Naoto Oshiman

Magnetotelluric imaging of fluids in intraplate earthquake zones, NE Japan back arc

Intraplate earthquake zones in the back arc of NE Japan were imaged by wide-band magnetotelluric (MT) soundings. The 90km long MT profile of 34 stations extends over the two topographic features, the Dewa Hills and the Ou Backbone Range, which were uplifted by thrust faults. MT data show two-dimensionality and strong TE/TM anisotropic responses at the periods around 100 s. After tensor decompositions with regional strike of N12°E, two-dimensional inversion was carried out where static shift was also a model parameter. The final model is characterized by conductive blocks in the mid-crust to account for the anisotropic responses. Correlation of the conductors to the seismic scatterers and to the low velocity anomalies suggests that the conductors represent fluids. High seismicity clustering near the rims of conductors suggests that the intraplate seismicity results either from the migration of the fluids to less permeable crust or from local stress concentration near the structural boundary.

Preliminary results of multidisciplinary observations before, during and after the Kocaeli (Izmit) earthquake in the western part of the North Anatolian Fault Zone

On August 17, 1999, a destructive earthquake occurred in the western part of the North Anatolian Fault Zone, Turkey. The earthquake source region has been designated as a seismic gap and an M7-class earthquake has been supposed to occur someday in the future so as to fill this seismic gap. So far we have undertaken various kinds of observations in this area and we could obtain some valuable data before, during and after the mainshock. Here we report some of the preliminary results of our recent studies, which include field work started in late July this year and continued during and after the earthquake occurrence just in the earthquake source region and its vicinity, in addition to seismic observations carried out for several years before the mainshock. Much emphasis is put on magnetotelluric field data acquired during the mainshock; in fact, large variations caused by seismic waves were recorded. Such variations could be interpreted in terms of electromagnetic induction in the conducting crust caused by the velocity field interacting with the static magnetic field of the Earth. In particular, the first motion of seismic wave could be identified in the records and used for precise determination of the hypocenter of the mainshock.

Resistivity structure in the western part of the fault rupture zone associated with the 1999 İzmit earthquake and its seismogenic implication

Magnetotelluric (MT) surveys were carried out along some profiles crossing the fault rupture zone associated with the İzmit earthquake which took place on 17 August 1999 in the western part of the North Anatolian Fault Zone (NAFZ). In this paper, we focus on the western part of the fault rupture zone where two different groups of seismicity followed the İzmit earthquake. One group was seen along a narrow belt and corresponds to aftershocks occurring along the fault rupture zone. The other was seen in a circular region and represents a swarm activity, presumably triggered by the occurrence of the İzmit earthquake. Two-dimensional inversion was performed for the MT data acquired along two profiles; one crosses the western end of the fault rupture zone and the other is located in the west of the swarm activity area. In the former case, aftershocks tended to occur in a resistive zone underlain by a moderately conductive zone, as was the case for the hypocenter area. In the latter case, the swarm activity tends to be confined in a conductive zone below a highly resistive zone. This activity is likely to be triggered through pore-pressure changes associated with the İzmit earthquake.

Small electric and magnetic signals observed before the arrival of seismic wave

Electric and magnetic data were obtained above the focal area in association with the 1999 Izmit, Turkey earthquake. The acquired data are extremely important for studies of electromagnetic phenomena associated with earthquakes, which have attracted much attention even without clear physical understanding of their characteristics. We have already reported that large electric and magnetic variations observed during the earthquake were simply due to seismic waves through the mechanism of seismic dynamo effect, because they appeared neither before nor simultaneously with the origin time of the earthquake but a few seconds later, with the arrival of seismic wave. In this letter we show the result of our further analyses. Our detailed examination of the electric and magnetic data disclosed small signals appearing less than one second before the large signals associated with the seismic waves. It is not yet solved whether this observational fact is simply one aspect of the seismic dynamo effect or requires a new mechanism.

Resistivity structure around the hypocentral area of the 1984 Western Nagano Prefecture earthquake in central Japan

We carried out magnetotelluric measurements in the southeastern region of Mt. Ontake, where the 1984 Western Nagano Prefecture earthquake occurred and earthquake swarms have been observed since 1976. Most of the earthquakes have focal depths shallower than 10 km. The depth of the hypocenters increases towards the west. Our purpose is to delineate the resistivity structures down to the focal depths of the earthquakes, because the resistivity structure is very sensitive to the free water in the crust. The regional strike was estimated as N60°E. We carried out a two-dimensional analysis over two profiles across the earthquake fault of the 1984 Western Nagano Prefecture earthquake: one is in the low seismicity region of the fault (A-A′) and the other, in the high seismicity region (B-B′). The resistivity structure along the A-A′ profile is reasonably homogeneous and shows a high resistivity of more than 300 ohm·m. The resistivity structure along the B-B′ profile has a clear boundary at the center of the profile. This boundary divides the structure along the B-B′ profile into two resistivity blocks and its location coincides with that of the earthquake fault. A conductor is detected at depths greater than 8 km to the northwest of the earthquake fault. The depth to the upper boundary of the conductor coincides with those of the seismic reflectors. This structure seems to be formed by the free water dehydrated from the magma.

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.

