Barry,

I don't have the NEIS data for 7/31, but the events in the Tonga-
Fiji arc are very deep, about the deepest in the world around
550 km. There was an aftershock this AM at 559 km. As the energy
from a deep event moves to the surface it is greatly attenuated,
and spread over a large area, so minimal surface waves are generated.
Kermadec Island region events are usually around 200 km deep.
Of course, the other influence on how well surface waves are
generated is the source function, or just how the earth moves at
the fault. The mechanism of the events at Tonga-Fiji is almost vertical.

Tonga-Fiji events are great for checking the polarity of vertical
sensors in a large network, since the propagation to a wide area
on the surface is quite uniform. We have used such events shortly
after installing networks to check for wiring reversals.

The other place for deep events, where the descending slab of the
plate remains solid (doesn't melt) to great depths, usually because
of its more rapid motion at the plate boundary convergence, is under 
Ecuador. Years ago there was an 8+ event under Quito that was felt 
from Panama to Argentina, but did no damage because the energy was 
so spread out.

Regards,
Sean-Thomas
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