Unheeded warnings contributed to death toll

The 921 earthquake was devastating enough for Taiwan, but in addition, it caused two mammoth landslides that each took more than 20 lives. One landslide occurred at Tsaoling mountain (草??s) in Kukeng township (古坑鄉) in Yunlin County; the other happened at a mountain in Kuohsing township (國姓鄉).

There are both similarities and differences in the two mountain slides. First the similarities. Many landslides have occurred at Tsaoling mountain in the recent past. A major earthquake struck the Chiayi-Tainan plain (1贖n|a震) on June 6, 1861, creating a precipice at an elevation of 800 meters. On Oct. 17, 1941, the Chiayi earthquake struck, measuring 7.1 on the Richter Scale. Landslides occurred at Tsaoling mountain again, blocking the Chingshui stream (2M?艩?/CHINESE>) and creating a "barrier lake." At 6am on May 18, 1951, the natural dam started to break up and water began to rapidly flow downstream at a rate of 5,400 cubic meters per second, killing 137 people.

A detailed geological survey of Tsaoling mountain and the landslides was undertaken in 1981. The research concluded that 400 million cubic meters of rock and debris would slide down the mountain at some point in the future, and that preventative measures should be implemented.

As predicted, more rockslides occurred at Tsaoling mountain after the 921 Earthquake. An aerial survey conducted by National Central University (??央?j學) showed that about 350 million cubic meters of rock slid down the mountain as a result of the quake, close to the volume estimated by scholars eighteen years ago.

The author still recalls that there were several families living near the peak of Tsaoling mountain when we were conducting our survey in 1981. We tried to convince them to move, but their attachment to the land was too strong, and they would not budge. Those who did not move lost their lives in the 921 earthquake. The main cause of the slide was the dip-slope morphology, interbeds of sandstone and shale and side-erosion by the Chingshui stream. The 921 earthquake was merely a catalyst for the landslide.

The landslide that occurred at Kuohsing township after the 921 earthquake was the first slide on record there. There was no evidence that a landslide was imminent before the quake. The topography and geology of the mountain is similar to that of Tsaoling mountain, but Setsai creek (澀仔溪) at its foot, is small and has not eroded the mountain's base. The author personally visited the mountain on a survey mission a few days before the earthquake and saw no evidence indicating a potential landslide.

The mountain's proximity to the earthquake's epicenter was perhaps the main cause of the slide. Strong seismic activity fractured and dislodged surface rock strata, sending tons of rock hurtling down the mountain and plugging up the stream below to form a barrier lake. Preliminary estimates of the volume of rock that slid into the valley is some 30 million cubic meters.

The two landslides were similar in that both involved sandstone strata breaking up and sliding down a steep mountain slope, damming up a stream and creating a huge lake. Both resulted in over twenty deaths, and the dams formed by the slides are both unstable and could burst at any time.

The mountain slide at Kuohsing township was purely an unexpected natural disaster. Tsaoling mountain, on the other hand, had a history of landslides. Many experts had warned of the strong possibility of such events recurring. We must examine why so many deaths occurred even after warnings were issued. Were laws lacking or was the administration lax in implementing the laws -- or did the government simply overlook expert opinion?