Robert W. Fleming

Structures associated with strike-slip faults that bound landslide elements

Abstract Large landslides are bounded on their flanks and on elements within the landslides by structures analogous to strike-slip faults. We observed the formation of thwse strike-slip faults and associated structures at two large landslides in central Utah during 1983–1985. The strike-slip faults in landslides are nearly vertical but locally may dip a few degrees toward or away from the moving ground. Fault surfaces are slickensided, and striations are subparallel to the ground surface. Displacement along strike-slip faults commonly produces scarps; scarps occur where local relief of the failure surface or ground surface is displaced and becomes adjacent to higher or lower ground, or where the landslide is thickening or thinning as a result of internal deformation. Several types of structures are formed at the ground surface as a strike-slip fault, which is fully developed at some depth below the ground surface, propagates upward in response to displacement. The simplest structure is a tension crack oriented at 45δ clockwise or counterclockwise from the trend of an underlying right- or left-lateral strike-slip fault, respectively. The tension cracks are typically arranged en echelon with the row of cracks parallel to the trace of the underlying strike-slip fault. Another common structure that forms above a developing strike-slip fault is a fault segment. Fault segments are discontinuous strike-slip faults that contain the same sense of slip but are turned clockwise or counterclockwise from a few to perhaps 20° from the underlying strike-slip fault. The fault segments are slickensided and striated a few centimeters below the ground surface; continued displacement of the landslide causes the fault segments to open and a short tension crack propagates out of one or both ends of the fault segments. These structures, open fault segments containing a short tension crack, are termed compound cracks; and the short tension crack that propagates from the tip of the fault segment is typically oriented 45° to the trend of the underlying fault. Fault segments are also typically arranged en echelon above the upward-propagating strike-slip fault. Continued displacement of the landslide causes the ground to buckle between the tension crack portions of the compound cracks. Still more displacement produces a thrust fault on one or both limbs of the buckle fold. These compressional structures form at right angles to the short tension cracks at the tips of the fault segments. Thus, the compressional structures are bounded on their ends by one face of a tension crack and detached from underlying material by thrusting or buckling. The tension cracks, fault segments, compound cracks, folds, and thrusts are ephemeral; they are created and destroyed with continuing displacement of the landslide. Ultimately, the structures are replaced by a throughgoing strike-slip fault. At one landslide, we observed the creation and destruction of the ephemeral structures as the landslide enlarged. Displacement of a few centimeters to about a decimeter was sufficient to produce scattered tension cracks and fault segments. Sets of compound cracks with associated folds and thrusts were produced by displacements of up to 1 m, and 1 to 2 m of displacement was required to produce a throughgoing strike-slip fault. The type of first-formed structure above an upward-propagating strike-slip fault is apparently controlled by the rheology of the material. Brittle material such as dry topsoil or the compact surface of a gravel road produces echelon tension cracks and sets of tension cracks and compressional structures, wherein the cracks and compressional structures are normal to each other and 45° to the strike-slip fault at depth. First-formed structures in more ductile material such as moist cohesive soil are fault segments. In very ductile material such as soft clay and very wet soil in swampy areas, the first-formed structure is a throughgoing strike-slip fault. There are other structures associated with strike-slip faults that are not ephemeral and tend to persist after many meters of displacement. These are extensional and compressional features, which are associated with steps and curves in strike-slip faults, and flank ridges, which form parallel to the strike-slip fault. Extensional structures form where a left- or right-lateral strike-slip fault steps or curves left or right, respectively. We monitored the formation of a pull-apart basin at a right step in a right-lateral strike-slip fault. The first crack formed between and connecting the strike-slip faults is diagonal and trends 45° to the traces of the faults. The upper few decimeters of the crack surface are rough, irregular, vertical and consistent with formation in tension. Below the tension crack, the surface dips downslope at about 60° and is slickensided and striated. The azimuth of the striations is parallel to the trend of the strike-slip faults but oblique to the diagonal fault surface. Thus, the upper part of the diagonal crack is a tension crack with an orientation consistent with published models for stresses that produce such cracks, but the lower part is an oblique-slip fault produced by displacement along the strike-slip faults upslope and downslope from the step. With continuing displacement, the diagonal crack is replaced by newly formed normal faults that are at right angles to strike-slip faults. The pull-apart basin becomes bounded on the uphill and downhill ends by normal faults, on one side by a strike-slip fault surface, and on the side within the moving ground by a monoclinal flexure. The monoclinal flexure develops a thrust fault at its hinge line where landslide debris moves laterally toward the basin. Compressional structures such as thrusts, folds and domes form where, for example, a left-lateral strike-slip fault steps right. These structures are similar to the small folds and thrusts formed between the ends of compound cracks, but they are much larger and tend to persist after many meters of landslide displacement. They apparently occur where segments of the upward-propagating strike-slip fault are grossly misaligned. The domes and folds occur within moving landslide debris. The thrust faults may result in displacement of landslide debris over adjacent, unfailed material. Flank ridges, which form parallel to a strike-slip fault, are up to several meters high, 1 to 3 decameters wide, and several decameters long. We recognized two fundamentally different types of flank ridges. One type, a depositinal flank ridge, forms on the outside of the strike-slip fault as a result of material spilling or being thrust over unfailed material. The other type, a deformational flank ridge, forms within the landslide debris, adjacent and parallel to the strike-slip fault. Some of the deformational ridges we examined contained intrusions and extrusions of highly plastic clay from deep within the landslide. At one flank-ridge site, the clay had been injected into the strike-slip fault as a tabular wedge and into adjoining landslide debris as anastomosing veins. Measurement of the growth of a ridge revealed that the crest of the ridge rises relative to adjacent material that is moving downhill as a result of displacement. Strain measurements revealed tension at the crest of the ridge and compression on the flank of the ridge but virtually no net strain accumulated across the entire cross section of a growing ridge. Therefore, the ridge cannot be the result of simple compression necessary for buckle folding. Field evidence for actively growing ridges includes tension cracks at the ridge crest, which are oriented oblique to the trend of the ridge, and small folds, which form around the downslope end and lower flank of a growing ridge. Indirect evidence for active ridge growth includes rupture of the ridge crest with exposure of fresh landslide debris and tilted trees.

