SURFACE FAULTING ALONG THE NEWPORT-INGLEWOOD ZONE OF DEFORMATION

By

PAUL D. GUPTILL and EDWARD G. HEATH, Geologists

Woodward-Clyde Consultants

INTRODUCTION

Faults of the Newport-Inglewood zone of deformation are predominately defined in the subsurface from oil-well data and ground-water data. Very little geologic evidence of surface faulting has been found within the zone and very few instances of documentation of surface faulting exist. Even following the 1933 Long Beach earthquake (ML 6.3), no evidence of surface faulting was found or reported. The authors have recently discovered evidence of surface faulting along the North Branch of the Newport-Inglewood zone at west Newport Mesa. There, the age relationships of faulted geologic units and man-made fill indicate that the surface faulting occurred very recently, probably during the 1933 earthquake.

REGIONAL GEOLOGY

The Newport-Inglewood zone of deformation is one of several, large predominately right-lateral strike-slip fault zones that parallel the San Andreas fault in southern California. The Newport-Inglewood zone of deformation has been intensely investigated in the subsurface since the early 1920s by the petroleum industry, which referred to it as the "Newport-Inglewood Uplift." Barrows (1974) refers to it as the Newport-Inglewood "structural zone." We have chosen to refer to it as the Newport-Inglewood zone of deformation in agreement with Hill (1971).

The zone is most popularly characterized as a classic wrench fault as defined by Wilcox and others (1973) and Harding (1973). The wrench-fault model includes a deep-seated strike-slip fault in the basement rocks that deforms overlying sedimentary basin deposits. Slip on the deep-seated fault causes a series of en echelon folds and faults in the sedimentary cover. Such a structural configuration is seen at the surface and in the subsurface along the Newport-Inglewood zone of deformation (Figure 1).

SEE FIGURE 1

Figure 1. Newport-Inglewood zone of deformation.

From north to south, the style of faulting in the sediments along the Newport-Inglewood zone of deformation varies from right-slip, with a moderate component of normal dip-slip associated with folding in the Baldwin Hills, to cross-trending short reverse faults in the Rosecrans and Dominguez Hills, and to several en echelon strike-slip fault segments in the Long Beach to Newport Beach area (figure 1). The left step from the Seal Beach fault to the Cherry Hill fault at Signal Hill is an area of compression between two en echelon right-lateral strike-slip fault segments, demonstrated by Segall and Pollard (1980).

The June 1920 Inglewood earthquake (ML 4.9, Richter, 1970) and the 1933 Long Beach earthquake (ML 6.3) are clear examples of the destructive potential of the Newport-Inglewood zone of deformation. The Long Beach earthquake resulted in 115 deaths and about $40 million of damage in 1933 (Sherburne, 1981). Since 1933, the population density has increased dramatically along the fault zone and the death toll could be significantly higher in the event of a repeat earthquake of a similar magnitude to the 1933 event.

The Federal Emergency Management Agency (1981) recently published results of their assessment of potential earthquake hazards in California. The results of their study show that the Los Angeles area could suffer greater property damage, injury, and loss of life from an earthquake on the Newport-Inglewood zone of deformation than from the "big quake" on the San Andreas fault system. This condition stems from the fact that the Newport-Inglewood zone passes through the heavily developed western edge of the Los Angeles Basin, whereas the San Andreas passes 35 miles northeast of Los Angeles. Thus, the Newport-Inglewood zone of deformation should be a high priority for city planners, and state and federal agencies in anticipating earthquake hazards.

FAULTING POTENTIAL

A recent study of the Newport-Inglewood zone of deformation (Woodward-Clyde Consultants, 1979) has helped in defining the earthquake hazard potential. The results of this work demonstrate that the geologic rate of strike-slip has been relatively constant at approximately 0.5 mm/yr since late Miocene and early Pliocene in Long Beach, Seal Beach, and the Baldwin Hills. The ratio of horizontal slip to vertical slip is on the order of 20 to 1, where folding is not a major contributor to the vertical component. First motion studies of the 1933 earthquake records, collected worldwide, indicate essentially pure right slip on a northwest trending slip surface. From the seismic moment calculations of the 1933 earthquake, the average slip on the fault is estimated to have been approximately 31 to 46 cm in the subsurface (Woodward-Clyde Consultants, 1979).

These data are valuable for characterizing future earthquakes, possible recurrence intervals, and the earthquake potential of the zone. However, more near-surface faulting data are still needed to fully define the prehistoric earthquake activity.

