What drives the occurrence of slow-slip events on subduction zones: “earthquakes”: that involve strain release over days and weeks rather than seconds? A new paper…doesn’t really answer that question, but it shows why it’s so complicated to answer.
The study uses seismic and drill core data to characterise what is entering the subduction zone off the coast of New Zealand, where multiple slow-slip events that involve the shallow part of the subduction thrust have been observed. The idea is that what is about to be fed in to the trench will be similar to what is now just subducted. By sampling what’s on the seafloor, we get some insights into what the rocks currently controlling the behaviour of the subduction thrust are like.
On a broad scale, there’s a lot of variability to what is being fed in. The incoming plate has a lot of basement topography – it’s an oceanic plateau with lots of volcanic seamounts of various sizes and heights, with volcaniclastic sediments in betweeen. Carbonate-rich deep sea sediments deposited after the plateau formed have buried some but not all of the volcanic topography.
The drill cores confirm this broader spatial variability: the sequences within the two cores reported, about 15 km apart, varies dramatically, mainly because one samples a seamount and one does not. But within each core, there is also a lot of much smaller-scale variation in the volcanic and carbonate sediments. Big changes in alteration, and therefore important properties like porosity and seismic velocity, vary significantly within a few cm up or down the core.
What all this means: the geometry of the subduction thrust, and the properties of the rocks involved in faulting, change a lot in quite small area. It’s the geological equivalent of ‘garbage in, garbage out’: feed a subduction zone a complex plate surface, and you get a complex fault zone with complex behaviour.
For more, also read this nice write-up at Eos
Nice plan for content warnings on Mastodon and the Fediverse. Now you need a Mastodon/Fediverse button on this blog.