The 15 January 2022 climactic eruption of Hunga volcano, Tonga, produced an explosion in the atmosphere of a size that has not been documented in the modern geophysical record. The event generated a broad range of atmospheric waves observed globally by various ground-based and spaceborne instrumentation networks. Most prominent is the surface-guided Lamb wave ( ≲ 0.01 Hz), which we observed propagating for four (+three antipodal) passages around the Earth over six days. Based on Lamb wave amplitudes, the climactic Hunga explosion was comparable in size to that of the 1883 Krakatau eruption. The Hunga eruption produced remarkable globally-detected infrasound (0.01–20 Hz), long-range (~10,000 km) audible sound, and ionospheric perturbations. Seismometers worldwide recorded pure seismic and air-to-ground coupled waves. Air-to-sea coupling likely contributed to fast-arriving tsunamis. We highlight exceptional observations of the atmospheric waves.

During volcanic eruptions, domes of solidifying magma can form at the volcano summit. As magma ascends it often forms a plug bounded by discrete fault zones, a process accompanied by drumbeat seismicity. The repetitive nature of this seismicity has been attributed to stick-slip motion(1) at fixed loci between the rising plug of magma and the conduit wall(2,3). However, the mechanisms for such periodic motion remain controversial(4-7). Here we simulate stick-slip motion in the laboratory using high-velocity rotary-shear experiments on magma-dome samples collected from Soufriere Hills Volcano, Montserrat, and Mount St HelensVolcano, USA. We frictionally slide the solid magma samples to generate slip analogous to movement between a magma plug and the conduit wall. We find that frictional melting is a common consequence of such slip. The melt acts as a viscous brake, so that the slip velocity wanes as melt forms. The melt then solidifies, followed by pressure build up, which allows fracture and slip to resume. Frictional melt therefore provides a feedback mechanism during the stick-slip process that can accentuate the cyclicity of such motion. We find that the viscosity of the frictional melt can help define the recurrence interval of stick-slip events. We conclude that magnitude, frequency and duration of drumbeat seismicity depend in part on the composition of the magma

ABSTRACT. We observed several swarms of repeating low-frequency (1-5 Hz) seismic events during a 3 week period in May-June 2010, near the summit of Mount Rainier, Washington, USA, that likely were a result of stick-slip motion at the base of alpine glaciers. The dominant set of repeating events ('multiplets') featured >4000 individual events and did not exhibit daytime variations in recurrence interval or amplitude. Volcanoes and glaciers around the world are known to produce seismic signals with great variability in both frequency content and size. The low-frequency character and periodic recurrence of the Mount Rainier multiplets mimic long-period seismicity often seen at volcanoes, particularly during periods of unrest. However, their near-surface location, lack of common spectral peaks across the recording network, rapid attenuation of amplitudes with distance, and temporal correlation with weather systems all indicate that ice-related source mechanisms are the most likely explanation. We interpret the low-frequency character of these multiplets to be the result of trapping of seismic energy under glacial ice as it propagates through the highly heterogeneous and attenuating volcanic material. The Mount Rainier multiplet sequences underscore the difficulties in differentiating low-frequency signals due to glacial processes from those caused by volcanic processes on glacier-clad volcanoes.

SUMMARY The purpose of this work is to gain insights into the 2011–2012 eruption of El Hierro (Canary Islands) by mapping the evolution of the seismic b‐value. The El Hierro seismic sequence offers a rather unique opportunity to investigate the process of reawakening of an oceanic intraplate volcano after a long period of repose. The 2011–2012 eruption is a submarine volcanic event that took place about 2 km off of the southern coast of El Hierro. The eruption was accompanied by an intense seismic swarm and surface manifestations of activity. The earthquake catalogue during the period of unrest includes over 12 000 events, the largest with magnitude 4.6. The seismic sequence can be grouped into three distinct phases, which correspond to well‐separated spatial clusters and distinct earthquake regimes. The estimated b‐value is of 1.18 ± 0.03, and a magnitude of completeness of 1.3, for the entire catalogue. B is very close to 1.0, which indicates completeness of the earthquake catalogue with only minor departures from the linearity of Gutenberg–Richter frequency–magnitude distribution. The most straightforward interpretation of this result is that the seismic swarm reached its final stages, and no additional large magnitude events should be anticipated, similarly to what one would expect for non‐volcanic earthquake sequences. The results, dividing the activity in different phases, illustrate remarkable differences in the estimate of b‐value during the early and late stages of the eruption. The early pre‐eruptive activity was characterized by a b‐value of 2.25. In contrast, the b‐value was 1.25 during the eruptive phase. Based on our analyses, and the results of other studies, we propose a scenario that may account for the observations reported in this work. We infer that the earthquakes that occurred in the first phase reflect magma migration from the upper mantle to crustal depths. The area where magma initially intruded into the crust, because of its transitional nature is characterized by high fracturing, thus favours anomalously high b‐values. The larger magnitude earthquakes recorded in the second phase may reflect relaxation around the magma reservoir that had fed the eruption and, thus, lower b‐values.