Volcano and Earthquake Hazards in the Crater Lake Region, Oregon
Summary
Crater Lake lies in a basin, or caldera, formed by collapse of the Cascade volcano known as Mount Mazama during a violent, climactic eruption about 7,700 years ago. This event dramatically changed the character of the volcano so that many potential types of future events have no precedent there. This potentially active volcanic center is contained within Crater Lake National Park, visited by 500,000 people per year, and is adjacent to the main transportation corridor east of the Cascade Range. Because a lake is now present within the most likely site of future volcanic activity, many of the hazards at Crater Lake are different from those at most other Cascade volcanoes. Also significant are many faults near Crater Lake that clearly have been active in the recent past. These faults, and historic seismicity, indicate that damaging earthquakes can occur there in the future. This report describes the various types of volcano and earthquake hazards in the Crater Lake area, estimates of the likelihood of future events, recommendations for mitigation, and a map of hazard zones. The main conclusions are summarized below.
VOLCANIC ERUPTIONS WITHIN CRATER LAKE CALDERA—The only volcanic eruptions in the Crater Lake area since the climactic eruption and formation of the caldera have taken place within the caldera itself. The most recent of these was about 5,000 years ago. Future eruptions may occur within the lake where interaction of magma(molten rock) and water may produce explosions that can eject ballistics (large rock fragments) and volcanic ash (rock and volcanic glass fragments smaller than 2 millimeters in diameter) outside of the caldera. Some of the ejected material would rise into the atmosphere along with expanding gas and result in blanketing of the area downwind by falling tephra(fragments of rock, frothy bits of magma, and finer-grained ash). Such explosions also can generate pyroclastic surges, ground-hugging flows of gas, steam, volcanic rock fragments, and ash moving at speeds that may exceed 100 meters per second (200 miles per hour) and which have the potential to devastate not only the area within the caldera (plate 1, Proximal Hazard Zone A) but also the valleys and upper slopes of Mount Mazama (plate 1, Proximal Hazard Zone B). Eruptions from vents in shallow water may be highly explosive while those in the deep lake would be expected to be much less violent. An eruption from a vent in the caldera wall itself also might be explosive because of the abundant groundwater within the mountain. Waves on Crater Lake several meters high could be associated with explosive eruptions within the caldera. Because postcaldera volcanoes are concentrated there, the west half of the caldera is considered the most likely site of future activity. The 30-year probability of renewed volcanic activity within or very near to the caldera is greater than one chance in 330, or 3×10-3. The area within the proximal hazard zones is entirely within Crater Lake National Park where access can be controlled and the potential for loss of life can be minimized by closure of appropriate areas at the onset of seismicity or other phenomena deemed precursory to volcanic activity. The possibility of explosive eruptions that may produce ballistic rock fragments or pyroclastic surges mandates that access to the caldera and the proximal hazard zones be controlled.
LAHARS—Lahars are rapidly-moving debris flows that originate at volcanoes and consist of rock fragments carried downslope in a matrix of clay or pulverized rock and water. Lahars can travel great distances from their sources. Most Cascade volcanoes (for example, Mount Rainier) have produced lahars in the past and are likely to continue to do so. Crater Lake differs from them in that no ice-clad summit or fragile mountaintop remains as a source of water and debris at high elevation. However, should an eruption occur within Crater Lake near the shoreline with sufficient violence to eject lake water from the caldera, abundant loose debris (left by the climactic eruption) on the upper slopes of Mount Mazama and in the valleys might be mobilized to form lahars. Alternatively, an eruption outside of the caldera that resulted in rapid melting of a thick snowpack similarly might produce lahars. Such lahars would be localized in low-lying areas and would tend to be confined to narrow canyons (plate 1, Lahar Hazard Zone). Because of this, and the lack of development within much of the lahar hazard zone, the degree to which communities outside the park need to prepare for inundation by lahars is limited to recognition that such a hazard exists in the drainages around Mount Mazama.
