TELESEISMIC SEISMIC ANALYSIS OF EPISODIC MAGMA HAMMERS IN THE RECENT CATACLYSMIC HTHH ERUPTION

To understand how the magma plumbing dynamics regulates the Jan-15-2022 Hunga Tonga-Hunga Ha'apai (HTHH) explosive eruptive activities on timescales of seconds to minutes, we estimated the source time function of the forces by stacking the teleseismic waveforms recorded by worldwide seismic stations. Basic characteristics of the forces can be obtained as follows. We stacked the waveforms in 24 azimuthal bins (each bin is of 15 degrees) to investigate radiate seismic P and S waves as a function of azimuth. For the P-wave azimuthal stacks, the 24 azimuthal stacks show highly similar waveforms (both shape and amplitude) with most energy appeared in the vertical component and weak energy on the radial component. For the direct S-wave azimuthal stacks, the energy on the transverse component was not significant and within the background noise level. Altogether, these azimuthal stacks suggest the volcanic forces ought to be rotationally symmetric. We examined several possible scenarios: a vertical single force, an explosion/implosion, a CLVD source, and a vertical force couple. Using full-waveform modeling, we can show that the single force scenario produces the best fit to the observed data. Other cases produce converted S waves around ~600-800s after the event origin time but such converted waves are not observed in data. In summary, the seismic data favor a vertical single-force mechanism. In our global P-wave stacks, one can see four similar seismic subevents, E1~E4, within a 5-minute time window in the intensifying early phase of the eruption. Each subevent is similar in waveform and duration and is characterized by a sequence of four stages labeled A~D. The first stage A (visible in E1~3; E4 is too close to E3), marked by a negative P-wave polarity, is best explained by an upward single force mechanism at the volcano. We call this upward force a magma-hammer force that is probably caused by flow blockage or constriction. The second stage B (downward single force down at volcano), followed by stage C (upward force) and D (downward force) magma hammers. Our proposed episodic magma hammer model, consistent with chemical, thermodynamical, and phase properties of the magmatic mixture, yields a remarkably consistent estimate of magma mass flow in the conduit with the vent discharge into the atmosphere based on the volcanic plume height.