**7.2. Tsunami height**

oscillation at BML1 was about 7 cm/s. There was no detection of the initial tsunami approach

From **Table 3**, the simulation test predicts that the tsunami velocity required for an initial detection at BML1 at a 5‐km range is ∼8 cm/s, which is approximately consistent with the observed velocity (7 cm/s) resulting in the initial detection of the Japan tsunami. The test indicates that larger tsunami velocities are required further from shore for detection to be

From **Figure 14**, the magnitude of the initial meteotsunami velocity oscillation observed at BRNT 6–12 km from shore was ∼10 cm/s. This value can be compared with the test results in **Table 3**: averaging from 5 to 11 km, the predictions indicate that the velocity required to produce a detection is ∼17 cm/s. This conservative estimate may reflect a difference between

In this section, we summarize five factors that can affect the ability of a radar site to detect and

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

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

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

possible. It also gives credibility to our simulation methodology.

the characteristics of a meteotsunami and an earthquake‐caused event.

**7. Factors affecting detectability**

provide warning of an approaching tsunami.

isobath as a convenient demarcation for likely detectability.

from the radar location, as described in Section 5.1.

further from shore.

106 Tsunami

*6.3.2. BRNT*

**7.1. Water depth**

m [19].

The orbital velocity and tsunami height of course increase with the severity of the tsunami, and this is a second factor determining detectability. We note that tsunami heights producing radar detections as measured by neighboring tide gauges were not large, varying between 0.05 and 2 m. Tsunamis with orbital velocity amplitudes of 5 cm/s have been detected [5]. Detection of larger tsunamis would of course be easier, for example, the tsunami waves that reached heights of 40.5 m in Miyako, Japan, following the 2011 Tohoku earthquake [20] would be very easy to detect.

The estimation of the tsunami height at the shore is of course of major interest and at present can only be estimated by tide gauge observations because of the breakdown of linear modeling very close to shore. More advanced modeling, as described in Section 5.1, should alleviate this problem.
