Researchers have long known that the mountain ranges surrounding the Central Valley have been rising faster than expected--a few millimeters every year for over a century. And over the same time, seismic activity in the area has also increased. According to a new study, both may be linked to the depletion of groundwater in the Central Valley. Colin Amos of Western Washington University is lead author on the study.
"We find that the mountains are rising surrounding the San Joaquin Valley where the greatest rates of groundwater withdrawal are happening. "
Amos and his colleagues estimate that over the last 150 years, an amount of groundwater equivalent to the volume of Lake Tahoe has been drawn out from underneath the southern San Joaquin Valley. And where a lot of mass is removed, the crust rises.
"The uplift of those mountains results in the reduction of the overall forces that are serving to keep faults clamped together and may be changing the rate of earthquakes that are on those faults. "
Researchers observed that the frequency of earthquakes not only increases annually, but fluctuates seasonally: fewer earthquakes in the wet season, more in the dry season. Most are too small to be felt by humans.
Amos stresses the paper is not intended to inspire change in water use and demand. But he points out that humans' impacts on the planet are significant.
"We're demonstrating a new and perhaps unappreciated link between what human beings are doing, taking water out of the ground…and changes to the solid earth. "
The study was published in the current issue of the journal Nature.