Discussion: LANDSLIDE

Discussion: LANDSLIDE

Discussion: LANDSLIDE

Land Use

Land use on the Palos Verdes Peninsula (Figure 1) consists mostly of single-family homes built on large lots, many of which have panoramic ocean views. The affluent residents of this area value a rural lifestyle, and the peninsula is zoned to permit horses, stables, and riding trails. Figure 2 is a topographic map showing the Portuguese Bend area.

By the time of the 1956 Portuguese Bend landslide, more than 100 homes had been built within the slide area, most of them south of Palos Verdes Drive. All of these houses were constructed with individual septic systems, generally consisting of septic tanks and seepage pits.

Geologic Setting

The topography of the peninsula is generally hilly, ranging from gently rolling to steep. A bluff exists along the coastline, varying between 30 and 60 meters (100 and 200 feet) in height above sea level, and elevations rise to over 460 meters (1500 feet) within 4.5 km (3 miles) of the coast. The preslide topography was characterized by a series of terraces that rose from the sea like giant steps. However, within landslide areas, terraces have been disrupted and the terrain now appears hummocky and irregular. Large arcuate scarps occur near the head of slide masses.

Within 1.5 km (one mile) of the coastline, the subsurface consists of volcanic and sedimentary rocks that dip toward the sea at about the same angle as the average slope of the land surface (Figure 3). Landslide slip surfaces occur near the base of a volcanic rock unit known as the Portuguese Tuff. This rock unit includes a layer of bentonite, a clay mineral that forms from the weathering of volcanic ash and is capable of absorbing large amounts of water.

Landslide Movement

Landslides have been active here for thousands of years, but recent landslide activity has been attributed in part to human actions. The Portuguese Bend landslide began its modern movement in August 1956, when displacement was noticed at its northeast margin. Movement gradually extended downslope such that the entire eastern edge of the slide mass was moving within six weeks. By the summer of 1957, the entire slide mass was sliding towards the sea.

The rates of slippage have varied through time, initially moving between 2 and 12 cm/day (1 and 5 inches/day) for the first two years, and then diminishing to less than 1 cm/day (0.4 in/day) over the next four years. The slide mass continued to move for almost 40 years, and the cumulative displacement exceeds 30 m (100 feet) in some areas.

Figure 1. Palos Verdes Peninsula, CA, 2016, Google Maps, DigitalGlobe.

Figure 2. Portuguese Bend, Topographic Map USGS Topographic Maps (zip file).

Figure 3. Landslide Cross Section. (Ozsvath)

Questions

1. What natural conditions in this area are conducive to landslides?
2. What specific type of mass movement is likely to occur in this geologic setting?
3. Is it possible that the 1956 Portuguese Bend Landslide was triggered entirely by natural causes?

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