Apocalyptic Glimpses II: The Big Ones

by Zbigniew

This is the second in a series of three posts looking at disasters that have befallen Vancouver in fact and fiction. Fire & the Flood considered a veritable buffet of historic catastrophes, from fire and structural failures, to landslides, debris flows and floods. This posting is dedicated to earthquakes and the next will look at fictional representations of disaster.

bc-earthquakes

Earthquakes occur when interruptions in tectonic plate movements produce sudden slips past one another, collisions or divergences, which release energy in the form of seismic waves that causes the ground to move and shake.

The region comprised of the southwest coast of Canada and the northwest of the United States is one of few areas in the world where all three of these types of plate movements take place, resulting in significant earthquake activity. As the geological and historical record makes clear, earthquakes in this area -including “The Big One”- are an existential reality.

The physics of earthquakes is too complicated for a venue dedicated to wiseassery and snark. What follows is an overview of some basic information to inform a consideration of the history and future of Pacific Northwest earthquakes.

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There is a confusing array of systems to measure the energy released by an earthquakes. Thankfully, most magnitude scales have been designed to give numerically similar results. Because these measures of magnitude are a base-10 logarithmic scale, an earthquake with a magnitude of 6.0 will be 10 times more powerful than one measuring 5.0, and a magnitude 7.0 quake will release 10 times more energy than a 6.0 and 100 times more energy than a 5.0. The earthquake that struck Japan in 2011 was 10,000 times more powerful than the one that shook the Pacific Northwest on December 29th 2015.

But this is just one consideration. Ultimately, a quake’s destructive capacity depends not only on its strength, but also on location, distance from the epicenter, depth, and duration.

The location below the earth’s surface where the earthquake originates is the hypocenter, while the location directly above it on the surface is the epicenter.

 

cascadia-subduction-zone

The “Ring of Fire” is an earthquake-prone basin surrounding the Pacific Ocean. Recent years have seen major earthquakes at various points along this ring, including Indonesia, Chile, and Japan. Several significant earthquakes have struck New Zealand in recent years, the latest on November 13th, 2016.

The Cascadia Subduction Zone also forms part of this ring; specifically, it lies west of Vancouver Island and extends from beyond the northern tip of the Island to northern California. The Juan de Fuca plate, which lies about 45 km beneath Victoria and about 70 km beneath Vancouver, is sliding (subducting) beneath the much larger continental plate at rate of about two to five cm/year.

csz-cross-section

There are three types of earthquakes that have struck -and will again strike- the Pacific Northwest.

The most common is the crustal or shallow crust earthquake . These originate in the North American plate at depths of approximately 30 kilometres. These shallow earthquakes account for the vast majority of the hundreds of small quakes that occur every year within the region.

However, these shallow quakes tend to be more damaging than deeper quakes of comparable magnitude. As seismic waves from deep quakes have to travel farther to the surface and lose energy along the way, small but shallow earthquakes can produce significant damage. This is evident in the August 24th, 2016, earthquake near Accumoli, Italy. The moderate-to-strong quake measured a magnitude of 6.1 but was very shallow, with a hypocentre only five kilometres beneath the surface. The quake produced extensive damage to the brick and mortar structures in the area, essentially levelled the town of Pescara del Tronto, and killed 300 people.

Occasionally, crustal quakes have large magnitudes. Locally they have been recorded up to 7.3.

Crustal quakes tend to produce aftershocks.

The second type is deep, or intraslab, earthquakes. These subcrustal earthquakes are concentrated in two areas within our region: 30 to 40 kilometres below the west coast of Vancouver Island and 50 to 60 kilometres below the Straight of Georgia, within the Juan de Fuca Plate but beneath the North American Plate. The larger earthquakes in the region tend to be intraslab, with a maximum magnitude of about 7. It’s rare for an intraslab quake to produce aftershocks.

