from California Geology, December 1987, Vol. 40, No. 12.
WHITTIER NARROWS EARTHQUAKES -- Los Angeles County
October 1 and 4, 1987
F. HAROLD WEBER Jr., Geologist
California Department of Conservation
Division of Mines and Geology
Photo 1. View of partially collapsed portion of three-level parking structure at "The Quad" shopping center just south of Whittier Blvd. and east of Painter Avenue, city of Whittier. No people were injured or killed. Photos by the author except as noted.
An earthquake of magnitude 6.1, located 10 miles east of the Los Angeles city Civic Center, occurred on Thursday, October 1, at 7:42 a.m. (Figures 1 and 2). The earthquake caused eight deaths and extensive damage, especially to older residential and commercial buildings in the populous epicentral region of the San Gabriel Valley and northern Los Angeles Basin. Particularly hard hit was the western part of the city of Whittier (Photos 1 and 2). Seismologists reported an unusually small number of aftershocks through Saturday, October 3. Then on Sunday, October 4 at 3:59 a.m., an aftershock of magnitude (M) 5.5 occurred slightly northwest of where the first earthquake struck. The two earthquakes caused at least $215 million in damage to 10,500 residential and business structures. These were the first damaging earthquakes to occur in the Los Angeles area since the M 6.4 San Fernando earthquake in 1971.
GEOLOGIC SETTING
The locations of the two principal earthquake epicenters are along the general strike of the Elsinore-Whittier fault zone (Figures 1 and 2). The Whittier fault segment of this zone is well defined at the ground surface in the Whittier area (Yerkes, 1972, Plate 1), but not to the northwest. Mapping by Lamar (1970) and Thompson and Stark (1970) in the Repetto Hills shows a complex pattern of inferred, discontinuous faults, striking northwest within bedrock in the northern Repetto Hills (Figure 2). To the north of the Repetto Hills faults lies the east-trending Raymond fault, part of a zone of faults that separates the east-trending Transverse Ranges geomorphic province on the north from the Peninsular Ranges province on the south. The epicentral area lies within the Los Angeles Basin (Yerkes and others, 1965), a geologic sub-province of the Peninsular Ranges that includes the Los Angeles physiographic basin (Figure 2) as well as the San Gabriel and San Fernando valleys. A thick sequence of marine sedimentary rocks was deposited in this basin from about middle Miocene to latest Pliocene time (15 to 3 million years ago).
Figure 1. Fault map of southern California and northwesternmost Mexico showing area covered by Figure 2. Faults: BP = Big Pine; E = Elsinore; G = Garlock; M-R = Malibu Coast-Santa Monica-Hollywood-Raymond; NI = Newport-Inglewood; RC = Rose Canyon; SM = Sierra Madre; SA = San Andreas; SG = San Gabriel; SJ = San Jacinto; SY = Santa Ynez; and W = Whittier.
Intense deformation of these and associated rocks over a period of several million years has created the regional terrain, shown on Figure 2, that consists of groups of hills and mountains separated by broad valleys. The deformation has also been a major factor in the concentration of significant petroleum deposits along faults and within folds. The deformation continues today and earthquakes, such as the recent October 1-4 earthquakes, are part of this process.
Figure 2. Compiled mostly from adjoining portions of the Long Beach, Los Angeles, San Bernardino, and Santa Ana sheets of the Geologic Map of California (Olaf P. Jenkins edition). Rock units: B = pre-Pleistocene bedrock making up mountains and hills; O = rocks and sediments of Pleistocene and Holocene age within valleys and Los Angeles Basin. Faults: H = Hollywood; N = Norwalk; NI = Newport-Inglewood; R = Raymond; SM = Sierra Madre; VE = Verdugo-Eagle Rock; W = Whittier; WH = Workman Hill. Fault symbols: solid heavy lines where well defined, dashed where less well defined, queried where inferred, dotted where concealed; hachures define north-dipping reverse faults; balls on Raymond (R) fault illustrate prominent south-facing scarp.
