SAN DIEGO—Carole Ziegler is a speaker for the San Diego County Office of Disaster Preparedness and a professor of Earth Science at Southwestern College. During an interview on Oct. 12, Ziegler explained some of the factors that influenced the landslide on Mount Soledad in La Jolla on Wed., Oct. 3.
Ziegler said that one factor that lent to the disaster is the Rose Canyon Fault, which runs through Downtown San Diego, then follows Rose Canyon and splits into two faults near Mt. Soledad. "Those two faults that split out are shoving upward, and pushing up Mt. Soledad," said Ziegler. "Mt. Soledad, in its old form was layers of rock. Now, by being pushed up, these layers of rock are at an angle, and these houses were sitting up on this angled rock."
Adding to the risk, explained Ziegler, was that the land that the homes were built on has a layer of clay minerals, which typically form in sheets and can be sheared off because of the way they are bonded together. "It's a standard 'slump' landslide. It's due to the underlying rock type, shale, which has a lot of clay minerals in it. Those clay minerals are slippery, and if you add weight such as a new home, it will increase what's called the driving force. At the bottom of these types of slopes, you have what's called the resisting force. So once you've overcome the resisting force, the hills start sliding down," Ziegler said.
Prior to the occurrence of the landslide local residents had reported water leakage and cracks appearing on the road.
Ziegler said that when clay gets wet it expands. Water is capable of dissolving soil, which could have played a part by dissolving the layers of clay rock that the homes were built on.
City geologists are still investigating whether long-term seepage from water pipes was the cause, or if earth movement caused the rupture of the pipes. Ziegler said that it's very likely that the beginning of the landslide caused the water pipes to rupture.
This is not the first time Mount Soledad has been the scene of this type of disaster. A similar landslide occurred on December 14, 1961 near the same site and destroyed eight homes.
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