athens, Greece e-mail: nbogris@atlas.uoa.gr, ceftax@atlas.uoa.gr Abstract One of the current problems in magnetotellurics is the effects of local near-surface inhomogeneities on the MT-impedance estimates. It is necessary to correct the MT-responses for the static distortions prior to any attempt to derive a conductivity model. In its more general form static distortions have been addressed by modeling their effects as 3D-galvanic bodies over a 2D-regional geoelectric structure. Bahr's and Groom and Bailey's decomposition methods are invoked in order to determine the appropriate model that best fits to the MT-data. In the present study Mohr circle analysis is also applied to define the dimensionality of the structure. This proves to be advantageous because we are able to resolve remaining multiplicities arising from a technique such as Groom and Bailey's decomposition, thus leading to the best choise of model that adequately describes the structure. Title: 3.22p A New Approach in the Determination of Characteristic Directions of the Geoelectric Structure Using Mohr Circles Authors: J. Makris, N. Bogris and K. Eftaxias Solid Earth Physics Institute Panepistimiopolis Zografou, GR-157 84 Athens, Greece e-mail: nbogris@atlas.uoa.gr, ceftax@atlas.uoa.gr Abstract Mohr technique is a very elegant graphical method widely used in structural geology for analysing stress variation and finite strain in bodies (Ramsay & Huber, 1987). The application of Mohr circles into magnetotelluric interpretation was initially introduced by Lilley (1976). By plotting Zxx-rotated versus Zxy-rotated (real and quadrature parts) we are able to obtain important information about the conductivity structure. In the case of strong local current channelling Mohr circles provide the polarisation directions of the local electric field. In this paper we suggest the following procedure for Mohr circle analysis: plotting Zxx-rotated versus Zyx-rotated (instead of Zxy-rotated) the principal axis system i.e., the strike angle, of the regional 2D-structure is resolved. The latter analysis is implemented to MT data from Ioannina area. Results from other interpretation techniques applied to the same dataset, such as decomposition methods, tipper and induction arrows analysis, provide strong evidence for the validity of the conclusions reached. These conclusions lead to a very convenient way fot the reduction of the MT-noise from the raw electric field records, thus allowing a better recognition of Seismic Electric Signals (SES). Title: 3.23p Volcano-hydrological Study and Magma Prospecting on Usu Volcano, Japan as revealed by Magnetotelluric Soundings Authors: N. Matsushima(1), Y. Ogawa(1), H. Oshima(2), Y. Nishida(2), S. Takakura(1), A. Tomiya(1), M. Utsugi(2), S. Tamura(2), H. Satoh(2) (1) Geological Survey of Japan, Tsukuba, Ibaragi 305-0046, Japan (2) Div. of Earth and Planet. Sci., Hokkaido Univ., Sapporo 060-0810, Japan E-mail: nishida@ares.sci.hokudai.ac.jp Usu is a stratovolcano located in Hokkaido, Japan. Paroxismal eruptions took place on August 8, 1977. The succeeding activity was characterized by phreatomagmatic eruptions on April to October,1978. On the summit crater, a new cryptodome, Usu- Shinzan, grew to about 180m in height during these periods. The intruded magma is inferred in position beneath the new cryptodome, although we have no direct information about it. Present study represents the results of broad-band magnetotelluric soundings on Usu Volcano. The objectives are (1) to study the volcano hydrological environment for the purpose of the prediction of phreatomagmatic eruption and (2) to detect the detailed position of the above-mentioned intruded magma. Results of 2-D inversion revealed as follows: (1) The low resistive layer (10 ohm-m or so) is sandwiched between the surface basaltic somma lava (1500〜2000 ohm-m) and the resistive Paleogene rocks (1000 ohm-m). This low resistive layer well collerates with Quaternary and Neogene rocks about 2 km in thickness. Considering the in-situ hydraulic conductivity measured by well tests at surroundings of Usu, the layer is identified as the water bearing one. (2) A relatively high resistive body (300 ohm-m or so) is burried at depths from 400 m to 800 m within a low resistive crater fill (about 10 ohm-m), suggesting the existence of the intruded magma. Title: 3.24p Investigation of correlation between magnetotelluric field and seismic electric signal using Kamchatka seismic zone physical model. Author: Moroz Yu. Insitute of Volcanic Geology and Geochemistry EB RAS Russua 683006 Petropavlovsk-Kamchatsky, Pyypa str.,9 E-mail: moros@ volgeo.kamchatka.su Kobzova V., Ladanivsky B. Carpathian Branch of Subbotin Institute of Geophysics NASU Ukraine 290601 Lviv Naukova str.3b E-mail va@carp.lviv.ua Sharp changes of telluric field before strong earthquakes have been observed over the past years in the result of continuous observations held on Kamchatka. This is supposed to be caused by electric charges generated in the earthquake source. 