Volume XXV - 1994

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 occ