Mw 8.4 Sumatra earthquake

There was a Mw 8.4 earthquake off the coast of Sumatra about three and a half hours ago. It generated a tsunami locally, at least.

The earthquake occurred on the same subduction zone as the 2004 Boxing Day earthquake that generated the huge, devastating tsunami, but it's on a segment further south and east:



(Map from USGS page... didn't copy well. Sorry.)

That means that the tsunami's propagation pattern should be different this time - and that different places are at risk.

The USGS pages for the earthquake have some good graphics showing a lot about the geology of the quake already. For instance, there's a cross-section of the seismicity that should be good for showing people what a subduction zone looks like, and the map on the map on the summary page already shows the focal mechanism. (Shallow thrust fault, dipping north... if you want to show anyone a shallow subduction zone focal mechanism, it's a really good graphic.)

From first glance, it seems that there has been a progression of subduction-zone earthquakes along that plate boundary: the big M 9 earthquake in 2004, then a good-sized (M8?) along the segment just to the south in 2005, and now another M8 along the next segment south. I'm curious to see where the slip on this earthquake initiated compared to the rupture zone of the 2005 earthquake, and whether the stress changes as the result of previous earthquakes predicted that this most recent rupture zone became closer to failure because of the earlier earthquakes.

[Edit - I'm getting e-mails from the USGS bigquake server, and it looks like they're recalculating the moment magnitude. Which might make this a good time for an explanation of the different ways of measuring magnitude, especially since Alessia has just posted a great explanation of the difference between earthquake magnitude and earthquake intensity. I can sort of explain the differences, but if there's anyone who does the real moment magnitude calculations, I would love to hear how they are done in practice.]