A subduction zone or megathrust earthquake is the rarest but by far the most damaging type. At the contact point of two plates, pressures can build and when this becomes excessive can be suddenly discharged. Subduction earthquakes can have magnitudes of 8.0 or larger. The 2011 Tōhoku earthquake of March 11, 2011, had a magnitude of 9.0, enough to move the main Island of Honshu 2.4 metres eastward. The quake caused 16,000 deaths, displaced a quarter of a million people, and produced extensive infrastructure damage. Loses are estimated at US $300 billion. Japan is a leader in earthquake preparedness.

As I write this, Earthquaketrack notes that for a magnitude of 1.5 or greater, across British Columbia there have been 494 earthquakes in the past year, 39 in the past month, 10 in the past seven days, and two earthquakes today. A quake with a magnitude of 3.0 and an epicenter off Port Townsend, Washington, manifested itself around 9:00 AM. (I recall feeling a slight shaking this morning -strong enough to dismiss truck traffic six stories below as the cause.)

While most of these earthquakes are too modest to be felt, on average our region is subjected to a magnitude 5.0 quake every five years, a 6.0 every 20 years, and a magnitude 7.0 every 40 or 50 years.

Subduction quakes appear to strike British Columbia -on average- about every 500 years. Or less. Or more. It’s unclear.

While earthquakes can’t be predicted with any accuracy, the historical record provides a good illustration of the kind of quakes that have hit this area and the damage produced. What follows is an inventory of those local quakes –that is, those tied to the Juan de Fuca plate and the area immediately adjacent on the continental plate. Excluded are several substantial earthquakes recorded near Haida Gwaii and Alaska. This inventory runs from a recent and modest quake -which will serve as a benchmark- to more significant quakes reaching back to 1700.

On December 29th, 2015, at 11:39 PM a 4.7 magnitude earthquake occurred, with an epicenter near Sidney Island off the Saanich Peninsula and a hypocenter about 50 kilometres underneath the Earth’s crust. This moderate intraslab quake was felt from the northern tip of Vancouver Island, south as far as Seattle, and east through the Fraser Valley. Although no damage or injuries were reported, it was the largest earthquake in the region in more than a decade.

Although I was on Bowen Island that night, the situation of my temporary abode on bedrock meant no Hollywood-style ripples disturbing my martini. Of the stories I heard on my return, my mother best quantified the power of the quake. She noted that several pictures were shaken off the walls of her sturdy Vancouver Special. “Moderate,” perhaps, but in her 50 plus years in Vancouver she had never experienced anything similar.

The February 28, 2001, the Nisqually (Ash Wednesday) earthquake near Olympia Washington was also an intraslab earthquake, this one measuring 6.8. Four hundred people were injured, with two indirectly related deaths. Although most of the property damage occurred near the epicenter, damage to the City of Seattle -some 90 kilometres away- totaled $36 million.

The April 29, 1965, Puget Sound earthquake, was an intraslab event measuring 6.7. It caused seven deaths and $12.5 million in damage.

An April 13, 1949, magnitude 7.0 earthquake near Olympia caused eight deaths, many injuries and property damage estimated at upwards of $25 million. At Olympia nearly all large buildings were damaged, water and gas mains were broken, and electric and telegraphic services were interrupted. Near Tacoma railroad bridges were thrown out of line and the quake produced a “tremendous” rockslide.

The 7.3 magnitude quake of June 23, 1946, had its epicenter at Forbidden Plateau near Courtenay. This crustal quake is the largest onshore quake ever recorded in Canada, and was felt from Prince Rupert to Portland. In Vancouver buildings oscillated dramatically, a gas line cracked, war veteran families housed in the Hotel Vancouver fled from an earthquake-induced fire, and the Lions Gate Bridge “swayed like a leaf.” The quake triggered more than 300 landslides, and there were numerous instances of liquefaction -the phenomenon of saturated or partially saturated soil behaving like a liquid– especially along the east coast of central Vancouver Island. Two deaths were attributed to the quake.

