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    February 1, 1971
    The Sylmar Earthquake


California Geology, April/May 1971, Vol. 24, No. 4-5., Special San Fernando Earthquake Edition

THE 1971 SAN FERNANDO EARTHQUAKE

An hour before sunrise, at 6:01 AM, PST, on February 9, 1971, the San Fernando region was struck by one of the most devastating earthquakes in California history. Although the Richter magnitude of the tremblor was 6.6, ranking it as moderate to large, but not great, it shook a wide, heavily populated area, leaving death and destruction in its wake.

 As we go to press one month later, 65 lives have been charged to it, and damage has been estimated at more than half a billion dollars. It was California's third worst earthquake in terms of lives lost (exceeded by San Francisco, 1906 and Long Beach, 1933) and second in terms of property damage (exceeded by San Francisco, 1906).

 The greatest damage was in the San Fernando area, near the front of the San Gabriel Mountains, where three hospitals were badly damaged (one of them accounting for the greatest loss of lives). Freeway interchanges collapsed (killing two men in a pickup truck under one fallen overpass), reservoirs were in danger of imminent failure, forcing people living below them to evacuate, and houses and commercial buildings collapsed or caught fire.

 The earthquake was not, or should not have been, unexpected. In the southern California area--from Point Arguello to Nevada and from the Mexican border to Owens Valley--seismic records suggest that 37 shocks of this magnitude or greater might be expected in a century; indeed, one might expect two shocks with a magnitude of 8.0 or more.

 By the time the sun had risen, ten geologists from the Los Angeles District Office of the California Division of Mines and Geology were in the field to appraise the geologic effects and to search for further potential hazards created by the earthquake. They paid particular attention to hillsides that might give way in aftershocks, causing more havoc than already existed. They were supported by aircraft: a fixed wing chartered plane and a helicopter provided and manned by the U. S. Marine Corps. Peter Fischer, of Whittier College, had the foresight and was instrumental in making arrangements for Marine Corps support.

 When it was apparent that the landslide potential of the hillsides, though great, did not threaten any urban areas, the geologists turned their attention to the study of geologic effects--so often ephemeral--and to other geologic hazards that may have been developed or increased by the earthquake.

 A preliminary report of their investigations is presented in this magazine. Besides those credited as authors, virtually all staff members in the Los Angeles office participated in the study: Wilma Ashby, Pat Caldwell, George Cleveland, Bill Edgington, Jim Evans, Don Fife, Cathy Govaller, Cliff Gray, Ed Kiessling, and Paul Morton. Charlie Bishop came from San Francisco, as well as several staff members from Sacramento-Quint Aune, Chief Wes Bruer, Bob Matthews, and Bennie Troxel. Messrs. Gray and Caldwell went immediately to the Los Angeles District Office where they maintained constant contact with field, administrative, and disaster staffs.

 Three Division geologists had an especial interest in the area, as well as especial expertise. Gordon Oakeshott, Deputy Chief had mapped part of the San Fernando 15-minute quadrangle in doing research for his doctoral dissertation in 1936; he revised and extended the map for publication in 1958 as California Division of Mines Bulletin 172, Geology and mineral deposits of the San Fernando quadrangle, Los Angeles County, California. By that time, much of the area had been built over; new base maps were available, and many new roads provided road cuts to give new looks at the geology. The 1971 earthquake has enabled him to add another facet to the study of the area he now knows so well.

In 1962, the Division inaugurated a study of the frontal fault system along the San Gabriel Mountains. As one portion of the study, Dick Saul mapped in detail the southeast quarter of the Oat Mountain 71/2-minute quadrangle, through which the Santa Susana thrust fault cuts obliquely. Upon revisiting the scene of his earlier mapping, Mr. Saul was able to recognize that movement had taken place on the Santa Susana thrust, in the Bee Canyon area.

 In 1969, the California Division of Mines and Geology published Map Sheet 15, entitled Preliminary reconnaissance map of major landslides, San Gabriel Mountains, California, by D. M. Morton and R. Streitz. The landslides they mapped included those in the eastern half of the San Fernando quadrangle. Dr. Morton's paper herein describes an additional 1,000 landslides generated by the earthquake--most of them west of the area previously mapped, but many in Big Tujunga Canyon, the site of already mapped slides.

 On the day following the earthquake, State Geologist Wesley Bruer convened a meeting of more than 40 geologists and seismologists engaged in studying the earthquake, so that all investigative activities might be coordinated. Among those attending, in addition to Division personnel, were representatives from the California Institute of Technology, the U S. Geological Survey, the National Ocean Survey (a part of the National Oceanic and Atmospheric Administration), the University of California at Santa Barbara, the University of Southern California, California State College at Los Angeles, and the University of Washington; from several state agencies-the Division of Oil and Gas, the Division of Highways, the Department of Water Resources; from Los Angeles County and from Los Angeles City; as well as from several private consulting firms.

 The California Division of Mines and Geology is continuing to act as an information exchange center for those who have been and are engaged in geologic, se is mo logic, and geodetic investigations of the earthquake. This early and continuing exchange has added significantly to the effectiveness of the investigations.

It was an expensive and heartbreaking lesson, but California has learned something, and can take heart. Olive View Hospital, a county-owned structure, is such a lesson. Although the county engineer has rated the medical care and treatment center and the psychiatric unit as total losses, for an estimated $31 million dollars, 615 patients and 300 staff members were safely evacuated. Only three lives were lost: two were patients who died when forcibly cut off from their positive pressure breathing equipment, and one staff member was killed by falling debris outside the building. The structure had been built according to earthquake-resistant code provisions; mapped faults, as shown by Dr. Oakeshott on his San Fernando quadrangle, were considered in the engineering design. There was no movement on the faults near the hospital; damage to the building has been attributed to the intense shaking to which it was subjected.

 But what we have learned is far surpassed by our astonishing luck. Had shaking of the endangered reservoirs continued for 2 seconds more, it has been estimated that there would have been no time to evacuate those below. Had the earthquake hit at rush hours, when the streets and freeways were full of traffic; had it hit during school hours, when schools were filled with children--our grief might be much greater.

We must hurry to learn what we can, and to put to use what we know, while our luck holds.... M.R.H.

