Scientists at Los Alamos National Laboratory have found that most moderate and large earthquakes are preceded by tiny earthquake clusters, sometimes even weeks before the main shock, according to the LA Times.
So much for the 'relieving pressure' argument.
"One of the biggest questions in earthquake seismology is how earthquakes get started," said the study's lead author, seismologist Daniel Trugman. "We’re finding that most, if not all, of [significant] earthquakes are preceded by foreshocks that we can detect" using 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. -LA Times
"Elevated foreshock activity is pervasive in Southern California," concludes the study.
"It is surprising" says co-author Zachary Ross, an assistant professor of geophysics at Caltech, adding "It’s 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?"
It sure looks that way - and according to the study, foreshock sequences can start anywhere from three days to 35 days ahead of the mainshock.
That said, it should be noted that just 5% of earthquakes are followed up by something worse.
"The vast majority of time that you have an earthquake," said Ross, "even if you see anomalous activity start up, it’s going to die down on its own — that’s most of the time."
But understanding how quakes get bigger can only help scientists get better at aftershock forecasting. That would help the public understand when there’s a greater risk, like 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 early moments after an earthquake has begun rupturing along a fault. -LA Times
The study was made possible by a breakthrough in detecting particularly small earthquakes - as small as magnitude 0 and 1, with some as small as magnitude negative 2, which is apparently possible. Unfortunately, this can't be done in real time yet - as past quake data is fed into a supercomputer which in turn crunches the data for several weeks.
"This new information is coming from the tiniest magnitude events that were basically invisible before," said Ross.
The team focused on 46 of the largest Southern California earthquakes between 2008 and 2017, finding that 33 of the 46 had a statistically significant increase in foreshocks vs. the normal rate of earthquakes for the region.
The March, 2014 5.1 magnitude La Habra earthquake, for example, had foreshocks in the 0 ane 1 mag range as early as 17 days before the main shock.
The 2010 Easter Sunday magnitude 7.2 earthquake widely felt in Southern California was not included in the analysis, since its epicenter was in Baja California. But that earthquake was preceded by a notable foreshock sequence.
The scientists could not determine a specific pattern to the foreshocks that would lead to a magnitude 4 or greater quake. Sometimes, it would appear as a burst of quakes near what would become the mainshock epicenter days or hours later. Other times, it would appear as a widespread increase in the earthquake rate in the general area before the mainshock. -LA Times
Also noted is that shallower mainshocks were preceded by more foreshocks, while areas with higher heat flow such as the Coso Volcanic Field and the Salton Sea experienced elevated foreshocks as well.
"It’s never just silent until the final failure," said Ross. "Physically, that seems a little difficult to imagine."