Properties of electromagnetic fields generated by tsunami first arrivals: Classification based on the ocean depth

Abstract

Tsunami flow coupled with the geomagnetic field generates electric currents and associated magnetic fields. Although electromagnetic (EM) tsunami signals can be used for analysis and even forecasting tsunami propagation, the dynamically self‐consistent effect of shoaling water depth on the fluid + electrodynamics has not been adequately clarified. In this study, we classify tsunami EM phenomena into three cases based on the ocean depth and find that the deeper ocean results in stronger self‐induction due to the increase in both tsunami phase velocity and ocean conductance. In this deep‐ocean case, the phase lead of the vertical magnetic variation relative to the sea surface elevation is smaller, while an initial rise in the horizontal magnetic component becomes observable prior to tsunami arrival. Furthermore, we confirm that the enhancement of tsunami height in shallower oceans shifts the ocean depth supplying maximum amplitudes of tsunami magnetic fields from approximately 2.0 km to 1.5 km.

Three‐Dimensional Time Domain Simulation of Tsunami‐Generated Electromagnetic Fields: Application to the 2011 Tohoku Earthquake Tsunami

“…In the three panels of Figure , readers may notice some important characteristics of the TGEM fields: (1) b z is in phase with the sea surface displacement in deep oceans (Figure a), (2) only the horizontal component parallel to the tsunami propagation direction is significant (Figure b), (3) the initial rise (e.g., Minami & Toh, ) in the horizontal components precedes the tsunami arrivals (Figures b and c). The first characteristic (1) is due to the strong self‐induction effect in deep oceans (Minami et al, ), while the second (2) is an important feature of TGM fields pointed out by Toh et al (). As for (3), we can recognize the light blue area with outward vectors in Figure b, and the pale red area in Figure c, on the eastern side of the leading tsunami wave, respectively.…”

“…This initial rise also appears in Figure as the first small positive peak in the eastward component at NWP (not clear at B14 due to the missing data). Minami and Toh () and Minami et al () pointed out that the initial rise in the horizontal component appears in deep oceans and precedes the tsunami leading wave by ~ T /4, where T is the dominant tsunami period of the leading wave. In fact, the panel of t = 100 min in Figure c clearly shows that the initial rise in b x arrives at NWP (indicated by a green square) before the arrival of the tsunami leading wave.…”

“…Relatively large errors in the amplitude (>1.5%) appeared at ( h , λ ) = (2.5 km, 320 km), for both two‐ and three‐layer meshes. Minami, Toh, and Tyler () revealed that the TGM fields varies dramatically with the ocean depth and have an amplitude peak around h = 2.0 km, where they reach twice the amplitudes at h = 6.0 km, provided that the tsunami wave height and period are kept constant irrespective of the ocean depth. The large errors at ( h , λ ) = (2.5 km, 320 km) may be due to combination of the large TGM fields and local roughness in the adopted mesh over the scale of the long wavelength.…”

Tsunami-induced magnetic fields detected at Chichijima Island before the arrival of the 2011 Tohoku earthquake tsunami

“…As such, the induced electric current near the coastline is substantially in phase with the tsunami. Minami et al (2015) discussed these phenomena in detail.…”

“…Ichihara et al (2013), Sugioka et al (2014), and Minami et al (2015) described a more extended formulation using a seafloor with finite electrical conductivity. Zhang et al (2014b) considered the electrical conductivity of the seafloor as a threedimensional multi-layer structure.…”