Characteristics of two tsunamis generated by successive M-w 7.4 and M-w 8.1 earthquakes in the Kermadec Islands on 4 March 2021

On 4 March 2021, two tsunamigenic earthquakes (M-w 7.4 and M-w 8.1) occurred successively within 2 h in the Kermadec Islands, offshore New Zealand. We examined sea level records at tide gauges located at similar to 100 to similar to 2000 km from the epicenters, conducted Fourier and wavelet analyses as well as numerical modeling of both tsunamis. Fourier analyses indicated that the energy of the first tsunami is mainly distributed over the period range of 5-17 min, whereas it is 8-32 min for the second tsunami. Wavelet plots showed that the oscillations of the first tsunami continued even after the arrival of the second tsunami. As the epicenters of two earthquakes are close to each other (similar to 55 km), we reconstructed the source spectrum of the second tsunami by using the first tsunami as the empirical Green's function. The main spectral peaks are 25.6, 16.0, and 9.8 min The results are similar to those calculated using tsunami-to-background ratio method and are also consistent with the source models.