While many on B.C.'s South Coast, were rattled out of their beds on Dec. 29 by a magnitude 4.8 earthquake, they probably were not aware that thousands of silent tremors have been shaking along the coast over the past few weeks.

Natural Resources Canada seismologist John Cassidy said the West Coast has been experiencing "episodic tremor and slip" recently.

"Normally Vancouver Island is moving towards the mainland very slowly [but] Vancouver Island will change direction roughly every 14 or 15 months, and will slip backwards towards the ocean," Cassidy told All Points West host Robyn Burns.

"There's a very slight movement back to the west, before Vancouver Island carries on its regular direction of being squeezed towards the mainland. Vancouver Island does a little bit of a dance."

A regular occurrence

Cassidy said this occurs regularly because the West Coast is on a subduction zone where one tectonic plate (the Juan de Fuca plate) is moving under another one (the North American plate).

cascadia subduction zone

The Cascadia subduction zone is one of the most seismically active regions of the world. The earthquake on Dec. 29, 2015 did not happen in the 'locked' section but rather in the Juan de Fuca plate as it moved underneath the South Coast. (USGS)

Episodic tremor and slip generally lasts a few weeks, and causes "thousands and thousands of tiny tremors" that are too small to be felt.

"They look almost like the recording of a train where energy will build up very slowly and shake for a few minutes , very subtly shaking, and then it rolls off again," said Cassidy.

The same phenomenon also occurs in parts of Japan, Mexico and Alaska.

Provides useful information for seismologists

Cassidy said the tremors can provide useful information about the large earthquake that is expected on the West Coast because of the plates moving under each other at the Cascadia subduction zone.

"This new information has been really very useful in helping us to identify where exactly that locked fault is, and that helps us to predict or better understand ground shaking we can expect during these future earthquakes."

Cassidy said it is unknown if the recent spate of tremors had anything to do with the Dec. 29 earthquake.

"At this point we don't know if there's a direct link," he said.


To hear the full interview listen to the audio labelled: Thousands of tiny tremors have been occurring recently along South Coast as part of regular geological event