 


California Geology, April/May 1971, Vol. 24, No. 4-5., Special San Fernando Earthquake Edition


SEISMOLOGIC AND CRUSTAL MOVEMENT INVESTIGATIONS OF THE SAN FERNANDO EARTHQUAKE

By Roger Greensfelder

SEISMOLOGY

Fault mechanism and aftershocks

The epicenter of the main quake was in the vicinity of Magic Mountain (map, page 62), about 10 km (6 mi.) north-northeast of Sylmar [2,3]. No seismograph stations were close enough to the epicenter to permit reliable determination of the quake's focal depth ; however a depth of about 12 km (7 mi.) is considered reasonable. The thousands of after-shocks that were recorded in the first few days following the earthquake were scattered over an area extending from the main shock's epicenter south to Sunland and southwest to Chatsworth [2,4] (map, page 62). About two dozen portable seismographs were deployed throughout the aftershock region during the week following the earthquake. Data collected with these instruments will permit the precise determination of locations (geographic position and depth) and focal mechanisms of many hundreds of aftershocks. This information will help to determine the position and mechanics of faulting associated with the main shock.

Preliminary solutions of the focal mechanism prepared by the National Center for Earthquake Research of the U. S. Geological Survey and the California Institute of Technology [2,3] indicate that the San Fernando earthquake was generated by oblique-slip reverse faulting on a plane probably dipping 50 to 60 N and striking between N64W and due west. The hanging wall (upper San Gabriel Mountains block) moved southwesterly or up and west relative to the foot-wall (lower San Fernando Valley block); the ratio of reverse to left-lateral slip was between 1:1 and 2:1 (diagram, page 64). Fault rupture reached the earth's surface, and the observed surface displacements represent a continuation of reverse faulting along the south flank of the San Gabriel Mountains which began less than 2 million years ago [5].

On the basis of the above information, we can conclude that fault rupture began at a point (the hypocenter) about 10 km (6 mi.) NE of Sylmar and 12 km (7 mi.) deep, and progressed upward toward the south and southwest, apparently spreading out along several faults as it approached the surface as suggested by the presence of multiple, near-parallel fault traces in a zone about 1 km wide (map, page 76 and 77).

Local and regional seismicity

Seismic activity (seismicity) in the Newhall-San Fernando region, and throughout the western San Gabriel Mountains, has been remarkably low in comparison with the rest of southern California during recent times. A strain release map of southern California for the period 1934 to 1963 [6] shows that this particular area had seismicity equivalent to less than four magnitude-3 shocks per 100 square km; other surrounding areas have shown activity more than 1000 times as intense, chiefly in the form of a single earthquake of magnitude 7 or greater.

Only one strong, destructive earthquake is known to have occurred in historic time previously in the San Fernando-Newhall area. This took place in 1593 near Pico Canyon, about 5 miles southwest of Newhall, causing landslides, rock-falls, and ground fissures; in Newhall and Saugus, chimneys were knocked down, and an adobe house was destroyed [1,5]. The shock was felt strongly in Mojave, San Bernardino, and Ventura, lightly in Los Angeles and Santa Ana, and was perceptible in San Diego. An intensity f ("Intensity" refers to the strength of earthquake ground motion at a particular locality, and ratings are based on observable effects on man-made structures and the ground. It is not to be confused with magnitude, which is an instrvmental rating proportional to an earthquake's energy, and is independent of the point of measurement.) Modified Mercalli Scale) of VIII to IX was assigned to this earthquake by Townley and Allen, apparently on the basis of its ground effects and felt area, as the reported minor to moderate structural damage suggests intensity VII to VIII. Severe structural damage caused by the San Fernando earthquake corresponds to an intensity of IX, and locally X, and so it appears that the 1893 shock was the lesser of the two.

Thus the San Fernando earthquake occurred in an area which has had relatively low seismicity and was not caused by movement on a major, historically active fault, such as the San Andreas or one of its branches. In fact, portions of the fault associated with this shock were not previously mapped, although they might have been partly inferred from the abrupt, linear scarp along the southern base of the San Gabriel Mountains between Big Tujunga and Lopez Canyons (see map). Several other destructive California earthquakes have occurred on faults which were either unmapped or not known to be seismically active. In 1892, the towns of Vacaville, Dixon, and Winters in the Sacramento Valley were severely damaged by an earthquake on an unmapped fault. In 1954, Eureka suffered moderate damage from a magnitude 6.6 shock, also on an unmapped fault. And in 1940, the Imperial Valley earthquake (magnitude 7.1), which caused moderate to severe damage in several towns, took place on a fault which was unknown before it ruptured and broke the surface at that time. The Kern County earthquake of 1952 (magnitude 7.7) took place on a tentatively mapped fault (the White Wolf) which was not known or suspected to be seismically active; other potentially destructive shocks have occurred similarly.

It is therefore evident that faults whose existence or seismic activity is unknown can be the source of destructive earthquakes. Indeed, since the great 1906 earthquake on the San Andreas fault, many destructive earthquakes in California have occurred on such faults. This clearly points out the need for the identification of mapped faults, and of those whose existence is only suspected, which could give rise to destructive earthquakes or surface rupture in and near urban areas. Better, more comprehensive information on the accumulation of strain could provide a better estimate of the frequency and magnitude of earthquakes in particular localities and on particular faults than can be obtained strictly on the basis of historical seismicity. While the historical sample of earthquake occurrence in a large region (tens of thousands of square kilometers) may be an adequate index of its future seismicity, apparently small regions (several thousand square km or less) often have not exhibited a valid sample of their long term seismic activity during historic time. More detailed studies of geomorphology (geologic analysis of topography) may allow this record to be extended far back into prehistoric time.

Magnitude and maximum ground motion

Many workers have studied the relationship between earthquake magnitude and intensity of ground motion, based on both instrumental and non-instrumental data; the rate of attenuation of ground motion with distance as a function of magnitude is a matter of special interest to structural engineers. Because very few strong-motion accelerograph recordings have been obtained near earthquake epicenters in the past, say within 25 km (15 mi.), we have little information on maximum acceleration as a function of magnitude.

The accelerograph at Pacoima dam, located about 5 km (5 mi.) south of the epicenter of the San Fernando earthquake, recorded a maximum horizontal acceleration somewhat greater than that of gravity (1.05 g on the S16E horizontal component) [9]. This is the greatest acceleration ever recorded during an earthquake, and is from two to ten times that expected at the epicenter of a magnitude 6.6 shock. The Pacoima dam accelerograph rests on a reinforced concrete abutment founded in crystalline basement rock. Therefore, the accelerogram should closely represent hard-rock motion, which is typically smaller than that observed on alluvium or soft sediments. It has been previously estimated that a magnitude 6.5 shock should produce an acceleration of 0.1 g on "better ground" [30].

