Seismic station near Mount Spurr (in background)	Crater Peak vent
Crater Peak is a small cone located on the south margin of the collapse caldera of Mount Spurr. Before its eruption in 1992, scientists had discovered evidence for at least 40 explosive eruptions from Crater Peak in the past 5,000 years. An explosive eruption of Crater Peak on July 9, 1953, sent an eruption column as high as 20 km and deposited ash 3 to 6 mm thick on Anchorage.

Eruption on August 18, 1992

Crater Peak erupted 3 times in 1992 on June 27, August 18, and September 16-17. The first sign of unrest, however, began in August 1991 when a swarm of earthquakes was detected beneath the volcano. Subsequent earthquake activity, including periods of continuous ground shaking known as tremor, was monitored by scientists of the Alaska Volcano Observatory, and it formed the basis of several eruption warnings and notifications in 1992.

This graph shows the number of earthquakes (events) that occurred beneath Mount Spurr every week between January 1991 and December 1992. The red lines indicate explosive eruptions on June 27, August 18, and September 16-17. Can you see a correlation between eruptions and the number of earthquakes per week?

The challenge scientists face in providing eruption warnings based on earthquake activity and other monitoring data is well illustrated by the 1992 activity of Mount Spurr. Before the June 27 eruption, there was a clear increase in earthquake activity. The next two eruptions were not preceded by a similar change in earthquake activity. And between October and December, swarms of earthquakes on three occasions did not result in eruptions at all. This apparent inconsistent relationship between earthquake activity and eruptions has also been observed at other volcanoes, which shows that the job of making accurate eruption predictions very difficult.

Eruption Warnings Issued by the Alaska Volcano Observatory

We've broken the activity of Mount Spurr into three time periods so that you can learn more about the eruption precursors and the eruption warnings and public statements issued by scientists of the Alaska Volcano Observatory in 1991-92.

• June 27: Eruption preceded by increasing earthquake activity
• Aug. 18 & Sept. 16-17: Eruptions preceded by little or no change in earthquake activity
• Oct.-Dec.: Earthquake activity does not lead to eruption

Reference

URL of this document: http://volcanoes.usgs.gov/About/What/Erupt/CaseSpurr.html
Last modified: December 17, 1998