Characteristics of consecutive tsunamis and resulting tsunami behaviors in southern Taiwan induced by the doublet earthquakes on 26 December 2006

Consecutive M w 7.0 and M w 6.9 submarine earthquakes occurred offshore Hengchun Peninsula, Taiwan on 26 December 2006. A small tsunami was generated, and recorded at tide gauge stations for the first time. This important event attracted public interest as it was generated by doublet sources and 15 demonstrated tsunami risk in Taiwan. This study analyzed tide gauge tsunami waveforms and numerical simulations to understand the source characteristics and resulting tsunami behaviors. The maximum wave heights at three stations were 0.08 m (Kaohsiung), 0.12 m (Dongkung), and 0.3 m (Houbihu), and only Houbihu recorded the first wave crest as the largest. The tsunami durations were 3.9 h at Dongkung, and more than 6 h at Kaohsiung and Houbihu. Spectral analyses detected dominant periodic components of 20 spectral peaks on tsunami waveforms. The period band from 15.5-26.6 min was identified as the tsunami source spectrum, and the approximate source area for the consecutive tsunamis was constrained to be 800 km 2 . Comparing the simulated tsunami waveforms to observed tsunami waveforms showed that the GCMT focal mechanisms explained the observations at all three stations better than the USGS solutions. Numerical simulations based on real and hypothetical bathymetry examined its influence on the transmission of 25 tsunami waves. The results revealed that wave trapping was connected to wave refraction caused by bathymetry. Trapped waves interfered with incident waves at shelf edges amplified tsunami waves and prolonged oscillations, which explained unusual observations recorded by the tide gauges. These elucidate the generation of the consecutive 2006 tsunamis, and consequent tsunami behaviors in southern Taiwan, contributing essential knowledge for tsunami warning and coastal emergency response in Taiwan to reduce 30 disaster risk. data of sea levels recorded at coastal sites provide one possible method to study tsunami patterns. To investigate the tsunami characteristics of the 2006 tsunami, the sea level records 100 from tide gauge stations were employed for analysis in the present study. For this purpose, the recorded data from three tide gauge stations (Kaohsiung, Dongkung, and Houbihu) located in southern Taiwan were obtained. These tide gauge stations are operated and maintained by the CWB, R.O. C. All stations recorded sea levels with a sampling rate of 6 min. In this doublet event, the first and second earthquakes occurred at 20:26:21 and 20:34:15 (Taiwan standard time), respectively. Hence, the tide gauge records for 28 h (from 105 as the sampling rate of the data was 6 min. The Fourier analysis was applied to background (i.e., -3-0 h data) and tsunami signals (i.e., 0-3 h and 3-6 h), separately, to identify significant changes in spectral energy associated with the tsunami. Additionally, the spectral ratio was computed for tsunami spectra to exclude the local modes of coastal sites from periodic components. Wavelet analysis was computed based on the 135 Morlet mother function, as suggested by Torrence and Compo, 1988. Wavelet analysis detects the periodic change in spectral peaks over time history. The length of tsunami data input in wavelet analysis was 15 h (15:00 on 26 December 2006 to 06:00 on 27 December 2006, Taiwan standard time). A similar method has In this study, two earthquake tsunami scenarios were constructed to examine the generation mechanism of these doublet tsunamis, accounting for the NP1 fault planes from each earthquake introduced by the GCMT and USGS. The first earthquake tsunami scenario was based on the GCMT moment tensor solution, 170 with a strike of 165°, dip of 30°, rake of -76°, and depth of 19.6 km for the first earthquake, and a strike of 151°, dip of 48°, rake of 0°, and depth of 32.8 km for the second earthquake. Another scenario applied by the USGS solution, with a strike of 171°, dip of 24°, rake of -61°, and depth of 25.5 km for the first earthquake, and a strike of 151°, dip of 48°, rake of 0°, and depth of 32.8 km for the second earthquake.