The vast majority of earthquakes we feel come soon after smaller ones, according to new research that provides unprecedented insights into how seismology works.

Sometimes days or even weeks before most temblors of at least magnitude 4.0, scientists have found, smaller ones start rippling beneath the Earth’s surface — activity that can be detected thanks to an advanced computing technique.

“One of the biggest questions in earthquake seismology is how earthquakes get started,” said the study’s lead author, Daniel Trugman, a seismologist at Los Alamos National Laboratory.

Previously, scientists observed that only half of all moderate quakes had smaller precursor events. This new study of earthquakes in Southern California of at least magnitude 4 between 2008 and 2017 found that at least 72% of them followed less-powerful quakes.

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