The year has begun with a series of earth-shaking events. Three earthquakes were reported in the Vancouver Island region on Jan. 2. The biggest, at magnitude 5.4, occurred 211 kilometres west of Port Hardy, while two smaller ones occurred west of Port Alberni. Five days later, a 4.8-magnitude quake west of Port Alice shook the coast.
They form part of a regional swarm of earthquakes that began late last year, as the tectonic plates beneath Vancouver Island released rock-bending pressure. To add perspective, about 4,000 earthquakes occur in B.C. every year. Of these, only a few — such as the larger Jan. 2 and Jan. 7 quakes — are felt by people.
As solid and secure as the ground beneath our feet seems, when the forces that shape our planet’s surface start squeezing it, the granites, basalts and sedimentary rock on which our region’s municipalities are built take on the consistency and strength of something like aged cheese.
Softer sedimentary rocks, such as those of the Georgia Basin, fare worse. Two University of British Columbia studies published last year show that when seismic waves pass through these layers of silt, sand and glacial deposits, the sediments take on wobblier characteristics that resemble gelatin. The softer rocks amplify the waves, causing greater shaking for longer periods of time and increasing risk for damage to buildings, bridges and other structures.
The seismically amplifying effects of the Georgia Basin’s sediments might apply to soft sedimentary rock and seafloor sediments elsewhere.
This underscores the complexity of our region’s geology. Geologists and seismologists must account for massive fragments of the Earth’s crust scraped off one tectonic plate onto another many millions of years ago, as well as the effects and deposits from Ice Age glaciation, types of bedrock, numerous fault lines, and movement, pressure and jostling of underlying tectonic plates. New theories are constantly being proposed, and new information uncovered.
For example, one University of Victoria geologist proposes that much of B.C. is made from the remains of an ancient continent bulldozed onto the edge of North America tens of millions of years ago. He says this theory, which is not accepted by the larger geology community, would explain many puzzling aspects of B.C.’s geology.
In another example, 10 significant new fault lines have been discovered in the Pacific Northwest in the last 15 years. The Leech River Fault, which follows the arrow-straight and steep-sided Leech River Valley, then passes beneath Langford and Colwood before disappearing into the sea-floor off Hatley Castle, is now believed to connect to two well-known U.S. faults.
One of these, the South Whidbey Island Fault, is considered a significant-risk fault by the U.S. Geological Survey. The other, the Devil’s Mountain Fault, extends from Victoria’s waterfront to the foothills of Washington’s Cascade Range. The faults might interact in ways we don’t yet understand — possibly even redistributing and relieving stress from one to another.
In another study, local researchers plotted the earthquake history of B.C.’s south coast by radiocarbon-dating sediments and sediment disturbances in Barkley Sound’s Effingham Inlet. The scientists identified 22 major earthquakes — damaging, mega-thrust events like the 2012 earthquake in Japan and the 2004 quake off Sumatra — occurring at 200- to 800-year intervals during the last 11,000 years. The last of the quakes, about 9.0 in magnitude, took place Jan. 26, 1700 — a precision verified by multiple sources of evidence, including historical tsunami records from Japan and First Nations accounts recorded by early British explorers to the coast.
What does all this new information tell us? Not much that’s truly helpful for individual area residents.
Yes, we’re due for another major earthquake, but it might happen a century or more from now. Yes, it would be best to avoid building highrises near faults that connect to active Pacific Northwest fault complexes. Yes, building codes need to consider the effects of underlying sediment and rock during earthquakes.
Most of all, it reminds us again that we all need to be prepared.
