|Start:||January 7, 2004 ||Observed|
|Seismicity with no confirmed eruption: ||
|Eruption Type:||Not an eruption.|
|Duration: ||12 hours ||
From Neal and others (2005): "On January 7, 2004, 90 earthquakes occurred over a period of 12 hours beneath Westdahl Volcano on Unimak Island in the eastern Aleutians. Since the short-period seismic network was installed on this volcano in 1998, the majority of background seismicity has occurred in the vicinity of Faris Peak, a young, intracaldera cone 4 km (2.5 mi) east-northeast of Westdahl Peak (fig. 21B.) The 2004 swarm consisted of earthquakes ranging in size from ML (local or Richter magnitude) = 0.2 to ML = 1.6. The largest earthquakes (ML = 1.6) occurred during the second half of the swarm. Depths ranged from sea level to 8 km (5 mi) below sea level. Over the next 10 months, AVO detected several deep, long-period events below the volcano. Given the location of this swarm beneath the general area of the most recent historical eruptions and the subsequent swarm of deep, long-period earthquakes, this seismicity most likely represents a magmatic intrusion. Due to the short duration and abrupt termination of this swarm, AVO did not raise the level of concern color code for Westdahl, nor did AVO mention the activity in its weekly updates."