Today’s USGS earthquake report shows no recorded seismic activity in Java at all during the past week, yet Mount Merapi in Java is slowly blowing itself to bits. I would have thought that a volcanic eruption of this size would have caused some recordable earthquakes. Any idea why not? Is it just because it’s mainly gas at the moment?
The USGS list only shows earthquakes of magnitude greater than 2.5. The seismic activity associated with the volcano is probably lower than that.
Is there even something causal happening? Normally Java looks like Sumatra on the map - covered in little and medium earthquakes. But this week, nothing (above 2.5 as you say). Could it be releasing magma pressure and thus reducing tectonic movement - or is that a stupid supposition?
No, that’s one of the current theories… a lot of little bangs keeps there from being a big one, kind of like anti-lock brakes, or telling somebody that his grandmother’s stuck up on the roof and we can’t get her down. Let things release slowly and everything stays calm. Same for plate shifts leading to general earthquakes; it’s just that with volcanoes, you’ve got a more direct connection through to the magma, so there’s more chance for pyrotechnics.
Well shit. I don’t know if this thread was prescient or coincidental.
My impression of a lack of activity has been suddenly reversed: a 6.2 right by Mount Merapi; the southern part of the city of Yogyakarta has been flattened, and hundreds are dead. 
I wonder if it is a situation similar to what Ethilrist said (though perhaps the outcome is the opposite of what he was saying), that the lack of minor activity I perceived in previous weeks built up the stress needed for a big one? Could this have had anything to do with Merapi’s activity? Was Merapi a precursor of the quake? Aren’t there any seismologists on the SDMB?
Sunfish gave me some good answers following the tsunami but I don’t think he renewed this time around.