Geological And Geotechnical Analysis Of a Submarine Slump, California Borderland
Authors: Edwards Brian D. , Field Michael E. , Clukey Edward C. 
Published: October 2008

Sediment on intercanyon segments of the southern California mainland slope south of Los Angeles is failing through repeated, localized rotational slides (slumps). We investigated the stability of this area with the use of shallow penetration (3.5 kHz, 0.2-0.8 kJ) high-resolution acoustic reflection records and with sediment cores up to 3.9 m long. High-resolution acoustic records for over 100 km of tracklines suggest failure within two narrow slumps. Failure of the normally consolidated, moderately sensitive, muddy sediment occurs on slopes of less than 4 °, and each slump is about 1.5 km across, 3.5 km long, and 50 m thick.

Routine laboratory evaluations of geotechnical and sedimentologic properties were made on recovered gravity cores. Instability due to residual stress state and seismic loading effects was evaluated by laboratory measurements of the consolidation and triaxial static and dynamic strength of piston cores paired with the gravity cores. Dynamic pore pressure response was an important part of this analysis.

The data show that cyclic shear strengths are up to 24 percent less than the static strengths of a comparable sample consolidated to the same stress level. stability analysis using Morgenstern's simplified approach (1967) shows that at the 3° to 4° slopes typical of the study area, horizontal earthquake accelerations of 0.13 g would be sufficient to cause failure. Accelerations of this magnitude can occur in the study area, and we conclude that seismic events are probably instrumental in triggering the observed failures.

INTRODUCTION

The role of gravity as a mechanism for transporting sediment from the shallow shelf downslope to the basin floor is well known (e.g. Moore, 1961; Nardin et al., 1979a). In deltaic environments, gravity-driven subaqueous landslides have been recognized on slopes as low as 0.01° (Prior and Coleman, 1978). On open slopes, however, such displacements occur more typically at declivities of 1° to 4° (Lewis, 1971), and on steeper slopes and within canyons. Zones of failure range from large features kilometers on a side through smaller features tens to hundreds of meters on a side, to very small displacements of locally contorted sediment. The importance of these small movements as a significant part of the slope molding and basin-filling process has recently been recognized, particularly in intercanyon areas (Field and Clarke, 1979; Field and Edwards, 1980). The significance of unstable slope sediment is amplified in light of the present development of offshore resources.

Studies conducted by the U.S. Geological Survey have revealed several areas of unstable slope sediment in the southern California borderland (Field et al., 1977). These regions were first located during reconnaissance environmental surveys focused on areas included in or adjacent to Outer Continental Shelf (OCS) petroleum lease sale areas. This article is a detailed analysis of a failure zone on an intercanyon segment of the southern California mainland slope (Fig. 1). The failure zone is located on a straight segment of the mainland slope off San Mateo Point, 70 km southeast of Los Angeles.