Crater Lake, Oregon: Historical Earthquakes

-- Excerpt from: Bacon, et.al., 1997, Volcano and Earthquake Hazards in the Crater Lake Region, Oregon: U.S. Geological Survey Open-File Report 97-487

The West Klamath Lake fault zone (WKLFZ), composed of several individual faults with lengths of up to 15 kilometers and an aggregate length of 50 to 70 kilometers, has been mapped through Crater Lake National Park west of the caldera. One of its constituent faults, the Annie Spring fault, passes less than 1 kilometer 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 of tectonic earthquakes as large as magnitude (M) 7 1/4. 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 around Magnitude 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 Epicenters and Magnitudes in the Crater Lake Region, 1920-1995

Earthquake hazards in the greater Crater Lake area are similar to those in other earthquake-prone areas, namely damage to structures, utilities, communication 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.

Prior to the 1993 Klamath Falls earthquakes, seismometers have been too few and too distant from Crater Lake to detect and accurately locate small earthquakes. There is, however, a sparse record of seismicity at Crater Lake and its vicinity. The largest event took place in 1920 before there were many seismometers in Oregon. It is known to have been felt at Intensity V (Modified Mercalli Intensity Scale V: "Felt outdoors; direction estimated. Sleepers wakened. Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open. Shutters, pictures move. Pendulum clocks stop, start, change rate."), and had an estimated magnitude of 4+. The earthquake's location is quite uncertain, though it is thought to have been near Crater Lake. In 1947 there was an event with an estimated magnitude of 3.7 south of Crater Lake near the town of Fort Klamath. One felt event in 1982 occurred near Crater Lake while a temporary array of seismic stations was deployed in Oregon (Kollmann and Zollweg, 1984). Relocation of this event by R.S. Ludwin (written commun., 1996) places it closer to Crater Lake and reduces its magnitude to 1.7 from the 2.5 calculated by Kollmann and Zollweg (1984). ...

Approximately 60 kilometers south of Crater Lake, two strong earthquakes, Magnitude = 5.9 and 6.0, occurred September 20, 1963, followed by hundreds of aftershocks during the succeeding weeks (the "Klamath Falls" earthquakes). The main events had hypocentral depths of approximately 9 kilometers and apparently took place on a north-northwest-trending normal fault inclined about 45 degrees to the northeast (Braunmiller and others, 1995). These earthquakes caused rock falls and small landslides (largest approximately 300 cubic meters) from road cuts, quarries, and steep bluff faces as far as 20 kilometers from the epicentral area (Keefer and Schuster, 1993). Subsequent to the Klamath Falls earthquakes of 1993, telemetered instruments were added to monitor ongoing seismicity (University of Washington, 1993), and locations and detection limits for earthquakes in the vicinity of Carter Lake improved. ...

In 1994 and 1995, there was a significant amount of seismicity near Crater Lake. Detection of the earthquakes of 1994 and 1995 may be partly a result of improved instrumentation, as the prior detection threshold for earthquakes that were not felt was probably at least a magnitude 3. ... In May, 1994, there were two events in the vicinity of the 1947 events near Fort Klamath. In December, there were three events (two felt) just south of Crater Lake. In August of 1995, there were three more events near Fort Klamath. It is possible that the recent magnitude 2-3 earthquakes represent a regional increase in seismicity related to the Klamath Falls earthquakes of 1993 because the number of events per year has declined each year since 1993. The area around Klamath Falls in the Klamath graben has had significantly more seismicity in the last 50 years than has Crater Lake (Sherrod, 1993).