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).
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