Limited data regarding surface faulting have been developed during studies of the Baldwin Hills Reservoir failure and during investigations within the Alquist-Priolo Special Studies Zones. Presently, several sections of the Newport-Inglewood zone of deformation are included in the Special Studies Zones. However, from Huntington Beach Mesa southward, the Newport-Inglewood zone has not been designated as part of the Special Studies ones mainly because of the lack of evidence for faulting in young sediments, (Hart, 1980).

SEE FIGURE 2

Figure 2. The study area at Newport Mesa showing the locations of observed faulting and sites of detailed studies (sites 1 and 2)

The Baldwin Hills area has several well developed fault scarps; surface faulting has been documented on subsidiary faults, which experienced slip leading to the 1963 Baldwin Hills Reservoir failure (Kresse, 1966). Unfortunately the surface faulting in the Baldwin Hills is closely associated with surface subsidence effects making assessment of the tectonic effects very difficult.

It is our understanding that a study under the Alquist-Priolo Act was recently completed and was successful in locating evidence of faulting at Landing Hill in Seal Beach. The discovery of near surface faulting at Seal Beach demonstrates that surface faulting along the Newport-Inglewood zone does exist and that other sites could yield valuable information.

NEWPORT MESA FAULTING

The authors have recently completed studies of the Newport-Inglewood zone of deformation where the North Branch crosses west Newport Mesa (figure 2) where offset terrace deposits and surface soils can be observed in several outcrops. The most obvious and best-known exposure of offset terrace deposits is at the intersection of Pacific Coast Highway and Superior Avenue (photo 1), where the terrace deposits are faulted down on the west against shales of the Monterey Formation. Unfortunately, at this location most of the terrace deposits and all of the soils have been removed by grading and excavation. Offsets of the terrace deposits were observed in at least seven localities along the previously mapped subsurface trace (figure 2). Two of those exposures (sites 1 and 2) were cleaned by hand and documented in detail. The strikes of individual breaks observed are about 10 to 15 degrees west of north, slightly off trend from the 40 degrees west of north for the trend of the North Branch fault. This relationship is suggestive of a series of en echelon surface breaks overlying the relatively continuous lateral fault in the near surface determined from oil-well data of Hunter and Allen (1956).

SEE PHOTO 1

Photo 1. View of south-facing road cut at the intersection of Pacific Coast Highway and Superior Avenue in Newport Beach. Terrace deposits (Qt) on left are faulted against Monterey shale (Tm) on right. Arrows indicate that fault trace, location 3.

NEWPORT MESA GEOLOGY

Newport Mesa is a Pleistocene wave-cut bench in Miocene and Pliocene marine shale overlain by Pleistocene marine terrace deposits and fluvial deposits. An assemblage of Pleistocene megafossils is usually observed at the base of the marine terrace sediments, and assigned to the substage 5a (Shakelton and Opdyke, 1975) high stand of sea level, approximately 120,000 years B. P. (Wehmiller and others, 1977). The terrace is traceable from south of San Onofre northward almost continuously to Dana point. From Dana Point to Newport Beach the terrace is traceable but is less continuous.

The terrace bench and associated fossil assemblage is present in the study area at west Newport Mesa. Near Superior Avenue a wave-cut bench on Monterey shale can be observed but farther to the west the bench plunges beneath the cliff exposures. Several beds of Pleistocene megafossils are interbedded with marine terrace deposits and possible fluvial deposits but these fossils are several feet above the wave-cut bench. A series of cross-bedded fluvial deposits and a buried soil horizon conformably overly the marine deposits in most places. Our interpretation of the west Newport Mesa area is that at the time of deposition, the area was at the interface where fluvial deposits were alternately being interbedded with marine sediments following erosion of the substage 5e platform. The subsequent drop in sea level, and coastal uplift along the Newport-Inglewood zone of deformation formed Newport Mesa, where the authors have now documented surface evidence that these young sediments have been off-set along the North Branch fault of the Newport-Inglewood zone of deformation.

The terrace deposits at west Newport Mesa are well exposed due to heavy grading for a once-proposed extension of Balboa Boulevard, a borrow area, and road construction for oil drilling. Two locations (figure 2, sites 1 and 2) along the North Branch fault at Newport Mesa were exposed by hand with pick and shovel to study the effects of faulting on surface soils. Site 1 is at a small gully eroded into a roadcut leading to Mobil's Banning well No. 320. Site 2 is on a cut slope east of the oil well pad.

SEE PHOTO 2

Photo 2. Site 1, exposure of faulted fluvial terrace deposits that are presented on figure 3. The 1.5 meter-high triangular wedge at the base of exposure is an apparent horst resulting from lateral slip.

SEE FIGURE 3

Figure 3. Site 1, log of faulted terrace deposits and solum on the north wall of a small gully across the fault trace.

