A brief explosive eruption of Mt. Spurr in southwest Alaska occurred on August 18, with little or no apparent precursory seismicity. Preliminary data suggested that the August 18 activity was similar to somewhat stronger than the previous explosive episode, on June 27 (see last 3 Bulletins). The June 27 ash had been carried north, away from nearby populated areas, but the August 18 ash fell on Anchorage, Alaska's largest city, 130 km east of Spurr, closing its internatioal airport and forcing most of its residents indoors.
The eruption was first reported at 1548 (all times are local: UT −8 hours) by an airplane pilot who saw a dark cloud, probably an ash plume, breaking through weather clouds. About 8 minutes of seismicity at slightly above background preceded the pilot report. No lightning pulses, which often accompany ash eruptions, were detected, but there were additional pilot reports of ash during the next half-hour. Seismicity increased markedly at 1641, and by 1645, NOAA C-band radar had detected a plume to almost 11 km altitude. The National Weather Service released a SIGMET, warning pilots of the ash plume, at 1653.