|Haida Gwaii earthquake 2012|
However, despite initial fears, there was minimal risk for a major tsunami, and the inhabitants of Hawaii could breathe more easily when the tsunami warnings were downgraded to advisories after a few hours.
Sliding Plates and Tsunamis:
If two tectonic plates simply slide past each other, major tsunamis are unlikely to happen:
The risk of a major tsunami from an earthquake in this transform fault is slim, the scientists point out, because the plates move sideways past each other. "You need quite a bit of vertical displacement on the ocean floor to generate a tsunami," Braunmiller said, "and earthquakes along the Blanco fault don't generate it."
This is what happened with our B.C. earthquake at Haida Gwaii – the plates simply slid past each other, and the tsunamis that resulted were smallish ones – less than 3 feet in height.
There is a greater likelihood of massive tsunamis being generated from the Canadian landmass if we have a large earthquake in the Jean de Fuca tectonic plate, which runs alongside Vancouver Island and into American waters.
Swarms of Earthquakes - Indicators of Disaster?
One interesting feature is the swarming of small earthquakes. In 2008 scientists recorded more than 600 quakes in 10 days off the Oregon coast not known for high seismic activity.
What's so unusual is that the swarming is taking place in the middle of the Juan de Fuca plate, and not at the edges of the tectonic plates, and is an indication of stress on the plate.
|Juan de Fuca Plate|
Could this be a sign that the Juan de Fuca plate might be preparing to break up into smaller pieces? Earlier swarms were brought on by volcanic activity, but not this one:
This is the eighth such swarm over the past dozen years, Dziak said, and the first seven were likely because of volcanic activity on the Juan de Fuca ridge. The plate doesn't move in a continuous manner and some parts move faster than others. Movement generally occurs when magma is injected into the ocean crust and pushes the plates apart.
“When it does, these swarms occur and sometimes lava breaks through onto the seafloor,” Dziak pointed out. “Usually, the plate moves at about the rate a fingernail might grow – say three centimeters a year. But when these swarms take place, the movement may be more like a meter in a two-week period."
But this eighth swarm may be different.
“The fact that it’s taking place in the middle of the plate, and not a boundary, is puzzling,” Dziak admitted. “It’s something worth keeping an eye on.”
The Diving Juan de Fuca Plate:
The Juan de Fuca plate lies just to the south of Vancouver Island. It is one of the smaller tectonic plates in the world, and is a remnant of the huge Farallon Plate, which has mostly dived under the North American Plate:
The Juan de Fuca plate itself has since fractured into three pieces, and the name is applied to the entire plate in some references, but in others only to the central portion.
The three fragments are differentiated as such : the piece to the south is known as the Gorda Plate and the piece to the north is known as the Explorer Plate. The separate pieces are demarcated by the large offsets of the undersea spreading zone.
This subducting plate system has formed the Cascade Range, the Cascade Volcanic Arc, and the Pacific Ranges, along the west coast of North America from southern British Columbia to northern California. These in turn are part of the Pacific Ring of Fire, a much larger-scale volcanic feature that extends around much of the rim of the Pacific Ocean.
The Jean de Fuca plate "subducts" – that means dives – under the North American plate off the Washington State coast. This area is known as the Cascadia subduction zone, and has large mega thrust quakes every 550 years or so on average.
The Birthing of Volcanoes in our Neighbourhood
There are some unusual features at the Cascade subduction zone. Where the Juan de Fuca Plate sinks beneath the North American Plate there is no deep trench, seismicity (earthquakes) are fewer than expected, and there is evidence of a decline in volcanic activity over the past few million years. The probable explanation lies in the rate of convergence between the Juan de Fuca and North American Plates. These two plates converge at 3-4 centimeters per year at present. This is only about half the rate of convergence of 7 million years ago.
The small Juan de Fuca Plate and two platelets, the Explorer Plate and Gorda Plate are the meager remnants of the much larger Farallon oceanic plate. The Explorer Plate broke away from the Juan de Fuca about 4 million years ago and shows no evidence that it is still being subducted. The Gorda platelet split away between 18 and 5 million years ago and continues to sink beneath North America.
The Cascade Volcanic Arc made its first appearance 36 million years ago, but the major peaks that rise up from today's volcanic centers were born within the last 1.6 million years.
More than 3000 vents erupted during the most recent volcanic episode that began 5 million years ago. As long as subduction continues, new Cascade volcanoes will continue to rise."
Slow Slips & troublesome Sticky Points in Cascadia:
The Jean de Fuca plate is exerting pressure on the plate it is sliding under, and this pressure goes down deeper into the earth than previously thought, up to 25 km deep. The result could be a mega thrust (read: massive) earthquake in the Cascadia subduction region:
The findings could have major implications for mega thrust earthquakes in the Cascadia subduction zone, an area along the West Coast from northern California to southern British Columbia. Megathrust events are huge earthquakes, often in the range of magnitude 9, that occur in areas where one tectonic plate is forced beneath another.
The slow slip events appear to be building stress on the mega thrust fault, where the Juan de Fuca plate is sliding beneath the North American plate, with the two locked together most of the time. That pressure is relieved when the plates slip during mega thrust earthquakes such as one determined to have occurred off the coast of Washington on Jan. 26, 1700, estimated at magnitude 9.2... In such events, the plates are locked together for hundreds of years and then slip past each other by sliding 50 feet or more during a mega thrust earthquake.
