I wonder if x is correlated with y. Here are some citations on z and y. If that's what you mean, sure. You win.

Here's some more data:

http://earthquake.usgs.gov/earthquakes/states/historical_sta...

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edit:

Big quakes are at once relatively rare and over time quite common. The last one happened under the ocean...

0.652°N, 124.692°W

18.2 miles below the surface.

in 2010