Tsunami Responses as Observed by Taiwan High-Frequency RadarNetwork

Tsunami-induced currents were detected by the Taiwan High-Frequency Radar (HFR) Network on April 3, 2024, following a magnitude 7.2 earthquake near Hualien, Taiwan. The earthquake triggered tsunamis that generated strong currents, leading to collisions between drifting vessels in two harbors along Taiwan's east coast. At Hualien Harbor, tsunami waves reached a height of 1.8 m, with near-field waves rapidly propagating along Taiwan’s eastern coastline and extending to southern Okinawa, Japan.
Since 2019, 19 HFR stations have been deployed along Taiwan's coastal regions. These stations are capable of detecting surface currents in water depths of up to 100 meters, with a minimum velocity sensitivity of 0.02 m/s. Observations revealed that tsunami-induced radial currents in Yilan Bay reached speeds of up to 0.2 m/s. Using ensemble empirical mode decomposition, three oscillation modes with periods of 8.8, 14, and 30 minutes were identified in the HFR data. The first two modes are closely linked to the continental shelf, which has an average depth of approximately 100 m and a width of 5 km. The 30-min mode, however, is more pronounced within 15 km offshore in Yilan Bay, where interactions between the northeastern Taiwan countercurrent, Kuroshio, and tidal currents likely influence these oscillations.
Understanding these dynamics is critical for the development of an integrated early warning system supported by real-time HFR monitoring. This study highlights the value of such remote sensing systems in providing crucial insights into tsunami-induced hazards in coastal environments.

Figure. Surface currents response to tsunami oscillations. 100 and 500 m water depth shown in dashed and solid lines, respectively.