Damaging historic earthquakes are known to have occurred along the Newport-Inglewood fault to the southwest of the Elsinore-Whittier fault, and along the San Jacinto fault to the northeast. No damaging historic earthquakes are known to have occurred along the east-trending Santa Monica-Hollywood-Raymond fault, but anomalous topographic features along its trace attest to its recency of activity and earthquake potential (Figure 2 scarp).
Elsinore-Whittier Fault Zone
The Elsinore-Whittier fault zone extends from Whittier Narrows southeast across the Santa Ana River, past Lake Elsinore, into western Imperial County and then into Mexico (Figure 1). The Whittier segment of the fault, northwest of the Santa Ana River, is a northeast-dipping reverse fault along which bedrock of the late Miocene Puente Formation, making up most of the Puente Hills, has been uplifted.
Southeast of the Santa Ana River, the Elsinore segment of the zone cuts diagonally across the northwestern nose of the Santa Ana Mountains and extends southeast along the northeast side of these mountains. Geologic mapping along the fault, and distribution of recorded epicenters, indicate that certain segments of the zone are reverse faults that dip moderately southwestward (Weber, 1977, Figure 2). The mapping suggests that total right-lateral offset along the Elsinore fault in the Corona-Elsinore area is about six miles.
Photo 2. Partially collapsed parking structure at "The Quad," in Whittier. Photo by J. F. Davis.
The principal strands of the northwestern Whittier fault are reported by Edward C. Sprotte (personal communication, October 1987) to dip 80°-85°•N to a depth of at least 10,000 feet. These data are based on his knowledge of petroleum exploration in the area. The principal strands in the northernmost Whittier area dip 80°•N to a depth of about 8,000 feet (Yerkes, 1972, plate 2, cross section B-B'). "Right-handed" en echelon folds along the Whittier fault within rocks of the Puente Formation indicate long-term right lateral offset. Deflected drainages that cross that portion of the fault southeast of Whittier are evidence for right-lateral movement continuing into recent geologic time (Yerkes, 1972, p. 29).
Surface excavation studies, mostly funded by the U. S. Geological Survey Earthquake Hazards Reduction Program, have been made on the Elsinore and Whittier faults to determine their recency of movement and potential for future earthquake occurrence. Results of studies along the Glen Ivy north segment of the Elsinore fault have been described by Lamar and Swanson (1981) along with Rockwell and others (1985). An earthquake estimated by Richter (1958, p. 533) as approximately M 6 occurred in the Glen Ivy-Lake Elsinore area in 1910. An earthquake of M 5.5 occurred in the same area in 1938.
The principal evidence for placing the central and southeastern parts of the Whittier fault within an Alquist-Priolo "Special Studies" zone is from a report by Hannan and others (1978), and is cited by Hart (1979). Colluvium offset along the fault at one locality southeast of Whittier contained charcoal that was dated at 2,000 years old by carbon- 14 methods (Hannan and others, 1978). Therefore the fault has been active during Holocene time (10,000-11,000 years old). The lack of a ''recently active strand," cited by Hart (1979, p. 4-5), to the northwest in Whittier was the reason given for not zoning it there (Hart, 1985, p. 17).
Geologic Evidence for Faults in the Epicentral Area
Seismologists at the California Institute of Technology preliminarily reported that the earthquake had a depth of 12 km (7.5 miles; Figure 3). They reported that data obtained from monitoring the main shock and aftershocks defined a thrust fault that dips 30°N. Such a fault projects to the ground surface at Norwalk, far south of the trace of the Whittier fault (Figure 2). This is near the northwestern terminus of the trace of the inferred "Norwalk fault," described by Richter (1958, p. 43, Figure 4-4) as the source of the M 4.7 Whittier earthquake that occurred in 1929. However, no subsurface geologic evidence for such a fault could be found by Yerkes (1972, p. 31).
To the north in the Whittier Narrows area and vicinity, geologists of the Division of Mines and Geology (DMG) and other organizations could find no ground rupture along the traces of the principal strands of the Whittier fault and along the trace of the Workman Hill fault (Figure 2). Some minor fault rupturing, however, may have occurred in the Puente Hills, across the 12700 block of Greenleaf Avenue in Whittier (Richard Greenwood, personal communication, Photos 3 and 4).