3-lauer scale model of Kamchatka seismoactive zone (500 x 500 km) has been developed in an electrolytic tank to check this idea. Deep electric signals have been modelled by electric dipoles of different dimentions, orientation, polarization and frequency. Geoelectromagnetic variations have been modelled by field of plane harmonic electromagnetic wave in the frequency range 0.1-10 mgc. The current of some tens of am at depth of about 20 km turns out to be enough for electric field to increase by tens thousands of mv/km as it is observed on the polygon deep electric processes are determined to be reprisented at MT sounding curves as charp overshoots in separate periods, displacements and changes of an inclination angle. Title: 3.25p STRUCTURE OF THE ERATH'S LOW FREQUENCY ELECTROMAGNETIC FIELD AND THE SEISMICITY Authors: Yu.F. MOROZ, Institute of Volcanic Geology and Geochemistry, Far East Division, Russian Academy of Sciences. Petropavlovsk-Kamchatsky 683006. Russia. E-mail: ivgg@svyaz.Kamchatka.su Researches have been carried out on the basis of electromagnetic and seismic monitoring at Kamchatka over the period 1990-1996. Electromagnetic monitoring is conducted around Petropavlovsk-Kamchatsky at 4 sites at the distance of 50 to 120 km between them. One of these sites comes nearer to the seismofocal zone. Information is recorded and transmitted by means of the radiotelemetric system. The scanning period is one minute. The structure of the electromagnetic field has been studied in the period range from few hours to a year and more. The spectral-temporal characteristics of the field, coherence, phase correlations and amplitude frequency functions between the components of the electro- magnetic field have been studied on the basis of multi- dimensional analysis. It has been determined that the diurnal harmonics (T=6,8,12,24 hours) dominate in the electric spectrum of the field. Yearly and diurnal vari- ations are more stable in space and time. These variations apparently carry the information of high seismicity in Kamchatka. Thus, 1n 1993 when large earthquakes of M=7.1 and 7.3 occured in the area of Kamchatka phase correla- tion of yearly telluric variations N-S and E-W changed to 90 grades. Canonical coherence of diurnal telluric variations (T=12 and 24 hours) and geomagnetic variations changed from the most common 0.8-1 to 0.3-0.5 values 10 to 30 days before the large earthquakes. Alongside with that bay-like disturbances from 3 to 4 days up to a month and a half duration, oscillating-type quasi-periodic variations with the time period of 1 to 6 days and the disturbed regular variations with the time period of few hours have been detected in the electrotelluric currents before the earthquakes. Title: 3.26p PHYSICAL MODELLING OF THE ELECTRICAL SOURCE IN SEISMOFOCAL ZONE Authors: Yu.F.Moroz, V.M.Kobzova, Ladanivsky B.T. Institute of Volcanic Geology and Geochemistry, Far East Division, Russian Academy of Sciences. Petropavlovsk-Kamchatsky 683006. Russia. E-mail:ivgg@svyaz.Kamchatka.su Institute of Geophysics, Carpathian Division, Ukrainian Academy of Sciences. L'vov. 290005. E-mail:va@carp.lviv.ua In Kamchatka due to monitoring the earth's electrical currents bay-like disturbances with the intensity of a few hundreds to 1500 mV/km were recorded at the observational sites 10 to 30 days prior to large earthquakes. A hypothesis exists that the disturbances are caused by the extensive electrical source. To check up this hypothesis physical modelling has been fulfilled at a tank apparatus. A simplified model corresponding to a natural geoelectric section was simulated. An electric dipole submerged into the lithosphere to a depth of 100 km was assumed to be the source of the electromagnetic field. The electric current frequency was 0.0004 - 0.000006 Hz. As a result of the experiment the regularities