HOMESTEAD VALLEY EARTHQUAKE SWARM San Bernardino County, California

By

H. G. HAWKINS and J. L. McNEY, Engineering Geologists

Southern California Edison Company

Rosemead, California

Photos by J. L McNey

A swarm of earthquakes occurred on March 15, 1979, shaking a large portion of southern California. There were five events between 12:17 p.m. and 3:17 p.m. with magnitudes (M) ranging from M 4.1 to M 5.2. At least 24 events were recorded by 12 p.m. March 16, 1979.

Other events which have occurred in the region since 1974 have been the Galway Lake earthquake (M = 5.2) on June 1, 1975; Goat Mountain earthquake (M = 4.6) on November 15, 1975; and Goat Mountain earthquake (M = 4.7) on December 14, 1975.

SEE PHOTO 1

Photo 1. Looking south along trend of breaks crossing University Road. There is no apparent vertical separation; tire tracks were made after the events on 15 March. The width of the zone at this location is about 15 meters. The cracks in the center of the road show flat slope where loose surface sands have been shaken and have fallen into the cracks. The location is west of the USGS 1/4 section monument on University Road.

SEE FIGURE 1

Figure 1. Location of Homestead Valley and Pipes Wash fault, San Bernardino County.

GROUND RUPTURE

An aerial inspection and field checking of the Homestead Valley area (photos 1-5) was made by the authors on the afternoon of March 15, 1979 with additional checking on March 17, to determine the extent of surface rupture and damage to utility facilities. The banks of Pipes Wash in Homestead Valley, San Bernardino County, displayed shatter-cracks along the crest that were en echelon and about 3 to 4 meters (m) in length spaced 10 to 20 centimeters (cm) apart (figures 1 and 2, photos 4 and 5). The cracks opened up several centimeters. At two locations on the bank, slumps were observed in portions of the 8-m-high slope (photo 4). Blocks of soil were intact at the bottom of the wash, apparently having been tossed off the crest. South along the slope, the cracks progressed down slope and disappeared within a few tens of meters (photo 5). The total length of the shattering and slumping was on the order of several hundred meters. The straight alignment of the east embankment of Pipes Wash with shattering following the crest and crossing down the slope suggests the presence of an unmapped fault trace. This fault trace on the east bank of Pipes Wash is 600 m northeast of the trace of a previously mapped unnamed fault (Jennings, 1975).

SEE PHOTO 2

Photo 2. Surface cracks in the soil crust show right lateral movement with the west (top of photo) side up. Voids on the right show pull-apart opening, while the left shows compression. The folding ruler is open 18 inches.

SEE PHOTO 3

Photo 3. Shaking in the epicentral area caused the soil in the low slope to fail. Note the dirt clod by hammer handle has fallen from the crest.

To the west from Pipes Wash fault, surface rupture was seen at three locations: (1) Severe cracking was seen near a house at the east end of University Road. The cracks trended north-northwest, subparallel to the alignment of Pipes Wash fault. The ground surface was displaced up to 1 cm horizontally with little or no vertical displacement. (2) Approximately 1 kilometer (km) west of the southeast corner of Section 23, T3N, R5E cracks which cut through the road were observed (photo 1). The trend of the cracks is essentially north-south extending about 60 m south and 120 m north from this location. Cracks were then found about 80 m south of University Road and slightly west of the cracks intersecting the road. Cracks at this location trend north-northeast and have an en echelon pattern. The zone is approximately 45 m wide. A low creek bank less than 25 cm high was shattered (photo 3). (3) On March 17, cracks and surface displacement were found to the south of University Road about 300 m and east of the cracks described that intersect the road. Cracks at this location trend en echelon north 15° west and in a zone about 45 m wide. The cracks were right-lateral offsets with strike-slip displacement of 2 cm horizontally and less than 1 cm of vertical displacement. At one isolated location, the crack had up to 5 cm vertical displacement and 1 cm of horizontal displacement.

SEE PHOTO 4

Photo 4. Looking north along Pipes Wash. Shatter-cracks on the east side (right) of Pipes Wash. A large slump occurred in the center of the slope with severe shattering south of the slump (right-hand side of the photo). The slope is about 8 meters high.

SEE PHOTO 5

Photo 5. Looking north along Pipes Wash. Shatter-cracks south of the slumping. The cracks trend parallel to the slope crest and are about mid-height on the slope face, suggesting that the east side of the wash represents a previously unmapped fault. There is apparent vertical separation here, based upon the tire tracks.

Recent rains had created a thin soil crust resistant enough to show cracks. The cracks ranged from a few centimeters to several meters long, were subparallel and en echelon, being spaced 1 to 3 m apart. They vary from hairlines to nearly 1 cm in width and have a horizontal displacement of about 1 cm (photo 2). One occurrence of right lateral pull-apart of 1 cm was noted. Noticeable scarps were sparse with local occurrences of soil pushed up about 1 cm. The cracks representing surface rupture begin at a distance of about 800 m south of University Road, near the west center of Section 26 and trend north toward the W ½ Section 23, T3N, R5E.

GROUND SHAKING EFFECTS

Two home owners within one mile of the Pipes Wash fault reported that objects were knocked from shelves, bottles and other items were overturned, and furniture shifted; this indicates a modified Mercalli intensity of VI. At a home on the east end of University Road where ground cracking was seen, a Franklin stove slid about 15 cm southwest. Nearby telephone poles were moved about 7 cm northwest, and a mobile home was moved off its supports for a distance of 15 to 20 cm in a northwest direction. These movements would be consistent with shear wave motion for a northerly trending strike-slip fault.

A Southern California Edison Company strong-motion instrument located some 30 km northwest of the area recorded acceleration of 0.04 g peak-to-peak. No significant damage to the utility facilities was observed, other than local, short-term outages.

SEE FIGURE 2

Figure 2. Surface rupture, Homestead Valley earthquake, March 15, 1979. Geology from Dibblee, 1970.

SUMMARY

The March 1979 earthquake swarm had five earthquakes of magnitudes greater than M = 4.0 in the initial 24 hours. The composite focal mechanism provided by Mr. Carl Johnson, California Institute of Technology (C. I. T.) Seismological Laboratory, suggests a near-vertical, right-lateral strike-slip fault striking N15°W. This is consistent with the field evidence based on orientation of the mapped cracks. Calculations by C. I. T. suggest hypocentral depths on the order of 1 km (personal communication, Dr. David Hadley, Sierra Geophysics, Inc.). Preliminary locations of the epicenters fall on a line parallel to Homestead Valley some 1.5 km west of the surface cracking.

REFERENCES

Dibblee, T. W. Jr., 1970, 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.

Jennings, C. W., 1975, Fault map of California with locations of volcanoes, thermal springs, and thermal wells: California Division of Mines and Geology, scale 1:750,000.