A magnitude 9 earthquakealreadymeasurablyalterstherotationoftheEarth; thetwomagnitude 9-plusearthquakesthiscenturybothalteredthelengthofthedayby a tinybutmeasurablefractionof a second.
Inordertogettomagnitude 10, you'd need a rupturedfaultlinethreeand a halfthousandkilometerslong, and a magnitude 11 quakewouldrequire a faultthatwenthalfwayaroundtheworld!
A magnitude 15 earthquakewouldbe a milliontimesmorepowerfulthanthat, andinvolvethereleaseofenoughenergytoevaporateallthewateronearth, though, ifthathappened,
Going a fewstepsfurther, a magnitude 18 earthquakewouldreleasenearlytentothethirty-twoJoulesofenergy, whichisroughlythegravitationalbindingenergyoftheEarth.