in behaviour of the electromagnetic components have been detected which depend on the dipole dimensions, power and frequency of the current in it and on the distance from the obser- vation sites to the dipole center. The cases have been determined where signals were manifested only in one orthogonal component and were not expressed in another one at various elimination from the electrical source. It has been found that the electrotelluric field variations in the periods of 0.0004 to 0.000006 s with the intensity of hundreds to a few thousands mV/km occure at a distance of 200 km from the center of dipole (length is 200 km, depth is 100 km), the value of electric current in it being a few ampere. The approximate estimates suggest that the overall electric current of such value probably occures due to the electrokinetic (eletcrofiltrative) effects in the litho- sphere blocks of a few thousands sq.km in section and a few hundreds in length. Title: 3.27p Surveying the Vesuvius by transient electromagnetics Authors: M. Mueller, A. Hoerdt, F. M. Neubauer (IGM, Cologne, Germany) Martin Mueller Applied Geophysics Institut fuer Geophysik LOTEM and Volcanology und Meteorologie Universitaet Koeln Phone: 0049 (0)221 470-2841 or Albertus-Magnus-Platz 0049 (0)221 470-5479 D-50923 Koeln Fax: 0049 (0)221 470-5198 Germany Email:mamue@geo.uni-koeln.de In April 1997 a two week long-offset transient electromagnetics (LOTEM) survey was conducted at Vesuvius volcano. LOTEM is an active method designed to determine resistivity distributions down to a penetration depth of approx. 15 km. 23 stations, each equipped with a magnetic and an electrical field sensor, were set up on a profile of 10 km length. The main aims of this first LOTEM application on an active volcano were the analysis of the specific conditions in volcanic environment and the investigation of the conductivity structure of Vesuvius. One particular aspect for measurements on volcanoes is the strong topography. We estimated the influence of topography using 3D finite difference modelling and found that it is mainly an early time effect. Another feature of the data is the strong distortion by powerline noise. The signal to noise ratio was significantly improved using new digital filters. The 1D inversion results show a good conducting layer in depths of 0.5 to 1.5 km below the surface. Conductivities of salt water and typical porosities of volcanic rock support the idea of fluids as source of the good conducting layer. 3.28p HYDROLOGICAL MODEL OF THE CHILECITO RESERVOIR INCLUDING GEOELECTRICAL AND MAGNETIC DATA Ana Osella, Alicia Favetto and Patricia Martinelli Departamento de Fosica - Universidad de Buenos Aires - Conicet Ciudad Universitaria - Pab. 1 - 1428 - Buenos Aires -Argentina Aquifer-bearing intermontane sedimentary basins of the Sierras Pampeanas in the North-West of Argentina are in general very deep and narrow and contain economically important deposit of Tertiary sediments. This paper presents a study to characterize the Chilecito aquifer system. This system is located at ~29 S and surrounded by Sierra de Famatina-Sa+/-ogasta to the west and Sierra de Velasco on the east. A geoelectrical survey was performed along a 40 km-EW profile. Correlating these results with geological data, borehole information and previous magnetic and audio-magnetotelluric data, a description of the characteristics of the aquifer has been performed. These results, together with hydrological and meteorological data allow to make an estimation of the recharge cycle and circulation of ground water. 3.29p CORRECTION OF ELECTROMAGNETIC METHODS DATA BECAUSE OF TIDES PHENOMENA INFLUENCE AT THE SEISMIC ACTIVITY MONITORING A. Saraev 1, M. Pertel 1, Z. Malkin 2, B. Pugachev 1 1: St. Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg, 199034, Russia. E-mail: aks@aks.usr.pu.ru 2: Institute of Applied Astronomy, 8 Zhdanovskaya st., St. Petersburg, 197110, Russia. E-mail: malkin@ipa.rssi.ru The diurnal variations of apparent resistivity Ra of 7% value are detected by results of measurements of ELF radio station electromagnetic field impedance. The measurements were carried out with ACF-2.2 device on Karelia isthmus at distance 950 km from the source, located on Kola peninsula. Frequency of 80 Hz was used, and the measurements were carried out during day through each 20 min. The obtained curve Ra with a high degree of a correlation (0.95) was