Masaaki Mishina

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.

Electromagnetic heterogeneity of the seismogenic region of 1962 M6.5 Northern Miyagi Earthquake, northeastern Japan

Magnetotelluric surveys were carried out to in- vestigate the electrical resistivity structure of the seismo- genic region of the 1962 Northern Miyagi Earthquake (M6.5) in Miyagi Prefecture, northeastern Japan. A 2-D inversion reveals the existence of a deep conductive zone and overly- ing re sistivezonein theuppe r crust. A positiveae romag- netic anomaly is also recognized above the resistive zone, which is interpreted as a buried Kitakami granitoid pluton. By comparison with these ismicity, wefound that themi- croearthquakes occur just above the deep conductor and in the resistive zone, and that several S-wave reflectors are just above the deep conductor. Since the S-wave reflectors sug- gest the existence of fluid beneath them, the deep conduc- tor can be interpreted as a fluid-filled zone. Moreover, we suggest that the seepage of the fluid from the conductive fluid-filled zone to the resistive granitoid pluton can become a trigger of the earthquakes.

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.

Three-dimensional magnetotelluric imaging of crustal fluids and seismicity around Naruko volcano, NE Japan

We analyzed the 3-D resistivity structure beneath Naruko volcano, northeastern Japan, with the aim of imaging 3-D distribution of fluids in the crust for its volcanic and seismogenic implications. The data were recorded at 77 sites in total: 30 sites are new and are arranged in an approximately 5 × 5 km grid whereas the remaining older sites constitute two separate east-west profiles. We ran a 3-D inversion using full components of impedance tensors in the period range between 0.13 and 400 s. The resulting model showed that a sub-vertical conductor exists a few kilometers below Naruko volcano. The conductor extends from the surface of the volcano and dips towards the south, away from the volcano towards the backbone range. High levels of seismicity are observed in the upper crust above and around the conductors. We suggest that the seismicity is fluid driven and that a fluid trap is created by the precipitation of quartz owing to a reduction in solubility at shallow depth. The Quaternary volcanic front is characterized by a sharp resistivity contrast and a high-resistivity zone and extends 10 to 15 km towards the east. A fore-arc conductor was observed at mid-crustal levels even farther towards the east. The sub-vertical conductors along the arc and the fore-arc conductor have resistivities of 1 to 10 Ωm. Assuming a Hashin-Shtrikman model with saline fluids of 0.1-Ωm resistivity, a porosity of 1.5% to 15% is required to explain the observed conductive anomalies.

Magnetotelluric investigations for the seismically active area in Northern Miyagi Prefecture, northeastern Japan

The ELF- and the ULF-MT surveys were carried out in the northern part of Miyagi Prefecture, northeastern Japan. This area is one of the most seismically active areas in this region, where hypocenters of microearthquakes are distributed on a fault plane at depths from 2 to 16 km. The aim of the present study is to investigate the relationship between electrical resistivity structure and the hypocentral distribution of microearthquakes in the area. The calculated impedance tensor at each site has been obtained from the observed data and decomposed to remove galvanic distortion, provided that the regional strike is N32?E to obtain the 2-D apparent resistivity and phase responses. The resistivity structure obtained by the inversion process using smoothness constraint shows that the relatively electrically conductive layer at depths from 4 to 10 km corresponds to the zone where the microearthquakes occur. The fact that the conductive zone correlates with the hypocentral zone is probably attributed to fluids in the crust. Another more conductive block is found at depths from 1 to 3.5 km and the bottom boundary of this conductor appears to restrict the uppermost depth where the microearthquakes occur. This subsurface conductor is interpreted as a marine sediment deposited during the Tertiary period. In the lower crust, the relatively conductive blocks (lower than 5Ω m) exist below a depth of 15 km.

Crustal strain, crustal stress and microearthquake activity in the northeastern Japan arc

Abstract Crustal movement and seismic activity in the northeastern Japan arc are produced by a reaction of the Pacific Plate sinking under the continental plate. We have investigated the time variation of crustal strain obtained from analysis of extensometer installed at eleven crustal-movement observatories in the northeastern Japan arc. We then investigated, using the finite element method, the spatial distribution and time variation of the crustal strain and stress caused by plate subduction. We finally compared the results with microearthquake activity observed by the seismic network of Tohoku University. The spatial distribution and time variation of microearthquake seismic energy were elucidated in order to know the seismic circumstances around the eleven observatories. A contour map of the seismic energy distribution was obtained, showing a lineation of the seismic activity. The time variation of various strain components from 1967 to 1981 were revealed. With the accumulation of additional data, it is possible to recognize the propagation of the maximum shear strain in the direction S50°E at 40 km/year from the Pacific Ocean side to the Sea of Japan. A similar time variation of areal-dilatation was found at the five observatories located at a distance of more than 100 km. The directions of the compression axes derived from the composite focal mechanism solution of microearthquakes coincide well with those of principal strain obtained from data gathered using extensometers, indicating E–W compression in most of the observatories and N–S compression around MYK. The spatial distribution and the accumulation of crustal stress caused by plate subduction were investigated using the finite element method, and a comparison with a hypocenter distribution of microearthquakes was performed. The distribution of the crustal stress is closely related to the crustal structure, but the amplitude of the observed strain is affected by local structures around the stations.

Caldera structure inferred from gravity anomalies west of Nagamachi-Rifu Fault, Northeast Japan

A gravity survey was conducted in and around the Nagamachi-Rifu Fault. The density for both terrain and Bouguer corrections was chosen to be 2,300 kg/m3, because volcanic rocks are dominant over the whole measurement area and the surface layer density is estimated to be low from geological considerations. The Bouguer anomalies are characterized by a low anomaly similar to those of a caldera and the basement structure inferred from two and three-dimensional analysis shows that the depth is often more than 1 km in the caldera region and that there is a circular structure. The gradient of basement is steepest at the southern margin, and it resembles to rim of funnel-shaped caldera.