Preliminary report on regional resistivity variation inferred from the Network MT investigation in the Shikoku district, southwestern Japan

The Network MT method was used in the eastern part of the Shikoku district, southwestern Japan, and a total of thirty-nine MT impedances (64 to 2560 sec) were obtained. These MT impedances had their values averaged over a triangular element, whose sides were a few kilometers long with geomagnetic observatory data from the Kakioka Geomagnetic Observatory. Well-determined MT impedances varied from north to south with the greatest differences being at the Median Tectonic Line, which is consistent with the surface geology in the area. In addition, very large or very small values of apparent resistivity were obtained in some triangular elements. These triangles were located on a cape or near an estuary, with effects of three-dimensionality clearly apparent. Stable MT impedances were not obtained for three groups of triangular elements: (1) those where one or two sides of the triangular element cross the coast; (2) those where the electric field was contaminated by severe artificial noise, these were mainly in the northwestern part of the survey area; (3) those where the triangles had an extremely acute- or obtuse-angle.A resistivity cross section was derived from the TM-mode data for a profile crossing the eastern part of the area. The shallower layer, which approximately corresponds to the crust, was divided into three blocks. Two resistive boundaries coincide with the geological tectonic lines and the strong horizontal contrast found at the Median Tectonic Line. The northernmost block is the most resistive, and the block to the south is the most conductive. Beneath these blocks, the subducting Philippine Sea plate was represented by a thick and highly resistive north-dipping layer. A highly conductive thin layer was found above the resistive layer on the southern side of the Median Tectonic Line. This layer is only found beneath the southern side of the Median Tectonic Line and is probably caused by pore water and/or sediment at the upper plane of the subducting Philippine Sea plate. Another conductive layer was found under the highly resistive north-dipping layer.The resistivity structure from the lower crust to the upper mantle is firstly obtained using the Network-MT method. However, further developments are needed in methods of data analysis, which are robust to artificial electric noise, in order to clarify the spatial distribution of MT impedances in the complete study area.

Three-dimensional electromagnetic imaging of upwelling fluids in the Kyushu subduction zone, Japan

A three-dimensional (3-D) lithospheric-scale electrical resistivity model, developed using network-magnetotelluric (network-MT) data, contains structures associated with arc magmatism beneath Kyushu Island in the Southwest Japan arc. Kyushu Island, where the Philippine Sea Plate (PSP) subducts beneath the Eurasian plate, can be divided into northern and southern volcanic regions separated by a nonvolcanic region. Many active Quaternary volcanoes occur along the volcanic front (VF) associated with the PSP in the two volcanic regions. Our 3-D electrical resistivity model shows three different shapes of upwelling fluid-like conductive anomalies, indicative of either slab-derived aqueous fluid and/or partial melt beneath the volcanic and nonvolcanic regions. A conductive anomaly in the northern volcanic region, located at some distance from the subducting PSP, extends from the surface to depths of 70 km depth, extends from the surface to depths of >100 km. In the nonvolcanic region, the upper region of a relatively conductive anomaly extends upward to a depth of ~50 km along the subducting plate. The degrees of magmatism and the relative contribution of slab-derived fluids to the magmatism vary spatially in the one nonvolcanic and two volcanic regions.

A strategy of tectonomagnetic observation for monitoring possible precursors to earthquakes in the western part of the North Anatolian Fault Zone, Turkey

Abstract In order to investigate changes in the geomagnetic field associated with seismic activity in the western part of the North Anatolian Fault Zone (NAFZ), seven continuous measurement stations and eighteen repeated-survey sites were set up in the Inik-Geyve region, taking into account the fault trace and its features. Simple differences in the total intensity between these continuous stations have been derived for detecting possible anomalous phenomena associated with earthquake occurrences. Theoretical studies imply that changes in the total intensity associated with fault activity are larger in the case of non-uniform distribution of rock magnetization than those in the uniform case. Meanwhile, inversion analyses of magnetic anomalies at the North Anatolian Fault Zone (NAFZ) suggest a dyke-like structure extending parallel to the fault strike. This structure enables us to test our strategy, based on non-uniform magnetization, for detecting pre- and/or co-seismic changes in the geomagnetic field. We made a theoretical estimation of geomagnetic changes for the case of a magnetic dyke structure intruding a non-magnetic body parallel to the strike of a fault. It turns out that, unlike the uniform case, the magnetic field arising from stress-induced magnetization is enhanced. A tectonomagnetic observation system talcing into account such a non-uniform magnetization structure in the Inik-Geyve region of the NAFZ is also described.

Modification of the Network-MT method and its first application in imaging the deep conductivity structure beneath the Kii Peninsula, southwestern Japan

The Network-Magnetotelluric (NMT) method is well-suited for investigating deep and large-scale conductivity structure; however, application of the method is strongly dependent on the availability of telecommunication facilities (specifically, metallic transmission cables). To overcome the problem posed by the progressive replacement of metallic transmission cables with fiber cables, we developed a modified NMT (modified NMT) method consisting of purpose-built electrodes, making use of local metallic telecommunication lines, without a transmission cable. We first applied this modified NMT method over the Kii Peninsula, southwestern Japan, undertaking two-dimensional conductivity modeling along a transect across the central part of the peninsula. The model is characterized by a large (∼20 km wide and depths of 10–60 km) and highly conductive (< 10 Ω m) zone in the central part of the peninsula between the Conrad discontinuity and the upper surface of the Philippine Sea slab. This zone contains the hypocenters of many deep low-frequency tremors but regular earthquakes are rare. The zone also corresponds to a high-Vp/Vs area. The presence of fluid in the zone plays a key role in the absence of regular earthquakes, occurrence of deep low-frequency tremors, and elevated Vp/Vs values, as well as enhancing conductivity.