“Devastating megathrust earthquake” a “substantial hazard” for Haida Gwaii, Canada-US study warns

A “devastating megathrust earthquake” could hit Haida Gwaii sometime in the future, according to Canadian and US studies carried out after the magnitude 7.8 earthquake off Haida Gwaii on Oct. 27, 2012 and the 7.5 magnitude quake off Craig, Alaska, a few weeks later on Jan. 5, 2013.

The 2004 Indian Ocean earthquake and the 2011 Tōhoku earthquake in Japan, both accompanied by major tsunamis are recent examples of “great” (higher than magnitude 8.0) megathrust earthquakes. Most of the concern on the west coast has been the likelihood of a megathrust earthquake on the Cascadia Fault on the Juan de Fuca plate that stretches from northern California to the middle of Vancouver Island.

New Douglas Channel geological studies near completion

Is Kitimat ready for a “big one?”

The 2012 Haida Gwaii main shock was the second largest seismic event in Canada since the establishment of a modern seismograph network. The first was the 1949 Haida Gwaii/Queen Charlotte earthquake with a magnitude of 8.1 That 1949 Haida Gwaii earthquake was a strike-slip event, where the plates move side-to-side, similar to the 1906 San Francisco earthquake and other quakes on the San Andreas Fault in California.

The 2012 Haida Gwaii earthquake is characterized in the studies as a “mini-megathrust” event, where part of the crust is pushed upward, meaning that a larger megathrust could have much more destructive consequences from both the earthquake and a possible tsunami.

A diagram of the situation  off Haida Gwaii that triggered the October 2012 "mini megathrust" earthquake seen at the lower centre, while the 1949 slip strike   earthquake is seen at the top. (Bulletin of the Seismological Society of America)
A diagram of the situation off Haida Gwaii that triggered the October 2012 “mini megathrust” earthquake seen at the lower centre, while the 1949 slip strike earthquake is seen at the top. (Bulletin of the Seismological Society of America)

Complex system of faults

The new studies show that the Pacific and North America plate boundary off the coast of British Columbia and southeastern Alaska creates a system of faults capable of producing very large earthquakes. The scientists conclude that while the two earthquakes in 2012 and 2013 released strain built up over years on the tectonic plates, those events did not release strain along the Queen Charlotte Fault off the west coast of Haida Gwaii. That means the fault remains the likely source of a future large earthquake.

Map showing the pattern of earthquakes along the Queen Charlotte Fairweather Fault system and the location of the Queen Charlotte Terrace.  (Bulletin of the Seismological Society of America)
Map showing the pattern of earthquakes along the Queen Charlotte Fairweather Fault system and the location of the Queen Charlotte Terrace. (Bulletin of the Seismological Society of America)

A special issue of the Bulletin of the Seismological Society of America (BSSA), released Monday, April 6, 2015, contains 19 scientific and technical papers, outlining the results of the work carried out over the past two years.

The team estimated the rupture dimension of the 2012 Haida Gwaii earthquake to be about 120 kilometres long at a depth of about 30 kilometres.

The Craig earthquake ruptured the Queen Charlotte fault over a distance of more than 100 kilometres and at a depth of about 20 kilometres.

The two areas are joined in what is called the Queen Charlotte Fairweather Fault System. To the south the Queen Charlotte Fault also interacts with the Juan de Fuca plate that stretches from Vancouver Island to northern California.

“The study of these two quakes revealed rich details about the interaction between the Pacific and North America Plates, advancing our understanding of the seismic hazard for the region,” said Thomas James, research scientist at Geological Survey of Canada.

Two faults off Haida Gwaii

The studies conclude that the interaction between the plates off Haida Gwaii is much more complex than previously believed. Before the 2012 earthquake, the Queen Charlotte Fault, a strike-slip fault similar to the San Andreas Fault in California, was believed to be the dominating tectonic structure in the area. The 2012 tremor confirmed the existence of a previously suspected thrust fault beneath what is called the “Queen Charlotte Terrace,” to the west of the Queen Charlotte Fault, where the Pacific plate is sliding at a low angle below the North American plate.

