Causes of the landslide

Physical factors
1. Steep slopes

Steep slopes are abundant because ongoing tectonic deformation pushes up rock that is too weak to support the steep slopes. Some of the uplifted material is geologically very young sediment that has not consolidated into more resistant rock. However, even older and harder rocks have been sheared and fractured by tectonic movement along the plate boundary, and thus the landslide occurred.

2. Precipitation

Excess water from precipitation that does infiltrate through to the water table drains along the path of least resistance and follows the force of gravity; the faults that exit through the cliff face provide this path of least resistance. These fault and their scarps can be seen in the photo. The rainfall that caused the landslide must be of sufficient duration and strength to raise the field capacity of the soil, the point where under gravity infiltration will equal percolation. At this point, the soil layer will become saturated with additional infiltration. With an abrupt increase in infiltration that exceeds percolation, the pore-pressure between soil particles decreases, and the upper layers of soil become saturated. As water replaces air between the soil granules, the resistance of soil layers to shear stress decreases and the mass of the saturated soil tumbling down the slope creates the landslide.

3. Type of rock

Geologic stability of subsurface soil layers and the slope of the hillside are important factors to consider. The area of the landslide may be underlain by weak, geologically young rock that is being uplifted and severely deformed by compressional forces generated along the tectonic plate boundary. The strata of weak clay and shale may have caused the landslide.

4. Earthquakes

As California is located near plate boundaries, it is often prone to earthquakes. However, in this case, it is unlikely that the cause of the landslide is an earthquake as the houses are still intact.

Human factors
1. The construction of roads
As seen from the photograph, a road has been constructed along the slope. This form of slope modification reduces the shear strength of the slope, hence making the slope more vulnerable to slope failure. The movement of cars along the road may cause vibrations while the mass of the cars may add weight to the slope, increasing the vulnerability of the slope to slope failure.

2. Building of houses
The houses built at the toe of the slope may add weight to the slope, thus increasing shear stress and causing the downward movement of soil particles.

3. Irrigation by people living at the toe of the slope
Irrigation has the tendency to raise the water table and potentially cause weakness in hill slopes. In the area around the toe of the slide water may emanate from beneath the cliffs.