A detailed log of the faulting exposed at site 1 is shown in figure 3. The undisturbed solum (A and B horizons of modern soil) adjacent to the site is on the order of 1.8 to 2.4 meters thick and has formed on fluvial terrace deposits. However, where the exposed fault intersects the ground surface, the top 1.2 to 1.8 meters has been eroded away and only the basal portion of the solum remains. The log illustrates a complex pattern of faulting with many splays reaching the present ground surface. The base of the solum is vertically offset down to the west a total of about 51 centimeters across the entire zone of the shears. At the base of the gully a horst block in terrace deposits is uplifted about 60 centimeters between two shears, although the net apparent vertical offset is only 15 centimeters on either side of the horst. The offset is measurable on two markers, one a buried argillic soil horizon (unit 2) and the second a gravel bed within unit 1 overlying cross-bedded sands (figure 3 and photo 2). The gravel bed, although a distinct horizon, is represented across one of the shears by slightly different facies varying from coarse gravel within the horst to coarse-grained sand in the block to the northeast. The presence of the horst, the fact that facies are different across one of the shears, and the fact that thicknesses of units vary across the zone all indicate that lateral slip has also occurred along this shear zone. However, the sense and total amount of lateral slip is indeterminate from the data gathered in this exposure.

TABLE 1 SITE ONE - NEWPORT MESA SOIL AND LITHOLOGIC DESCRIPTIONS

SEE PHOTO 3

Photo 3. Site 2. the fault trace (indicated by arrows) is visible on a southeast-facing slope of marine terrace deposits. The top arrow shows where evidence for the 1933 rupture was found.

Site 2 is located on a 7.5 meter high east-facing cut slope (photo 3). Although the majority of the terrace surface in the area of this exposure has been excavated, the surficial soils at this location have been spared from heavy grading because it is the site of an oil-well pump pad. However, at least two different ages of artificial fill from surficial grading are present where the fault intersects the ground surface. The fault break can be traced from the base of the slope to the top where it vertically displaces the solum and older fill by 30 centimeters down to the west (photo 4). Bedding in the terrace deposits at the base of the slope is offset approximately 45 centimeters in the same sense, down to the west.

The offset solum at site 2, shown in photo 4 and 5 is documented in figure 4. Figure 4 illustrates numerous shears disrupting the solum and in particular a distinctive laminated, very fine-grained silty sand. The sand is offset 30 centimeters down to the west. The sand is distorted near the offset and a fragment of it clings to the upthrown hanging wall of the shear surface. The sand appears to have been deposited on a nearly flat surface, probably in an ephemeral puddle in a swale, during the rainy season or following a heavy rain storm. Thus the laminated sand represents the ground surface at one time prior to faulting.

Results from a palynology analysis of the sand tend to confirm the environment of deposition. The analysis indicates spores from fungi and algal debris such as would be found in a short-lived puddle in a prairie environment (Anderson, Warren and Associates, 1981 personal communication). The pollen type in the sample tested include thistle, oak and grass. Modern fill was found to rest directly on the laminated sand across the fault break. On the down-thrown side, debris consisting of wood fragments, tarred roofing material, newspaper fragments, and a brick were contained within the fill. The location of the brick and other debris with respect to the fault is shown in photo 6. The brick is embossed with the brick makers symbol "Fireback". With help from the Masonry Institute, we found that Fireback bricks were first manufactured in the area during the late 1920s. Thus, the brick could pre-date the Long Beach earthquake.

This possible age of the fill combined with the field relationships indicate that the ground surface was involved in faulting probably after the man-made debris was dumped on the ground surface. We believe that these data document historical surface faulting on Newport Mesa. The only historical earthquake large enough to have caused that surface faulting is the 1933 Long Beach earthquake with an epicentral location offshore from Newport Beach.

SEE PHOTO 4

Photo 4. Site 2 as it appeared before exploratory excavation. The base of the soil horizon (indication by arrows) is offset across the fault down to the west (left).

CONCLUSIONS

Realizing the possibility that the Newport-Inglewood zone poses a threat to the Los Angeles and Orange County areas, geologists should devote more effort to evaluation of the surface and near-surface evidence of prehistoric earthquakes. This type of investigation would clarify the locations of fault traces along the Newport-Inglewood zone and it would help define the surface faulting potential. Although the zone is virtually in many geologists' backyards, few investigations of its surface-faulting history have been attempted in contrast to the many sophisticated investigations made along the San Andreas fault. In general, geologists have been deterred from surface investigations of the Newport-Inglewood zone because of the striking lack of evidence for surface faulting and the heavy urbanization along its traces, making the zone particularly difficult to analyze.

