Seismologists record more than 3,500 ‘tiny tremors’ on Vancouver Island in nine days

Seismologists record more than 3,500 ‘tiny tremors’ on Vancouver Island in nine days

Natural Resources Canada seismologist John Cassidy, otherwise known as Earthquake Guy, says these are not earthquakes but rather a good reminder that we live in an active subduction zone.

Seismologists have recorded more than 3,500 tiny tremors on Vancouver Island, but don’t panic, these aren’t earthquakes. That’s the last thing B.C. needs in 2020.

Natural Resources Canada seismologist John Cassidy, otherwise known online as the Earthquake Guy, posted a tweet early Thursday about the event. He says while these tremors don’t provide answers to when a major earthquake will hit, they “certainly remind us that we live in an active subduction zone.”

The cluster of tiny tremors is likely an ETS event, an episodic tremor and slip, said Cassidy. ETS is a process that occurs deep below the Earth’s surface along faults that form the boundaries of tectonic plates. It involves repeated episodes of slow fault slip of a few centimetres over a period of several weeks, accompanied by seismic tremors.

ETS events can help scientists better understand the quake hazard by studying how it affects the subduction fault beneath the Island, which is about 30 kilometres beneath the surface, and in Vancouver, at 60-to-70 km beneath the surface.

“This is helping us to map out where the subduction fault is completely locked and storing energy,” he said. “These look very different from earthquakes but we can get a better understanding of the process … There are still a lot of questions.”

During ETS events there is also a “slip,” where the region moves to the west over the course of a few weeks.

“This is quite remarkable when you think about Vancouver Island moving a few millimetres,” he said. “But it’s not clear whether this will impact that subduction fault.”

ETS events are a regular occurrence on the Island, and happen about every 14-to-15 months. The last ETS event on southern Vancouver Island was one year ago, said Cassidy.

“These tiny tremors that we locate … have a very different appearance on our seismographs (than quakes). Energy builds up slowly and drops off slowly. They are not felt but can be detected and located by our very sensitive seismographs,” he said. “It’s the slip that we are really watching for.”

The Cascadia Subduction Zone, which runs from northern Vancouver Island to Northern California, is Canada’s most active earthquake region.

As for a major quake, Cassidy says scientists still can’t say when this will happen or how large the next one will be, but huge strides have been made in understanding temblors and how often they strike an area. Unlike 20 years ago, seismologists now know that B.C. has seen magnitude-nine earthquakes and that they occur roughly every 500 years or sometimes as short as 200 years between quakes.

“We know the last one struck on Jan. 26 of the year 1700. So we know the exact date and time,” said Cassidy.


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