B.C. quakes raise fears of Big One
STRING OF TREMORS OVER HOLIDAYS CAUSES SCARE FOR RESIDENTS – AND ROYAL VISITORS
When Prince Harry went to Vancouver Island for Christmas he undoubtedly felt like he was getting away from all the rumbles of home. The Queen herself said as much in her Christmas address. It was a tumultuous year, mainly because of Andrew, capped off with a health scare for Philip, and personally difficult for the Sussexes, who sued a tabloid for intrusive coverage of Meghan.
But even as Yuletide fell on their Pacific Northwest holiday near Victoria, B.C., the royals were menaced by a more natural upheaval.
They were smack in tsunami range as the Cascadia fault seemed to threaten the Big One.
The day before Christmas Eve was seismically unusual in the waters off Vancouver Island, alarmingly so to the layperson, trained to expect the worst. A cluster of earthquakes was underway about five kilometres below the seafloor, one every couple of hours.
The first came at 8:44 a.m. local time, just off the northern tip of Vancouver Island, with a magnitude of 5.1. Then at 11:13 was a 5.6, and at 11:49 a 5.8. They were getting larger and more frequent. A 6.2 on Christmas Eve was enough to escalate the panic. Another on Christmas Day seemed fateful. All told, there were nine within a little more than two days, and not just any two days.
The potential consequences of these things are still fresh in the mind. Exactly 15 years ago, on Dec. 26, 2004, an earthquake off the coast of Indonesia triggered a tsunami that killed nearly a quarter of a million people.
But this was not that. Citizen reports started coming in to Natural Resources Canada, nearly 40 in all, of mild shaking, but no damage, nothing even knocked off shelves.
People sent reports from as far away as Richmond, where the sedimentary ground of the Fraser River Delta has a way of amplifying low frequency earthquakes enough that they can be felt in upper floors of tall buildings, according to Andrew Schaeffer, research scientist with Natural Resources Canada.
Frightening as these reports were, given the general understanding that a major quake in Cascadia is overdue, these clustered quakes were not heralds of the Big One. Small quakes do not trigger big quakes, except in rare circumstances, Schaeffer said. They are just not big enough.
It is still unusual to have so many large earthquakes in such a short span of time. But they are not unheard of. In the past 30 years, there have been 130 quakes of magnitude 5 or so in the same area.
“Statistically there’s nothing abnormal,” Schaeffer said.
These quakes were not even on the main fault between the North America and Pacific plates, which is where the expected catastrophic quake will occur. This recent sequence came from a smaller fault under the sea, known to be very active. It is in a transitional area between two major fault lines, Cascadia to the south, Queen Charlotte to the north.
There are several quakes here each week, but many are in the range of 2, so they get no attention, and are not even felt by anything other than the seismometers installed on the mainland and Vancouver Island, which can calculate location and intensity, with additional input from a network of seafloor devices.
These latest quakes were also caused by plates sliding and slipping against each other laterally, not falling vertically as one is forced underneath another. The first type shakes the ground, and can move the surface, as is visible for example along the San Andreas Fault in California. The second type causes tsunamis.
This is the great terror and promise of the Cascadia Subduction Zone, the area to the west of Vancouver Island and south into the U.S., where the eastward-moving Juan de Fuca plate will eventually slip and fall as it is pushed under the stationary continental plate of North America.
Vertical movement of the Earth’s crust under the ocean instantly displaces the water above, and it has to go somewhere. This is the wave. It will radiate eastward toward North America, where it will swamp coastal areas a few minutes later, not as a great curling wave, but as a rushing tide, full of debris.
It will also go west, where Japan and other lands across the Pacific will have longer to flee. There is nothing else to do.
Analysis of the long-term frequency of big subduction quakes in Cascadia puts it at a little more than 200 years, so in theory it is about due for another one. The last was in 1700, which was famously discovered via Indigenous oral history and confirmed with Japanese documentary records.
New research out of Oregon State University to be presented Friday at the American Geophysical Union suggests a quake in Cascadia can be “triggering” to the more southerly San Andreas Fault, meaning the entire west coast of North America could suffer a cataclysmic earthquake all at the same time.
But subduction events of the sort that threaten coastal Cascadia are not all that common. Strike-slip earthquakes of the kind that just happened are far more frequent. The Juan de Fuca plate is relatively young, and therefore relatively warm, and therefore relatively pliable, able to withstand greater distortion without falling, as compared to the older plate that causes the more common subduction earthquakes in Japan.
So for Prince Harry and all others in the Pacific Northwest, the Christmas 2019 burst of earthquakes is cause for wonder more than terror.
“It’s a reminder to just be prepared,” Schaeffer said.