The Alaska Volcano Observatory (AVO) reported that low-level unrest at Spurr was ongoing. An extended outage of the seismic network occurred during February-April; elevated seismicity was already occurring when the network returned on 3 April. The rate of about 20 earthquakes per week and their magnitudes did not significantly change through mid-October . Over 900 located earthquakes were clustered in two areas; the earthquakes in one cluster were located at depths of 0-10 km beneath the volcano and those in the second cluster were located at depths of 20-35 km beneath an area SE of Crater Peak vent. The largest earthquake, a M 2.4, was recorded on 6 October. Inflation began to be detected in March by Global Navigation Satellite System (GNSS) stations on the flanks and has continued at a steady rate. A total of 4 cm of uplift was recorded by the stations closest to the source of deformation. The deformation was also confirmed in satellite radar data. A small lake formed in the summit crater sometime between 15 May and 15 June. During a 23 June overflight scientists observed the lake and estimated that it was approximately 3,800 square meters in area. Gas measurements detected low levels of sulfur dioxide and carbon dioxide at levels similar to the background levels measured in previous years. The summit crater lake grew to around 5,670 square meters by 18 September, was blue-green in color, and partially covered by ice. Active steaming was visible from vents along and above the NE lake shore and crater wall, and from a pit in the ice on the NE crater rim and had not changed in vigor during the current period of unrest.

The observations suggested that a new batch of magma had been emplaced beneath the volcano, with preliminary models indicating an area of pressurization at 3-5 km depth and about 3-4 km W of Spur. AVO noted that the current increase in earthquake activity has some similarities to past episodes of increased earthquake activity at Spurr. These periods include August 1991 to June 1992 that preceded the 1992 eruptions and 2004 to 2006 when increased earthquake activity accompanied an episode of heating at the summit that melted a substantial portion of the ice cap. The Volcano Alert Level remained at Normal (the lowest level on a four-level scale) and the Aviation Color Code remained at Green (the lowest color on a four-color scale).

Geological Summary. Mount Spurr is the closest volcano to Anchorage, Alaska (130 km W) and just NE of Chakachamna Lake. The summit is a large lava dome at the center of a roughly 5-km-wide amphitheater open to the south formed by a late-Pleistocene or early Holocene debris avalanche and associated pyroclastic flows that destroyed an older 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-collapse cones or lava domes are present. The youngest vent, Crater Peak, formed at the southern end of the amphitheater and has been the source of about 40 identified Holocene tephra layers. Eruptions from Crater Peak in 1953 and 1992 deposited ash in Anchorage.

Source: US Geological Survey Alaska Volcano Observatory (AVO)