The Parkfield earthquake of 1966, which had a magnitude of 5.5, yielded a peak acceleration of 0.5 g at a distance of about 100 meters from the fault rupture near Cholame. Again, this acceleration is much greater than had been expected from a magnitude 5.5 shock. Apparently moderate earthquakes may result in quite strong and locally very damaging ground motion near their epicenters. In terms of potential damage, perhaps the chief difference between a shock of magnitude 6 and one of magnitude 8 lies in duration of shaking and size of the area affected, rather than in the intensity of ground motion very near the epicenter.

Structural damage

Death and injury in an earthquake are caused primarily by partial or total collapse of man-made structures. In the San Fernando earthquake, the number of deaths (65), of people sustaining serious injuries (over 1,000) and amount of major structural damage was almost entirely confined to portions of San Fernando and Tujunga Valleys, and the Newhall-Saugus area (map, page 62). Beyond these areas, damage was minor to moderate and mostly limited to old, unreinforced masonry structures as far away as downtown Los Angeles, 40 km (25 mi.) south of the epicenter.

Zones of extreme damage are closely correlated with those of surface faulting, lurching, compression-ridging, cracking, and other "permanent" ground deformation. Intense lurching, heaving, and cracking of ground generally parallel the contact of alluvium and bedrock along the southwestern base of the San Gabriel Mountains (see map, page 62).

At Olive View Hospital, a number of newly constructed reinforced concrete buildings were very seriously damaged or collapsed, causing the death of three persons. A two-story building "pancaked," its second floor dropping to ground level. Fortunately, no one was on the ground floor. Four five-story stairwell-wings pulled away from the main building, three of them toppling over (photo, on cover). Nevertheless, considering that several hundred people were inside the main building at the time of the earthquake, the building was relatively successful in terms of safety.

Forty-four people were killed at Sylmar Veterans Hospital when two unreinforced tile masonry buildings collapsed; these structures were built more than 40 years ago, before the general adoption of modern building codes which require earthquake-resistant design.

The majority of modern residential and commercial structures in the Newhall and northern San Fernando Valley areas escaped serious harm; however, it has been estimated that more than 1300 buildings and 1700 mobile homes suffered major damage. Old, unreinforced masonry walls, and new chimneys and concrete block fences were knocked down or cracked over a large portion of San Fernando and Tujunga Valleys, and in downtown Los Angeles. Had the earthquake occurred a few hours later, many people might have been seriously injured or killed by bricks falling into the street.

Schools built since the passage of the Field Act in 1933, which requires earthquake resistant design and construction of school buildings, performed well in the earthquake. However, a number of schools built before 1933 were hard hit, and in some cases had to be condemned.

New high-rise buildings in downtown Los Angeles and southern San Fernando Valley suffered no structural damage. However, this cannot be taken to be proof that these structures will not be damaged under maximum expectable earthquake ground motion at their critical periods of vibration. These periods are a function of a building's height and rigidity, and can roughly be said to fall in a range of from 1 to 5 seconds. The San Fernando shock did not produce very strong motion in this period range compared with that expected from a magnitude 7- to 8- earthquake. However, the numerous strong-motion accelerograph recordings obtained for the first time in many new high-rise buildings should be very useful in estimating their response to stronger ground motion.

The old, hydraulic earth-fill dam on Lower Van Norman Lake suffered major sliding on its upstream face, posing such a threat of failure that a large area below had to be evacuated for a four-day period. People were allowed to return to their homes only after the water level in the lake had been lowered and careful inspection had shown that immediate danger of failure had passed. The dam on Upper Van Norman Lake was also severely damaged, and it, as well as the lower dam, will have to be rebuilt.

Modern freeway roads and bridges in the Sylmar area were severely damaged. A number of bridge spans collapsed; one collapsing span killed two men in a pickup truck. Eight-inch thick concrete pavement slabs were compressed into ridges and thrust over one another, apparently as a result of land-shortening in a north-south direction. Lateral spreading affected some sections of freeway underlain by fill material. Practically all bridge collapses were in the Interstate Highway 5/210 interchange and pavement was ridged between there and the Interstate Highway 5/405 interchange, about 2 miles to the south. A fault disrupted the Foothill Freeway (1-210) just west of MacLay Street; preliminary measurements indicate 5 feet (1.5 m) vertical and 41/2 feet (1.3 m) horizontal (shortening) displacement across the main zone of rupture [12].

Utility lines--gas, water, sewer, telephone and electricity-were disrupted in the areas of most intense ground motion. Pipes failed where they crossed the zones of surface faulting.

Oil field facilities and related structures were relatively little damaged. There was minor damage to tanks, roads, pipelines, and a few wells in Aliso Canyon, Cascade, Castaic, New-hall, Newhall-Potrero, Oak Canyon, Placerita, and Ramona oil fields.

CRUSTAL MOVEMENT

Preliminary measurements of fault displacements across several traces indicate that the cumulative oblique reverse slip across the 1 km (0.6 mi.) wide zone exceeds 1 m (3 ft.). Left-lateral displacement appears to be smaller than vertical offset throughout most of the fault zone. This indicates that a western block of the San Gabriel Mountains moved south to south-westward and upward relative to the San Fernando Valley block (diagram, page 64).

Precise regional (geodetic) and local survey measurements are required to determine the magnitude and direction of crustal block movements. The City and County of Los Angeles, the U. S. Geological Survey (USGS), the National Ocean Survey (NOS), the California Division of Mines and Geology, and the State Division of Highways are engaged in triangulation, leveling, and laser ranging programs for this purpose.

In order to determine vertical movement, the City and County of Los Angeles will resurvey a first-order level line from the Los Angeles City Hall north to Newhall or Gorman, and another line east from San Fernando toward Pasadena. These lines were last surveyed in 1969, as a part of the Southern California Cooperative Level Net. In order to detect both initial and possible continuing vertical movements in the fault zone, the USGS is making repeated observations on several short level lines. Two lines are located in Lopez and Little Tujunga Canyons, and another goes through the town of San Fernando.