The Queen Charlotte Terrace, which is about a kilometre below the surface of the ocean, is built up of layers of sediment, several kilometres thick, scraped off the oceanic plate as it subducts under the North American plate. It may also include some fragments of oceanic crust. For most of the terrace, it is “present as a clearly defined linear feature,” but the study adds: “north of about 53.5° N, a complex pattern of ridges and valleys appears.”

The earthquake was “essentially a mini-megathrust earthquake along the dipping plate interface of a subduction system,” one of the scientific papers says. The epicenter of the Haida Gwaii main shock was located about five kilometres landward (northeast) of the Queen Charlotte Fault. That probably means that the rupture was near the bottom of the locked plates, where the plate motion’s side to side movement is also thrusting downward. Significant aftershocks appeared to cluster on the periphery of the main rupture zone with most of the aftershocks occurring seaward to the west.

The scientists used GPS observations of crustal motion to locate the earthquake’s rupture offshore to the west of Haida Gwaii.

The situation off Haida Gwaii is complex because while the Pacific plate is converging with the North American plate at a rate of 15 to 20 millimetres a year, at the same time the two plates are slipping by each other toward the north northwest at angle of about 20 degrees at a rate of about 50 millimetres a year.

Honn Kao, a seismologist with the Geological Survey of Canada said, “This was an event the thrust interface of the plate boundary system, confirming that there is a subduction system in the Haida Gwaii area.

“The implication of a confirmed subduction zone is that in addition to the Queen Charlotte Fault, we now have another source which can produce devastating megathrust earthquakes in the area,” said Kao.

The study of the Haida Gwaii tremor looked at the causative faults, the rupture processes and depths of the main shock and sequence of strong aftershocks.

The Haida Gwaii earthquake generated a significant tsunami that left deposits indicating run-up exceeding 3 metres (maximum 13 metres) in a number of bays and inlets along about 230 kilometres along the west coast of Haida Gwaii. In Hawaii, a 0.8 metre wave was measured on a tide gauge.

In Queen Charlotte City perceptible shaking lasted for one and half to two minutes, with very strong shaking for about 30 seconds. The earthquake was felt as far away as Yukon Territory, Alberta, and Montana.

The study says “Damage was limited, in part owing to the sparse population, but also because of the seismic resistance of the generally low rise, wood-frame buildings on the islands. Felt intensities were at expected values close to the source zone, but regional intensities were smaller than predicted.”

The Haida Gwaii rupture also shook southeastern Alaska. The northwest direction of ground motion then may have influenced the timing of the Craig earthquake a few weeks later in January 2013. That earthquake occurred farther north in southeast Alaska, where relative plate motion is nearly parallel to the Queen Charlotte fault.

Aftershocks
.

Map showing the pattern of aftershocks following the October 2012 Haida Gwaii earthquake. (Bulletin of the Seismological Society of America)
Map showing the pattern of aftershocks following the October 2012 Haida Gwaii earthquake. (Bulletin of the Seismological Society of America)

The Haida Gwaii aftershocks clustered around the periphery of the rupture zone, both on the seaward and landward side of the plate boundary and reflected what the study calls “normal faulting behavior–caused by the bending, extending or stretching of rock– rather than the thrust faulting of the main shock.” The pattern of aftershocks is similar to those observed after the 2011 Japanese megathrust earthquake.

“Our observations of normal faulting imply that the main shock of the Haida Gwaii earthquake dramatically altered the stress field in the rupture zone, especially in a neighboring region,” Kao said.

The distribution of aftershocks occurred to the north of a previously identified seismic gap where large earthquakes have not occurred in historic times. The gap is located to the south of the where 1949 magnitude 8.1 Queen Charlotte earthquake ruptured.

Though the Haida Gwaii earthquake may have activated some part of the Queen Charlotte Fault, Kao said, it was limited and did not relieve stress along the seismic gap.

The study concludes:

The Haida Gwaii event confirmed substantial seismic and tsunami hazard from large thrust events on the plate margin along the southern Queen Charlotte fault. It occurred where relatively young oceanic lithosphere under thrusts North America and in some ways is an analog for the much larger megathrust earthquakes known to occur on the Cascadia subduction zone to the south, where the young Juan de Fuca plate and other small plates subduct beneath North America. The Haida Gwaii earthquake had a complex pattern of main shock rupture and aftershocks and a large tsunami.

