EFFECTS OF IMPERIAL VALLEY EARTHQUAKE

October 1979, Imperial County, California

By

CHARLES R. REAL, Seismologist

RICHARD D. McJUNKIN, Geologist

EDDIE LEIVAS, Geologist

California Division of Mines and Geology

All photos by C. R. Real unless otherwise noted

The following article summarizes information collected by the authors shortly after the Imperial Valley earthquake of 15 October 1979. The observations presented here are preliminary and may be modified by more detailed studies .....Editor.

INTRODUCTION

A moderate magnitude earthquake (M = 6.6 Caltech Seismological Laboratory) occurred at 16:16 Pacific Daylight Time on 15 October 1979 in the southern Imperial Valley of California. The main shock was located at 32.62°N latitude and 115.33°W longitude (Caltech Seismological Laboratory). The earthquake had a shallow focal depth and was generated by right-lateral slip on the NW trending Imperial fault. Faulting produced approximately 30 km of surface rupture.

SEE COVER PHOTO

Damage from the earthquake, estimated to be $30,000,000, was most evident in residential areas of southern Imperial County and northeastern Baja California. The foundation and structure of the multi-million dollar Imperial County Services Building in El Centro was extensively damaged (see cover). Most of this six story building was supported by piers that failed (photo 1) and allowed partial collapse of the east portion of the building.

The agriculture industry also suffered high dollar losses from the earthquake. There were extensive lateral slope failures along many irrigation canals, including the All American canal (photo 2). The amount of subsequent crop damage from insufficient irrigation is uncertain. Many fields cut by the 15 October faulting must have the subsurface drain tile replaced and/or be releveled to facilitate flood irrigation.

SEE PHOTO 1

Photo 1. Collapsed columns on east side of the Imperial County Services Building. Columns failed as a result of rocking motion of the building. Note bent concrete reinforcing bars in the columns and cracks in the cross members of first floor ceiling (top of photo).

GEOLOGIC SETTING

The region most affected by the 15 October 1979 earthquake is situated in the southern Salton Trough. The Salton Trough is an extension of the Gulf of California (Sharp, 1976; Morton, 1977) formed along strike of the southern San Andreas fault zone. The basin is extensively filled with alluvium intercalated with lake bed deposits and volcanic rocks. The lowest portion of the trough is approximately 84 meters below sea level and is inundated by the Salton Sea.

SEE PHOTO 2

Photo 2. Earth lurching along banks of the All American Canal. Right-lateral displacement in this area is reported to be as much as 19 feet during the 1940 earthquake (Richter, 1958). Lurching seriously disrupted irrigation throughout the region.

A zone of right lateral strike-slip faults in the Salton Trough defines the San Andreas transform boundary between the Pacific and North American plates (Atwater, 1970; Atwater and Molnar, 1973; Silver, 1974; Hileman and Hanks, 1975). Most of the faults are poorly exposed in the lake beds and valley alluvium they cut.

Strike and alignment of the Imperial fault are nearly concordant with traces of the San Jacinto fault northwest of the Salton Sea. The San Jacinto fault is historically more active than the San Andreas fault south of the Transverse Ranges (Real and others, 1978a, 1978b). General alignment and historic activity suggest that the Imperial fault may be a southern splay of the San Jacinto fault. A scarp 1 to 2 meters high is locally prominent along segments of the Imperial fault indicating this zone of rupture has been prehistorically active.

The Brawley fault is located approximately 7 km east of the Imperial fault (figure 1). Small scale movement was observed on the Brawley fault during post earthquake investigations by the authors.

SEE FIGURE 1

Figure 1. Area of surface faulting, Imperial Valley earthquake, 15 October 1979.

Johnson and Hadley (1976) and Sharp (1976) describe synchronous offsets of the Brawley and Imperial faults during the 1975 earthquake swarm in this region. Movement of these faults and subsequent down-dropping of the central block between them is interpreted by Johnson and Hadley (1976) for creation of the Mesquite Depression (figure 1).

