**3.2. Radial velocity components**

The radars had a transmit frequency of 42 MHz and a range increment of 0.5 km. The water depth over the entire radar coverage area is less than 200 m. The 42 MHz frequency band is used for high‐resolution, short‐range current observations and results in a radar range less than 15 km, due to significant attenuation of the surface wave passing across the sea at these

The direction and strength of the flow were measured at approximate 4‐min intervals with a cell resolution of 0.5 km × 0.5 km. **Figure 2** shows total current‐velocity maps, the first

**Figure 2.** Tsunami current velocity vectors with the tsunami height shown in colors. Radar measurements from A087 (blue circle) and A088 (red circle) made on March 11, 2011 (a) the tsunami arrival at 15:53 JST and (b) outward flow

higher HF frequencies.

82 Tsunami

occurring later at 21:00 JST.

**3.1. Total velocity current maps**

As would be expected, the tsunami signal is also visible in the radar returns from a single radar site [3]. To simplify the analysis of the data with the aim of developing objective detection criteria, we group the radial velocities into rectangular area bands 2‐km wide approximately parallel to the depth contours. The radial velocities are resolved into components perpendic‐ ular and parallel to the area bands. These components are averaged over the band; the averages are termed "band velocities." A time series of the band velocities is then formed, which displays the characteristic oscillations produced by the tsunami.

**Figure 3** gives an example of measured radial vectors with a 4‐min time resolution and the 2‐ km wide bands used in the analysis.

**Figure 3.** Radial current velocities from the Usujiri, Hokkaido radar (A087) and the area bands used in the analysis which are 2‐km wide and approximately parallel to the depth contours.

**Figure 4** shows band velocities observed at Kinaoshi, Hokkaido (A088), for three 2‐km bands ranging from 6 to 12 km from the shore.

**Figure 4.** Band velocities from Radar A088. Distance from radar: 6–8 km (blue), 8–10 km (red), and 10–12 km (black). Time span: March 11, 2011 (a) surrounding the tsunami arrival, from 12:46 to 19:46 JST and (b) close to the tsunami arrival, from 14:16 to 15:16 JST.

The arrival of the tsunami is indicated by the commencement of distinctive oscillations in velocity with a period of about 40 min, which are strongly correlated over range.

Further examples of band velocities from radars on both sides of the Pacific close to the tsunami arrival time are shown in **Figure 5**.

**Figure 5.** Time series of band velocities showing the typical appearance around the tsunami arrival time for three adja‐ cent 2‐km area bands. (a) A087 (Hokkaido, Japan), (b) YHS2 (Oregon, USA), (c) BML1 (California, USA), and (d) PREY (California, USA).