On Friday, December 6, 1918, a crustal quake measuring approximately 7.0, with an epicenter near the Estevan Point and Nootka lighthouses and was followed by at least 14 aftershocks. While the quake was felt as far away as Kelowna, no injuries and little damage was reported –in part the result of the limited population residing in the area.

On the afternoon of January 11, 1909, an earthquake measuring 6.0 and centered in the Gulf/San Juan islands was felt throughout the Puget Sound area, including Vancouver, Victoria and Seattle. No damage reported.

On December 15th, 1872, an earthquake with an estimated magnitude of 6.5 to 7.0 struck with an epicenter near Lake Chelan in Western Washington. The quake was felt from British Columbia to Oregon and from the Pacific Ocean to Montana. While the quake occurred in a sparely populated area with few structures, the impact was extensive. Landslides were reported throughout the Cascades, including one that blocked the Columbia River for several hours. Fissures also occurred throughout the area -one produced sulfurous water and another generated a 9-meter-high geyser for several days.

fault_labeled

These events pale in the face of an earlier cataclysm.

Late on January 26th, 1700, waves reaching a height of five meters struck the east coast of Japan, destroying homes and other structures, generated fires, flooding fields, and other damage and loss of life. A tsunami, but without an earthquake –at least not a local one.

The oral histories of the First Nations along the British Columbia coast –those that survive the population decimations that came with European contact- refer to a cataclysmic event that occurred many generations previously -histories supported by geological evidence.

In the 1980s and 1990s, scientists found substantial evidence that in the early 1700s coastal lands in the Northwest dropped suddenly and were inundated by tsunami waves and mud. From southern Washington to Northern California, carbon dating of spruce, peat and fossilized plants attributed their sudden death to high waters. Pursuing these and other clues, researchers have determined that at about 9:00 pm on January 26, 1700, a 1,000 km stretch of the Cascadia fault “broke”, producing a subduction earthquake with a magnitude of 8.7 to 9.2. The energy released generated waves that crossed the Pacific and hit Japan nine hours later.

Evidence also suggests that 19 megathrust quakes have struck the British Columbia coast over the last 10,000 years.

Another will strike. The Juan de Fuca and Cascadia plates are currently locked and pressure is building. When the tectonic plates on the Cascadia Subduction Zone suddenly slide past each other, the Cascadia plate could spring 10 to 15 metres westward through four or five minutes of intense shaking.

When exactly this energy will be released is uncertain. The odds of such a quake occurring tomorrow are very low. The chances of a such a quake happening in the next 500 years are very high. Most estimates suggest a 10% to 15% likelihood over the next 50 years, although others rate the danger as high as 33% to 37% for the same period.

If a megathrust doesn’t get us, a crustal or subcrustal might. Natural Resources Canada argues that there is a 30% chance of an earthquake big enough to cause significant property damage hitting southwestern B.C. within the next 50 years.

bc-earthquake-hazard-map

What kind of damage could we expect form a major earthquake? The immediate impact would be intense shaking and, in some places, liquefaction. The associated impact would include almost all the potential disasters highlighted in the previous post on this subject –structural failure and fire, landslides, and coastal flooding.

During an earthquake buildings of unreinforced masonry can collapse or shower bricks to the ground below, while the older generation of brittle-concrete buildings will not flex but crack or break. Seismic building codes were first introduced in Canada in 1953, with modern standards introduced in 1973. Despite all the subsequent development, 60 per cent of Vancouver’s buildings were constructed before the modern building codes were introduced, including such landmarks as the Sun Tower, St. Paul’s Hospitals, Waterfront Station, and Holy Rosary Cathedral, as well as numerous single room occupancy hotels in the Downtown Eastside, many apartments in the West End, and buildings along Kingsway and Broadway and in the Fraser River industrial and commercial zone. And schools: as of a year ago, 342 British Columbia schools still required seismic upgrades, with 128 yet to participate in the upgrade plan. There are 68 high-risk schools in Vancouver alone.

