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 Montains.

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).