LOS ANGELES 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 the Earths 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 studys 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.
Elevated foreshock activity is pervasive in Southern California, the study concluded.
It is surprising, said study coauthor Zachary Ross, an assistant professor of geophysics at the California Institute of Technology. Its important for understanding the physics of earthquakes. 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 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 earthquake sequences can grow, not unlike a spreading disease epidemic. In fact, the study shows the foreshock sequences ranged from starting three days to 35 days ahead of the mainshock.
The finding doesnt mean we should suddenly be worried about small quakes. Statistically speaking, only 5% of earthquakes are followed by something worse. It also doesnt mean researchers are any closer to predicting the exact timing and epicenters of big earthquakes.
The vast majority of time that you have an earthquake, Ross said, even if you see anomalous activity start up, its going to die down on its own thats most of the time.
But understanding how quakes get bigger can help scientists get better at aftershock forecasting. That would help the public understand when theres a greater risk, such as when the chance of a large quake rises from a background risk of 1-in-10,000 odds to 1-in-1,000 odds, based on a previous quake.
We are definitely moving toward forecasting that is statistical in nature, Trugman said.
The discovery could also help improve the speed of earthquake early warning systems, Ross said. If the computer has detected microquakes close to a major fault, and knows that most major quakes are preceded by smaller foreshocks, that can help speed up the decision by the system to issue a warning in the moments after an earthquake has begun rupturing along a fault.
The breakthrough in the study, published earlier this summer in the journal Geophysical Research Letters, was made possible by the discovery of a new technique to find very small earthquakes quakes as small as magnitudes 0 and 1, and some as small as magnitude negative 2.
