Improved models of the piezomagnetic field for the 2011 Mw 9.0 Tohoku-oki earthquake

To assess the feasibility of observing changes in the magnetic field produced by the piezomagnetic effect, an improved model of the piezomagnetic field corresponding to the Mw 9.0 Tohoku-oki earthquake is presented. In contrast to an earlier study, the proposed model explicitly considers the spatial distribution of slip on the seismic fault, and the results from this new model differ significantly from those of the previous model where slip distributions were ignored. Quantitative aspects of the piezomagnetic effect are discussed through comparisons of data and models. One feature clarified is that, because the fault rupture is so far offshore, the expected amplitudes are quite small at onshore existing observation sites; consequently, there would have been little chance of observing sizable piezomagnetic signals at inland sites during the Tohoku-oki earthquake. Nevertheless, piezomagnetic signals were reportedly detected at a few sites, possibly indicating that the stress sensitivity or the initial magnetization was larger (by several factors) than assumed. On the other hand, relatively large variations in the magnetic field of up to 10 nT may have occurred offshore. This means that if ocean-bottom sensors had been installed, larger piezomagnetic signals would have been detected. Moreover, the piezomagnetic field in offshore areas is sensitive to the detailed slip distribution, suggesting that observations of the magnetic field at ocean-bottom sites might provide important constraints on determination of slip models.