Horizontal movements will be determined by first-order triangulation and high-precision laser-ranging programs which are now underway. The area covered extends from the San Andreas fault on the north to the Santa Monica Mountains on the south. The NOS and County of Los Angeles are performing large-area triangulation, reinforced by Geodimeter (a laser ranging device) distance measurements on selected lines. A small trilateration network spanning the fault zone in the Sylmar-San Fernando area has been established and is currently being monitored by the USGS, using a Geodolite (also a laser ranging device). The California Division of Highways is re-observing triangulation networks which extend along Interstate Highways 5 and 210, and State Highway 14, from San Fernando Valley to the Newhall area.

A number of small survey figures were set up during the week following the earthquake in order to detect a possible continuation of fault movements, or creep, at the surface. At this writing, it appears that very little creep has taken place. At a site about 1 mile east of Lopez Canyon, vertical movement of about 5 cm (3 in.) was detected between February 11 and February 14; however, it is uncertain how much of this was due to fault creep [11] As yet, no significant movement at other sites has been reported.

It is of considerable interest to discover whether or not crustal movements accompanying the earthquake produced a measurable or significant strain change in the vicinity of the San Andreas fault from Gorman, 56 km (35 mi.) to the northwest, southeastward to San Bernardino. This portion of the San Andreas fault has apparently been "locked," showing essentially no seismic or aseismic slip since the great (magnitude 8+) 1857 Fort Tejon earthquake. It is believed that the fault slipped more than 6 m (20 ft.) and surface rupture extended more than 320 km (200 mi.) from Cholame Valley to Cajon Pass during that earthquake. Many geologists and seismologists believe that a great earthquake (magnitude 8+) is likely to occur on this portion of the San Andreas fault, and in the not too distant future-perhaps before the end of this century.

In consideration of this problem, the Division of Mines and Geology has reobserved 12 Geodimeter lines near or crossing the San Andreas fault between Gorman and San Bernardino. Eight of the lines form a small closed figure near Gorman, and their re-observation will allow calculation of the strain change in it since the last observation in 1966. The other four lines do not form closed figures, and re-observation of them will allow only rough estimation of strain change.

At this writing, the lengths of 10 of the above lines have been calculated from the field observations; however time has not permitted more than a cursory interpretation of the data. Nevertheless, it is significant that all lines except one are now longer than when last observed, irrespective of their azimuth. Three lines which cross the San Andreas fault between Little-rock, about 13 km (5 mi.) southeast of Palmdale, and San Bernardino were last observed in December 1970: one line (Tenhi-Ward) near Littlerock and another near Cajon Pass (Phelan-Sevaine) had become 2 cm (0.8 in.) longer when reobserved about 10 days following the earthquake; the line north of San Bernardino (Sevaine-Strawberry) showed no change. The change of +2cm on lines Tenhi-Ward and Phelan-Sevaine suggests regional dilatation (expansion) on the order of 1 part per million (ppm), and no movement on the fault. Another line (Sawmill-Thumb) which crosses and is nearly perpendicular to the San Andreas fault about 5 miles west of Lake Hughes is about 22 cm longer (1 ppm) than it was during the period 1960-1963. Preliminary data on five out of eight lines in the Gorman figure suggest dilatation on the order of 2 or 3 ppm since 1966.

On the basis of the above information, it appears that the San Fernando earthquake may have caused a small relaxation of stress in the region of the San Andreas fault between Gorman and Cajon Pass. The data do not indicate either slip or a significant change in shear strain on the fault.

 

REFERENCES

1. Allen, C. R., St. Amand, P., Richter, C. F., and Nardquist, J. M., 1965, Relationship between seismicity and geologic structure in the southern California region: Bulletin of the Seismologicol Society of America, vol. 55, no. 4, p. 782.

2. Lee, W. H. K., 1971, personal communication.

3. Nardquist, J. M., 1971, personal communication.

4. Allen, C. R., 1971, personal communication.

5. Oakeshatt, G. B., 1958, Geology and mineral deposits of the San Fernando quadrangle, California: California Division of Mines Bulletin 172, p. 92.

6. Allen et al., op. cit., plate 1.

7. Townley, S. D., and Allen, M. w., 1939, Descriptive catalog of earthquakes of the Pacific Coast of the United States, 1769 to 1928: Bulletin of the Seismological Society of America, vol. 29, no. 1, p. 92.

8. Perrine, C. D., 1894, Earthquakes in California in 1893, U.S. Geological Survey Bulletin 114. p. 13-16.

9. Hudson, D., 1971, personal communication.

10. Gutenberg, B., and Richter, C. F., 1956, Earthquake magnitude, intensity, energy, and acceleration (second paper): Bulletin of the Seismology Society of America, vol. 46, no. 2, p. 130.

11. Burford, R. o., 1971, personal communication.

12. Parmer, A., 1971, personal communication.

ACKNOWLEDGEMENTS

Thanks are due to Dr. Clarence Allen, Dr. Donald Hudson, and Mr. John Nordquist of the California Institute of Technology, and to Dr. Willie Lee and others of the U. S. Geological Survey, who kindly provided the basic seismological data presented in the report; also to Dr. Robert Burford and Dr. Robert Nason, who provided information on fault creep and displacements. Thanks are also extended to Mr. Al Parmer of the California Division of Highways for summarizing the damage to highways and bridges.

 

California Geology, April/May 1971, Vol. 24, No. 4-5. Special San Fernando Earthquake Edition

THE GEOLOGIC SETTING

By Gordon B. Oakeshott

Although data are incomplete and "conclusions" are certain to be modified later, the big picture of what happened, geologically, is emerging. It appears likely that a block of the western San Gabriel Mountains, including a surface area of at least a hundred square miles, moved relatively upward and southwestward several feet-perhaps about 4 feet vertically and a couple of feet horizontally. Movement originated at a focal depth of about 12 km (7 to 8 miles), with the epicenter about l0 km (6 miles) north-northeast of Sylmar. Displacement probably took place along a highly irregular fault surface dipping about 40 - 60 northward from discontinuous surface fault ruptures appearing as several traces of very much disrupted soil and sedimentary rock striking about N 80 W in the northern part of the San Fernando Valley. Thus, this earthquake originated in displacement along a reverse or thrust fault--like Arvin-Tehachapi in 1952--in contrast to all other known California earthquakes which have resulted from movement on strike-slip or normal faults.

