A parametric study of the January 2006 explosive eruptions of Augustine Volcano, using seismic, infrasonic, and lightning data: Chapter 4 in The 2006 eruption of Augustine Volcano, Alaska

Abstract

A series of 13 explosive eruptions occurred at Augustine Volcano, Alaska, from January 11&ndash;28, 2006. Each lasted 2.5 to 19 minutes and produced ash columns 3.8 to 13.5 km above mean sea level. We investigated various parameters to determine systematic trends, including durations, seismic amplitudes, frequency contents, signal characteristics, peak acoustic pressures, ash column heights, lightning occurrence, and lengths of pre-event and post-event quiescence. Individual tephra volumes are not known. There is no clear correlation between acoustic peak pressure and ash column height or between peak seismic amplitude and duration. However, several trends are evident. Two events, January 11 at 0444 AKST (1344 UTC) and January 27 at 2337 AKST (0837 UTC) are short (180 and 140 seconds) and have very impulsive onsets and high acoustic peak pressures of 93 and 105 Pa, as well as high peak seismic amplitudes. We interpret these to be mainly gas releases. Two of the largest events followed quiescent intervals of 3 days or longer: January 17 at 0758 AKST (1658 UTC), and January 27 at 2024 AKST (January 28 at 0524 UTC). These two events had reduced displacements ( D R ) of 11.4 and 7.5 cm 2 , respectively. Although these D R values are typical for eruptions with ash columns to 9 to 14 km, most other D R values of 1.6 to 3.6 cm 2 are low for the 7.0 to 10.5 km ash column heights observed. The combination of short durations, small D R and high ash columns suggests that these events are highly explosive, in agreement with Vulcanian eruption type. Several events had long durations on individual seismic stations but not on others; we interpret these to represent pyroclastic or other flows passing near the affected stations so that tractions or momentum exchange from the cloud or flow adds energy to the ground only near those stations. The eruption on January 27 at 2024 AKST had more than 300 lightning flashes, whereas the following eruptions on January 28 at 0204 AKST and 0742 AKST had only 28 and 6 lightning flashes. The 2024 AKST eruption had a longer duration (1,180 versus <460 seconds), a higher ash column height (10.5 versus 7.0&ndash;7.2 km) and higher acoustic peak pressure (83 versus 66 and 24 Pa). The data suggest that the lightning-rich 2024 AKST eruption produced more tephra than the following eruptions, hence there were more charge carriers injected to the atmosphere. Seismic signals preceded the infrasound signals by 0 to 5 seconds with no obvious pattern in terms of the above groupings. The explosive eruption phase overlapped with the subsequent continuous phase by about 2 days. Parametric data may be useful to estimate eruption conditions in near real time.