AVO reported that a minor increase in seismicity at Spurr was detected at about 0500 on 25 June, lasted about 45 minutes, and was characterized by several discrete M1earthquakes. The signals recorded were consistent with seismic energy generated by an energetic flow of water, possibly indicating a glacial outburst flood on the lower S flank. The next day seismic levels had declined to near background and no additional flowage signals were observed.

Geological Summary. The summit of Mount Spurr is a large lava dome constructed at the center of a roughly 5-km-wide horseshoe-shaped caldera open to the south. The volcano lies 130 km W of Anchorage and NE of Chakachamna Lake. The caldera was formed by a late-Pleistocene or early Holocene debris avalanche and associated pyroclastic flows that destroyed an ancestral edifice. The debris avalanche traveled more than 25 km SE, and the resulting deposit contains blocks as large as 100 m in diameter. Several ice-carved post-caldera cones or lava domes lie in the center of the caldera. The youngest vent, Crater Peak, formed at the breached southern end of the caldera and has been the source of about 40 identified Holocene tephra layers. Eruptions from Crater Peak in 1953 and 1992 deposited ash on the city of Anchorage.

Source: US Geological Survey Alaska Volcano Observatory (AVO)