Why British Columbia's "Tsunami Warning" Never Really Happened

Yesterday, Canadians living along the coast of British Columbia were shocked awake in the early morning by a tsunami warning

Sirens blared in the dark after a 7.9 magnitude earthquake struck an area southeast of Chiniak, Alaska at 1:31am, alerting citizens of the impending danger. 

But the warnings were just that, warnings. Nothing ever came to be, as no tsunami (thankfully) never formed and devastated B.C. coastal communities. 

The question remains: why the heck not? 

Based on general tsunami data, yesterday’s earthquake should have caused a gigantic sea wave. Earthquakes that are 7.5 and above on the rector scale and not too deep into the water tend to create massive tsunamis, and that’s exactly how yesterday’s earthquake has been described. 

So, again, if the earthquake met the standards for tsunami-generation, why did nothing happen?

A CBC seismologist has the answers. 

Johanna Wagstaffe explained that yesterday’s earthquake was of the “strike-slip” variety. That’s when two tectonic plates (the giant hunks of rock that make up the earth’s crust and are constantly moving) hit together and move horizontally. 

Unlike a “megathrust” earthquake, Wagstaffe said, strike-slips don’t cause a lot of vertical displacement. Basically, the plates crashing together didn’t cause a “punching up” effect, so ocean water wasn’t sent careening upwards. 

Instead, water was moved less intensely by the horizontal motion caused by the strike-slip earthquake. 

Interestingly, if the earthquake occurred only 90km to the west, then the opposite would have likely occurred. A megathrust ‘quake would have happened and there may have been a gigantic tsunami, said Wagstaffe. 

For a more full explanation of the earthquake-tsunami effect, complete with diagrams, head over to the CBC article quoting Wagstafffe here