![]() When the backfill collapsed, a hole was left more than 40 feet deep in places and 15 feet wide in others. When the rain hit it, water flowed in and undermined the backfill. When Chandler Heights was developed, the fissure was backfilled in places. It was visible on aerial photos back in 1969. A fissure opened up from just north of the San Tan Mountains, crossed the Hunt Highway and San Tan Boulevard, curved west and stopped just short of Sossaman Road. On the night of July 21, 2007, a thunderstorm pounded Chandler Heights with 2 inches of rain in an hour. When you draw down water in these basins, the land surface sinks and gets torn apart from the bedrock. Drive to Palm Springs, California, or Tucson and you’ll have a perfect illustration of it: wide, deep alluvial basins interspersed with solid bedrock mountains. ![]() The desert Southwest is mostly basin and range topography. And so over time that causes the ground surface elevation to lower because basically the space in between the grains has been removed.” ![]() “When you remove that water, sometimes those sediments rearrange, and that core pressure that was keeping the grains apart has collapsed. ![]() “(Many) aquifers hold water in between the core space of a sediment,” she said. Though people picture aquifers as cracks in fully formed rock - and that is often the case in Arizona - that’s not how all aquifers work, Carlson said. Carlson has done research on local subsidence. The science behind it is simple to explain, said Grace Carlson, an Arizona State University postgraduate researcher who earned her master’s degree in geoscience at ASU's School of Earth and Space Exploration. It’s called subsidence, and it’s caused by groundwater pumping. ![]()
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