Geologic relationship in Tertiary bedrock underlying alluvial sediments in the Whittier Narrows apparently are structurally complex (Stark and Thompson, undated). California Department of Water Resources geologists (1966, p. 42, 47) state, however, that the flow of ground water through the Whittier Narrows, including the flow across the trace of the Workman Hill fault, is not impeded. Thus the bedrock surface underlying the alluvial sediments does not appear to be displaced by faulting.
Tertiary age marine sedimentary rocks were deposited over the basement rocks in the region of the earthquake epicenters. A subsurface 'structural high' is shown above the buried ancestral Tertiary ground surface (Stark and Thompson, undated maps). This structural high is referred to as the Elysian Park anticline (Yerkes and others, 1965, p. 50) and extends from the Puente Hills northwestward through the epicentral area (Figures 1-3).
The top of the basement rocks is about 4,000 to 6,000 feet below ground level in the epicentral area (Figure 3). The hypocenters of the earthquakes (the rupture points in the earth's crust where the earthquakes originated) are about 6 miles below the ground surface in the epicentral area. Exploratory wells drilled for Petroleum in the region have reached basement rock at depths ranging from 4,000 to 7,000 feet (California Division of Oil and Gas, 1982).
Figure 3. Pictorial cross section illustrating the major known and inferred geologic structural relationships in the epicentral area. Compiled from several sources including Yerkes and others, 1965, p. 4, Figure 2. The sedimentary rocks shown in the cross section range in age from Late Cretaceous to Pliocene. The sediments are Pleistocene and Holocene in age.
Photo 3. View east of crack crossing street, 12720 Greenleaf Ave., Whittier western Puente Hills. Crack occurred on day of earthquake, October 1. It could not be traced beyond this locality. Additional cracks cross the street and occur in the general vicinity. Photo by Richard Greenwood.
Surface and inferred subsurface geologic relationships indicate that the Whittier Narrows earthquakes did not occur on a previously known fault. A study of the seismicity of the northern Los Angeles Basin determined that concentrated seismic activity occurs near the junction of the Hollywood-Raymond fault and the projected trace of the Whittier fault (Real and others, 1978).
DAMAGE AND OTHER EFFECTS
More than 9,100 residential and business structures were damaged by ground shaking caused by the two earthquakes, almost all within 10 miles of the two epicenters. Of the 82 incorporated cities in Los Angeles County, the cities receiving the most damage were Whittier, Monterey Park, Montebello, El Monte, South El Monte, Santa Fe Springs, Norwalk, Alhambra, Pasadena, San Gabriel, Arcadia, Monrovia, Rosemead, and Pico Rivera. Several unincorporated areas in east Los Angeles also received damage. Cities in northern Orange County, including La Habra, also were damaged. Most seriously damaged were older, unreinforced structures --- some constructed prior to 1900 --- located as far away from the immediate epicentral area as Pasadena to the north and central Los Angeles to the west (Photos 1, 2, 5-7). Historical buildings badly damaged included the San Gabriel Mission.
Many parked automobiles were damaged by falling walls and bricks. Severe structural cracks within the foundation of the interchange between Interstate Highways 5 and 605, which normally handles 400,000 vehicles each weekday, caused CalTrans officials to close it for the day for temporary repairs. Houses were partially shaken from their foundations in Whittier and chimneys were damaged at least as far away as Arcadia. Helicopter surveys of dams for flood control and water storage taken immediately after the earthquake showed no significant damage. No apparent effects of liquefaction were reported to have occurred, based on field investigation by DMG staff immediately after the first earthquake. Small surficial slides could be observed in previous landslide terrain in Turnbull Canon in the northern Whittier area (Photo 8). In general, however, the terrain was much too dry for the ground shaking to have activated deep-seated landslides. Dust clouds rose over the southern flank of the west-central San Gabriel Mountains caused by rock falls and surficial sliding from road cuts.