coincided with calculated curve of vertical tide deformation dH of an earth surface. The value of deformation dH was 0.2 m. An equation of line correlation connection for the observation point was obtained: Ra=1067+240dH (Ra, Ohm*m; dH,m). For increase of earthquakes prediction reliability the correction of measurements data is necessary because of tides phenomena influence is comparable with Ra changes prior to earthquakes. Called by tides phenomena of Ra variations can be calculated with the use of equations, similar showed above. The tides deformation increases in southern latitudes (on equator vertical deformation of the earth surface is 0.6 m). Therefore these effects should be taken into account also at carrying out of MT, AMT and others sounding methods. The work was performed with the support of "Integration" grant, # 326.66, and "University of Russia" program, # 2140. Title: 3.30p POSSIBLE SEISMO-MAGNETIC SIGNALS DETECTED AT THE SURLARI OBSERVATORY Authors: A. Soare, G. Cucu In tectonic-active areas some changes of rock magnetisation can appear due to piezomagnetic effects, put in evidence by particular modification of the geomagnetic field, recorded on the ground, named "seismo-magnetic signals". The Surlari Geophysical Observatory is situated in the proximity of intramoesic fault, well-know by the surface earthquake frequency, and not far from the important seismic zone - Vrancea.This is a good opportunity to detect some signals on continuous magnetic recordings realised at the observatory, statistical correlated with main seismic events. Preceding the earthquakes, the seismo-magnetic signals are important because they can be taken into account as eventual prediction factors.This work has in view to develop some started researches after the major earthquake in Vrancea, on March, 4, 1977, in order to find a separation way of signals sent by two seismic sources. Title: 3.31p LABORATORY STUDIES OF ELECTROMAGNETIC EMISSION FROM ROCKS UNDER STRESS AND DESTRUCTION Michae Stefaniuk* *University of Mining and Metallurgy, al.Mickiewicza 30, 30-059 Cracow, Poland e-mail: stefaniu@galaxy.uci.agh.edu.pl Abstract Laboratory tests of electromagnetic emission from rocks subjected to stress and destruction are part of a wide-range investigations of natural radiofrequency radiation generated in a rock medium. The objective of the study was to identify the genesis and process of this phenomenon that could be advantageous in understanding electromagnetic effects accompanying earthquakes, landslides and rockbursts. Despite numerous studies made for the last 25 years, basic questions of the genesis and characteristics of electromagnetic emission from rocks remains still open. Since different measurement techniques and data processing methods were employed in different research centres, comparing their results is questionable. The objective of the presented studies was to examine a possibility of applying the electromagnetic emission from rocks into predicting rockbursts and other local geodynamic processes connected with the deformation and destruction of rocks. Electromagnetic emission time series were recorded with a two-channel equipment consisting of an EM field detector, set of amplifiers and filters, and two digital recorders. Maximum amplitudes and spectra of signals were then analysed. Results of the studies included a comparative analysis of EM emission characteristics for granite, syenite, sandstone, quartzite, marble, limestone, graphite, and black coal samples. Title: 3.32p LONG-TERM MONITORING OF MAGNETOTELLURIC FIELDS IN A SEISMO-ACTIVE AREA Authors: Svetov B.S., Odintsov V.I., Kuksa Yu.I., Balandina S.E. (Geoelectromagnetic Research Institute RAS, Troitsk, Russia) P.O.B. 30 City: TROITSK, Moscow Region. Postal code: 142092. Country: RUSSIA Phone +-7(095)3340906, Fax:+-7(096)7514400 Email: SVETOV@GEO.IGEMI.TROITSK.RU 2. SESSION 3 (Electric and magnetic approaches to seismic and volcanic activity studies). 