Use of longitudinal strain in identifying driving and resisting elements of landslides

Observations of deformation at the surfaces of landslides in Utah and Hawaii indicate that the upslope parts of the land-slides have stretched and the downslope parts have shortened parallel with the direction of movement. The maximum displacement of each landslide occurs in a relatively undeformed zone between the zones of shortening and stretching. The pattern of deformation at the surface of these landslides may be useful in analyzing their mechanics by helping to constrain the longitudinal forces in limit-equilibrium stability analysis. We used earth-pressure calculations to determine the range of possible longitudinal forces (per unit width) for active failure in the zone of stretching and for passive failure in the zone of shortening of one of the Hawaiian landslides. Longitudinal forces computed by stability analysis, assuming homogeneous strength, exceeded the possible forces in much of the upslope half of the landslide. Consequently, we assumed inhomogeneous strength and adjusted shear-strength parameters at each segment of the slip surface until the longitudinal forces computed by stability analysis agreed with those computed by earth-pressure theory, and the factor of safety approached unity. The distribution of longitudinal forces computed for inhomogeneous strength indicated that the boundary between driving and resisting elements of the landslide is near the thickest part of the slide, in agreement with a simple formula for the location of the boundary.

Rates of seasonal creep of silty clay soil

Summary Profiles of displacement caused by seasonal creep of an expansive silty clay soil were measured at several times of the year with a tiltmeter and flexible pipes installed in two areas near Stanford University in central California. The displacement profiles typically are convex upward and net creep displacement at the ground surface of an undisturbed slope with a gradient of 12 to 14 per cent is about 0.4 in/yr (1 cm/yr) for a layer of soil of five feet (1.5 m) thickness The soil apparently expands and creeps on wetting of the soil and while the soil is at a high moisture content, but the creep rate diminishes with time, as predicted by laboratory tests by Mitchell and Singh. On drying, a point on the surface of a slope contracts roughly along a normal to the slope but directions of horizontal displacements during drying appear to be random and to depend on the location of the point with respect to local shrinkage cracks.

Right-lateral-reverse surface rupture along the San Andreas and Sargent faults associated with the October 17,1989, Loma Prieta, California, earthquake

The Loma Prieta earthquake produced bewildering arrays of surface fractures of all possible modes in the epicentral area. Preliminary reports of the fracturing, though, suggested that there was no evidence of right-lateral surface slip above the fault as it is defined seismically. Through detailed mapping of selected localities along the San Andreas and Sargent faults, however, we have been able to show that strands of the San Andreas and Sargent faults ruptured to the surface with right-lateral-northward thrust components of slip in amounts of 10 to 20 cm.

Landslide Hazards and Their Reduction

Abstract Landslides are a widespread and costly problem in many parts of the United States. Although reliable estimates of the costs of landslide damage are difficult to obtain, a conservative estimate of the present-day direct and indirect costs of slope failures in the United States exceeds

Growth of a tectonic ridge during the Landers earthquake

1 billion annually. Losses from landslides can be significantly reduced by the cooperative effort of geologists, engineers, and planners, clearly demonstrating the value of loss mitigation measures. A program to reduce the losses from landslides in the hillside areas of Los Angeles has been in effect since 1952 and, by one method of evaluation, has reduced losses by nearly a factor of 50.