One of the eight deaths directly caused by the October 1 earthquake involved shifting earth that trapped a Southern California Edison employee in a construction excavation for an electrical power-line transmission standard in the San Gabriel Mountains. A second death was caused when a large slab of concrete from a parking structure fell onto a student at California State University, Los Angeles. This university may have received more than $22 million in damage. Two additional deaths were caused by falling debris, at least three by heart attack, and one by electrocution. In addition to these eight deaths associated directly with the October 1 earthquake, a man was reportedly killed when he jumped, from fear, out of the window on the second floor of a building. One person was reported to have died from a heart attack as a result of the second earthquake.
Photo 4. Close-up view north of feature shown in Photo 3. Pavement is up about 0.5 inches on north side of the crack. Photo by Richard Greenwood.
Photo 5. Damage to older unreinforced brick buildings on the west side of Fair Oaks Avenue, just south of Green Street, Pasadena. Two persons in the small building in the foreground that collapsed saved their lives by running out of it when the shaking began. Most authorities, however recommended that persons remain inside buildings in a doorway until the shaking subsides, then leave the building. Photo by Edmund Kiessling.
The north, older "uptown", section of Whittier sustained serious damage. Early reports stated that 200 residences and 30 businesses were badly damaged, and most of the damage was to older structures. More than $25 million recently had been spent modernizing the business section of this part of the city prior to the earthquake. Unfortunately, most of the modernized structures were not upgraded for maximum earthquake safety. The modern, 9-story Hilton Hotel in this area was not structurally damaged.
Initial strong motion data of the earthquakes released by the DMG indicate that ground shaking was intense at localities in the western San Gabriel Valley and eastern San Fernando Valley. Damage was severe along the trace of the northwestern portion of the Whittier fault in the northern "uptown" section of Whittier, and along Whittier Boulevard, two miles to the south. (Photos 1 and 2.) The apparent south-trending pattern of the most severe damage in Whittier is similar to the pattern of intensity that took place there during the M 4.7 Whittier earthquake of 1929 (Richter, 1958, Figure 4-4, p. 29).
HISTORIC PERSPECTIVE
The earliest earthquake in southern California for which reliable historic data are available occurred July 13, 1769, only 219 years ago. Geologists know, however, that multitudes of earthquakes have had to occur over the previous millions of years in southern California while the regional geologic framework evolved. Members of the Portola expedition recorded an earthquake in 1769 while camped along the Santa Ana River, a short distance from the Elsinore-Whittier fault zone. The causative fault for the earthquake, however, has not been determined. Since 1769, many damaging and potentially damaging earthquakes have occurred in southern California at widely separated localities on land and offshore. Little is known of the effects of the earlier of these earthquakes because the area was sparsely populated until about 1875, and there were no earth scientists to study them. For example, little data was recorded from a strong earthquake that struck the Los Angeles region in 1855.
The first urban earthquake in the populated Los Angeles region to cause widespread, serious damage and death was the M 6.3 Long Beach earthquake that occurred near Long Beach at 5:54 p.m. on March 10, 1933. The earthquake killed 120 people. Among legislation passed after this earthquake was the Field Act which regulates construction of schools, and the Riley Act which regulates construction of buildings for human occupancy by more than two families. This earthquake occurred on the Newport-Inglewood fault zone, preceded by a smaller, less damaging earthquake in Inglewood in 1920. Although earth cracks caused principally by liquefaction of watery sediments occurred in 1933, no tectonic ground rupture along faults was determined to have occurred (Barrows, 1974).
The M 6.4 San Fernando earthquake of February 9, 1971, caused many surprises. It occurred on a north-dipping thrust fault that ruptured the ground surface. For the most part, the fault had not been defined and mapped by geologists. For the first time on record, ground acceleration of 1 gravity (g) was reached during an earthquake: the maximum acceleration at Pacoima Dam was 1.25 g. All four hospitals in the epicentral area were severely damaged, including the newly constructed Olive View Hospital. Twenty-six persons were killed, and the cost to repair or replace structures damaged by ground shaking, liquefaction, and fault rupture probably exceeded $1 billion. State legislation enacted following the earthquake strengthened requirements for construction of hospitals. The collapse of freeway bridge segments during the earthquake caused CalTrans to alter its engineering design of these structures.
Photo 6. Severely damaged unreinforced brick building at the corner of Del Mar and Garvey avenues, in Rosemead. Walls and ceiling of upper story of two-story building partially collapsed.
