Trying to determine when an earthquake begins is challenging. Complete characterization of subsurface faults is not possible and lab experiments to differentiate frictional properties between two materials are less than optimal. However, observational studies do provide some insight into the physical conditions that exist when an earthquake occurs.

Using such data from the 2012 magnitude 8.6 earthquake that occurred offshore of Sumatra, Carl Tape and his colleagues at the University of Alaska Fairbanks and the Indian Institute of Technology in Kharagpur pinpointed the beginning of the magnitude 3.9 earthquake that occurred in central Alaska immediately following the Sumatra quake.

The team discovered that the Alaskan quake was preceded by a 24-second-long foreshock signal that grew exponentially. They interpreted this activity as the nucleation phase or beginning of the Alaskan quake and suggested that transient stresses during nucleation may influence subsequent earthquakes.

The nucleation findings will provide much needed observational data that can be used to constrain theoretical models of earthquake nucleation, thereby providing insights into the physics of earthquakes.

A report on this work was published in Earth and Planetary Science Letters.