Analecta of structures formed during the 28 June 1992 Landers-Big Bear, California earthquake sequence (including maps of shear zones, belts of shear zones, tectonic ridge, duplex en echelon fault, fault elements, and thrusts in restraining steps)

The formation of tectonic ridges by localized vertical uplift along strike-slip faults has long been suspected, but the actual growth of a tectonic ridge during an earthquake has never been documented. During the 1992 Landers, California, earthquake sequence, an awl-shaped, dome-like topographic ridge along the Emerson fault zone increased its height at least 1 m concurrently with 3 m of right-lateral shift across the fault zone containing the ridge. Five deformation vectors within the ridge reveal dilatant behavior in addition to the uplift and shift on boundary faults. 10 refs., 5 figs.

Growth of a tectonic ridge

The June 28, 1992, M{sub s} 7.5 earthquake at Landers, California, which occurred about 10 km north of the community of Yucca Valley, California, produced spectacular ground rupturing more than 80 km in length (Hough and others, 1993). The ground rupturing, which was dominated by right-lateral shearing, extended along at least four distinct faults arranged broadly en echelon. The faults were connected through wide transfer zones by stepovers, consisting of right-lateral fault zones and tension cracks. The Landers earthquakes occurred in the desert of southeastern California, where details of ruptures were well preserved, and patterns of rupturing were generally unaffected by urbanization. The structures were varied and well-displayed and, because the differential displacements were so large, spectacular. The scarcity of vegetation, the aridity of the area, the compactness of the alluvium and bedrock, and the relative isotropy and brittleness of surficial materials collaborated to provide a marvelous visual record of the character of the deformation zones. The authors present a series of analecta -- that is, verbal clips or snippets -- dealing with a variety of structures, including belts of shear zones, segmentation of ruptures, rotating fault block, en echelon fault zones, releasing duplex structures, spines, and ramps. All of these structures are documented with detailed maps in text figures or in plates (in pocket). The purpose is to describe the structures and to present an understanding of the mechanics of their formation. Hence, most descriptions focus on structures where the authors have information on differential displacements as well as spatial data on the position and orientation of fractures.

Winnetka deformation zone: Surface expression of coactive slip on a blind fault during the Northridge earthquake sequence, California. Evidence that coactive faulting occurred in the Canoga Park, Winnetka, and Northridge areas during the 17 January 1994, Northridge, California earthquake

The 28 June 1992 Landers, California, earthquake of M 7.6 created an impressive record of surface rupture and ground deformation. Fractures extend over a length of more than 80 km including zones of right-lateral shift, steps in the fault zones, fault intersections and vertical changes. Among the vertical changes was the growth of a tectonic ridge described here. In this paper the authors describe the Emerson fault zone and the Tortoise Hill ridge including the relations between the fault zone and the ridge. They present data on the horizontal deformation at several scales associated with activity within the ridge and belt of shear zones and show the differential vertical uplifts. And, they conclude with a discussion of potential models for the observed deformation.

Economic Losses and Fatalities Due to Landslides

Measurements of normalized length changes of streets over an area of 9 km{sup 2} in San Fernando Valley of Los Angeles, California, define a distinctive strain pattern that may well reflect blind faulting during the 1994 Northridge earthquake. Strain magnitudes are about 3 {times} 10{sup {minus}4}, locally 10{sup {minus}3}. They define a deformation zone trending diagonally from near Canoga Park in the southwest, through Winnetka, to near Northridge in the northeast. The deformation zone is about 4.5 km long and 1 km wide. The northwestern two-thirds of the zone is a belt of extension of streets, and the southeastern one-third is a belt of shortening of streets. On the northwest and southeast sides of the deformation zone the magnitude of the strains is too small to measure, less than 10{sup {minus}4}. Complete states of strain measured in the northeastern half of the deformation zone show that the directions of principal strains are parallel and normal to the walls of the zone, so the zone is not a strike-slip zone. The magnitudes of strains measured in the northeastern part of the Winnetka area were large enough to fracture concrete and soils, and the area of larger strains correlates with the area of greater damage to such roads and sidewalks. All parts of the pattern suggest a blind fault at depth, most likely a reverse fault dipping northwest but possibly a normal fault dipping southeast. The magnitudes of the strains in the Winnetka area are consistent with the strains produced at the ground surface by a blind fault plane extending to depth on the order of 2 km and a net slip on the order of 1 m, within a distance of about 100 to 500 m of the ground surface. The pattern of damage in the San Fernando Valley suggests a fault segment much longer than the 4.5 km defined by survey data in the Winnetka area. The blind fault segment may extend several kilometers in both directions beyond the Winnetka area. This study of the Winnetka area further supports observations that a large earthquake sequence can include rupture along both a main fault and nearby faults with quite different senses of slip. Faults near the main fault that approach the ground surface or cut the surface in an area have the potential of moving coactively in a major earthquake. Movement on such faults is associated with significant damage during an earthquake. The fault that produced the main Northridge shock and the faults that moved coactively in the Northridge area probably are parts of a large structure. Such interrelationships may be key to understanding earthquakes and damage caused by tectonism.