Location of eruption-related earthquake clusters at Augustine Volcano, Alaska, using station-pair differential times

Abstract

Families of similar earthquakes at shallow depths occurring over multiple timescales have been identified prior to and during the 2005–2006 eruption of Augustine Volcano. The use of conventional and double-difference location methods failed to result in stable locations, due primarily to noisy site conditions as well as unfavourable station geometry for the Alaska Volcano Observatory stations. Previous work found that relatively robust estimates of the median arrival time difference between station pairs could be determined and used a constant velocity half-space model, true station elevations and a variant of the method of hyperbolas to estimate locations for four of the clusters. We have carried out a more extensive location analysis, including data from nine clusters and incorporating a linear gradient velocity model obtained from preliminary forward modelling of the data. Our location results show a progressive shallowing of the clusters with time. Clusters prior to 2005 December lie near sea level, whereas those in 2005 December and 2006 January concentrate about 500 m above sea level (asl) prior to the explosive eruptions and about 500 and 1200 m asl, respectively, (essentially at the surface) afterwards. These results are consistent with geodetic observations and modelling.