OK it was a 5.9, but that’s not what gets me excited about the quake, it was 575 km deep. Like real deep, way down there. That’s the mantle, supposedly so deep that anything from the surface melted long ago, but, if you plot the earthquakes you clearly see that it’s the subducted pacific plate quaking at almost 600km below the surface.
What is happening there, we can only guess at. Well educated guesses based on what little we do know about the mantle and well yeah, guess. It’s melted rock, which is under a lot of pressure and with strong, but slow currents caused by convection, from heat from the earths core. And the subducted pacific plate is pushing into this plastic like hot goo and breaking up or cracking as it goes and then melting as well.
And the surface rock is different than the mantle, different types of rock and there is embedded organics so there may be some reactions there so yeah it’s complicated stuff, but why is it making earthquakes. And this isn’t the surface rock from a few days ago, this rock must be down there centuries, or longer, 5cm per year, 600 km = 600,000 /.05=a long time 12 million years.
You would have thought it would have melted by now, but if it had how could it quake? Who wants to did a big hole and find out?
I think that it takes a long time to melt through 40 km of crustalrock, and that the mantle while hot and plastic doesn’t pass that heat very well, so the crust may actually pick up some solidified mantle as it goes before it gets deep enough that it starts to melt. And while the mantle is rock and melted rock that doesn’t mean it’s liquid like water, more like honey or even oobleck, a non-Newtonian fluid. So with the crust pushing into it it can sometimes act like a solid rather than a liquid and pressure builds against the subducting crust and it has to push through it sometimes resulting in a quake of this size or larger. (Is it me or is the line where the inner and outer mantle divide too coincidental with the last depth of known deep earthquakes?)