How does this behavior fit into what we know of the geology of the area? What has been the history of development of this great mountain range which lies in the central part of the Transverse Ranges of southern California? Particularly, what has been the late history of faulting, folding, and mountain-building in this region?

The rock

Rock formations in the San Gabriel Mountains include most major rock types in great variety, ranging from Precambrian igneous and metamorphic rocks to Holocene (Recent) alluvium. The Precambrian crystalline rocks consist principally of another site and related types, radiometrically dated as 1.2 billion years old, which have intruded the 1.4 billion-year-old Mendenhall Gneiss. The Precambrian rocks are found mainly north of the San Gabriel fault. Mesozoic granite rocks, at least some of which have been dated at about 70 million years old, crop out both north and south of the San Gabriel fault. South of the fault they carry some inclusions of Paleozoic(?) igneous and metamorphosed sedimentary rocks.

The crystalline rocks which form the central core in the highland part of the San Gabriel range are flanked on the north, west, and south by overlying younger Tertiary sedimentary and volcanic rock formations. Small bodies and fragments of Paleocene (60-70 million years old) marine sandstone and conglomerate have been sliced into the San Gabriel fault zone. Paleocene to middle Eocene marine sandstone and conglomerate occur at the extreme western end of the range.

The later Tertiary and Quaternary sedimentary rocks around the western San Gabriel Mountains were deposited in two principal basins, both representing arms of the eastern end of the great Ventura Basin: the Soledad basin northeast of the San Gabriel fault, and the San Fernando basin a few miles southwest of that fault. In the Soledad basin, late Eocene to late Miocene time is represented by about 14,000 feet (4200 m) of non-marine and volcanic rocks of the Vasquez, Tick Canyon, and Mint Canyon Formations. Remnants of the continental and nearshore middle Miocene Topanga Formation are found in the San Fernando basin. The marine middle to upper Miocene Modelo Formation is thickest in the Little Tujunga syncline in the San Fernando basin but is also present overlying the Mint Canyon Formation in the Soledad basin. Some of the fault traces of the February 9th earthquake appear as bedding plane faults in sandstone and shale of the Modelo Formation at the northern margin of the San Fernando basin. Lower Pliocene marine beds and upper Pliocene shallow-water marine to brackish-water beds are found in both basins. Continental gravel and coarse sandstone of the lower Pleistocene Saugus Formation are very widely distributed but the formation reaches its maximum thickness of over 6,000 feet on the south flank of the range in upper Lopez Canyon. Coarse reddish-brown breccia of the middle Pleistocene Pacoima Formation underlies the northern margin of the San Fernando Valley in the area from the Olive View Sanatorium to the Veterans Hospital. A succession of nearly flat-lying Pleistocene river terrace gravel s flanks the mountains. Quaternary sediments (Alluvium, Terrace deposits, and Pacoima and Saugus Formations) reach extraordinary depths along the northern margin of the San Fernando basin: an exploratory petroleum well drilled about half a mile west of the Olive View Sanatorium revealed that Quaternary(?) gravels reach a depth of at least 12,000 feet (4000m)!

 

The structure

Structural features of the western San Gabriel Mountains are dominated by the near-vertical, right-lateral, strike-slip San Gabriel fault, which extends obliquely across the mountains on a strike of about N65 W. The pattern of faulting comprises (1) a principal series of steeply north-dipping fault planes along the San Gabriel fault; (2) north of the San Gabriel fault, a series of complementary left-lateral strike-slip faults trending approximately N65 E; and (3) in the block south of the San Gabriel fault, the series of discontinuous north-dipping reverse faults of the Santa Susana and Sierra Madre fault zones. The third series includes the fault ruptures of the February 9th earthquake.

The most prominent fold structure of the Soledad basin is the northeast striking, west-plunging Soledad basin syncline. In the San Fernando basin, the dominant fold is the Little Tujunga syncline whose axis lies close to the northern margin of the Quaternary sedimentary rocks. The axis of this fold closely parallels the general trace of the Sierra Madre fault zone, following it with a west-northwest trend from Tujunga to the Veterans Hospital at Loop Canyon where it seems to have been overridden by crystal-line rocks along the Hospital fault. Continuing westward, the synclinal axis turns southwest, paralleling the northeastern end of the Santa Susana fault zone.

The Sierra Madre reverse-fault zone

The Sierra Madre fault zone comprises the series of discontinuous reverse faults extending about 12 miles from the northeast end of the Santa Susana fault on the west to the Rowley fault across Big Tujunga Canyon on the east. These are accurate, convex-southward reverse faults which separate the pre-Tertiary crystalline rocks on the north from the Tertiary and Quaternary sedimentary formations on the south. The faults are discontinuous, with dips ranging from 150 to vertical; all dip northward with the crystalline rocks thrust upward toward the south over sediments as young as the mid-Pleistocene Pacoima Formation. Displacement has been essentially of the dip-slip type and has been calculated to range from 200 to 4,000(60 - 1200m) feet on individual faults in the zone. Although the Pacoima Formation has clearly been both folded and faulted, no positive viled evidence was found to indicate that latest Pleistocene terrace deposits were offset.

Mid-Pleistocene to present mountain building

 

The mid-Pleistocene orogeny was the major event in the building of the modern San Gabriel Mountains. At that time the lower Pleistocene Saugus Formation and all older formations were intensely folded by north-south directed compressional forces. Large movements-probably both vertical and horizontal-took place in the San Gabriel fault zone; the Sierra Madre zone of reverse faulting was developed; initial movements on some faults, and renewed movements on others, took place along a system of northeast trending faults in the north block of the San Gabriel fault zone; and the central crystalline rock block was elevated several thousands of feet.

Continuation of vertical uplift is indicated by the development of very steep-sided canyons like Big Tujunga and Pacoima, as much as 3,000 feet deep, which have cut head-ward into the pre-Saugus surface of erosion but have not yet destroyed it. Essentially the same pattern of uplift appears to be continuing today, as evidenced by the February 9th fault movements and earthquake!

REFERENCES

Hill, Mason L. 1930. Structure of the San Gabriel Mountains north of Los Angeles, California: University of California Department of Geological Sciences Bulletin, vol. 19, pp. 137-170.

Jennings, C. W. and Strand, R. G. 1969. Los Angeles sheet: Geologic Atlas of California: California Division of Mines and Geology.

Oakeshott, G. B. 1958. Geology and mineral deposits of San Fernando quadrangle, Los Angeles County, California: California Division of Mines Bulletin 172, 147 pp.