Further study needed

The Geological Survey of Canada plans further studies to understand the formations off Haida Gwaii.
One question to ask is if there are any records of major earthquake events in the past history of Haida Gwaii. The study notes that the impact of the tsunami was relatively minor “in this region with steep rocky coastlines.” That means there are limited sources of coastal sediments that can be checked for past events. It adds: “Low-elevation lakes, ponds, and bogs may offer the best opportunities for paleotsunami studies” warning that large earthquakes in the past that produced tsunamis may have left little evidence in the “paleoseismic record of Haida Gwaii and similar settings worldwide.”

 

 


Megathrust

Megathrust earthquakes occur at subduction zones at destructive plate boundaries where one tectonic plate is subducted (forced underneath) by another. These interplate earthquakes are the planet’s most powerful, with moment magnitudes that can exceed 9.0. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes. During the rupture, one side of the fault is pushed upwards relative to the other, and it is this type of movement that is known as thrust. The displacement of the ocean in a thrust can trigger a tsunami.

Transform fault
A transform fault is one where the motion is predominantly horizontal. Those faults end abruptly and are connected on both ends to other faults, ridges, or subduction zones. The best-known (and most destructive) are those on land at the margins of tectonic plates. Transform faults are the only type of strike-slip faults at plate boundaries show strike-slip or side-to-side in movement.

Queen Charlotte Terrace
The Queen Charlotte Terrace is a 25 kilometre wide zone of built up marine sediment immediately west of the active Queen Charlotte fault. The crust is about 12 kilometres thick at the terrace. On Haida Gwaii, the earth’s crust is 18 kilometres thick at the eastern edge. On the BC mainland the crust is in excess of 30 kilometres thick.

Historic earthquakes.
The 1949 Haida Gwaii quake was one of the largest in the recorded history of North America.

The largest known earthquake along the coast was the megathrust event on the Cascadia fault on January 26, 1700 where the Juan de Fuca plate ruptured for about 1,000 kilometres along from what is now northern California to Vancouver Island, estimated at magnitude 9.0. The dating is based on a tsunami that hit Japan that had no associated local earthquake as well studies of tree rings from the remains of trees downed in the tsunami.


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New Douglas Channel geological studies near completion

Three studies of the geology of Douglas Channel are near completion and publication, according to Natural Resources Canada. That news comes as studies, released today, warn of a major megathrust earthquake on the fault west of Haida Gwaii.

Northwest Coast Energy News asked the Geological Survey of Canada if there were any recent updates available after the agency said that a survey had located a “possible fault” on Hawkesbury Island during studies for the Enbridge Northern Gateway Joint Review Panel.

“Devastating megathrust earthquake” a “substantial hazard” for Haida Gwaii, Canada-US study warns

Is Kitimat ready for a “big one?”

Natural Resources Canada responded with a statement: “NRCan continues to conduct research studies in the area, including study of possible faults. Three scientific expeditions have been completed on board Coast Guard research ships. The first two reports are in the final stages of editing, and will be published in the coming months. The third expedition was just completed; therefore the third report will be available later.”

The initial joint survey by the Geological Survey of Canada and Fisheries and Oceans in September 2012, found a tsunami hazard and a possible seismic fault in Douglas Channel near Kitimat.

Fault zone map Douglas Chanel
A 2012 map from the Geological Survey of Canada showing the line of a possible seismic fault on Douglas Channel (Geological Survey of Canada)

The follow up study by Fisheries and Oceans, released about three weeks after the Haida Gwaii earthquake, in November 2012,  identified two slope failure events on the southern end of Hawkesbury Island during the mid-Holecene period, between 5,000 and 10,000 years ago. That study showed that the tsunami from the slope failure reaching Kitimat would have been about 60 centimetres or 23 inches. Hartley Bay would have been hit by a wave height of 7.5 metres or about 25 feet.

