Researchers have another clue that could help predict the “big one,” and it has to do with the large earthquakes that strike Vancouver Island almost annually.
Every 14 months, Islanders experience the equivalent of a magnitude-7 earthquake — it just happens so slowly and so deep that you can’t feel it.
“Slow earthquakes” occur over a period of days or weeks like clockwork, in contrast to the damaging earthquakes that happen in seconds or minutes.
Now scientists believe they know why slow earthquakes occur with such regularity, adding to the understanding of the earthquake cycle as a whole.
“Every time you have a slow earthquake just below, the part of the fault that will slip in the next big earthquake gets a little bit of a push,” said Roland Bürgmann, a seismologist at the University of California, Berkeley.
“The question everybody is asking now is: Will the next big earthquake happen when one of those small, slow earthquakes happens below? And more importantly, is there going to be something different about these slow earthquakes right before the next big one? Are they going to start behaving differently?”
Bürgmann co-wrote a study published Wednesday in Nature with University of Ottawa geophysicist Pascal Audet, offering insight into the regularity of slow earthquakes.
Slow earthquakes were only identified for the first time about a dozen years ago, on Vancouver Island.
Audet, who was the lead author on the study, said he began his research with an observation: Slow earthquakes occur like clockwork.
But while they happen every 14 months in the Vancouver Island area, they occur every six months in Japan and every two years in New Zealand.
“What was mysterious was this difference in repeat times of these slow earthquakes,” Audet said.
“Our study tries to address this problem.”
The answer, he believes, lies in the circulation of fluid through the fault zone.
Audet compared seismic data, as well as the characteristics of tectonic plates at several slow earthquake sites.
Areas with frequent slow earthquakes had higher concentrations of quartz in the crust, which forms when fluids rise and cool quickly.
“So [slow earthquake zones] have to involve lots of fluids and the amount of fluids changes the way these slow earthquakes behave,” Bürgmann said.
They believe the fluid builds up until the plate slips.
More data and observation is needed to fully understand the complex relationship between slow earthquakes and more damaging ones.
But the change in fault behaviour between regular earthquakes and deeper ones helps paint a clearer picture of what’s happening below the surface.
“We can use that information in computer simulation on how earthquakes happen, in order to improve our understanding of the earthquake cycle as a whole,” Audet said.
