Measured and Perceived Shaking from a Vancouver Island Earthquake

Victoria, British Columbia experienced a tiny earthquake this afternoon. It poses no threat, but its location within a dense seismograph network and proximity to population centers make it an ideal case study for comparing measured and perceived shaking.

The earthquake was located just off the southeast tip of Vancouver Island at 1:37 pm local time, adjacent to Victoria and within a populated island group. The magnitude 3.0 or smaller earthquake is so small as to pose no threat to people or infrastructure. The region is densely populated with seismometers installed to track any motion in the Cascadia fault zone.

If you live within the region, please fill out a Did You Feel It? report, whether you felt it or not. Null reports are important to keep from over-estimating perceived shaking, while positive reports are useful for understanding how superficial geology and building design amplify or dampen seismic waves.

The seismic signal from the earthquake is well-recorded, with nothing all that unusual about it. It's being reported as a M2.3 or M3.0 by different sources. An earthquake that small is roughly equivalent to the rumbling of a truck going past, with the shaking lasting only a few seconds.

Measured and Perceived Shaking from a Vancouver Island Earthquake

Seismic signal of the M3.0 earthquake near Victoria, BC on June 16, 2014.

At the time of writing, the Did You Feel It? map is based on 141 responses.

For the most part, it makes sense: people farther away didn't feel it at all. It does have one interesting feature for the moment, in that an island farther from the epicenter apparently felt the earthquake more strongly than the closer islands, and more strongly than the magnitude of shaking. This could be an over-estimate from insufficient data points, an exaggerated report from a freaked-out island, or could indicate that the island has more shake-amplifying sediment than most of the glacial-scoured bare-rock Gulf Islands.

The Pacific Northwest is bordered by the Cascadia subduction zone. The Juan de Fuca plate is just offshore of the region.

Measured and Perceived Shaking from a Vancouver Island Earthquake

The plate terminates in triple-plate junctions with the Pacific and North American plates just offshore of northern California and the north tip of Vancouver Island. To the west, the Juan de Fuca meets the Pacific plate at a divergent junction, where a mid-ocean ridge produces tiny earthquakes and new oceanic crust. On its eastern boundary, the Juan de Fuca is subducting beneath the North American Plate, a plate boundary capable of producing massive magnitude 8 or even magnitude 9 earthquakes.

This earthquake was within the Cascadia subduction zone, but is not from the main fault, which is offshore of Vancouver Island. Its location and shallow depth indicate it's a minor shift on some smaller fault. I've previously worked mapping seismic fault lines in British Columbia. While I can't tell you anything specific about the maps I've made (not even to identify if this earthquake lines up to a known fault or not), I can tell you that I was shocked and saddened by how scarce the data is when compared to similar maps for California. Mapping the exact location of faults in Western Canada is a tricky business from both a scientific and a political standpoint.

From a scientific perspective, earthquakes in subduction zones are generally less frequent and more intense than earthquakes in transform boundaries (like California). Without many earthquakes, it's difficult to actually identify where active faults are. We can identify some historic faults that may still be active by looking for areas of offset, but sediments deposited since the last Ice Age can bury and hide faults. Western Canada is also a temperate rainforest, with a lot of trees and not much exposed bedrock for geologists to map. Unless they move, those hidden faults can lurk undetected until they activate with a new earthquake.

The next problem is political: Canada is a country with a small population, and a correspondingly small budget. Worse, it lacks a central agency tasked with disaster mitigation, so the task of studying, anticipating, and responding to natural catastrophes is split amongst many agencies with variable cooperation and communication.

While Washington state had the resources and organization to fly a LiDAR survey using lasers to create detailed topography maps and spy out offsets that identify faults, the other side of the border lacks this kind of mapping. For example, check out how many of the red fault lines terminate at the national border in the USGS basemap for their shakemap [left].

While geologists and seismologists gossip about possible faults under the Strait of Georgia, within the Fraser River's delta, hidden in the thick forests of Vancouver Island, or cutting below the major metropolitan centers of Victoria and Vancouver, we don't have the tools, resources, support, or money to actually look for those potential faults. British Columbia has a fault map, but it could really use some dedicated love, attention, and funding, preferably before a major earthquake strikes.

If you live in Victoria, southern Vancouver Island, or the Gulf Islands, give your friendly neighbourhood geologists a hand. Fill out a Did You Feel It? report, and help us gather the data that we otherwise don't have the tools to collect. And if you're a bit surprised that we don't have a detailed fault map for British Columbia, feel free to nudge your political representatives that we really shouldn't wait until The Big One to get a proper geophysical investigation of the seismic hazard near one of the biggest cities in Canada.