One recent discovery shows that there are episodic tremors and slips (also called slow slips) in the area as the two plates slide past each other:
For most of a decade, scientists have documented unfelt and slow-moving seismic events, called episodic tremor and slip, showing up in regular cycles under the Olympic Peninsula of Washington state and Vancouver Island in British Columbia. They last three weeks on average and release as much energy as a magnitude 6.5 earthquake... Because the two tectonic plates are locked together, stress builds at their interface as they collide with each other at a rate of about 4 centimeters (1.6 inches) a year. The slip events and smaller tremors ease some of that stress locally, Creager said, but they don't appear to account for all of it.
"Each one of these slip events puts more stress on the area of the plate boundary where mega thrust earthquakes occur, which is shallower and farther to the west, bringing you closer to the next big event," he said. "There's nothing to tell you which one will be the trigger."
Since the slip events and intervening small tremors don't accommodate all of the stress built up on the fault, scientists are getting a better idea of just what the hazard from a mega thrust earthquake is in the Seattle area.
These slip events surprisingly show that they also reverse their direction:
Earthquake scientists trying to unravel the mysteries of an unfelt, weeks-long seismic phenomenon called episodic tremor and slip have discovered a strange twist. The tremor can suddenly reverse direction and travel back through areas of the fault that it had ruptured in preceding days, and do so 20 to 40 times faster than the original fault rupture.
The depth at which the relief of the stress caused by the diving plate and stationary plate colliding with each other differs - in so-called "locked zones", there are dangerous sticky points that build and build and build pressure:
Houston noted that episodic tremor and slip occurs at a depth of 22 to 34 miles, where high temperatures have made the tectonic plates more pliable and thus more slippery. At a substantially shallower depth, perhaps 12 miles, the plates are not slippery and so are tightly locked together.
In the locked zone, the tectonic plates can hold the buildup of stress for hundreds of years, rather than just 15 months, but when the interface ruptures it can unleash a great mega thrust earthquake such as the one that struck off the coast of Japan in March. Such earthquakes occur in the Cascadia subduction zone every 500 years, on average, and the last one -- estimated at around magnitude 9.0 -- happened in January 1700. Houston noted that the region is within the large time window when another mega thrust earthquake could occur.
One key question still to be answered, she said, is what is happening on the plate interface between the locked zone and the depth where tremor occurs.
The last big one in Cascadia:
The last time a mega thrust quake took place to relieve the accumulating pressure was in 1700:
The slow slip events appear to be building stress on the mega thrust fault, where the Juan de Fuca plate is sliding beneath the North American plate, with the two locked together most of the time. That pressure is relieved when the plates slip during mega thrust earthquakes such as one determined to have occurred off the coast of Washington on Jan. 26, 1700, estimated at magnitude 9.2. That quake was similar to the great Sumatra-Andaman Islands earthquake the day after Christmas in 2004, which also measured 9.2 and triggered a devastating Indian Ocean tsunami.
In such events, the plates are locked together for hundreds of years and then slip past each other by sliding 50 feet or more during a mega thrust earthquake.
Most large quakes take place at subduction zones, where one plate dives below another one.
Wikipedia has more information on the great 1700 Cascadian earthquake and massive tsunami.
Goldfinger puts odds of Huge Earthquake in Northwest at 1 in 3 in next 50 years
Marine geologist Chris Goldfinger of Oregon State University spells out the odds of a huge earthquake – but points out that the Vancouver area chance is 10% to 15% in the next 50 years, compared to more than 1 in 3 for the areas further south:
New analyses by Oregon State University marine geologist Chris Goldfinger and his colleagues have provided fresh insights into the Northwest’s turbulent seismic history – where magnitude 8.2 (or higher) earthquakes have occurred 41 times during the past 10,000 years. Those earthquakes were thought to generally occur every 500 years, but as scientists delve more deeply into the offshore sediments and other evidence, they have discovered a great deal more complexity to the Cascadia Subduction Zone.
“What we’ve found is that Cascadia isn’t one big subduction zone when it comes to major earthquakes,” Goldfinger said. “It actually has several segments – at least four – and the earthquake activity is different depending on where a quake originates. The largest earthquakes occur in the north and usually rupture the entire fault. These are quakes of about magnitude-9 and they are just huge – but they don’t happen as frequently.
“At the southern end of the fault, the earthquakes tend to be a bit smaller, but more frequent,” he added. “These are still magnitude-8 or greater events, which is similar to what took place in Chile, so the potential for damage is quite real.”
Based on historical averages, Goldfinger says the southern end of the fault – from about Newport, Ore., to northern California – has a 37 percent chance of producing a major earthquake in the next 50 years. The odds that a mega-quake will hit the northern segment, from Seaside, Ore., to Vancouver Island in British Columbia, are more like 10 to 15 percent.“Perhaps more striking than the probability numbers is that we can now say that we have already gone longer without an earthquake than 75 percent of the known times between earthquakes in the last 10,000 years,” Goldfinger said. “And 50 years from now, that number will rise to 85 percent.”
Enter the Sun and the Moon:
Even the sun and the moon can influence earthquakes:
The faint tug of the sun and moon on the San Andreas Fault stimulates tremors deep underground, suggesting that the rock 15 miles below is lubricated with highly pressurized water that allows the rock to slip with little effort, according to a new study by University of California, Berkeley, seismologists...
"Tremors seem to be extremely sensitive to minute stress changes," said Roland Bürgmann, UC Berkeley professor of earth and planetary science. "Seismic waves from the other side of the planet triggered tremors on the Cascadia subduction zone off the coast of Washington state after the Sumatra earthquake last year, while the Denali earthquake in 2002 triggered tremors on a number of faults in California. Now we also see that tides -- the daily lunar and solar tides -- very strongly modulate tremors."
Fascinating things, earthquakes.