SEISMIC HISTORY

Regional strain in the Imperial Valley, estimated by geodetic measurements to accumulate at a rate of 8 cm per year (Whitten, 1956), is being relieved both seismically and aseismically as evidenced by the historical record of earthquakes (Richter, 1958; Hileman, and others, 1973) and observations of fault creep (Sharp, 1976; Goulty, and others, 1978). Instrumental records throughout the region indicate that the occurrence of earthquakes is confined to the upper 8 km of the crust presumably because of the high geothermal gradient in the Salton Trough and the associated plastic behavior of crustal rocks at greater depths. Using this relatively narrow fault width with conventional methods of computing the slip rates from earthquakes, it can be shown that nearly all of the observed slip in the Imperial Valley is accounted for by earthquakes (Brune, 1968).

Earliest reports of earthquakes in the region date back to the middle 1800s and are based on felt reports from San Diego, Yuma, and San Bernardino. More reliable accounts of earthquakes began around the turn of the century when the area became inhabited. On November 29, 1852, based on reported effects in distant towns, a large earthquake is believed to have occurred in the northern Salton Trough, probably in the Imperial Valley. A magnitude of about 6.5 is estimated for this event (Toppozada and others, 1979). On April 18, 1906, only hours after the Great San Francisco earthquake, a large earthquake occurred in Imperial Valley, estimated to be M = 6.0 (Toppozada and others, 1978). No damage was reported as the area was still sparsely populated at the time. However, on June 22, 1915, two earthquakes of M = 6.3 occurred, killing six persons and causing $900,000 in damages in Imperial Valley (Coffman, 1969). Further south in the Salton Trough, below the International Border, on December 30-31, 1914, two large earthquakes (M = 6.5 and 7.1)occurred along what was believed to be the southern extension of San Jacinto fault. It was not until 1940, however, that the Imperial fault was recognized.

On May 18, 1940, at 8:36 p.m. a M = 6.7 (Hileman and others, 1973; M = 7.1 --- Richter, 1958; M = 6.4 --- Trifunac and Brune, 1970) earthquake occurred accompanied by over 40 miles of ground surface rupture extending from northern Baja California to just south of Brawley. This rupture became recognized as the Imperial fault (Buwalda, Richter 1941). Right lateral displacements of up to 19 feet and vertical displacements up to 4 feet were reported between Highway 98 and the International Border (Richter, 1958). Nine persons were killed during this event and structural and agricultural losses accumulated to $6,000,000 (Coffman, 1969).

Interim seismic activity from 1940 to 1979 is characterized by earthquake swarms which occurred in 1950, 1955, 1966, 1973, 1975, and 1976 (Hill and others, 1975; Johnson and Hadley, 1976; Person, 1977). The 1975 Brawley swarm was studied in detail and shed new light on the complex interaction of movements between the Imperial and Brawley faults (Johnson and Hadley, 1976; Sharp, 1976).

SEE PHOTO 3

Photo 3. Left-stepping en echelon fractures along the Imperial fault near State Highway 111. The west side of the fault (right side of photo) has moved upward relative to the east side. The house incurred no structural damage.

An earthquake occurred on March 4, 1966, with a magnitude of only 3.6 and was accompanied by surface rupturing consisting of en echelon cracks along a 10 kilometer stretch of the Imperial fault (Brune and Allen, 1966). This event was very unusual in that this is the smallest earthquake ever associated with surface faulting. Other earthquakes which have produced ground rupture in the Salton Trough since 1940 include the 1951 Superstition Hills, the 1968 Borrego Mountain earthquake, and the ground cracking associated with the 1975 Brawley earthquake swarm. Sympathetic movement occurred on the Imperial, Superstition Hills, and San Andreas faults during the 1968 Borrego Mountain earthquake (Allen and others, 1972). There have been no great earthquakes (M> 8) in the Imperial Valley in historic time. It has been suggested that the rather frequent occurrence of moderate size earthquakes is sufficient to release the accumulated strain (Allen and others, 1965).

1940-1979 COMPARISON

There are striking similarities between the May 18, 1940, and October 15, 1979, Imperial Valley earthquakes. Although of lesser extent, the October 15, 1979, ground rupture followed the same trace as the 1940 event, and showed many of the same features and characteristics. Both ruptures appeared to have maximum lateral displacement near the International Border, and predominant vertical displacement near the Mesquite Depression east of Imperial. Activity shifted to the north with both events having damaging aftershocks near Brawley. Also, like the 1979 event there is evidence that the Brawley fault underwent sympathetic movement in 1940 (Sharp, 1976). The similarities also extend to the distribution and types of damage as described for the 1940 earthquake (Richter, 1959; Sylvester, 1979). In the 1940 event the structural damage was most severe in Brawley, and probably would have been so in the 1979 earthquake if it were not for the failure of the multi-million dollar County Services Building in El Centro.

