We use the techniques we’ve learned on Earth to understand our world to now understand Mars. But this isn’t to say we actually understand Earth. One particularly terrifying event 1700, for instance, is now getting new attention. 

On January 26, 1700, an earthquake killed the population of large swaths of Vancouver Island and the surroundings, and also sent a tsunami sweeping across the ocean all the way to Japan where it was still able to do damage. This was triggered by a slip in the Cascadia fault line. Concern that this fault line could once again trigger an earthquake around magnitude 9 influences building construction in the northwest of the United States and Canada and is part of the justification for earthquake and tsunami alert systems. These systems, however, assume all the damage was triggered by a single quake. 

Now, researchers modeling the damage spoken of in oral histories of the First Nations people in that region and marked in the geological record are considering the possibility that this event wasn’t just one horrifying earthquake but may actually have been a series of strong earthquakes over several days and years. According to study author Diego Melgar: The tradition has been only ‘a mega-quake explains everything,’ and what I found is that’s not true. A megaquake still can explain everything, but so can a sequence of events.

This is decidedly not good news. While a single massive magnitude 9 or so earthquake is horrifying, a series of magnitude 8 earthquakes would continue to damage and re-damage the region, making rebuilding efforts perhaps feel futile during the decade of devastation. To understand which of these two scenarios – a megaquake or a series of massive quakes – matches reality, a detailed study of the coastal rocks is needed. 

It is well understood that the coastline changed as a result of the quake or quakes with the coastline subsiding, or dropping, as the faultline shifted. If the shift occurred all at once, the record of small organisms living on the rocks will show a sudden change along the length of the coast, while a series of quakes and piecemeal subsidence would cause the organisms on the rocks to change over time.

Either way, I’m fascinated, and Pamela continues to refuse to move to Vancouver.

I do admit to being really worried about that faultline. When the Cascadia fault slipped, ocean water was sloshed into the mountains where it created saltwater marshes and killed trees. While I love the science of this, I choose to live where there are tornadoes instead of tsunami. We all have our limits.

More Information

Was a humongous Cascadia earthquake just one of many? (Live Science)

“The 26 January 1700 Cascadia Earthquake as Part of an Event Sequence,” Diego Melgar, 2021 April 20, 2021 Annual Meeting (abstract)