Toru Mogi

Resistivity imaging across the source region of the 2004 Mid-Niigata Prefecture earthquake (M6.8), central Japan

Across the source region of the 2004 Mid-Niigata Prefecture earthquake, wideband magnetotelluric (MT) survey was performed just after the onset of the mainshock. Owing to the temporal stop of the DC powered railways around the area together with intense geomagnetic activity, we obtain MT records with excellent quality for both short and long period data, as long as 10,000 s. Two dimensional regional strike is evaluated with the aid of the Groom-Bailey tensor decomposition together with induction vector analysis. As a result, N15°W is determined for the strike. This strike is oblique to the local geological trend and also to the strike of the main shock source fault together with aftershock distribution of N35°E. Two dimensional resistivity structure is determined with the aid of an ABIC inversion code, where static shift is considered and estimated. Characteristics of the structure are as follows. (1) About 10 km thick sedimentary layer exists on the top. (2) A conductive body exists in the lower crust beneath the source region. The mainshock occurred at the boundary of the conductive sedimentary layer and a resistive basement beneath it and aftershocks occurred in the sedimentary layer. From geological studies, it is reported that the sedimentary layer was formed in the extensional rift-structure from Miocene to Pleistocene and has been thickened by compressional tectonic regime in the late Quaternary. Interstitial fluids or clay minerals, which reduce the sedimentary layer resistivity, control the reactivation of the normal fault as the mainshock thrust fault and aftershock activity. The second conductive body probably indicates existence of fluids in the depths as well. Such a conductive layer in the lower crust was also revealed by previous MT experiments along the Niigata-Kobe Tectonic Zone and probably plays a main role in concentration of strain rate along the zone.

A new computation method for a staggered grid of 3D EM field conservative modeling

A new three-dimensional (3D) MT modeling scheme conserving electric current and magnetic flux is developed. The scheme is based on finite difference (FD) staggered rectangular non-uniform grid formulation for the secondary electric field with continuous components of tangential electric and normal magnetic fields, in contrast to existing FD algorithms with a discontinuous E-field at the face of the cells. The scheme leads to a sparse 13-band complex symmetrical system of linear equations, which is effectively solved by fast and stable conjugate gradient (CG) methods. The preconditioning procedure was used to decrease the condition of a number of an ill-conditioned matrix system by several orders and stably and quickly solves the matrix system. The special module for the correction of divergence-free current J greatly increased the speed of convergence and accuracy, especially at low frequencies and for high-contrast resistivity or conductivity structures. A special procedure was developed to improve the accuracy of tangential magnetic and vertical electrical components at the Earth’s surface and at the interface with a large conductivity contrast. The validity of the new algorithm was demonstrated for difficult models with high-contrast resistivity structures including topography and for COMMEMI project models.

Grounded electrical-source airborne transient electromagnetic (GREATEM) survey of Mount Bandai, north-eastern Japan

Airborne electromagnetics (AEM) is a useful tool for investigating volcanic structures because it can survey large and inaccessible areas. Disadvantages include lower accuracy and limited depth of investigation. The Grounded Electrical Source Airborne Transient Electromagnetic (GREATEM) survey system was developed to increase the depth of investigation possible using AEM. The method was tested in a survey at Mount Bandai in north-eastern Japan. Mount Bandai is an andesitic stratovolcano that rises 1819 m above sea level. An eruption in July 1888 left a hoof-shaped collapsed wall in its northern crater and avalanche debris at its base. Previous surveys of Mount Bandai allow for comparisons of data on its structure and collapse mechanism as obtained by GREATEM and other geophysical methods. The results show resistive structures in recent volcanic cones and conductive structures in the collapsed-crater area. Conductive areas around the collapsed wall correspond to an alteration zone resulting from hydrothermal activity, supporting the contention that a major cause of the collapse associated with the 1888 eruption was hydrothermal alteration that structurally weakened the interior of the volcanic edifice.

Geoelectric potential difference monitoring in southern Sumatra, Indonesia — Co-seismic change—

Five geoelectric potential difference (electric field, here after) monitoring stations have been in operation since September 1997 in anarea near Liwa town, southern Sumatra, Indonesia, to examine the relationship between electric field changes and earthquakes. Short-term electric field variations were found to correspond mainly to geomagnetic activity, while long-term variation was mostly gradual shift and was clearly correlated neither precipitation nor ground water level variations. Co-seismic electric field changes ranging between 1 and 8 mV were observed for five mb > 5 earthquakes at multiple stations during September ∼ December 1997. The epicenters of the earthquakes were in the Indian Ocean within about 170 km from the monitoring sites.

Magnetotelluric observations around the focal region of the 2007 Noto Hanto Earthquake (Mj 6.9), Central Japan

On 25 March 2007, a damaging earthquake (Mj 6.9) occurred near the west coast of the Noto Peninsula, Central Japan. A wideband magnetotelluric (MT) survey was carried out in the onshore area of the source region immediately after the mainshock, with the aim of imaging the heterogeneity of the crustal resistivity structure. The final observation network had consisted of 26 sites. As a preparatory step for imaging three-dimensional features of the resistivity around the focal region, we constructed two-dimensional resistivity models along five profiles using only the TM mode responses, in order to reduce three-dimensional effects. Four profiles are perpendicular to the fault strike, and a fifth profile is parallel to the strike through the mainshock epicenter. Significant characteristics of the resistivity models are: (1) beneath the mainshock hypocenter, there is a conductive body which spreads to the eastern edge of the active aftershock region; (2) a resistive zone is located in the gap of the aftershock distribution between the mainshock hypocenter and the largest eastern aftershock; (3) one of the largest aftershock occurred at the boundary of the resistive zone described above. These results suggest that the deep conductors represent fluid-filled zones and that the lateral heterogeneity could have controlled the slip distribution on the fault plane.

