from California Geology, May 1979, Vol. 32, No. 5.

EARTHQUAKES NEAR HONEY LAKE LASSEN COUNTY, CALIFORNIA

By

WILLIAM A. BRYANT, Geologist

California Division of Mines and Geology

SEE PHOTO 1

Skedaddle Mountains (left of center) and Amedee Mountains (right of center) viewed eastward across Honey Lake Valley toward Nevada. Thick sections of andesite flows and layered pyroclastic rocks form the main mountain masses. These are flanked by basaltic flows which appear near the edges of view. Photo by C. W. Chesterman.

A moderate earthquake (M 5.2, Berkeley) occurred at 7:57 am on February 22, 1979 in Lassen County. The epicenter was located in the southeast portion of Honey Lake Valley about 80 kilometers (km) north of Truckee (figure 1).

SEE FIGURE 1

Figure 1. Honey Lake Valley is a part of the Basin Ranges geomorphic province. The epicenter of the February 22, 1979, earthquake is located near Doyle. The earthquake was felt as far away as Sacramento.

REGIONAL GEOLOGY

Structure

The Honey Lake Valley region is a part of the Basin Ranges geomorphic province. Honey Lake Valley is a graben wedged between the northern end of Sierra Nevada granitic mountains, and the southern end of Modoc Plateau volcanic terrane (Macdonald and others, 1968). Spectacular faults form the Sierra Nevada front and bound the Fort Sage Mountains block at the southeast end of the valley (figure 2). Displacement along the Honey Lake fault zone is thought to be at least 2,000 feet (Diller, 1908). The presence of tufa-building hot springs along the northeastern edge of the valley suggests the hidden extent of province bounding faults.

SEE FIGURE 2

Figure 2. Generalized geology of Honey Lake Valley and surrounding region. Geology from portions of Westwood and Chico sheets of Geologic Map of California.

Lithology

Physiographic features of Honey Lake Valley are typically Basin Ranges, but the lithology is characteristic of the Sierra Nevada province to the south and the Modoc Plateau province to the north (figure 2). Mountains west and southwest of Honey Lake Valley are composed of Mesozoic granitic rocks ranging in composition from granodiorite to quartz diorite (Diller, 1908; Gianella, 1957). Tertiary volcanic rocks (andesite breccia, mudflows, and tuff) overlie the granitic rocks. The Fort Sage Mountains are similarly composed of Mesozoic granitic rocks overlain by Tertiary volcanic rocks. The mountains north of Honey Lake Valley are formed principally of Pliocene andesitic and basaltic rocks typical of the Modoc Plateau region (photo 1).

The floor of Honey Lake Valley is underlain by folded Tertiary sedimentary rocks of lacustrine origin (photo 2). These rocks are overlain by valley fill and late Pleistocene to Holocene lake deposits.

SEE PHOTO 2

Photo 2. Typical outcrops of Tertiary lake beds (foreground) and granitic rocks (background) south of Doyle, looking east.

Seismicity

Earthquakes of magnitude 4 and greater have occurred since 1900 south and east of Doyle (figure 3). A magnitude 5.6 earthquake occurred in 1950 in the Fort Sage Mountains a few miles north of Doyle.

Ongoing research by California Division of Mines and Geology (CDMG) of historical seismicity of this region prior to 1900 indicates that the Honey Lake region is seismically active. Strong earthquakes (M 5.5 and greater) occurred in 1875 (M 6.0), 1885 (M 5.6), and 1889 (M 5.5) near Susanville and Janesville (Toppozada and others, 1979). The regions of greatest intensity were along the foot of the Honey Lake escarpment, generally near Janesville.

FEBRUARY 22, 1979 EARTHQUAKE

At 7:57 a.m. local time February 22, 1979, a magnitude 5.2 earthquake occurred in the southeastern portion of Honey Lake Valley, near the town of Doyle (39.99 N, 120.12 W). Damage in the epicentral area was mild; telephone service was temporarily disrupted, desks were moved and lamps swayed, but no bottle or window damage was reported. The earthquake was strongly felt in Reno, Nevada (60 km southeast of the epicenter) and was felt by a few individuals as far away as Sacramento (200 km southwest of the epicenter) (figure 1). People within the epicentral region reported hearing the earthquake. The sound was said to resemble a sonic boom.

SEE FIGURE 3

Figure 3. Epicenters of earthquakes (M>4) within a 100 km radius of Doyle that have occurred between 1900-1974. The February 22, 1979, epicenter is also shown. From Real and others, 1979, Earthquake epicenter map of California.

