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

Abstract

On 4 March 2021, two tsunamigenic earthquakes (Mwg 7.4 and Mwg 8.1) occurred successively within 2g h in the Kermadec Islands, offshore New Zealand. We examined sea level records at tide gauges located at g 1/4100 to g 1/42000g 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 5-17g min, whereas it is 8-32g 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 (g 1/455g 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 9.8g min. The results are similar to those calculated using tsunami-To-background ratio method and are also consistent with the source models.