3. Poster In 1993-1994 continuous recording of 5 component magnetotelluric variations (Hx, Hy, Hz, Ex, Ey) was carried out in Bishkek (Kirgizstan), with a sampling rate equal to 0.1 Hz. This data set has been used for determination of time domain characteristics of transfer functions. The components of MT fields are connected by convolution of such type: Hz(Ex,y)(t)=K1(t)*Hx(t)+-K2(t)*Hy(t)+-D(t) This equation is solved by iterative (adaptive) procedure. As a second stage, time variations of transfer functions have been analyzed. Averaged data characterizing these variations are presented in the form of spectral-time section versus time. Sampling rate was equal to 6 minutes, 1 hour and 1 day. Considerable variations of these values, with duration ranging from several hours to several months have been discovered. They are connected either to changes of the ionosphere current system responsible for recorded geomagnetic variations, or to changes in time of the geoelectrical section or to generation of electromagnetic fields in the Earth interior related to geodynamic processes. We found no stable correlation between variations of MT transfer functions and individual seismic events (earthquakes). On the other hand correlation between low pass filtered (three-month filter) data and energy of geodynamic process in the vicinity of measuring site has been established. Energy was determined using seismic events within 200-km radius around the MT site. This conclusion is preliminary because it was made on the basis of limited data series (+AH4-1.5 year). It is illustrated by measured data and results of their processing. INTAS (INTAS project 94-1304) and RFBR (grant 97-05-64103) supported these investigations. TITLE: 3.33p Temporal variations in the electrical resistivity near the Hanaori fault, Japan, observed by the Network MT method AUTHORS : Kengo Tanimoto 1)*, Naoto Oshiman 1), Norihiko Sumitomo 1), Satoru Yamaguchi 2) and Yasunori Nishida 3) 1) Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University 2) Faculty of Science, Kobe University 3) Graduate School of Science, Hokkaido University *Now at Graduate School of Science, Hokkaido University. 1) Gokasyo, Uji 611-0011, Japan 2) 1-1, Rokko-dai, Nada-ku, Kobe 657, Japan 3) N10 W8, Kita-ku, Sapporo 060-0810, Japan Email: kengo@ares.sci.hokudai.ac.jp A network magnetotelluric (Network MT) method is a technique which enables a broad profiling of the resistivity structure by using the telephone line network with dipole length ranging from several to several tens kilometers (Uyeshima,1990). Hanaori fault located in southwestern part of Japan was chosen as the subjective area to the study, and the Network MT method has been applied to monitor the crustal resistivity changes accompanied by the seismic activities. The electric fields in the study area were severely polluted by the artificial electric noises mainly caused by the electric leakage current from the railway, Kosei line. Under such circumstances, we precisely examined the detectability of the Network MT method in the study area by using observed data set of two years period from 1995 to 1996. In particular, we evaluated our Network MT monitoring system from the view points of 1) accuracy and stability of detecting resistivity changes and 2) temporal resolution power in the time domain. Annual stacked values of the apparent resistivities and the phases were determined stably within the averaged standard errors, 5.3% and 1.5 degree, respectively. The results assured stable monitoring of resistivity and phase changes even in the electrically noisy area. Title: 3.34p Radon emanation and electric potential variations associated with transient deformation in the vicinity of reservoir lakes. Authors: Trique, M. (1) [trique@ldg.bruyeres.cea.fr], Perrier, F.(1) [perrier@ldg.bruyeres.cea.fr], Richon, P. (2) [patrick.richon@ipsn.fr], Sabroux, J.C. (2) [sabroux@basilic.ipsn.fr] & Avouac, J.P. (1) [avouac@ldg.bruyeres.cea.fr]. (1) Departement Analyse et Surveillance de l'Environnement, Laboratoire de Detection et Geophysique, B.P.12, F-91680 Bruyeres-le-Chatel, France. (2) Institut de Protection et Surete Nucleaire, 60-68, avenue du General Leclerc, F-92265 Fontenay aux Roses, France. Abstract: Electric potential variations, radon emanation and deformation are monitored since november 1995 in the French Alps in the vicinity of two artificial lakes. The levels of the two lakes vary both by more than 60 meters on a yearly cycle inducing stress variations and fluid percolation in the surrounding rock formations. Reproducible electric potential variations and radon emanations are observed in relation with transient deformation induced by lake level variations. Meteorological parameters are also recorded on the site, and they show no correlation with the radon emanation or the electrical signals. A preliminary interpretation is presented, where