On November 4, 1987, at press time for this issue, CALTECH announced revised magnitudes for the October 1987 Whittier Narrows earthquakes: October 1, M 5.9; October 4, M 5.3 . . . . editor.
Photo 7. Detail of damaged, unreinforced brick building in uptown Whittier. Much destruction occurred to such buildings in the area.
The location and geologic circumstances of the Whittier Narrows earthquakes have demonstrated once again that damaging earthquakes occur almost anywhere in the Los Angeles region. Each earthquake teaches its own lessons. However, the historic earthquake record is still far too short. In addition, the pattern of faulting in the Los Angeles Basin is very complex (Figure 2) compared to the San Francisco Bay region. On the other hand, studies of areal and subsurface geology have advanced greatly since serious geologic studies involving earthquakes began after the San Francisco earthquake of 1906. In the last 15 years, studies funded by the U. S. Geological Survey Earthquake Hazards Reduction Program, investigations related to Alquist-Priolo Special Studies Zoning, and other engineering geology studies have greatly increased the knowledge related to potential for earthquakes and accompanying ground rupture on faults in the Los Angeles region. Much of this knowledge has been summarized in a report by the U. S. Geological Survey (Ziony, 1985). Nevertheless additional, detailed, surface and subsurface geologic studies are needed in much of southern California to understand the relationship of geologic structures to earthquakes. Especially important are further studies of the Malibu-Santa Monica-Hollywood-Raymond fault zone which extends through highly developed Santa Monica, Century City, Beverly Hills, Hollywood, Pasadena, and Arcadia areas.
The damaging effects of the two Whittier Narrows earthquakes were very moderate when compared with the effects of the 1933 Long Beach and 1971 San Fernando earthquakes. The obvious major lesson learned once again is that older unreinforced multi-story buildings are very vulnerable to even moderate ground shaking. Such buildings had been described as a major earthquake hazard in the "seismic safety element" for the city of Whittier* (Leighton and Associates, 1974, p. 23-24). These kinds of buildings received extensive structural damage in the central Los Angeles area 10 to 12 miles west of the epicenter. The M 8.3 + earthquake assumed to occur on the San Andreas fault (Davis and others, 1985), and the many large aftershocks that presumably should follow, could cause many deaths to people working or living in such buildings.
* A seismic safety element includes the identification and appraisal of such seismic hazards an the susceptibility to ground shaking, and the susceptibility to ground rupture from faulting.
Photo 8. Small surficial slide, Turnbull Canyon area, city of Whittier, that apparently was caused by the earthquake. Such slides were sparse.
Some radio and television staff while reporting on air during the aftermath of the two Whittier Narrows earthquakes tended to speculate about factors involving the history and causes of earthquakes. For example, many historic earthquakes in southern California have occurred early in the morning. Some reporters gave the erroneous impression that there may be some scientific reason for their early morning occurrence. These reporters, however, failed to acknowledge that many historic earthquakes in southern California have occurred during other times of the day --- for example: 1812 Santa Barbara, 10:00 a.m.; 1893 Newhall, 11:40 a.m.; 1933 Long Beach, 5:54 p.m.; 1855 Los Angeles, 8:15 p.m.; and 1941 Santa Barbara, 11:15 p.m. Because many television viewers and radio listeners receive a major part of their knowledge of earthquakes during the aftermath, it is important that the information given to them is accurate.
The following people in the Los Angeles Regional office of the Division of Mines and Geology contributed to this article: Allan Barrows, Richard Greenwood, Venice Huffman, Richard Kaumeyer, Edmund Kiessling, Michael Perkins (volunteer), Shavonda Rhodes, and Jerome Treiman. Additional contributors included Clarence Allen of the California Institute of Technology, who provided preliminary seismic data on the October 1 earthquake; Edward Sprotte, a retired employee of the Division of Mines and Geology, who provided information on the Whittier fault; and Colleen Rice.
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Ziony, J. L., 1985, editor Evaluating earthquake hazards in the Los Angeles region --- an earth-science perspective: U. S. Geological Survey Professional Paper 1360, 505 p.