Fig. 1.
Left: schematic representation of tsunami-related-GW propagation with its vertical 
and horizontal 
group velocities, where k
z
and k
z
are the vertical and horizontal k-vector, g the gravity, N the Brunt-Väisälä frequency and 
Following Occhipinti et al. (2006, 2008a), we assume that the spatial and temporal dependency of wavefield is expressed in the form exp (k
x
x – k
z
z – ωt), where the dispersion equation take the form 
. As V
H
-igw is lower than the tsunami speed 
, the IGW accumulates a delay (dt) with respect to the tsunami at the sea level during the upward propagation. The vertical and horizontal group velocity profiles for IGW of period T1 = 14 minutes (solid lines) and T2 = 26 minutes (dashed lines) are shown in the middle and right panel respectively; gray color indicates the speed dependence of ocean depth H (m). At the altitude of 250 km the delay dt is on order of 6 min and 3 min for T1 and T2 respectively. An IGW needs around 80 min and 145 min to reach the altitude of 250 km for T1 and T2 respectively. Once the IGW reaches ionospheric altitude, the delay dt between the IGW and tsunami wavefronts is constant during the oceanic propagation for a constant oceanic depth. The effect of the bathymetry variation can reduce, vanish or invert the delay dt.