Mantle upwelling revealed by genetic algorithm inversion of the magnetovariational anomaly around Kyushu island, Japan

Abstract

The spatial extent of an outstanding mantle upwelling in the west of Kyushu island, Japan, has been estimated by a genetic algorithm inversion. Here we show how the mantle upwelling manifests in modeling of observed anomalies of temporal geomagnetic variations, i.e., magnetovariational anomalies around southwest Japan. Observed ratios of the vertical to horizontal geomagnetic components at short periods are known as indicators of subsurface electrical contrasts. However, they are also sensitive to the presence of oceans and continents because of much more conductive seawater than land rocks. To circumvent the difficulty, we used three‐dimensional nonuniform thin sheet approximation to subtract out the conductive as well as inductive effects of the ocean on the magnetovariational anomalies. The observed anomalies were then inverted by a genetic algorithm inversion for conductance (the product of electrical conductivity and thickness) with the known bathymetry as a priori information. The derived conductance model has particular sensitivity to the northeastern portion of spatial extent of the mantle upwelling probably centered at the East China Sea. The upwelling may result in horizontal flows of the uppermost mantle, which induce the horizontal stress field that can trigger hazardous shallow intraplate earthquakes of large magnitude, such as the earthquake off the west coast of Fukuoka Prefecture that occurred on 20 March 2005. The northeastern rim of the mantle upwelling thus revealed can also be related to an unidentified boundary between the Amurian plate and the Eurasia plate.