How Earthquakes Lead to Flooding
An Example from the Great China Quake of 2008
Earthquakes are a dangerous and often fatal natual disaster. The trembling earth buckles structures and causes landslides that take the lives of whomever is unfortunate enough to be in range. But some of the danger of earthquakes, such as flooding, can occur days or even a week later. How do earthquakes cause floods?
Earthquakes Can Trigger Floods
Earthquakes can cause flooding and the formation of new lakes when rivers and streams are dammed. Landslides, boulders, and other loose debris enter a water system, clogging the flow and creating new dams, backing the water up behind it and flooding low lying areas. So while one region may have survived the earth shaking, it may have to deal with flooding a few days later. Water flowing down stream meets obstacles that weren't there the week before and overflows its banks onto riverfront towns.
Additionally, the newly created dams pose a danger other than the threat of flooding behind it. The threat of flooding in front of the dam occurs once the dam starts to crumble because of its helter-skelter creation. The water pushing behind it may overflow the top or break holes in the dam. Aftershocks can rearrange the structure of these natural dams and cause the water to again flow down in a torrent.
The Example of China's 2008 Earthquake: New Lakes Create Floods
On May 12, 2008, at 6:28 Universal Time, a 7.9 earthquake rocked Eastern Sichuan, China. It was 2:28 local time at the epicenter, just 45 miles from the city of Chengdu. Those who lived through the earthquake may have felt that the worst was over. But for some local residents, the greatest destruction for them was still to come.
In the images here, the build-up of flood waters behind newly created earthen dams through earthquake debris is evident. Two days after the earthquake, on May 14, the river is still flowing normally. Visible in the picture are roads running alongside the river along with communities on each riverbank. A bridge connects the roads and crosses the river at the far right.
One day after the May 14 image, the river has begun to spill its banks. On May 15 a large portion of the road below the river is washed out, and much of the farm community above the river is underwaer. The bridge connecting the two communities is completely washed out.
Four days after the last image and one week after the earthquake, the last image shows the flood waters, which have continued to rise as they formed a new lake, named Yansai Lake. In this image, the roads and communities have disappeared under the flood waters from the earthquake. This one spot in China was not a unique circumstance; by May 19 more than 20 new lakes had formed throughout the region.
Source: NASA Earth Observatory
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