TABLE 2 SITE TWO - NEWPORT MESA SOIL AND LITHOLOGIC DESCRIPTIONS

This study at Newport Mesa demonstrates that surface faulting has occurred and is recognizable along the Newport-Inglewood fault zone. However, the evidence of youthful faulting and surface faulting is subtle along the zone; discovery of such evidence requires particularly careful, detailed investigations.

SEE PHOTO 5

Photo 5. Site 2, exposure showing faulted terrace deposits and solum. Arrows indicate the main fault break.

SEE PHOTO 6

Photo 6. Site 2, close-up view of the brick found at the base of fill resting on laminated sand (indicated by arrows).

SEE FIGURE 4

Figure 4. Site 2, log of exposure showing relationship of fill and modern debris (D) to faulted solum. Arrows indicate main fault break.

REFERENCES

Barrows, A. G., 1974, A review of the geology and earthquake history of the Newport-Inglewood structural zone, southern California: California Division of Mines and Geology Special Report 114, p. 115.

Federal Emergency Management Agency, 1981, An assessment of the consequences and preparations for a catastrophic California earthquake: Findings and actions taken, Washington, D. C., 59 p.

Harding, T. P., 1973, Newport-Inglewood trend, California an example of wrench style deformation: American Association of Petroleum Geologists Bulletin, v. 57, no. 1, p. 97-116.

Hart, E. W., 1980, Fault hazard zones in California: California Division of Mines and Geology Special Publication 42 Revised Edition, 37 p.

Hill, M. L., 1971, Newport-Inglewood zone and Mesozoic subduction, California: Geological Society of America Bulletin, v. 81, p. 2957, 1962.

Hunter, A. L., and Allen, D. R., 1956, Recent developments in west Newport oil field: Summary of Operations, California Oil Fields: California Division of Oil and Gas, v. 42, no. 2, p. 11-18.

Kresse, F. C., 1966, Baldwin Hills Reservoir failure of 1963, in Engineering Geology in Southern California, edited by R. Lung and R. Proctor: Association of Engineering Geologists Special Publication, p. 93-103.

Lang, H. R., and Dreesen, R. W., 1975, Subsurface structure of the northwestern Los Angeles basin, in California Division of Oil and Gas Technical Papers, Report No. TP01, 33 p.

Morton, P. K., Miller, R. V., and Evans, J. R., 1976, Environmental geology of Orange County, California: California Division of Mines and Geology Open File Report 79-8.

Morton, P. K., Miller, R. V., 1973, Geologic Map of Orange County, California, in Geo-environmental Maps of Orange County. California: California Division of Mines and Geology Preliminary Report 15.

Richter, C. F., 1970, Magnitude of the Inglewood, California earthquake of June 21, 1920: Seismological Society of America Bulletin, v. 60, p. 647-649.

Segall, P., and Pollard, D. D., 1980, Mechanics of discontinuous faults: Journal of geophysical research, v. 85, no. B8, p. 4337-4350.

Shakelton, N. J. and Opdyke, N. D., 1975, Oxygen-isotope and paleomagnetic stratigraphy of Pacific core v-28-239, late Pliocene to latest Pleistocene: Geological Society of America Memoir 145, p. 449-464.

Sherburne, R. W., 1981, Seismology program: CALIFORNIA GEOLOGY: v. 34, no. 1, p. 3-6.

U. S. Geological Survey, 1981, Scenarios of possible earthquake affecting major California population centers, with estimates of intensity and ground shaking: Open File Report 81-115, p. 36.

Wehmiller, J. F., Lajoie, K. R., Kvenvolden, K. A., Peterson, E., Belknap, D. F., Kennedy, G. L., Addicott, W. O., Vedder, J. G., and Wright, R. W., 1977, Correlation and chronology of Pacific coast marine terrace deposits of continental United States by fossil amino acid stereochemistry-technique evaluation, relative ages, kinetic model ages and geologic implications: U. S. Geological Survey Open File Report 77-680, 106 p.

Wilcox, R. E., Harding, T. P., and Seely, D. R., 1973, Basic wrench tectonics: American Association of Petroleum Geologists Bulletin. v. 57, no. 1, p. 97-116.

Woodward-Clyde Consultants, 1979, Report of the evaluation of maximum earthquake and site ground motion parameters associated with the offshore zone of deformation, San Onofre Nuclear Generating Station: Report prepared for Southern California Edison, Rosemead, California, on file at Mission Viejo Library.

Yeates, R. S., 1973, Newport-Inglewood fault zone, Los Angeles Basin, California: American Association of Petroleum Geologists Bulletin, v. 57, no. 1, p. 117-135.