Research shows Cascadia Subduction Zone reacts to tidal forces; ‘slow slip’ building up pressure on fault

April 2015 SEATTLE — As the death toll rises from Nepal’s magnitude 7.8 earthquake, new research from the University of Washington shows that a similar fault off the Pacific Northwest coast is so weak and slippery in places that minuscule tidal forces produced by the sun and moon are enough to trigger tremors. The quakes involved are so tiny they’re imperceptible to people. But they’re part of a recently discovered process called slow slip that ratchets up pressure on the offshore fault called the Cascadia Subduction Zone — slightly boosting the risk of a megaquake and tsunami. The next slow-slip event is expected to begin sometime this fall or winter. The Cascadia fault and the one responsible for Nepal’s devastation are both boundaries where tectonic plates collide.  In Nepal, the Indian subcontinent is being forced under Central Asia, while in the Northwest, the seafloor is diving — or subducting — under North America. Even though the faults aren’t identical, geologists say the Cascadia analysis could help improve the broader understanding of when and how such plate boundaries rupture, generating some of the world’s most powerful quakes.
For the study published Monday in the journal Nature Geoscience, UW geophysicist Heidi Houston analyzed tens of thousands of faint tremors under Washington’s Olympic Peninsula and Vancouver Island between 2007 and 2012. The tiny quakes occur when tectonic plates slip slowly past each other 20 miles or more underground. These slow-slip events last for several weeks and can propagate 100 miles or more north and south. The pattern repeats roughly every 12 to 14 months. Each slow-slip event releases as much energy as a magnitude 6.5 quake, but the motion is so slow it’s detectable only on sensitive seismometers and GPS stations, Houston explained. While the slow slip relieves strain deep on the fault, it adds a little pressure to the shallower portion — called the locked zone — that will unleash a quake of up to magnitude 9 the next time it ruptures. “It’s very possible, if not likely, that when we have the next very big earthquake it will happen when one of these slow earthquakes happen,” said University of California, Berkeley, researcher Roland Bürgmann, who wrote a companion piece to Houston’s paper titled Diary of a wimpy fault.
Cascadia 2Scientists and self-styled seers alike have long been intrigued by a possible link between earthquakes and gravitational and tidal forces, said UW earth sciences professor Ken Creager, who was not involved in Houston’s study. The idea is that fluctuating tidal forces should raise and lower the strain on faults, causing them to snap. But despite decades of study, the impact on regular quakes has proved negligible. Creager and his colleagues were among the first to show that the story is very different with slow slip. They found a clear link between tremor rates and tide cycles in the complex topography of the Pacific Northwest. During some phases, the minute changes in pressure encourage tremor; during other phases, tremor is inhibited. Houston delved into the data in more detail and found that when a portion of the fault first begins to slip, it’s not very sensitive to tidal effects. But as the tremor continues over several days, the fault weakens and the tidal pull becomes a much more dominant factor. “We think the stress causes the fault to start slipping faster,” Houston said.
As a result, little spots where the plates are stuck together break, generating the tiny quakes called tremor. The best explanation for the deep fault’s sensitivity to such small forces is that it must contain far weaker, more slippery rock than anyone imagined, Houston said. The presence of fluid is also key, allowing rocks to slide at depths where the pressure is so great it would otherwise clamp the fault shut. Overall plate motion during slow-slip events adds up to only a little more than an inch. And even when tidal forces are at their peak, as during king tides, the risk of a damaging quake remains very low, Houston said. But new insights into the frictional properties of the deep fault will help researchers develop better models to explain what’s going on down there and devise better experiments to simulate what might happen during different conditions.
The ultimate concern, of course, is the shallower portion of the fault, where plates can jolt past each other by 60 feet or more in what’s called a megathrust quake. Japan’s 2011 Tohoku quake and tsunami was preceded — and likely triggered by — shallow, slow slip on an offshore fault. There’s some evidence of similar slip before a recent quake off the coast of Chile. There’s no proof yet that slow slip or tremor occur on the fault that caused Nepal’s weekend quake, but it wouldn’t be surprising, Creager said. Houston and other researchers are closely tracking Northwest slow-slip events, looking for potential danger signs like more intense tremor or motion close to the locked zone. “There is a possibility — but no guarantee — that there will be some enhanced tremor or slow slip prior to the (Cascadia) megathrust,” Houston said. Researchers are even discussing whether they should issue warnings if such worrisome signs appear. “Maybe we should tell people: We don’t quite know what this means, but we’re fairly certain the probability of an earthquake is bigger now,” Bürgmann said. But 315 years have passed since the last time the Cascadia fault snapped in 1700. Hundreds of slow-slip episodes — and countless tidal cycles — have played out since then without triggering a seismic cataclysm. –PDN
This entry was posted in Avalanche, Black Swan Event, Civilizations unraveling, Dormant fault activation, Earth Changes, Earth Watch, Earthquake Omens?, High-risk potential hazard zone, Human behavioral change after disaster, Infrastructure collapse, Potential Earthchange hotspot, Prophecies referenced, Seismic tremors, Signs of Magnetic Field weakening, Strange high tides & freak waves, Tectonic plate movement, Time - Event Acceleration, Volcanic Eruption, Volcano unrest, Volcano Watch. Bookmark the permalink.

10 Responses to Research shows Cascadia Subduction Zone reacts to tidal forces; ‘slow slip’ building up pressure on fault

  1. STLloyd says:

    Yep the big one is coming!


  2. Helen Parks says:

    so, imagine what the reaction to the incoming mini solar system will be! All of the Earth’s quaking and moving is a result of the incoming celestial bodies, and all of the warring is the “head-turner” to take our minds to the more “urgent” individual matters. Russia seems to be the only country which has provided safe places for all of the population when things do become worse


  3. Dennis E. says:

    Possible 9.2 in the future, off shore.


  4. Jimmy G says:

    Would suggest everyone study plate tectonics themselves. There really isn’t evidence it exists but in reality there is evidence it does not exist. The earth instead is a plaster covered balloon that is expanding and the quakes are when the dried crust cracks. Look at that 1/2 mile line split in the crust along US/MEX border. Why would that appear out of nowhere? Don’t mock me until you study this,


  5. Chris says:

    I actually had a dream of this quake it was at least a 9+ I have been in a 7 before and this was much bigger.


  6. Justin says:

    The only safety I feel in this post besides the constant preaching of doom and destruction is that over 315 years constant tidal waves and fault slips have occurred. Which means that in the next decade we MIGHT not have an earthquake because what are the chances out of 325?


  7. Larz says:

    9.5 Cascadia fault quake is locked and ready to explode! Will this happen soon? One scenario I have seen is a combination with the San Andreas breaking south to north and out to sea where Cascadia is waiting. That could cause a 10.0+ quake. Now that would definitely be the “Big-One!”


  8. Hunter says:

    Your ‘Cascadia: Ticking Time Bomb’ image is not credited to the actual creators of that image: University of Washington’s Center for Environmental Visualization. You’ve actually put your logo on the image which you did not create. So not cool.


    • Hunter, we credit images when the notations are available. The logo is on every picture on the blog, not because we claim ownership or rights to the picture but because they are part of the blog post. However, proper accreditation has been listed.

      Thanks for the update and your concern.


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