OK for these 3, but a lot of aftershocks are appearing extremely close in time and close to each other from the animation.

OK, I don't know whether Japan already has cabled sea floor observatories. Let's assume they don't have any extensive network that might have seen tremor or some other indicator of something unusual going on after the M7.2 foreshock. The Japanese are spending a half billion $ drilling a number of IODP coreholes into the Nankai trough including through a major fault above the megathrust that they thought was the type of fault that caused big tsunamis (was not this type of fault March 11 though).

Would they learn more spending $500 million on cabled sea floor observatories? Or, would it only cost a couple of tens of millions? These would include cabled OBS (seismometers) and who knows what else (monitors of fluid flow on higher-angle faults that come near sea floor?)? HW was a P.I. on a couple of rejected NSF proposals to do this on the North Anatolian fault offshore of Istanbul.

Of course, you could do both given the disaster...

And, John: same for Cascadia?

Chris