03 Introduction

Volcano and Earthquake Hazards in the Crater Lake Region, Oregon


Crater Lake National Park is visited by about 500,000 people each year, with heaviest use during the summer months. Crater Lake partially fills a type of volcanic depression called a caldera that formed by collapse of a 12,000 foot volcano known as Mount Mazama during an enormous pyroclastic eruption approximately 7,700 years ago. Although this Cascade volcano does not directly threaten large population centers, it does pose a hazard to facilities and people at Crater Lake National Park and to the major transportation corridor east of the Cascades. The ultimate causes of volcanic activity at all Cascade volcanic centers are linked by common processes. However, Crater Lake is unique in many regards, and potential effects of future activity cannot be anticipated by analogy with past eruptions there or at other Cascade volcanoes. Profound changes occurred at Crater Lake about 7,700 years ago that affect the type of eruptions that can occur and the consequences of such events for the surrounding area. This report attempts to forecast what may happen in such future, but largely unprecedented events.

The caldera-forming or climactic eruption of Mount Mazama changed the landscape all around the volcano. Pyroclastic flows devastated the surrounding area, including all of the river valleys that drained Mount Mazama to as far as 70 km away, and a blanket of pumice and ash fell to the northeast of the volcano at least as far as southern Canada. Erosion removed much of this material, feeding rivers that carried it far from its source, ultimately into the Pacific Ocean. Prior to the climactic event, Mount Mazama had a 400,000 year history of activity more like that of other Cascade volcanic centers such as Mount Shasta. Since the climactic eruption, there have been several less violent, smallerpostcaldera eruptions within the caldera itself. In addition, many short-lived volcanoes have erupted at various times in the Crater Lake region, most recently about 10,000 years ago. We recognize that volcanic hazards at Crater Lake fall into two main categories: eruptions within the caldera, reflecting reawakening of the Mazama system, in which Crater Lake itself plays an important role in determining eruptive violence, and eruptions from new vents in the surrounding region.

Volcanic eruptions are not the only geologic hazards at Crater Lake. The Crater Lake region is cut by many faults, some of which are capable of producing damaging earthquakes (e.g., Klamath Falls, September 1993). Not only do earthquakes pose direct hazards to people and structures but they also can cause rockfalls and landslides which, if they entered the lake rapidly, could produce life-threatening waves. Consequently, this report contains information about faults, seismicity, and possible effects of earthquake shaking in addition to an evaluation of volcano hazards.

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