Resistivity structure around the focal area of the 2004 Rumoi-Nanbu earthquake (M 6.1), northern Hokkaido, Japan

The Rumoi-Nanbu earthquake (M 6.1) occurred in northern Hokkaido, Japan, on December 14, 2004. We conducted MT surveys along three profiles in and around the focal area to delineate and decipher the structural features of the seismogenic zone. The inverted 2-D resistivity images of the three sections comprised two layers: an upper conductive layer and a lower resistive layer. The boundary of these layers lay at a depth of approximately 3–5 km. A comparison with the surface geology and drilling data revealed that the upper conductive layer and the lower resistive layer corresponded to the Cretaceous—Tertiary sedimentary rocks and older basement rocks, respectively. A clear upheaval of the layer boundary was found along the profile at the center of the focal area. In addition, borehole data indicated an obvious increase in the Young’s modulus toward the lower layer. Therefore, the elastic properties with a complex geometry around the focal zone tended to vary; this probably depicts the zone of stress accumulation that triggered the earthquake.

ANN reconstruction of geoelectrical parameters of the Minou fault zone by scalar CSAMT data

Scalar controlled source AMT data collected in a northern part of the Minou fault area (Kyushu Island, Japan) are interpreted by means of the ANN Expert System MT-NET in terms of 3-D earth macro-parameters. A number of synthetic responses created in advance by means of forward modeling in typical 3-D geoelectrical models (conductive and resistive local bodies, fault, dyke, etc.) formed sequences for teaching an artificial neural network (ANN). MT-NET, once taught to the correspondence between the data images and the model parameters, is able to recognize unknown parameters given even incomplete and noisy data. The results of ANN reconstruction are compared with the resistivity distribution obtained for the same area using fast 3-D imaging based on synthesis of 1-D Bostick transforms of the apparent resistivities beneath each site as well as on 2-D TM mode inversion along four profiles. The best-fitting model reconstructed by ANN belongs to the guessed model class formed by ‘‘dykes buried in the two-layered earth’’, on the one hand, and to the equivalence class formed by all models giving rms misfit less than the noise level in the data, on the other hand. D 2002 Elsevier Science B.V. All rights reserved.

Tectonomagnetic study in the eastern part of Hokkaido, NE Japan (II): Magnetic fields related with the 2003 Tokachi-oki earthquake and the 2004 Kushiro-oki earthquake

We examined short-term geomagnetic changes related with the 2003 Tokachi-oki earthquake (M 8.0) and the 2004 Kushiro-oki earthquake (M 7.1) in Hokkaido, Japan. However, we could not find the precursory and co-seismic signals above several nT at a magnetic station whose epicentral distances were about 120 km and 50 km, respectively. Model calculations showed that co-seismic piezomagnetic fields did not amount to 1 nT at the station in both cases when we assumed the relevant fault parameters, in-situ Curie temperature depth, subsurface magnetic structure and stress sensitivity of rocks. Therefore, it may be reasonable that we could not detect the piezomagnetic signals at the station. We also made model calculations to forecast the piezomagnetic amplitudes caused by M 7.9 and M 8.5 earthquakes which have been expected to occur along the southern Kurile trench in the future. The model calculations reveal the piezomagnetic fields up to about −4 nT and −7 nT are expected in the eastern part of the Hokkaido island for the M 7.9 and M 8.5 earthquakes respectively, encouraging magnetic observations hereafter.

Resistivity and density modelling in the 1938 Kutcharo earthquake source area along a large caldera boundary

We present the crustal structure around the fault zone pertaining to the 1938 Kutcharo earthquake (M 6.0), northern Japan, to consider why large earthquakes have occurred around calderas. The study was based on gravity anomalies and magnetotelluric and direct-current (DC) electrical-resistivity survey data. The density structure obtained from gravity anomalies indicated that the fault plane corresponded to the main depression boundary of the Kutcharo caldera. The resistivity section, based on audio-frequency magnetotelluric surveys, indicated that the estimated fault plane was located along the boundary of resistivity blocks, which also corresponded to the depression boundary. A detailed resistivity section in the ruptured zone revealed by a DC electrical-resistivity survey showed a discontinuity of layers, implying cumulative fault displacements. These results indicate that the 1938 earthquake was an abrupt slip along the main depression boundary of the Kutcharo caldera. The most likely hypothesis pertains to fluid intrusion along the depression boundary. However, additional seismic and geodetic studies are required to identify other feasible earthquake mechanisms.

Grounded electrical-source airborne transient electromagnetics (GREATEM) survey of Aso Volcano, Japan

Grounded electrical-source airborne transient electromagnetics (GREATEM), a type of semi-airborne electromagnetics, was used to examine Aso Volcano in south-west Japan, to verify its applicability to surveying deep subsurface resistivity structures. Comparison of the GREATEM resistivity values with those of ground-based transient electromagnetics (TEM) data, repeated GREATEM survey results at the same and different flight heights, and lithologic descriptions indicated that GREATEM can successfully identify underground structures as deep as ~800 m in rugged mountainous areas. An active volcanic region (Naka-Dake crater) was mapped as a low-resistivity zone from the surface to a depth of 100 m. This low-resistivity zone extended to the west-north-west, implying future volcanic activity in this area. Therefore, the GREATEM method is useful for surveying deep structures in large, inaccessible areas, such as volcanic provinces, in a quick, cost-effective way.