Where coastal tsunami hazard is governed by near-field sources, such as Submarine Mass Failures (SMFs) or meteo-tsunamis, tsunami propagation times may be too small for a detection based on deep or shallow water buoys. To offer sufficient warning time, it has been proposed to implement early warning systems relying on High Frequency (HF) radar remote sensing, that can provide a dense spatial coverage as far offshore as 200-300 km (e.g., for Diginext’s Stradivarius radar). Shorebased HF radars have been used to measure nearshore currents (e.g., CODAR SeaSonde® system (http://www.codar.com/), by inverting the Doppler spectral shifts, these cause on ocean waves at the Bragg frequency. Both modeling work and an analysis of radar data following the Tohoku 2011 tsunami, have shown that such radars could be used to detect tsunami-induced currents and issue tsunami warning. However, long wave physics is such that tsunami currents will only raise above noise and background currents (i.e., be at least 10-15 cm/s), and become detectable, in fairly shallow water, which would limit direct HF radar detection to nearshore areas, unless there is a very wide shelf.
Here, we use numerical simulations of both tsunami propagation (in the Mediterranean basin) and HF radar remote sensing to develop and validate a new type of tsunami detection algorithm that does not have these limitations. This algorithm computes correlations of HF radar signals at two distant locations, shifted in time by the tsunami propagation time computed between these locations (easily obtained based on bathymetry). We show that this method allows detection of tsunami currents as low as 5 cm/s, i.e., in deeper water, beyond the shelf and further away from the coast, thus providing an earlier warning of tsunami arrival.