Magnetotelluric imaging of fluids in intraplate earthquake zones, NE Japanback 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 statio ns extends over the two topographic features, the Dewa Hills and the Ou Bac kbone Range, which were uplifted by thrust faults. MT data show two-dimensi onality and strong TE/TM anisotropic responses at the periods around 100 s. After tensor decompositions with regional strike of N12 degreesE, two-dime nsional inversion was carried out where static shift was also a model param eter. The final model is characterized by conductive blocks in the mid-crus t to account for the anisotropic responses. Correlation of the conductors t o the seismic scatterers and to the low velocity anomalies suggests that th e 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 concen tration near the structural boundary.