Earthquakes that originate at intermediate depths are an underappreciated seismic risk, according to a study of a June 2014 earthquake in the western Aleutian Islands off Alaska.

The epicentre of the magnitude-7.9 quake was approximately 100 kilometres deep, making the quake the largest in this depth range — between about 70 and 200 km down — in the past century. Thorne Lay of the University of California, Santa Cruz, and his colleagues analysed the earthquake and found that the energy release was weak at first but became strong during its final 25 seconds.

Other regions with a similar tectonic structure, such as Japan and Indonesia, should be aware of how big and powerful intermediate-depth quakes can be, the authors warn.

Geophys. Res. Lett. http://doi.org/v45 (2014)