2:00 PM - 2:15 PM
[SEM16-02] Broadband MT data compilation in the central Tohoku region
Long-period MT observations in the central Tohoku region have been carried out on a 20-km grid over the entire central Tohoku region (Ichiki et al., 2015) in investigating the mantle resistivity structures. On the other hand, crustal structure studies using the broadband MT method have been vigorously conducted since 1990. However, each has been limited to a 20 km × 20 km spread area for volcanic and inland earthquakes (e.g., Ogawa et al., 2014), partly because of the computer memory limitation required for inversion. This study aims to compile the data of 590 stations obtained from previous broadband MT observations to elucidate the detailed resistivity structure from the mantle to the crust. As a preliminary step, we compile the existing broadband MT data to understand its properties over a wide area using different parameters for a wide range of periods.
We mapped the responses of electric and magnetic fields for four typical periods (0.4, 4, 40, and 400 seconds) with various parameters. We then qualitatively investigated the distribution of the subsurface resistivity structure.
First, we assumed hypothetical events where the ionospheric currents were polarized northward and westward, respectively, and mapped the corresponding electric field distributions calculated from the impedance tensors. We found that the directions of the electric currents follow those of the ionospheric currents in a short period. However, in the long period (400 seconds), the observation points where the electric field existed only in a specific direction, independent of the direction of the ionospheric current, were distributed over a wide area. In particular, when the ionospheric current is directed westward, the high-resistivity Kitakami Mountains block the electric currents from passing through. As a result, the electric currents are heavily deflected and are flowing in two "anomalous" paths. One is a northward path through the northward channel of the sedimentary layer along the Kitakami River. Another path is along the sedimentary layer to the south of the Naruko Volcano. Such behaviors of the deflected electric currents are also harmonious with the distribution of long-period (400 s) induction vectors, where electric currents are concentrated in much shallower structures (~3 km) than their skin depth (~100 km).
This current concentration corresponds to the distribution of the phase tensor, which measures the three-dimensionality of impedance, with βexceeding 5 degrees, and the longer the period, the more the area of observation points with large β around this current concentration area expands.
From the distribution of phase (Phi_2) analysis, the short period (0.4 seconds) shows a high phase area corresponding to the distribution of the sedimentary layer. At 4 seconds, there is a high phase area corresponding to the crustal fluid and melt distribution in the Naruko and Sanzugawa calderas. There is also a high phase region in the southern Kitakami Mountains. At the long periods (40, 400s), the contrast between the low phase on the frontal arc side and the high phase on the dorsal arc side is visible along the volcanic front.
From the alignment of phase tensors at 4 to 40 seconds, the hypocenters of the large historical inland earthquakes seem located at the edge of the resistivity structure boundary.
We mapped the responses of electric and magnetic fields for four typical periods (0.4, 4, 40, and 400 seconds) with various parameters. We then qualitatively investigated the distribution of the subsurface resistivity structure.
First, we assumed hypothetical events where the ionospheric currents were polarized northward and westward, respectively, and mapped the corresponding electric field distributions calculated from the impedance tensors. We found that the directions of the electric currents follow those of the ionospheric currents in a short period. However, in the long period (400 seconds), the observation points where the electric field existed only in a specific direction, independent of the direction of the ionospheric current, were distributed over a wide area. In particular, when the ionospheric current is directed westward, the high-resistivity Kitakami Mountains block the electric currents from passing through. As a result, the electric currents are heavily deflected and are flowing in two "anomalous" paths. One is a northward path through the northward channel of the sedimentary layer along the Kitakami River. Another path is along the sedimentary layer to the south of the Naruko Volcano. Such behaviors of the deflected electric currents are also harmonious with the distribution of long-period (400 s) induction vectors, where electric currents are concentrated in much shallower structures (~3 km) than their skin depth (~100 km).
This current concentration corresponds to the distribution of the phase tensor, which measures the three-dimensionality of impedance, with βexceeding 5 degrees, and the longer the period, the more the area of observation points with large β around this current concentration area expands.
From the distribution of phase (Phi_2) analysis, the short period (0.4 seconds) shows a high phase area corresponding to the distribution of the sedimentary layer. At 4 seconds, there is a high phase area corresponding to the crustal fluid and melt distribution in the Naruko and Sanzugawa calderas. There is also a high phase region in the southern Kitakami Mountains. At the long periods (40, 400s), the contrast between the low phase on the frontal arc side and the high phase on the dorsal arc side is visible along the volcanic front.
From the alignment of phase tensors at 4 to 40 seconds, the hypocenters of the large historical inland earthquakes seem located at the edge of the resistivity structure boundary.