(CBS SF) — One hour before a 4.1-magnitude earthquake shook the Napa area Thursday night, a rare 4.2 quake woke up people living near London.

Was it a pure coincidence or some connected seismologic event?

“The timing of these two magnitude-4 events must be complete coincidence,” UC Berkeley earth and planetary scientist Roland Burgmann told CBS SF. “We are very interested in how great earthquakes can sometimes trigger other events at such great distances, but it appears to take events of a magnitude less than 7 to accomplish that. That is more than 30,000 times the energy of a magnitude 4.”

A 2012 study published in the journal Nature found that the massive 8.6 Sumatra earthquake in April 2012 triggered other global earthquakes within a few hours and continued to generate temblors up to six days later.

The findings are a reminder that those living along active fault lines could also be at risk from a large earthquake from the opposite end of the globe.

“Until now, we seismologists have always said, ‘Don’t worry about distant earthquakes triggering local quakes,’” said Burgmann, a coauthor of the study. “This study now says that, while it is very rare – it may only happen every few decades – it is a real possibility if the right kind of earthquake happens.”

Thursday’s tremor stuck near the English town of Sandwich at 6:52 p.m. PDT Thursday, about an hour before the  Yountville quake. No damage or injuries were reported in either quake.

About 200 earthquakes take place in the United Kingdom every year but not all are as large as this tremor, according to British Geological Survey seismologist David Galloway.

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