Why some subduction zones have few earthquakes

Why do some places have plenty of colliding plates but hardly any earthquakes? That is the puzzle we need to understand. The opening first gives us the familiar explanation: the earth's outer layer consists of plates moving over the mantle below. When two plates collide, one is forced underneath the other. This process, subduction, can build up energy that is suddenly released as an earthquake. The opening says this grinding explains most earthquakes, so we need not treat it as the only possible cause.

That seems to fit the hot zones, where subduction and earthquakes are both common. But it doesn't explain the quiet zones, where intense subduction occurs with little shaking. Perhaps the plates interact with less grinding in those places. We don't yet know how, but the observation already tells us why the amount of subduction alone cannot explain everything.

The phrase “until recently” points us toward a proposed answer. The scientists focus on the nature of the collision. Picture two plates moving toward each other. Now picture both moving in the same direction, with one going faster. The faster plate can catch up to the slower one. Both situations involve collisions; the second isn't a case in which the plates never meet.

Start with that second situation, the one associated with quiet zones. The faster plate catches the slower plate, and the faster plate's leading edge goes underneath. Track that carefully: the plate doing the overtaking is also the one being subducted. Its front edge descends into the mantle.

Why does this produce less shaking? The collision zone moves relatively quickly over the mantle. The mantle pushes back against the descending plate, forcing it down at a steep angle. The oar analogy helps make that resistance concrete: dip an oar into the water while the boat is moving, and the water resists it. Here, resistance from the mantle changes the angle at which the plate descends.

Now follow the consequence of that angle. A plate descending steeply moves down and away from the other plate more sharply. Less of the two plates remains pressed together, so there is less earthquake-producing friction. The explanation isn't simply that moving faster prevents earthquakes. The motion matters because of its effect on resistance, descent, contact, and ultimately friction.

What changes when the plates move toward each other? The collision zone is relatively motionless over the mantle. The descending plate therefore encounters less resistance from the mantle and goes down at a shallow angle. Instead of dropping steeply away, it remains in contact with the other plate over a much larger area. More contact gives the plates more opportunity to grind against each other.

That's the point of the two sheets of sandpaper. Press them together and try to slide them: they resist each other. The author uses that image to explain why the shallow-angle collision generates so much friction.

There are two different kinds of resistance here, and keeping them separate makes the science much easier. Resistance from the mantle pushes a plate steeply downward and reduces its contact with the other plate. Resistance between the plates is the friction associated with earthquakes. More of the first can therefore mean less of the second. Treating both as simply more resistance would miss the mechanism.

The final paragraph turns the explanation into a warning. A region with little subduction isn't automatically safe. If that subduction occurs at a shallow angle, the plates can still have substantial contact and produce considerable friction. The type of collision matters, even when its overall amount is low.

So our central idea is more than just subduction can occur without earthquakes. The scientists offer a reason why: different collisions create different angles of descent, contact areas, and amounts of friction. The scientists are explaining an apparent exception within plate tectonics. Their proposal accounts for relative quiet, without requiring the underlying plate movement or collision to disappear.