Tadashi Nishitani

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.

Titanomagnetites : Oxidation and Magnetic Properties

Titanomagneties, Fe3-xTixO4, are the most important carriers of magnetic remanence in terrestrial igneous rocks. In subaerial effusive rocks such as basalts, titanomagnetite often undergoes a high-temperature oxidation ; i. e., Ti-rich spinel → Ti-poor spinel + Ti-rich ilmenohematite, the latter being exsolved into (111) planes of the host spinel forming a lamella structure. Another form of oxidation, low-temperature oxidation, proceeds if a titanomagne-tite sample is heated to a moderately high temperature ( 1μm) tend to deveolop rhombohedral phase due to oxidation even at low temperatures. However, significant variations remain even after the grain size effect is taken into account. One possibility is that the starting materials may be subtlely different because of the difference in sample preparation procedures.