 


California Geology, April/May 1971, Vol. 24, No. 4-5.  Special San Fernando Earthquake Edition

SAN ANDREAS BLAMED FOR SAN FERNANDO QUAKE

A 50-mile bend in the San Andreas fault north of the San Gabriel Mountains was ultimately responsible for the destructive San Fernando earthquake of February 9, even though there apparently was no movement along the big fault.

This was the opinion expressed recently by Don Anderson, director of the Seismological Laboratory of the California Institute of Technology.

"The bend tends to block and jam the general northwesterly movement (at the rate of about 2 inches a year) of that part of California that lies west of the San Andreas fault in relation to the rest of the state east of the fault," Dr. Anderson said "The fault runs in virtually a straight line both north and south of the ben& In those areas this general northwesterly movement is punctuated by occasional horizontal slipping along the San Andreas and its associated faults, accompanied by earthquakes.

"But near the bend the horizontal slipping gets hung up. Compression builds up and instead of horizontal movement you have overthrust faulting in that region, with land thrusting over land along fault breaks, triggering earthquakes. The uplifted land eventually forms mountains."

During the February 9 earthquake, the San Gabriel Mountains back of San Fernando were lifted several feet. There was considerable thrust faulting in the area, just as there was earlier during the 1952 Tehachapi and Bakersfield earthquakes.

"I think this is the way the land west of the San Andreas fault gets around the bend - by overthrusting and making mountains," Dr. Anderson said.

The San Andreas fault slices through western California for more than 600 miles. It extends virtually in a straight line southeastward from the Mendocino County coast to the southern San Joaquin Valley, where the jog occurs, then extends southeast again along the north flank of the San Gabriel Mountains. Branches of it eventually reach the Gulf of California.

"Although the bend tends to build up compression in much of southern California, the San Fernando earthquake decreased the overall compressional strain, but not very much," the geophysicist said.

Records of Caltech strain-measuring instruments at Isabella Lake and at the Nevada (atomic) Test Site showed that the earthquake did relieve compression over a wide area. Rocks at IsabelIa expanded about one millionth of an inch between the piers of a quartz strain measuring instrument. The expansion was much greater in the vicinity of the 'quake's epicenter.

Under the widely accepted geological theory of continental shift the San Andreas fault serves as part of the border of two great land masses, or plates. The theory says that the earth's land masses have been moving apart since the time when they were all in one piece. Great plates that include entire continents, and more, are moving over a plastic layer.

The massive drifting, which has been going on for millions of years, is related to the upwelling of molten rock beneath the great mid-ocean ridges. The material moves up vertically, solidifies and then spreads out to form the ocean floor, moving laterally away from the ridges as huge plates.

When two plates meet head-on, one usually is driven under the other. As a result, mountains form, deep oceanic trenches exist, the earth quakes, and volcanoes erupt. In some places, the two plates slide horizontally against each other.

The San Andreas fault and the Gulf of California are thought to separate two major plates. One extends from the mid-Atlantic westward to the San Andreas. The other extends westward from the San Andreas to the Asiatic shoreline. Part of California is on one plate, and part of it is on the other....Caltech


California Geology, April/May 1971, Vol. 24, No. 4-5.  Special San Fernando Earthquake Edition

GEOLOGIC SURFACE EFFECTS OF THE SAN FERNANDO EARTHQUAKE

By J. E. Kahle, A. G. Barrows, F. H. Weber, Jr., and R. B. Saul

The San Gabriel Mountains rise for thousands of feet above heavily populated Los Angeles and the northeastern part of the San Fernando Valley. Although the abruptness of the mountain front and the ruggedness of the topography are clear physical indications of the youthfulness of the San Gabriels, much more definitive evidence of the recency of uplift that created the range can be found along its southern edge, especially in the walls of deep stream canyons. There, the ancient rocks that comprise the bulk of the mountains overlie very young alluvial fan deposits composed of debris derived from the ancient rocks. These are exposed thrust faults: surfaces along which older rocks have ridden over the young fan deposits here dipping north under the range. The so-called "frontal fault" system along the southern boundary of the San Gabriel Mountains is a complex of several interrelated thrust faults.

These steep, high mountains have been raised by earth processes that have included innumerable intermittent movements along faults during the past few million years. Because movement along faults is considered to be one of the major causes of earthquakes, the dissimilarity of the rocks now juxtaposed across the faults implies that there were unnumbered earthquakes during the long span of time it took to lift up the mountains. Even during the time represented by recorded history in southern California, numerous strong tremors have jolted the land and terrified and perplexed the inhabitants. The San Fernando earthquake of February 9 is only the latest step in the continuous process of elevating these mountains.

Although the earthquake brought tragedy, it also provided an impressive amount of information about the earth-the study of which will help expand and refine man's knowledge of earthquakes. While seismologists have been busy registering and analyzing the data they can derive from deep within the earth's crust, geologists have been observing and recording the surface geologic effects before man or nature can erase or obscure them. These surface effects include surface faults, ground breakage other than faults, slope failures such as landslides or rockfalls, and shattered ridge tops. They are discussed and illustrated on the following pages.

Surface faults

Surface breaks caused by faulting during the earthquake extended from the Bee Canyon area of the Santa Susana Mountains eastward across the San Fernando-Sylmar area in the north San Fernando Valley to Big Tujunga Wash in the Sunland area, a distance of about 121/2 miles (20 km). Most of the breaks are surface expressions of thrust faulting, whereby land on the northern sides of the breaks was lifted above that on the south and shoved or thrust obliquely toward the southwest. We do not know whether the north side was truly the block that moved or whether the southern block dropped, or both moved, as the field evidence shows only that the north or San Gabriel block is now higher and farther southwest, relative to the south or San Fernando Valley block.

The most spectacular breaks, because they are in populated areas and helped to cause great damage, extend from near the intersection of Hubbard and Glenoaks Boulevard in Sylmar eastward across a residential neighborhood of San Fernando (see photos, page 74), across the new Foothill Freeway (see photo, page 68), to the San Fernando Industrial Park. The breaks continue eastward along Foothill Boulevard, where just west of Paxton the northeast sidewalk and curb were lifted abruptly several feet relative to the pavement in front of the Foothill Nursing Home (photos, page 85). The breaks continue eastward and are well exposed across the mouths of Lopez, Kagel, and Little Tujunga Canyons (localities 12, 15, and 16 on map, pages 76 and 77).

