Modeling of atmospheric normal modes excited by the 15 January 2022 Hunga Tonga eruption

  • Venue:

    Online

  • Date:

    15.11.2022

  • Speaker:

    Rongjiang Wang

    GFZ Potsdam

  • Time:

    9:00 am

Abstract

The Hunga Tonga-Hunga Ha’apai submarine volcano eruption on 15 January 2022 produced a huge vertical plume more than 30 km tall. Then global-scale air waves, ionosphere anomalies, tsunamis and seismic waves are observed by different kinds of instruments over the world. Among these observations, long-period and very-long-period seismometers worldwide recorded a type of low-frequency, long-lasting, harmonic Rayleigh waves. In this talk, we first show the observations of these signals in the seismic waveform profile. The spectra of seismic Rayleigh waves show clearly three atmospheric resonance peaks at 3.7, 4.6, and 6.0 mHz, while their dispersion curve is in good agreement with the fundamental spheroidal modes for the solid Earth. In contrast, the globe-circling air waves, also called Lamb waves, do not show the three resonance peaks, except for stations at small epicentral distances of up to a few hundred kilometres. To understand the coupling mechanism between the atmosphere and the solid Earth, we use the QSSP tool (Wang et al. 2017) to calculate synthetic barograms and seismograms based on the reference solid Earth model PREM combined with the U.S. standard atmosphere. The spectra of near-field synthetic barograms reproduce the observed resonance peaks associated with the vertically oscillating acoustic waves exited by the eruption. The induced pressure variations in the eruption area generate seismic waves in the way like multiple hammer slows on Earth’s surface, explaining why the teleseismic Rayleigh waves exhibit the same resonance peaks as the near-field air pressure. In comparison, the global-scale Lamb wave propagating in the atmosphere has a different excitation mechanism; it is developed from multiple acoustic waves through superposition and interference. To better understand the physics of the Lamb wave, we investigate the normal modes of an isothermal atmosphere using both analytical and numerical (QSSP) approaches. For the isothermal U.S. standard atmosphere model, our results reveal that the Lame wave is constituted from the fundamental gravity modes in the lower frequency band (≤ ca. 4.0 mHz) and the fundamental acoustic modes in the upper frequency band (≥ ca. 6.0 mHz), and it is slightly dispersive. In the transition frequency around 5.0 mHz, there seems to be a small gap (velocity minimum) of the Lamb wave. This finding is likely supported by the background Lamb waves previously derived from the array micro barometer data. Many other issues related with agreements and disagreements between the synthetics and observations will be discussed.