The eruption of the Hunga Tonga-Hunga Ha'apai volcano on 15 January 2022 as observed at seismic stations in Germany

On 15 January 2022 at 04:15 UTC, an enormous explosive eruption of the Hunga Tonga-Hunga Haʻapai submarine volcano (short: Hunga) occurred in the Tonga-Kermadec volcanic area in the southern Pacific Ocean. It was one of the strongest volcanic eruption within the last 150 years. The eruption column reached a height of more than 50 kilometres causing heavy atmospheric turbulences. A strong Lamb and a tsunami wave were generated. Besides these phenomena also seismic waves could be observed on seismic stations all over the world.

Consequently, seismic body and surface waves of the Hunga main explosion could be clearly recorded at seismic stations in Germany. After about 19 minutes, the PKP phase was the first arriving body wave reaching the broadband stations of the German Regional Seismic Network and the Gräfenberg Array. Using the short-period WWSSN-SP filter it was possible to determine the onset times of relatively weak PKPbc phases at several stations. The onset times as well as slowness and azimuth determined by array methods allowed an unambiguous assignment to the Hunga event and an epicenter localization deviating approximately 1 to 1.5 degrees from the volcano.

While the PKP phase is only weakly visible in short periods it shows up clearly in the long-period range (SRO-LP filter). The onset times determined therein were still accurate enough to provide a localization similar to that obtained in the short-period range. Furthermore, at least one additional event is detected on the long-period seismograms about 4 minutes after the main event.

To assign a seismic magnitude to the Hunga event, we analyzed the surface wave trains. The Ms magnitudes vary between 5.8 and 6.3 within the individual stations of the GRSN, with a mean value of 6.0.

The Tonga-Kermadec subduction zone is characterized by strong earthquake activity. This allows us to compare the seismic recordings of the Hunga event with those of earthquakes from the same area with shallow focal depths and comparable magnitudes. It turns out that PKP phases of the Hunga eruption have significantly smaller amplitudes in the short-period range than for the compared earthquakes but similarly strong in the long-period range. We conclude that a long-period excitation is characteristic for the seismically relevant focal process of the Hunga event.