the observed signals result from a permeability enhancement in the undrained mode, inducing ground water circulation, gas discharge and electrical potential variations. A careful investigation of these phenomena should be undertaken before considering their application to earthquake prediction. Title: 3.35p Seismomagnetic Effect considering the Inhomogeneity of Crustal Magnetization Authors: Mitsuru Utsugi and Yasunori Nishida Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan mithuru@ares.sci.hokudai.ac.jp nishida@ares.sci.hokudai.ac.jp We attempt to evaluate the seismomagnetic effect considering the inhomogeneous distribution of the crustal magnetization. The crust may be divided into a number of cubic compartments. The seismomagnetic fields due to the magnetically inhomogeneous crust can be represented by the superposition of that due to each cubic compartment. Therefore, we consider in the present study that a cubic region in the crust is uniformly magnetized, while outside of the cube has no magnetization. The magnetic changes due to the piezomagnetic effect are written in the form of surface integral of the stress-induced magnetization distributed on the surface of magnetized region. This 2-D integral involves 1st, 2nd and 3rd kinds of elliptic integrals which can be easily evaluated by Landen and Gauss transforms. Therefore, the 2-D integral is reduced to 1-D analytically. We performed the numerical calculation of the 1-D integral and evaluated the seismomagnetic fields derived from a uniformly magnetized cube. Title: 3.36p Change of Resistivity Before and After Zhangbei-Shangyi Earthquake (M=6.2, 10/1/1998) Authors: Guoze Zhao Ji Tang Jijun Wang Wenjun Li Zhao Jiang Yan Zhan (Institute of Geology, State Seismological Bureau, Beijing 100029, China) Zhao Guoze, Institute of Geology State Seismological Bureau Beijing 100029 China Tel: (08610) 6202 3377-615 (office) (08610) 6202 3377-735 (home) Fax: (08610) 6205 0634 or 6202 8617 Email:zhaogz@public.bta.net.cn Zhangbei-Shangyi earthquake with magnitude of M=6.2 occurred in the northwest corner of Hebei province at 11:50 of January 10, 1998. The epicenter located in sub-tectonic unit of Zhong-Chao platform (North China platform), Nei-Mongolia earth*s axis. Nei-Mongolia fold system is to it's north. There are some sub-tectonic units of North China platform to it*s south. After main-shock magnetotelluric measurement was carried out at epicenter named as site MMC and at site 511 that is 26 km distant from epicenter to south-east. At site 511 MT data were obtained on June 1, 1994 and observed repeatedly on 13 and 20 of January of 1998. It is found that Rho(yx) for TE polarization corresponding to NNW direction decreased on 13 and 20 of January with comparing to original data observed on June 1, 1994 for periods range from 0.2 sec to 2 sec and from 10 sec to 200sec. The maximum amount of decrease is about 30% for period 0.5sec. The Rho(yx) for 20 of January is bigger than that for 13 of January. Rho(xy) for TM polarization to NEE direction post main shock is bigger than that for June 1, 1994. MT data were measured continuously at site MMC from 16 to 19 of January. No measurement had been carried out at site MMC before main shock, but a change of apparent resistivity was also observed. Resistivities for both of TE and TM polarization at site MMC (epicenter site) were gradually increased from 16 to 20 of January for similar period range. Suppose the resistivities decreased before and during the main-shock, the resistivity increase after main shock can be regarded as resumption of resistivity. The direction NNW of resistivity decrease before and/or during main shock time nearly coincides with that of extension for site 511 according to regional stress field. In the direction of compression resistivity increased, which shown an anisotropy of resistivity change. For site MMC decrease of resistivity of both polarizations before main shock is possibly caused by strong fracture for both direction and anisotropy is much weaker than that for 511. The period ranges of resistivity decrease in NNW direction are corresponding to depths 1-2 km and 10-20 km respectively for site 511. The later is corresponding to the low resistivity layer in the crust. The low resistivity layer is at depth of about 16 km for MMC. Main shock and large post-shock occurred at the depth above 15 km, that is, in the upper brittle layer of the crust.