The Cascadia Subduction Zone is so weak and slippery in places that minuscule tidal forces produced by the sun and moon are enough to cause tremors.
Yes, deep part of Cascadia fault is so slippery that sun and moon 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 — boosting the risk of a megaquake and tsunami.
What are slow slip events?
- They occure when tectonic plates slip slowly past each other 20 miles or more underground.
- They last for several weeks and can propagate 100 miles or more north and south.
- The pattern repeats roughly every 12 to 14 months.
- Each releases as much energy as a magnitude 6.5 quake, but very slowly (faintly detectable).
- 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.
Moreover, a clear link exists 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.
So what’s the deal?
There is a possibility — but no guarantee — that there will be some enhanced tremor or slow slip prior to the Cascadia megathrust.
The last time the Cascadia fault snapped was in 1700. This means that hundreds of slow-slip episodes — and countless tidal cycles — have played out since then without triggering a seismic cataclysm.
The next slow-slip event is expected to begin sometime this fall or winter.