SEE PHOTO 4

Photo 4. Mole track delineating 15 October 1979 Imperial fault rupture, trends across highway and extends beneath the end of stacked bales. Only bales directly above the fault trace at the end of the stack were knocked over. Mole tracks in the immediate foreground have been destroyed by vehicular traffic. The small ridge which defines the trace of the fault is termed a mole track in that it resembles the track of a gigantic mole. See also photo 5.

SEE PHOTO 5

Photo 5. Linear mole track trending diagonally across field defines the trace of 15 October 1979 Imperial fault rupture. Offset is approximately 30 cm of right-lateral slip at this location.

GROUND EFFECTS

The 15 October 1979 fault rupture extends from about 4 km north of the International Border to about 4 km south of Brawley. Maximum lateral displacement is about 55 cm in Heber Dunes and the maximum vertical displacement is 19 cm in the vicinity of Mesquite Depression southeast of Brawley. Lateral displacements are characterized by left stepping en echelon cracks (photo 3) and mole tracks (photos 4 and 5). Vertical offsets show a clean scarp at the base of the preexisting Imperial fault scarp (photo 6). Secondary features observed included sand boils (photo 7) and lateral spreading along the southern extent of the Imperial fault (photo 8) and lurching along canal banks (photo 2). Other structures damaged included mobile homes, a concrete block wall, bridge abutments, and metal grain elevators (photos 9-12).

ACCELERATION DATA

Twelve accelerograph stations, instrumented by the Strong Motion Instrumentation Program (SMIP) of the California Division of Mines and Geology, were triggered and recorded the 15 October earthquake. Records from these stations provide valuable acceleration data that will be used to evaluate damage-level motion from the earthquake. The Imperial County Service Building in El Centro is the SMIP station that recorded the accelerogram (figure 2) of ground motion and structural response showing times of the most extensive damage and subsequent partial collapse. This accelerogram is the first central recording system (CR-1) record ever generated from an extensively damaged building in California. The structure remains instrumented by SMIP to record aftershock accelerations which provide comparative data. A SMIP freefield station is instrumented approximately 75 m east of the building to record ground motion free of structural input from the heavy structure.

SEE FIGURE 2

Figure 2. Strong-motion accelerograms from Imperial County Services Building, El Centro, California. The record showing the ground motion and structural response is the first central recording system (CR-1) record ever generated from an extensively damaged building in California (see cover photos). Measurements and scaling of these accelerograms are not recommended for use in detailed studies because of possible photo copy distortions during reproduction.

PROFESSIONAL RESPONSE

Other groups which responded with studies in this area include California Institute of Technology, University of Southern California, Earthquake Engineering Research Institute and the U. S. Geological Survey. Post-earthquake investigations being conducted by various individuals from these groups include damage surveys, mapping of surface faulting, mapping of secondary effects, strong-motion studies, aftershock studies (photo 13), geodetic survey measurements and earthquake intensity studies.

SEE PHOTOS 6, 7, 8, 9, 10, 11, 12 and 13

Photo 6. Vertical movement along Imperial fault on 15 October 1979 at base of a preexisting scarp. Surface rupture also occurred at this location during the 1940 earthquake.

Photo 7. Sand boils extending across carrot field. Sand boils are an indication of liquefaction at depth. Water saturated sediments flowed upward along fractures. Water was reported by local resident to have spouted from sand boils 2 m above ground surface during the earthquake.

Photo 8. Lateral spreading along Heber Road at the north end of Heber Dunes. This type of spreading is probably generated by liquefaction at depth. Photo by David R. Fuller.

Photo 9. Damage to mobile home in north El Centro. The mobile home was knocked off its support piers. This mobile home was not anchored to the ground.

Photo 10 Damaged concrete block wall in north El Centro. Reinforcing bar was placed in the wall during construction only as high as the portion which remains standing.

Photo 11 Abutment damage to State Highway 86 (west bound lanes) over New River from rotational motion of the bridge during the earthquake.

Photo 12. Grain silo in Holtville, damaged by rocking motion. Photo by David R. Fuller.

Photo 13. CDMG scientist Chris H. Cramer and Steven P. Bezore installing a telemetered seismograph station near the Imperial fault. The seismograph at this station will record earthquake information.

REFERENCES

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