**7.1. Water depth**

Tsunami orbital velocities are small in deep ocean basins, with orbital velocity below the detectability threshold for HF radars. In the deep ocean, tsunami wave amplitude is always less than 1 m and wavelengths may be hundreds of kilometers. For example, fishermen 30 km offshore did not detect the huge Japanese tsunami of June 15, 1896. The tsunami height was only about 40 cm, but when the tsunami arrived at the shore, its height after run‐up was 38.2 m [19].

As the tsunami moves onto the continental shelf, both velocity and height increase. Velocity, however, grows more rapidly with decreasing depth which gives advantage to the radar sensor. As the depth decreases, the orbital velocity can exceed a detection threshold. Based on our experience with small to moderate tsunamis in 21 HF radar detections, we use the 200‐m isobath as a convenient demarcation for likely detectability.

The offshore bathymetry has a strong influence on HF radar tsunami detectability that is highly radar site dependent. It must be studied in simulations that guide the selection of detectability thresholds and other parameters for any tsunami warning methodology. This can be done using numerical models for near‐field tsunami propagation based on the bathymetry offshore from the radar location, as described in Section 5.1.
