Electromagnetic field generated by tsunamigenic seabed deformation

Recent observations by submarine geomagnetic observatories revealed the existence of a characteristic electromagnetic signature anticipating the arrival of earthquake-generated tsunamis in several locations around the world. This interesting phenomenon arises from the dynamo effect, where the motion of conductive seawater through Earth’s primary magnetic field gives rise to a small electromagnetic (EM) field. This work presents an analytical model combining potential flow and dynamo theories to explain the observed phenomenon. We introduce a mathematical framework via the governing Cauchy–Poisson boundary-value problem associated with surface gravity waves and the EM field arising from a disturbance in the seabed. Through the application of asymptotic analysis, we demonstrate that the EM signal, observed at significant distances from the epicentre, can be decomposed into two components. The first term is proportional to the Airy function, propagating concurrently with the surface gravity wave. The second term is proportional to the Scorer function, displaying a phase lag relative to the surface gravity wave. This phase lag provides an explanation for the observed time discrepancy between the arrival of the EM signal and the surface gravity wave resulting from seabed deformation, as evidenced in field measurements. Further parametric analysis reveals the role of water depth and lateral propagation in two horizontal dimensions in amplifying (or otherwise reducing) this phenomenon. This work is in collaboration with Dr M. Mazza (Loughborough University), Dr S. Michele (University of Rome Tor Vergata, Italy) and Dr J. Sartori Ziebell (UFRGS, Brazil).