Less well exposed breaks continue from Little Tujunga Canyon to Big Tujunga Wash (see photos, pages 78 and 79), across Oro Vista Avenue (locality 24). Between Little Tujunga and Big Tujunga Canyons, breaks can be traced back into the hills, where they form prominent scarps in Oliver and Schwartz Canyons (localities 19 and 20, also photos on pages 68 and 78).

The most westerly break noted is less spectacular because it is in an area in which construction has not yet started-in undeveloped Bee Canyon west of northern Balboa Boulevard (locality 1; see also article by R. B. Saul, page 83). Movement along this break was left lateral, and occurred along the Santa Susana thrust fault, an important fault of the Santa Susana Mountains.

A break in the Mission Wells area of Sylmar strikes generally northeast, toward the major zone (locality 8); it also has left-lateral horizontal offset, and seems to dip moderately to the northwest. Houses and a trailer park were damaged along its extent.

A type of fault in which the dip and strike of the fault parallel the dip and strike of enclosing strata is known as a bedding-plane fault. A fault of this type is to be seen in Lopez Canyon, where a very prominent, partly overhanging 32-inch (80 cm) scarp was formed, which strikes east and dips north, as does the bedding of the sedimentary rocks (locality 11 and photos, pages 78 and 79). Another such fault transects Rajah Street (locality 9), near the Sylmar Veterans Hospital; it dips 700 to the north, and is geologically notable because it is 2 miles north of the major zone of surface breaks. The height of the south-facing fault scarps averages about 18 inches (0.5 m) in the region, although in places (localities 13, 22, and 23) it exceeds a yard (1.0 m). The scarps of the thrust fault are asymmetrical compression features that resemble, in many places, a wave breaking upon a beach. In other places, especially where the scarps cross grassy fields, a feature resembling a mole track has developed.

Other typical features along the scarp are overlapping or tucked under slabs of pavement or, more commonly, clumps of sod that were overridden or overturned during the south-westward thrusting of the upper block. Determinations of the precise amount of shortening of the earth's crust, implied by compressional features, can only be obtained from geodetic and careful local surveying; however, because there were orange groves at Middle Ranch (localities 15 and 16) near the mouth of Little Tujunga Canyon, the approximate amount of shortening across the fault could be measured. A fault scarp transected several regularly spaced rows of trees. The trees on opposite sides of the trace of the thrust fault are now closer together than they were originally as indicated by the other rows of trees. The average shortening, derived from the measurement of five rows which were crossed by the fault, is 3.6 feet (1.1 m).

The dip of the faults at the surface, where measured, ranged from 10 at locality 14 to 60 at locality 23. The low-angle dip of much of the main thrust fault can be observed where the trace of the fault curves upstream across stream canyons (localities 13,14, and 10).

Multiple scarps may be seen in Little Tujunga Canyon (locality 17) and elsewhere. Those in the alluvium of Big Tujunga Wash (see map) were first noted by Richard Cook, Jr., and Mark H. McKeown, geologists with the Metropolitan Water District of Southern California, who had previously mapped the Lakeview thrust fault in this vicinity (locality 18). Movement of rocks above the Lakeview thrust triggered numerous landslides along the trace of the fault. They may be seen in the hillslopes north of Foothill Boulevard near Oliver and Schwartz Canyons.

Some types of faults other than thrust faults were active during the earthquakes. At several places (localities 4 and 7), geologists noted high-angle faults with small displacement that cut across bedding.

Cracks in the ground are abundant at many places within the area of most intense shaking (see map). Most of the fractures, called lurch cracks or simply fissures, are not the direct result of surface faulting. They are not faults marking displacement along rock masses, but are instead formed when weak or unconsolidated earth material is subjected to intense shaking, and is incapable of responding elastically. Others are at the boundaries of contrasting surface materials which respond differently to ground motion. It is possible that some cracks may represent the surface expression of near- surface faulting, masked by a soil or alluvial cover.

Tension cracks (sometimes called "openings") are ubiquitous within the first few hundred feet to the north of the zone of thrust fault scarps. They are also to be found in areas that are away from zones of unambiguous surface faulting, where many of them, as well as accompanying compression ridges ("closings") and buckled and broken sidewalks and pavements, exhibit either a right- or left-lateral horizontal component of offset.

Except for a predominant left-lateral component of offset in the zone of thrust faults, the sense of offset across cracks appears to be controlled by local conditions. For example, the general pattern for groups of cracks in the vicinity of the Los Angeles County Juvenile Hall is right-lateral offset across the western ones and left-lateral offset across the eastern ones (locality 6), suggesting southwest movement of the land between the groups of cracks. The abundant tension cracks and compression ridges on the grounds of the Olive View Hospital, while exhibiting both right- and left-lateral offset, reflecting control by the nature of the ground surface and by local geologic conditions on the sense of the movement.

Slope failures

Landslides and rockfalls triggered by the San Fernando earthquake are both common and widespread in the foothill areas. A discussion of these features, based upon interpretation of aerial photographs, is given elsewhere in this issue (see page 81).

Shattered ridge tops

Some ridge crests in the foothills have a striking "exploded" appearance. Such shattering is most common along those crests underlain by sandstone and conglomerate strata of Tertiary and Quaternary age (see cross section) that have a soil cover commonly less than 2 feet (2/3 m) thick, and probably is the result of intense shaking.

The surface of the ridges resembles plowed fields where the soil looks as if it had been heaved upward by a sharp blow from below. Small chunks of pulverized subsoil are exposed between the larger tilted blocks (see photo, page 67). These features are not landslides. The tops of blocks involved in landsliding generally tilt toward the centerline of the ridge; not so the blocks of soil cover on shattered ridge tops-they tilt invariably away from the centerline. Good examples of shattered ridge tops may be seen near Grapevine Canyon (locality 5); west of Balboa Boulevard; in the area between the junction of Lopez and Indian Canyons east of Bartholomaus Canyon; along the hills north of Olive View Hospital; and along the ridges underlain by conglomerate above Oliver and Schwarts Canyons (locality 21).

