Oil and gas exploration could be triggering earthquakes in California, where natural quakes might mask the induced ones.

The injection of millions of litres of wastewater into underground oil and gas reserves has been linked to earthquakes around the world, including in central parts of the United States, such as Oklahoma. Thomas Goebel of the University of California, Santa Cruz, and his colleagues analysed three quakes of magnitude-4 and higher that happened on the same day in September 2005 along southern California's White Wolf fault. They found unusual seismic patterns and, using a hydrogeological model, concluded that nearby fluid injection that began just months before was probably responsible.

More analysis is needed to identify human-induced earthquakes in California, the authors say.

Geophys. Res. Lett. http://doi.org/bb6k (2016)