Reports of audible sounds accompanying local earthquakes are common. Studies indicate that earthquake sounds are generated mainly by seismic P-waves (compressional or longitudinal body waves). Upon striking the earth's surface, the P-wave generates an acoustic wave in the atmosphere (Hill and others, 1976). The ground around an observer acts like an enormous loudspeaker during a nearby earthquake. Earthquake sounds are normally heard only in an area near the epicenter because the higher, audible frequencies generated by P-waves are filtered out as the distance from the epicenter increases. For earthquakes of magnitude 2 to 3, audible waves begin within a fraction of a second after the first P-wave arrival. Perceptible shaking follows the sound, accompanying the later arrival of the S-wave (shear wave). The S-wave is usually felt during an earthquake because it is the seismic wave of maximum amplitude. For stronger earthquakes, the P-wave sometimes may be felt before the arrival of the S-wave. In this case, audible earthquake sounds usually coincide with the onset of shaking.

The February 22 earthquake was preceded by a foreshock (M 3.5) at 11:17 p.m. February 21. A magnitude 3.7 aftershock occurred at 7:41 p.m. February 22. This event was the largest in a small series of aftershocks. Aftershock activity quickly diminished after February 23.

The focal depth of the February 22 earthquake was about 12 km (University of Nevada, Reno). The causitive fault for this event may have been the Honey Lake fault or the Fort Sage fault. However, a reliable determination of the causitive fault could not be made at the time of this

writing because of the tentative epicenter location and the lack of fault plane solutions.

1950 EARTHQUAKE

A magnitude 5.6 earthquake occurred December 14, 1950 in the Fort Sage Mountains. Ground shaking was strongly felt in the epicenter region and damage was sustained in Doyle and the Sierra Ordnance Depot at Herlong.

SEE PHOTO 3

Photo 3. Surface fault rupture on the Fort Sage fault, about 3 km northeast of Doyle. This fault scarp, formed in alluvium, was subsequently destroyed by normal erosional processes. Photo taken January 27, 1951, by F. A. Riddell.

Surface fault rupture occurred along the Fort Sage fault, located on the south side of the Fort Sage Mountains. The fault scarp produced during the 1950 event consisted of two segments that trend north-south (figure 4). The maximum displacement along the southern portion of the fault was about 20 centimeters (cm) (photo 3), and displacement on the northern segment was about 13 cm (Gianella, 1957). Movement along the fault was in a normal sense with the downthrown block to the west; evidence of lateral movement was not observed. A parallel fault located about 400 meters west of the northern fault segment produced a scarp about 8 cm high (figure 4). Downthrow was to the east, thus producing a small graben.

SEE FIGURE 4

Figure 4. Faults associated with surface rupture during the Fort Sage Mountains earthquake. December 1950. Portion of Doyle 15' quadrangle.

The fault scarps produced during the 1950 earthquake were in alluvial material. Normal processes of erosion have subsequently destroyed these scarps.

CONCLUSION

The northeastern portion of California north of Lake Tahoe is a seismically active region. Historical earthquakes have occurred on the spectacular frontal faults of the northern Sierra Nevada and along normal faults in the Basin Ranges province. Continued earthquake activity within this region, such as the December 1950 and February 1979 earthquakes, can be expected.

REFERENCES

Burnett, J. L. and Jennings, C. W., 1962, Chico sheet, geologic map of California: California Division of Mines and Geology, scale 1:250,000.

Diller, J. S., 1908, Geology of the Taylorsville region, California: U. S. Geological survey Bulletin 353, 128 p.

Gianella, V. P., 1957, Earthquake and faulting, Fort sage Mountains, California, December, 1950: Bulletin Seismological Society of America, v. 47, no. 3, p. 173-177.

Hill D. P., Fischer, F. G., Lahr, K. M., and Coakley, J. M., 1976, Earthquake sounds generated by body-wave ground motion: Bulletin seismological Society of America, v. 66, no. 4, p. 1159-1172.

Lydon, P. A., Gay, T. E., and Jennings, C. W., 1960, Westwood sheet, geologic map of California: California Division of Mines and Geology, scale 1:250,000.

Macdonald, G. A., Gay, T. E., Stewart, J. H., and Ross, D. C., 1968, Geology of the Basin Ranges: Mineral Information service, v. 21, no. 9, p. 131.

Real, C. R., Toppozada, T. R., and Parke, D. L., 1978, Earthquake epicenter map of California, 1900-1974: California Division of Mines and Geology Map Sheet 39, scale 1:1,000,000.

Toppozada, T. R., Real, C. R., Bezore, S. P., and Parke, D. L., 1979, Compilation of pre-1900 California earthquake history: California Division of Mines and Geology Open-file Report 79-6 SAC.