On July 15th a magnitude 7.8 earthquake struck the Fjiordland region of the southern island of New Zealand. On September 29th a magnitude 8.0 earthquake occurred near the Samoa Islands region triggering a tsunami which killed at least 150 people. On September 30th a magnitude 7.6 earthquake occurred near Sumatra and at least 1,100 deaths have been reported. Then, on October 7th a magnitude 7.8 earthquake occurred near Vanuatu, also in the South Pacific.

Is there a pattern here?

“The most likely scenario is that they are not related,” said Dr. John Cassidy, research scientist with the Geological Survey of Canada (GSC Pacific). “We still have a lot to learn about earthquakes. We are still learning about earthquake triggers and different parts of the global tectonics.”

According to Cassidy, on average there is one magnitude 8 or greater earthquake a year and 10 or 12 magnitude 7 or greater quakes/year somewhere in the world. Since 1900 there have been five megaquakes magnitude 9 or bigger. British Columbia’s biggest was an 8.1 in 1948 off Haida Gwaii. The fact is, though, the earth is quivering beneath our feet all the time. British Columbia experiences between 300 and 400 largely unfelt tremours a year.

The geology of plate tectonics is still a mysterious, complex and emerging science but some data is beginning to suggest a relationship between quakes even years apart in the same region.

“A change in the local stress field leads to changes in the rocks that could lead to future earthquakes,” explained Cassidy. “In Turkey we saw a big earthquake (magnitude 7.4 in 1999) and then a few years later we saw another earthquake further along the fault.”

Shock waves from a giant earthquake move through a region and trigger hundreds of tiny earthquakes elsewhere. “In the Indonesia region the plate tectonics is very complicated,” said Cassidy. “The plates are moving at 90 mm/year.”

“There is mounting evidence that a very large earthquake at a subduction or transform plate boundary can set up a subsequent quake centered on an adjacent part of the same fault,” agreed Dr. John Clague, SFU research professor and Canada Research Council chair in natural hazard research. “I think there might be a link between the Indonesian quakes and the megaquake in 2004.”

U.S. seismologists at Rice University in Texas have suggested the link might stretch even further. Their research, published in the journal Nature, claims that the magnitude 9 Indonesian quake in 2004 weakened a portion of California’s San Andreas fault.

“An unusually high number of magnitude 8 earthquakes occurred worldwide in 2005 and 2006,” said study co-author Fenglin Niu, associate professor of Earth science. “There has been speculation that these were somehow triggered by the Sumatran-Andaman earthquake.”

They examined more than 20 years of seismic records from Parkfield, California, which sits astride the San Andreas Fault. They zeroed in on a set of repeating micro quakes originating at almost exactly the same location and found that the fault strength changed markedly three times in that period. They related two of them to regionally nearby earthquakes. But they narrowed the onset of the third shift “to a five-day window in late December 2004 during which the Sumatran quake occurred”, hypothesizing a link between the two.

Much more research will be needed to clarify the complex interrelationships between one earthquake and another and a central part of that data-gathering is the constant monitoring of the tremours and jolts beneath our feet. The GSC, which has approximately 200 seismographs and accelerographs across the country, records every nuance of movement around the clock.

For a fascinating look, you can see them, live, on GSC’s seismogram at www.earthquakescanada.nrcan.gc.ca.