Possible Detection of Magnetic Anomaly During Tsunami Events in Indonesian Regions Using Global Magnetogram Records

Ocean flow generates secondary, weak magnetic signals relative to the main field induced by the Earth spinning motion, where the secondary signals lead to magnetic anomaly. The anomaly were apparently observed as short-lived variation in secondary field components, namely the vertical b z and horizontal components b H , respectively, during tsunami occurrence. In this study, maximum amplitudes associated with these components were determined using theoretical approaches and field records on global magnetogram provided by INTERMAGNET and BCMT. The roles played by a depth ratio of h/L where h and L are the ocean depth and characteristic length, respectively, and a speed ratio of c/c s where c and c s are the speed for linear long wave solution and the complex speed involving ocean diffusion, respectively, are here examined using Indonesian case studies of tsunami with respect to trans-Pacific tsunamis as reference. For cases with advection dominance, it was found that frozen-flux theory can be used to estimate b z and b H, consistent with values provided by the global magnetic institutions. In short, whereas b z is a measure of water surface elevation and hence tsunami height offshore, b H is an indicator for tsunami propagation direction. Detection of magnetic anomaly prior to tsunami arrivals at coastal zones is thus possible, making it crucial for tsunami early warning.