Locating the Coseismic Ionospheric Disturbance from the 2010 Mw7.8 Mentawai, Indonesia, Tsunami-Earthquake

Abstract. Forecasting tsunamis using seismological and geodetic observations remains challenging. However, tsunamis generate acoustic waves that travel into the atmosphere and affect the ionosphere, creating disturbances in total electron content (TEC). These disturbances, known as Coseismic Ionospheric Disturbances (CIDs), can be observed using GNSS data. Identifying the source location of the CID could provide valuable information about whether a tsunami has been triggered. In this study, we attempt to determine the source location of the CID following the 2010 M_w7.8 Mentawai tsunami-earthquake, using onset time measurements of the CID on vTEC waveforms. Our approach combines acoustic wave propagation within a realistic atmosphere alongside a Bayesian inversion framework that considers uncertainties in the observations and the forward problem itself. Our approach search for the effective height of the ionosphere rather than fixing its altitude, as is commonly done. Our results suggest an effective height of the ionospheric of around 250 km, which is below the F-layer peak. Furthermore, we show that the inferred source location of the CID does not coincide with the zone of maximum seafloor uplift or the area where the tsunami initiates. An alternative model based on the assumption of a homogeneous atmosphere locates the source of the CID within the area where the tsunami initiates. While this might imply that the realistic atmosphere model does not allow acoustic waves to propagate at the correct speed, our findings demonstrate that both models have the same apparent acoustic wave speed. Therefore, we argue that the approach using a realistic atmosphere provides an accurate location of the CID source because it takes the complexity of propagation paths into account. We offer several explanations as to why the source location the CID does not match the area where the tsunami initiates, such as the effect of the spatial extent of the actual source, more complex tsunami initiation dynamics due to bathymetry effects, or potential errors in the earthquake source models that could shift the area where the tsunami initiates. This work demonstrates the potential of using ionospheric observations to complement existing tsunami monitoring strategies.