Catastrophes!: Earthquakes, Tsunamis, Tornadoes, and Other Earth-Shattering Disasters

4 Landslides: Gravity Always Wins

4 Landslides Gravity Always Wins The mountain is coming down!” he shrieked. Standing directly underneath the tumbling hillside, hearing its terrible crackle and roar and watching a plume of earth spew toward the sky, the men broke and ran for their lives . . . He lost sight of the others as the hillside bore down. Out of the corner of one eye, he could see a house and a trailer in hot pursuit. —Los Angeles Times, January 2005 A Surfer’s Nightmare: La Conchita, California, 2005 The winter of 2004–2005 was wet in Southern California. Many places in the steep mountains behind the urban belt had flooded and experienced landslides . Huge amounts of rain had fallen in the last weeks of December and the first weeks of January. In the sleepy coastal town of La Conchita, there was no reason to think that the winter rainy season would be unlike any other. La Conchita consisted of a few dozen houses with about 300 residents, located right on the coast on Highway 101 between the wealthier communities of Santa Barbara and Ventura. La Conchita was much more laid-back and inexpensive, 108 Catastrophes! with small beach cottages inhabited mostly by retired surfers, artists, beachcombers , and hippies who savored their pleasant beachfront life without Santa Barbara’s high prices and congestion. It was also located north of the freeway and railroad tracks, at the base of a steep cliff made of loose sandstones and mudstones once deposited in ancient seas (fig. 4.1). These ancient sedimentary rocks were then uplifted by faults to heights more than 150 m (500 feet) above sea level. The cliffs were prone to landslides up and down the coast, from Malibu to the sea cliffs west of Ventura . They were formed of softer sedimentary rocks, with lots of clays that soaked up water, expanded, and became slippery when they were saturated. The bluffs above the town had many landslide scars, showing a long history of instability. In March 1995, part of the hillside had given way, covering up the houses on the street against the base of the cliff. The 1995 slide measured 120 m (400 feet) wide, 330 m (1,100 feet) long, and spread across 4 hectares Fig. 4.1. The scar and earthflow deposits of the 2005 La Conchita landslide. The slide covered most of the town at the base of the slope. (Photo by the author) Landslides 109 (10 acres). It was greater than 30 m (100 feet) deep, with an estimated volume 1.3 million cubic meters (1.7 million cubic yards). This had also been a wet year, with 390 mm (15 inches) of rain in the two weeks before the slide. No one was killed in the event, because the landslide moved relatively slowly and residents were warned to evacuate, but a number of houses were destroyed. Afterward, large retaining walls were built at the base of the slide debris to stop further movement. Still, the residents of La Conchita were pretty mellow during the rains of 2004–2005 and did not expect anything different from previous rainy winters. Early on the morning of January 10, small mudflows started moving down the nearby canyons. Highway 101 was closed, and emergency officials and TV crews monitored the mudflows. Then at 12:30 in the afternoon, the cliff gave way, as the TV news cameras were rolling. (Video footage of this event is online and amazing to watch.) A mass of earth 350 m (1,150 feet) long and 80–100 m (260–330 feet) wide, remobilized from the unstable 1995 landslide, quickly moved downslope at 10 m/sec (33 feet/sec), faster than anyone could outrun it. Before people in the houses below could react, it had overrun almost half the town, burying and destroying 13 houses, damaging 23 others, and burying dozens of people. Many of the houses were pushed forward as if by a mighty bulldozer and then torn apart before being buried. Emergency workers and townspeople rushed to aid people trapped under their smashed houses and rescued quite a few. Still, some houses were so deeply buried and crushed that there was no way to dig down without rescuers endangering their own lives, and there were no sounds or other signs of life. The rescuers eventually gave up, and these bodies remain buried in the slide mass, with memorials marking the site where they vanished (plate 7...