In May 1997, a massive earthquake trembled the Kermadec Islands region in the South Pacific Ocean. Roughly over 20 years later, in September 2018, a second earthquake struck the exact location, with waves of seismic energy originating from the same region.
“Though the earthquakes occurred two decades apart because they occurred in the same region, they’d be expected to send seismic waves through the Earth’s layers at the same speed, “said Ying Zhou. He is a geoscientist at the Virginia Tech College of Science Department of Geosciences.
However, in data reported at four of more than 150 Global Seismographic Network stations that log seismic tremors in real-time, Zhou discovered an abnormality among the twin events. During the 2018 earthquake, a set of seismic waves recognized as SKS waves traveled about one second quicker than their counterparts experienced in 1997.
According to Zhou, whose discoveries were recently published in Communications Earth & Environment, the one-second discrepancy in SKS wave travel duration provides an essential and remarkable glimpse of what’s occurring more profoundly in the Earth’s interior, in its outer core.
Scientists have only been able to theorize about the source of gradual shifts in strength and direction of the magnetic field that has been identified, which likely encompasses changing flows in the outer core.
“If you look at the north geomagnetic pole, it’s currently moving at a speed of about 50 kilometers [31 miles] per year,” Zhou explained. “It’s moving away from Canada and toward Siberia. The magnetic field is not the same every day. It’s changing. Since it’s changing, we also speculate that convection in the outer core is changing with time, but there’s no direct evidence. We’ve never seen it.”