Fortunately the epicenter, or the place where the seismographs indicate the shocks are focused, turned out to be the Mountain Lakes Wilderness Area, in 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
The vast area extending from south central 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 tending 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 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-Sierra 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 form 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 well where the top is tilted away while the bottom is pulled closet 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.

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 line 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 Mt. Mazama - the mountain which holds Crater Lake.

A pyroclastic eruption

Volcanic actvity 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 Mt. Mazama) can erupt explosively and eject lethal particulate matter and or gases. 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 signalling 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 the prevailing lake temperatures near bacterial mats and "blue pools" indicated that Mazama's heat could play a role in perpetuating what we see as Crater Lake.

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, these hypothesized vents provide no clues as yet to the so-called burp which occurred in 1945. Bluish-gray clouds of smoke or gas appeared over the lake several times form September to December of that year. 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 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 form 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.

Admitting we do not have all the pieces to the puzzle is, however, a base 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.

From Reminder of Uncertainty by Steve Mark and Ron Mastroguiseppe