Awaiting Seismic Upgrades - Metro Vancouver SchoolsCBC.ca

In one City of Vancouver scenario, a 7.3-magnitude earthquake in the Strait of Georgia is estimated to damage 90,000 buildings in Metro Vancouver, 20,000 of them beyond repair. Despite the benefit of the updated seismic codes, those figures could be conservative.

As a BBC documentary on the potential Pacific Northwest megathrust quake notes, building codes have never been tested by such an event: “The lessons haven’t been learned yet.”

Of the nearly 400,000 buildings destroyed in the 2010 Chile earthquake, most were newer high-rises, buildings designed with fewer and thinner “shear walls” -internal concrete bracing that resists shaking– in order to fit more spots in underground parking garages and create more interior space. Many new high-rises in Vancouver and across North America are built to similar standards. A University of British Columbia Earthquake Lab study found buildings made with six-inch concrete support walls aren’t as resistant to earthquakes as previously believed, despite being fully compliant with building codes.

And how well have those codes been respected? The Lower Mainland’s construction boom has brought with it all kinds of shady dealings, from dirty money to shadow flipping, all in the service of a quick buck. What would prevent an unscrupulous developer -a few of those about- from cashing in by cutting corners? Defraying costs to future generations is the scourge of our times.

Furthermore, the effects of an earthquake will not be evenly distributed. “Seismic microzonation” means that some geographic pockets can be up to five times more prone to shaking, liquefaction and damage. Consider all those towers built on loose Richmond silt, or False Creek infill, exactly the kind topography that experienced the most damage in the 1989 San Francisco earthquake.

Assuming that codes are robust enough and structures are built on solid ground, there are sill issues. No building is earthquake proof, only earthquake resistant –structures are designed to hold together long enough for you to GET OUT.

Unfortunately, if you do manage to escape your luxury 25th floor Coal Harbour condominium (at least no one else will be there to plug up the stairwells) you’ll have to contend with falling glass.

(How much falling glass is uncertain. If I understand correctly, Japanese towers include thin wire mesh embedded in glass panels to limit shattering, while local codes have no such requirement. For what it’s worth, it’s often been said that in the event of a major earthquake downtown Vancouver will be buried in glass.)

Of course, other structures will be affected, too. Skytrain lines and overpasses are vulnerable. While most bridges will simply sway off the sudden application of energy, not all are so resistant. The Patullo Bridge in particular does not meet modern seismic standards and most of Metro Vancouver other bridges were constructed before the introduction of modern seismic codes.

While older wood frame buildings are likely to survive even a strong quake, many run the risk of collapsing chimneys or coming off their foundations to which they may be improperly anchored.

Although the City and region have invested in upgrading the resilience of water and other infrastructure, this process is not complete and there is likely to be extensive disruptions in their carrying capacity.

Severed gas lines and associated fires are likely to be widespread.

A strong quake will probably trigger landslides. Assuming our local damns are not damaged, slides could contaminate our local drinking supplies.

And then there are tsunami. It’s widely assumed that as a megathrust earthquake will take place to the west of Vancouver Island, and that that landmass will act as a shield and spare Vancouver the worst of the resulting flooding.

However, anything is possible given the strength and epicenter of a quake.

On March 27th, 1964, the second largest earthquake ever recorded (9.2) struck off the coast of Alaska. The resulting waves funnelled up the 40 km (25 mile) Alberni Canal and a tsunami struck Port Alberni, lifting houses off foundations and upending cars.

port-alberni-tsunamiVancouver Sun

Regardless, river channels and low lying areas –Richmond, again- are in a hazard zone.

Pacific Northwest Seismic Network

Collapsed buildings, damaged bridges, debris impeding emergency routes, fires, disrupted water supplies, landslides, and coastal flooding are all bad enough, but the worst may come after the shaking has ceased. A 2014 Auditor General’s report states that Emergency Management BC inadequately prepared to manage the consequences of a major earthquake.

Nor are the rest of us, I’ll wager. Just ask family, or a friend, or neighbour if they have an earthquake kit.