Reminders of Uncertainty - Vol. 25, 1994, Nature Notes from Crater Lake

Illustration by L. Howard Crawford, Nature Notes from Crater Lake, 1934.

An earthquake is one of the most unnerving experiences that a person can have at Crater Lake. Quakes registering magnitudes of 5.9 and 6.0 brought thoughts of Mount Mazama's reawakening to park residents on the evening of September 20, 1993. This heralded over 2500 aftershocks in the following three months, most of which could be detected only by seismograph.

Fortunately the epicenter (or the place where seismographs indicate the shocks are focused) turned out to be the Mountain Lakes Wilderness Area, an old caldera located 40 miles due south of Crater Lake. These tremors did not portend volcanic activity, but are periodic reminders that Crater Lake sits on the edge of a place where the earth's crust is expanding. A restless sea of mountains called the Basin and Range is shoving the great Cascade-Sierra Nevada chain westward.

Characteristics of the Basin and Range Province

The vast area extending from southcentral Oregon to Utah and encompassing most of Nevada is testimony to 20 million years or more of movement. It is called Basin and Range because comparatively flat areas of drainage alternate with north-south trending mountain ranges. Most of the basins do not drain to the sea, but one at its western edge does. Despite that hydrographic anomaly, the Klamath Basin south and east of Crater Lake National Park is characteristic of the larger Basin and Range region. Like so many others, the wide basin seen from Rim Village or Dutton Ridge is defined by mountain ranges running roughly parallel to each other -- often with one range forming a steep rise, or scarp, away from the basin.

Earthquakes can occur as the Basin and Range pushes against the Cascade-Sierran wall. Where the earth's upper crust snaps and breaks, fracture lines called faults are produced. In this most recent earthquake, a "normal" fault permitted one side of the fracture to drop down and pull away from the other. Some northwest- trending faults, so typical of Basin and Range seismic activity, produced the steep scarps on the east side of Upper Klamath Lake which can be seen so readily while traveling on Highway 97 from Crater Lake to Klamath Falls. Like a ramp which lets one side slip slowly down and slightly away, the plane of a normal fault is similar to a slanted wall where the top is tilted away while the bottom is pulled closer to you.

Although the earthquakes of 1993 have been attributed to this type of activity, the fault responsible is difficult to define. Some seismographs suggested that the fault skirts Upper Klamath Lake and Howard Bay, but aftershock epicenters have been placed some distance away. Since many faults do not reach the surface or may not be vertical, it is not surprising that the September earthquakes do not appear related to any known fault.

Earthquakes in the Klamath Basin

Since 1945 at least 12 earthquakes have occurred in the Klamath Basin. Geologists estimate that the fault zone around Upper Klamath Lake is capable of generating quakes as large as magnitude 7.25. An army officer's report from Fort Klamath in 1873 described one earthquake as severe enough to have knocked people and animals to the ground. Two hard shocks lasting some five to ten seconds each in this quake broke every pane of window glass at the fort, but the wooden frame buildings there appeared to have suffered little damage.

With relatively little property damage resulting, the September quakes seemed less intense at Fort Klamath than the ones 120 years earlier. Park structures at Crater Lake seemed unaffected, but Klamath Falls reported severe damage. As a small village called Linkville in 1873, Klamath Falls had comparatively little experience with quakes of 5.9 and 6.0 magnitude. Some of its brick buildings, being relatively rigid, appeared susceptible to the shock waves that moved outward from the epicenters of these earthquakes. Several structures had to be demolished, adding to the toll of several million dollars in property losses.

A Volcano's Warning Signs

As much as the Klamath Basin's dramatic landscape has been affected by earthquakes, most visitors cannot fail to notice the imprint of a related phenomenon. For the most part, volcanic activity is concentrated in the same areas as seismic activity. Vulcanism occurs where magma reaches the earth's surface through a long crack (fissure) or central vent. An example of the former is Lava Beds National Monument at the southern end of the Klamath Basin, while the latter is amply demonstrated by Mount Mazama -- the mountain which holds Crater Lake.

Volcanic activity can release ejecta (debris which range from large chunks of lava rock to glowing ash), liquid lava, and gases. Volcanoes located in the Cascade Range such as Mazama can erupt explosively and eject lethal particulate matter and/or gasses. Swarms of local earthquakes, which generally increase in size and number, usually precede such an event, as they did before Mount St. Helens erupted in 1980.

