Scientists finally know how big earthquakes start: with many smaller ones

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

The finding offers unprecedented insight into what happens before moderate and large earthquakes — and scientists are finding that the vast majority of them occur after smaller earthquakes start rippling underneath the ground, sometimes days or even weeks before the main shock.

“One of the biggest questions in earthquake seismology is how earthquakes get started,” said the study’s lead author, Daniel Trugman, seismologist at Los Alamos National Laboratory. “We’re finding that most, if not all, of [significant] earthquakes are preceded by foreshocks that we can detect” with a new computing technique.

Previously, scientists observed that only half of all moderate quakes had precursor smaller events. Now, this new study of earthquakes in Southern California of at least magnitude 4 between 2008 and 2017 finds that at least 72% of them had earlier, smaller quakes.

https://www.latimes.com/california/story/2019-08-19/earthquakes-foreshocks-seismology-new-study