 


California Geology, April/May 1971, Vol. 24, No. 4-5.  Special San Fernando Earthquake Edition

Seismically Triggered Landslides Above San Fernando Valley

By Douglas M. Morton

The interpretation of aerial photographs has indicated that more than 1,000 landslides, distributed over a 100-square-mile (250 sq. km) portion of the hilly and mountainous terrain above San Fernando Valley were triggered by the earthquake and its aftershocks. Overall distribution of the landslides was determined primarily by the intensity of ground shaking; the local density of landslides reflected differing local geology. Where ridge tops have been severely fractured, abundant landslides may develop later when saturated with water.

The photos used in this preliminary study were a set of U. S. Geological Survey black-and-white aerial photographs taken on February 9 by American Aerial Surveys, Inc. (scale 1:10,000) and small scale color photographs and infrared photographs taken on February 9 by aircraft assigned to the National Aeronautics and Space Administration, and color photographs taken February 18 by the U.S. Air Force, scale 1:20,000. Photo coverage was adequate for the area (see figure, page 80), with the exception of the northeast part, where clouds largely obscured the ground.

Landsliding took place in response to the shaking of the ground during the primary earthquake shock, although some rockfalls that continued intermittently for several days along steep canyon walls (e.g., Pacoima and Little Tujunga Canyons) may have been, in part, triggered by aftershocks. Landslides were concentrated in a 100-square-mile (250 sq. km) area of foothills and mountains above San Fernando Valley south-southwest of the epicenter of the main shock, adjacent to the zone of surface faulting and within the area of principal aftershocks.

Approximately 1,000 landslides, ranging in length from about 50 feet to more than 1,000 feet (15 to 300 m) were mapped from the photographs and a few were checked in the field. One of the larger landslides, 600 feet (200 m) across is shown in the photo on page 84. Rockfalls, soil falls, debris slides, debris avalanches, and slumps were the principal types of landslides triggered by the earthquake. Surficial debris slides (6 inches to 3 feet 0.2 to 1 m-thick) and avalanches are probably the most widespread and common type of failure and were especially pronounced in areas underlain by sedimentary mlrs. Rockfalls were common on steep canyon walls cut in well fractured basement rock, but also occurred in particularly resistant sedimentary beds which formed a cliff. Soil falls occurred mainly on the steeper faces of recent stream terrace deposits along major drainages. Slumps were largely limited to reactivation of older, pre-earthquake, slumps.

Landslide distribution was controlled primarily by the intensity of ground shaking. Within the area of intense shaking, variation in landslide density reflects local geologic conditions, such as type of rock units and degree of fracturing, including pre-existing landslide deposits and fault zones. As might be expected, very steep areas generally produced more landslides than more gentle topography.

The relation of the amount of landsliding to the intensity of shaking is particularly clear in the area underlain by eastward-striking Pico, Modelo, Towsley, and Saugus Formations west of San Fernando Valley. There, the number of landslides decreases markedly along strike westward from the valley away from the areas of intense shaking. The control of landslide distribution by rock units is best seen in the vicinity of Lopez and Bartholomaus Canyons (see figure, page 82).

Landslides were also common in man-made excavations. Numerous roads were blocked by landsliding in cut slopes, and innumerable rock falls occurred in road cuts throughout the San Gabriel Mountains.

No new large rotational or complex landslides, such as are characteristic of the San Gabriel and Santa Susana Mountains, were triggered in this earthquake. Apparently different circumstances have been responsible for slides of these kinds in the past. There are however, features along the valley margin near the San Fernando Ranger Station that may be indicative of incipient landsliding, perhaps related to the numerous fractures in the hills above the ranger station. If this is, or becomes a slide, it will be a large one.

The ground was fractured over a wide area, as shown in the figure on page 80. Ridge tops were shattered, as discussed on page 79; it is possible that, when saturated with water, these fractured ridge tops may give rise to more sliding than took place during and immediately after the earthquake.

 


California Geology, April/May 1971, Vol. 24, No. 4-5.  Special San Fernando Earthquake Edition

Effects of the San Fernando Earthquake in the Oat Mountain Quadrangle

By R.B. Saul

Detailed geologic mapping in the southeast quarter of the Oat Mountain quadrangle was a part of the California Division of Mines and Geology's program of urban mapping between 1966 and 1968, in cooperation with the Los Angeles County Flood Control District and the Los Angeles County Engineer. This work revealed details of a segment of the Santa Susana thrust, a fault along which older rocks are known to override Quatemary alluvium. Within the quadrangle, disruption of the ground surface in response to the earthquake of February 9, 1971 was confined mainly to a zone roughly 2 miles wide along the eastern boundary of the quadrangle (a line roughly coincident with Balboa Boulevard) from Rinaldi Street on the south, north to about San Fernando Pass.

Across most of the southeast quarter of the Oat Mountain quadrangle, the complexly imbricate surface zone of the Santa Susana thrust trends due east. In general, the sense of movement is from north to south. Furthermore, subsurface data indicate that this thrust is folded along an east-west axis. West of Bee Canyon no element of this thrust appears to have moved during the recent earthquake. East of Bee Canyon tectonic and/or lurching motion appears locally to have been coincident with or closely parallel to the sole of the Santa Susana thrust zone as previously mapped and may represent a local deflection of energy along existing planes of weakness. Where the base of the Santa Susana thrust crosses the road to St. Vincent de Paul Camp (locality 1) left-lateral movement offset a line on the pavement about 1 foot (30 cm), with no visible vertical component of movement. To the west of this road the trace of ground movement turns northwest into Bee Canyon, athwart the zone of the Santa Susana thrust, and appears to be absorbed along the strike of the bedding in the sedimentary rocks of the north wall of Bee Canyon. Northeast of St. Vincent De Paul Camp road, ground breakage follows the trace of the sole of the Santa Susana thrust (see photo, page 83) along a discontinuous and locally ambiguous series of breaks as far as the large road cut on Balboa Boulevard north of Van Norman Lake (localities 2 and 3).

In low-lying areas, surface cracks appear to be largely the result of lurching or settling of alluvium. In hilly areas, effects range from minor lurching of fills and soils to intensely shattered soil, landslides and rock falls. The most severe effects are principally confined to the square mile surrounding the mouth of Bee Canyon. Here, soil and weathered rock debris on ridge crests lies in cracked and jumbled disorder as though it had been heaved, and the flanks of most ridges are scarred by shallow landslides. In the Bee Canyon and Cascade Oil Field area, rocky ridges, road cuts, and the steep cliff along the north side of the canyon yielded rock falls.

 



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