RECENT EARTHQUAKES NEAR TWENTYNINE PALMS

SAN BERNARDINO COUNTY, CALIFORNIA

By ROBERT L. HILL, Geologist

California Division of

Mines and Geology

On 15 November and 14 December 1975, earthquakes of magnitude 4.6 and 4.7, respectively, occurred east of Goat Mountain 20 miles northwest of Twentynine Palms (figure 1). These earthquakes occurred near the Emerson fault, one of several major northwest trending faults in the Mojave Desert. In May 1975, an earthquake of magnitude 5.2 occurred near the Emerson fault in the vicinity of Galway (dry) Lake 18.5 miles northwest of Goat Mountain. The Galway Lake earthquake was accompanied by right-lateral surface rupture in a zone 4.2 miles long in the preexisting but previously unmapped Galway Lake fault zone (Beeby and Hill, 1975).

SEE FIGURE 1

Figure 1. Reconnaissance investigation of earthquake effects near Twentynine Palms, San Bernardino County, California, based on 3 days of field work during November and December 1975 by D. M. Morton, USGS, and J. E. Kahle and R. L.•Hill, CDMG. Topography from USGS Emerson Lake and Joshua Tree quadrangles, 15 minute series.

FOCAL DEPTH

Preliminary information from the Seismological Laboratory at the California Institute of Technology indicates that the 15 November earthquake had a focal depth of 5.6 kilometres. The 14 December event had a focal depth of 1.8 kilometres.

SEE FIGURE 2

Figure 2. Rockfall from a strongly jointed outcrop at locality R on figure 1. Displaced rocks in the central part of the photo are easily distinguished by the light grey to white surfaces that contrast with the darker surfaces of rock in place that are covered with a thin brown veneer of desert varnish. The rock is light colored where freshly broken, where impact scars have removed the dark veneer of desert varnish, and where surfaces are covered by a powdery layer of crushed rock. The two large boulders (a and b) near the base of the rockfall are shown in figures 3, 4, and 5. Inset: large boulder (a) in figure 2.

ROCKFALL

No evidence of surface rupture or shaking effects associated with the 15 November earthquake were observed during a geologic reconnaissance by foot and by helicopter on 18 November. Considering the shallow focal depth of the 14 December event, surface rupture was a likely occurrence. However, no surface rupture was observed during a reconnaissance by foot and by vehicle on 15 and 16 December and homes in the vicinity of the epicenter were not visibly damaged. The only observed effect of the earthquake is a rockfall at locality R on figure 1. The rockfall originated from a steep, strongly jointed outcrop (figures 2-6).

SEE FIGURE 3

Figure 3. Impact crater at base of large boulder (a) in figure 2. The boulder bounced from rocks to the right of the photo and dozed up a mound of sand and gravel visible in the lower left part of the photo.

SEE FIGURE 4

Figure 4. Mound of sand caused by impact of large boulder (b) in figure 2 against smaller boulder below.

Rockfalls, rockslides, debris slides and liquefaction slides are varieties of landslides that are commonly triggered by earthquakes. Because California is a seismically active region, these phenomena are potential hazards in many parts of the state where certain combinations of geologic, geomorphic, and hydrologic conditions suitable for their formation exist. These hazards should be considered not only in urban planning but also in planning of public utilities and public facilities, such as energy transmission lines and camp grounds in remote areas, as well.

SEE FIGURE 5

Figure 5. Close-up view of the rockfall. For scale note the geologist in the center of photo. Many precariously situated boulders near the top of the rockfall have lost their support, posing a potential hazard to unwary climbers.

SEE FIGURE 6

Figure 6. Close-up view of surface rock involved in the rockfall. The white to light gray, powdery layer of crushed rock on the weathered surface of the rock contrasts with its dark brown veneer of desert varnish. The powder was readily removed by blowing on the surface of the rock, indicating that the rockfall was no more than a few days old at the time it was observed two days after the earthquake of 14 December 1975. Moderate to strong winds that occurred in the area prior to the earthquake would have removed the powder. Note the photo lens cap for scale.

ACKNOWLEDGMENTS

The Division of Mines and Geology thanks Col. W. J. Hallisey, Jr., Chief of Staff, Marine Corps Base, and his staff for making it possible to travel on the U. S. Marine Corps Gunnery Range and for their assistance during the reconnaissance. The Division also thanks D. M. Morton of the U. S. Geological Survey for inviting Division geologists to participate in a reconnaissance by helicopter following the 15 November earthquake.

REFERENCES

Beeby, D. J., and Hill, R. L., 1975, Galway Lake fault -- a previously unmapped active fault in the Mojave Desert, San Bernardino County, California: California Division of Mines and Geology, California Geology, v. 28, no. 10, p. 219-221.

Dibblee, T. W., Jr., 1967a, Geologic map of the Emerson Lake quadrangle, San Bernardino County, California: U. S. Geological Survey Miscellaneous Geologic Investigations Map I-490, scale 1:62,500.

Dibblee, T. W., Jr., 1967b, Geologic map of the Joshua Tree quadrangle, San Bernardino and Riverside Counties, California: U. S. Geological Survey Miscellaneous Geologic Investigations Map I-516, scale 1:62,500.