*3.4.3. Chile*

A WERA radar system operating at 22 MHz at a site near Concepcion, Chile, observed the Japan tsunami [4]. Current components pointing toward/away from the radar were measured within beams formed by the receiving antenna array. The orbital velocity of the shallow-water tsunami wave is therefore part of the total signal, which also includes other background contributions such as tides and geostrophic flow. **Figure 11** shows radial components of the surface current velocity plotted as a function of range and time, and wave‐level readings from a tide gauge located at Lebu, Chile, about 50 km from the radar.

The observed background current velocities are quite variable for this site: it is the correlations between velocities in different bands that allow the tsunami to be detected by the pattern-

The Japan tsunami took approximately 10 h to reach the US West Coast; it moved down the coast from north to south. Tsunami and tide gauge arrival times are compared in **Table 2**. Radar

*Tide gauge Arrival time*

*(UTC)*

*Water-level change*

recognition algorithm described in Section 3.3.

*Arrival time (UTC)*

**US West Coast March 11, 2011 UTC**

*Radar (XMTR Freq)*

90 Tsunami

*3.4.3. Chile*

stations are listed in the order of the expected arrival of the tsunami.

STV2 (12 MHz) 15:32 Garibaldi 15:48 1.2 m SEA1 (12 MHz) 15:47 Garibaldi 15:48 1.2 m YHS2 (12 MHz) 15:45 South Beach 15:54 0.3 m TRIN (5 MHz) 15:34 Crescent City 15:48 0.5 m GCVE (14 MHz) 15:44 Pt. Reyes 16:00 0.5 m BML1 (12 MHz) 15:46 Pt. Reyes 16:00 0.5 m PREY (13 MHz) 15:49 Pt. Reyes 16:00 0.5 m COMM (13 MHz) 15:56 Fort Point 16:30 0.4 m ESTR (13 MHz) 16:04 Port San Luis 16:24 2.0 m LUIS (13 MHz) 16:05 Port San Luis 16:24 2.0 m

**Table 2.** Comparison of tsunami arrival times from radars and neighboring tide/wave gauges.

patterns derived from the analytical model, as discussed later in Section 5.1.

neighboring tide gauges were not large, varying between 0.3 and 2 m.

**Table 2** shows that listed arrival times obtained from the radar *q*-factors reported are normally in the correct order; thus, it arrives in Southern California after it gets to Northern California and Oregon. Arrival times measured by the radars preceded those at neighboring tide gauges by an average of 15 min, due to both the "quadrature relation" between velocity and height (discussed in Section 4.1) and the tsunami propagation delay between the two observations. We expect to be able to quantify the propagation delay using simulated tsunami velocity

We note from **Table 2** that the tsunami was detected even though water-level changes at

A WERA radar system operating at 22 MHz at a site near Concepcion, Chile, observed the Japan tsunami [4]. Current components pointing toward/away from the radar were measured within beams formed by the receiving antenna array. The orbital velocity of the shallow-water tsunami wave is therefore part of the total signal, which also includes other background contributions such as tides and geostrophic flow. **Figure 11** shows radial components of the

**Figure 11.** Reproduced with permission from Ref. [4]. Top: radial velocity in m/s of surface currents measured by the HF radar in Chile. Bottom: mean sea level measured by the tide gauge at Lebu, Chile, during the tsunami traveling near the Chilean coast.

**Figure 11** shows clear periodic disturbances produced by the tsunami in both radar and tide gauge observations. Obvious correlations in tsunami signatures can be seen for both meas‐ urements.

Dashed lines in **Figure 11** give the depths within the main radar beam pointed offshore. The depth contours define a short continental shelf, followed by a steep slope. From a depth of about 50 m at 5 km, the depth drops to 1000 m at a distance of 34 km, that is, a steeply sloping region within a ∼29‐km span. From there, the depth decreases slowly with distance beyond the shelf/slope region.

The tsunami component of these currents is identified from their typical periods that lie between 20 and 45 min, arriving at about 05:07 UTC on March 12, approximately 22 h after the Japan earthquake. This arrival time was confirmed by NOAA's tsunami model and the tide gauge data.

These observations have unexpected features as follows: (a) the tsunami peaks/troughs are seen out to 40‐km range at approximately the same time regardless of distance from the shore and (b) beyond the shelf, where depths change slowly, from 760 m at 25 km to 1510 m at 40 km, the Green's Law approximation described in Section 2.3.2 should be valid. However, the observed velocity is nearly constant, which contradicts Eq. (7). These effects may be due to signal aliasing, as discussed later in Section 7.
