Seismicity

The magma chamber is believed to be about 40 by 80 kilometers across, similar in size to the overlying Yellowstone caldera. The top of the chamber is about 8 km deep and the bottom is around 16 km deep.
There are about 1,500 potentially active volcanoes worldwide, aside from the continuous belt of volcanoes on the ocean floor. About 500 of these have erupted in historical time.
The Alaska Peninsula and the Aleutian Islands have about 80 major volcanic centers that consist of one or more volcanoes.  Learn More:
During the 9 hours of vigorous eruptive activity
More than 80 percent of the Earth's surface -- above and below sea level -- is of volcanic origin.
Since the most recent giant (caldera-forming) eruption 640,000 years ago, approximately 80 relatively nonexplosive eruptions have occurred.
Alaskan volcanoes have produced one or two eruptions per year since 1900.
Mount Shasta has erupted, on the average, at least once per 800 years during the last 10,000 years, and about once per 600 years during the last 4,500 years. The last known eruption occurred about 200 radiocarbon years ago.