ERUPTIONS OUTSIDE OF THE CALDERA— The Oregon Cascades include many small volcanoes around and between the large volcanoes such as Mount Mazama. These small volcanoes include cinder cones, fissure vents, lava domes, and shield volcanoes, each of which formed in a brief period of time. They are the result of regional volcanism. Hazards include slow-moving lava flows and viscous domes, and associated tephra falls, surges, and pyroclastic flows. If surges or pyroclastic flows occur, such as might be expected for an eruption in a low-lying (wet) location, the area affected by them likely would be only a few square kilometers. Tephra falls may be significant near the vent and for a few kilometers downwind. Lava flows will advance slowly enough that they will pose a threat only to property and structures. Because exact locations of future eruptions cannot be predicted, we have estimated annual and 30-year probabilities of an eruption occurring in a particular area. The two hazard zones for regional volcanism shown on plate 1 (RH and RL) indicate higher probabilities approximately west of the main axis of the Cascades and lower probabilities to the east. The probability of eruption of a new volcanic vent near Crater Lake is sufficiently small (30-year probability = 3×10-3 to 3×10-4) that potential hazards from regional volcanism need only be considered significant when even this small degree of risk to a specific facility is unacceptable.
VOLCANO-RELATED EVENTS OF HIGH CONSEQUENCE BUT LOW PROBABILITY— (1) A large pyroclastic eruption, such as the one during which the caldera formed or the (smaller) 1991 eruption of Mount Pinatubo, Philippines, is not considered likely for many thousands of years in the future because the magma reservoir which fed the climactic eruption of Mount Mazama has not had sufficient time to regenerate a large volume of gas-rich magma. (2) Sudden gas release from Crater Lake would seem to be a possibility by comparison with the lethal release of cold carbon dioxide gas from Lake Nyos, Cameroon, in 1986. However, natural mixing of deep water with near surface water in Crater Lake prevents volcanic carbon dioxide from accumulating near the lake bottom. As long as the natural mixing process continues, sudden gas release is not considered to be a significant hazard at Crater Lake. (3) Catastrophic draining of Crater Lake is an extremely unlikely event but one which would have disastrous consequences for downstream lowlands in the affected tributary drainages. There appears to be no mechanism, short of another caldera-forming eruption, that could either eject most of the water in the lake or cause the caldera wall to fail.
EARTHQUAKES—The West Klamath Lake fault zone (WKLFZ), composed of several individual faults with lengths of up to 15 km and an aggregate length of 50 to 70 km, has been mapped through Crater Lake National Park west of the caldera (plate 1). One of its constituent faults, the Annie Spring fault, passes less than 1 km west of Rim Village. All of the faults of the WKLFZ trend approximately north–south and have mainly dip-slip displacement such that the east side is dropped down relative to the west side. By determining the ages of lava flows that have been offset by the faults, the long-term rate of vertical displacement is known to be about 0.3 millimeters per year. The lengths of the faults and the measured displacements suggest that the WKLFZ is capable oftectonic earthquakes as large as magnitude (M) 71/4. The recurrence interval of large earthquakes is unknown but probably is between 3,000 and 10,000 years. Although few earthquakes have been recorded in the Crater Lake area, the known events are consistent with the WKLFZ being active. Moreover, the September 1993, Klamath Falls earthquakes (the two largest events were M » 6.0) occurred farther south along the same general zone. Many other potentially active faults are present east of the Cascades, notably along the east side of Klamath valley (East Klamath Lake fault zone). Local volcanic earthquakes would produce ground motion at Crater Lake but the likely maximum magnitude of such events is about 5, significant but far smaller than for tectonic earthquakes. An additional source of earthquakes is the Cascadia subduction zone, the fault zone that forms the boundary between the tectonic plates that contain the North American continent and the Pacific Ocean floor. Although distant, the potential for this zone to generate M = 8 – 9 earthquakes means that shaking of up to several minutes duration could occur at Crater Lake.
Earthquake hazards in the greater Crater Lake area are similar to those in other earthquake-prone areas, namely damage to structures, utilities, communication lines, and transportation systems. Rockfalls and landslides are significant hazards below steep canyon or caldera walls. Should a large mass of rock fall or slide rapidly from the caldera wall into Crater Lake, one or more large waves could be generated. Waves could be many meters high and travel across the lake in as little as two minutes, such as from Chaski Bay to the boat landing at Cleetwood Cove. Volcanic, local tectonic, or distant Cascadia subduction zone earthquakes all could produce shaking adequate to trigger sliding of the fractured and poorly consolidated rock of the caldera walls and talus slopes. Earthquake shaking alone, without rapid entry of slide material into Crater Lake, would not be expected to cause dangerous waves.
