http://quaketips.blogspot.com/2013/05/should-i-turn-my-gas-off-after.htmlThis and other sources suggest that turning off the gas should not be done in a knee jerk fashion. It may be a while before you can get the utility company to turn it back on. On the other hand, if you turn the gas off, it is indeed off.
The decision will vary with the circumstances. I would be just as aware of downed electric wires, among the many potential haards, as leaking gas.
The context of the discussion was "so unfortunately if there is a major earthquake." Perhaps then not everyone is in agreement of what a major earthquake is. Two times in my life, I have witnessed the disastrous effects of an unknown gas leak and don't wish to see a third and would not hesitate to turn off the gas in a "major earthquake."
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Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books.
John Lubbock