More recently there were slope failures nearer to Kitimat. The first slope failure occurred on October 17, 1974, triggering a 2.4 metre tsunami at low tide. Then on April 27, 1975 there was a second slope failure near low tide on the northeast slope of the Kitimat Arm that generated an 8.2 metre tsunami. The 1975 tsunami destroyed the Northland Navigation dock near Kitimat and damaged the Haisla First Nation docks at Kitamaat Village.

Thomas James, of the Geological Survey told Northwest Coast Energy news about the team’s finding on the Haida Gwaii earthquake: “The studies focused on the Haida Gwaii and Craig earthquakes which happened at the Pacific and North American plate boundary, west of Haida Gwaii, so east of Haida Gwaii there’s no comparable plate boundaries that gives rise of historic sieismisticity.”

As well as the fact that recent studies say the mainland margin coastal zone has had very little historical seismicity, it adds no currently active faults have been identified. A study ten years ago identified some very ancient faults which have not been active since the Eocene, about 33 to 56 million years ago.
GPS studies show that in northwestern British Columbia coastal block is moving northeast at the rate of just 5 millimetres a year.

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Is Kitimat ready for a “big one?”

Kitimat Emergency Coordinator Bob McLeod
Kitimat Emergency Coordinator Bob McLeod at the earthquake postmortem Oct. 29, 2012 (Robin Rowland/Northwest Coast Energy News)

Studies of the October 2012 magnitude 7.8 Haida Gwaii earthquake show that the region is vulnerable to a “major megathrust” earthquake along a newly confirmed fault line west of the islands.

That earthquake was felt in Kitimat and Kitamaat Village and a tsunami warning was issued within a few minutes.

So have the District of Kitimat, the Haisla Nation Council or Rio Tinto Alcan changed or upgrading their earthquake and tsunami plans in the past couple of years?

“Devastating megathrust earthquake” a “substantial hazard” for Haida Gwaii, Canada-US study warns

New Douglas Channel geological studies near completion

Bob McLeod, who recently retired as the District of Kitimat’s emergency coordinator, told Northwest Coast Energy News: “I think we’ve done quite a bit. One of the biggest issues in the first one was trying to get information out. We’ve come a long way on that. Whether you reach everyone or not, that’s another thing, because you never reach everybody. One of the critical things to me is getting the information out so you avoid all this Facebook, Twitter speculating and rumour. The communications aspect has improved a hundred fold.

“We did more work on the mapping and planning. Over the course of the last year, there were a lot of meetings with industry and various stakeholders, discussing emergency preparedness in general but touching on some of these other things as well.

“One of the things we did was to try to set up some shelter points. We have an agreement with the Baptist Church, the Catholic Church and the Seventh Day Adventists. They’re strategically located and could be gathering points for the various neighborhoods if necessary.

“We’ve also done quite a lot of work on Riverlodge as a group lodging centre, thinking in terms of an earthquake where there may be damage and you have to move people.

“We did look at the evacuation planning and we’ve had a couple of exercises involving that, looking strategically about how can you move people from certain neighborhoods, asking which neighborhoods would be at the most risk if you ended up with a tsunami situation.

As for tsunamis, McLeod said, “From everything we’ve heard and been told, tsunamis in extremely deep water like that is not going to be as dangerous as one in shallower water, but the possibility is still there.

“The thrusts are the killers when it comes to tsunamis, but there is a very good warning system on the tsunamis. We do get very very rapid feedback on the earthquakes.

“The only danger in that regard is if you have a severe earthquake and you have part of a mountain drop into the salt chuck, you’re going to get a massive wave and you’re going to get no warning whatsoever, like the Moon Bay collapse in the seventies.

“The emergency plan is in good shape. We scheduled a number of exercises last year through training programs.

“One of the things I personally push is personal preparedness. I think as a community, we fail greatly at that. That was evident even during the snowstorm. People are not just prepared to look after themselves, it’s unfortunate. You just have to keep chipping away.”

Last week, Northwest Coast Energy News asked Rio Tinto Alcan and the Haisla Nation Council if either could comment on updated earthquake or tsunami response plans. So we have received no answers.

 

Related links
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Geological Survey of Canada identifies tsunami hazard, possible fault line on Douglas Channel

Scientists identify major Japanses style tsunami hazard for west coast