Big quakes start with tiny shakes
United States
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 strike, scientists have found, smaller ones start rippling beneath 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 had observed that only half of all moderate quakes had smaller precursor events. The new study of earthquakes in southern California of at least magnitude 4 between 2008 and 2017 found that at least 72 per cent of them followed less powerful quakes.
‘‘It’s important for understanding the physics of earthquakes,’’ said study co-author Zachary Ross, an assistant professor of geophysics at the California Institute of Technology. ‘‘Are they silent until this big event? Or is there a weakening process of the fault, or some evidence that the fault is changing before this larger event?’’ The study shows that the answer is probably the latter explanation.
The discovery gives scientists a better understanding about how earthquakes are generated.
Knowing that even moderate quakes probably occur after a series of less powerful ones gives added weight to the idea that quake sequences can grow, not unlike a spreading disease epidemic. In fact, the study shows that the foreshock sequences started three to 35 days ahead of the main shocks.
The finding doesn’t mean researchers are any closer to predicting the exact timing and epicentres of big quakes. But understanding how quakes get bigger can help scientists get better at aftershock forecasting.
The discovery could also help to improve the speed of earthquake early warning systems, Ross said.
The breakthrough in the study, published in the journal Geophysical Research Letters, was made possible by the discovery of a new technique to find very small earthquakes – as small as magnitudes 0 and 1.
The results help to solve a long mystery that scientists had not been able to explain. In laboratory experiments where they simulated quakes with sensitive equipment, there would always be small quakes that came before the main quake. – LA Times