The September earthquakes did not, of course, follow the pattern signaling another chapter in Mazama's eruptive history. Nevertheless, it would be a mistake to assume that the mountain is dead or even dormant. The long-suspected presence of hydrothermal vents at the bottom of Crater Lake has been confirmed by researchers who piloted a submarine there in 1988 and 1989. Water significantly warmer than prevailing lake temperatures has been found near bacterial mats and features known as "blue pools" at the bottom of Crater Lake. What it suggests is heat from the volcano perhaps playing a role in perpetuating what we see from the surface.

The "Burp" of 1945

Although one might be so bold as to liken present levels of hydrothermal activity to a pilot light left on low, little is known about these vents. No clues have surfaced as yet about the so-called "burp" which occurred almost half a century ago. Bluish-gray clouds of smoke or gas appeared over the lake several times from September to December of 1945. Each time visitors and park staff saw these clouds, the day was calm and clear with no sign of fog or storm conditions. A cloud would form near the center of Crater Lake, rise sharply, mushroom out, and finally drift away with the prevailing breezes. By the time U.S. Geological Survey personnel arrived to monitor the lake with portable seismographs in January 1946, the strange phenomenon ceased.

Many people were willing to forget about the "burp" once formation of the clouds seemed to stop. The park superintendent at the time, however, noted that a strange disturbance affected several Oregon lakes in August 1919. Most pronounced was a marked discoloration and the destruction of more than 1000 fish in Diamond Lake from what observers took to be an underwater eruption. Newspapers mentioned disturbances of less intensity in Crater Lake, Upper Klamath Lake, and Marion Lake. An explanation has eluded geologists, but like the earthquake example, it is hard to be certain about phenomena of such short duration that occur underwater and/or underground.

Uncertainty is what drives some people to engage the scientific method, which is aimed at explaining or interpreting the phenomena observed. Science depends upon following a logical plan, beginning with fundamental observations. These are followed by the formulation of hypotheses so that a plan can be implemented by a way of collecting information or data for analyses. This method must allow for replication by other investigators and will determine if the initial hypotheses are to be accepted or rejected. At all phases, an open mind is required. If preconceived notions about the phenomenon to be studied affect the thinking of an investigator to deviate from the method, the effort becomes unscientific. Good scientific investigations generally yield more questions than were initially asked.

A Mysterious Buried Log

Radiocarbon dating of organic material such as buried wood is a valuable tool in attempting to understand the past. Trees destroyed by Mount Mazama's climactic eruption have been used to ascertain the date of that cataclysmic event at 7700 years Before Present. So many carbonized trees have been found in the Crater Lake region that the 7700 year age is the common assumption for all buried logs in or near the park.

A buried log five and one half feet below the present surface was discovered in 1984 by Marion Ribble near Spring Creek, southeast of Crater Lake. Water-saturated buried wood is safe from decay organisms, and may remain preserved for thousands of years in an oxygen-free environment. The simple interpretation of the Spring Creek log could have been that the tree was buried during Mazama's climactic eruption. After all, this was the story behind a highly-publicized discovery in 1991 northeast of the park, near Chemult. The "Mazama Tree" was found inside a vertical tree well entombed by a fiery avalanche deposit, and then covered by a 3 5 foot deposit of airfall pumice. This well-preserved log did indeed date to Mazama's climactic eruption 7700 years ago.

Map showing the location of the Spring Creek log

The Spring Creek log, however, was buried beneath redeposited ash, not airfall pumice. What led to the entombment of the Spring Creek log was not a fiery avalanche flow, but an event which might have made Spring Creek or its antecedent a reservoir for log debris. This log is not carbonized, bearing only surface bark char, displays intact cellular wood structure, and its widely-spaced tree rings provide evidence for favorable climactic conditions. These characteristics sparked enough curiosity to have the log radiocarbon dated. Its age (after calibration to correct for the difference between radiocarbon and calendar years) proved to be 7025 years Before Present. This means the tree first grew approximately 700 years after Mazama's climactic eruption.

The unexpected date for the Spring Creek log raises a number of questions. Could neighboring cinder cones have produced sufficient ashfall to create dams to the flow of water and allowed large quantities of debris to accumulate? Perhaps an oxbow lake of the ancient Williamson River--which might have predated Spring Creek -- served as a reservoir for log debris. Once these hypothesized dams broke, large lowland areas became inundated with deposited debris including buried log fragments.

At this point a reconstruction of events on Spring Creek is, of course, only conjecture. But admitting that we do not have all the pieces to the puzzle is, however, a bedrock of scientific inquiry. We always live with the possibility that our interpretations based on limited evidence may be wrong. With this in mind, the next earthquake should not have to be a reminder that we live in a very complicated and sometimes inexplicable world.