Merriam Webster - 11th edition

something resembling a fault : SPLIT, RIFT *a major conceptual fault line in foreign policy — Morton Kondracke*

Mike :)

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Note added at 6 mins (2004-07-22 03:02:41 GMT)
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In New York and LA, where generally people are more liberal and less isolated, the exposure of Janet Jackson\'s breast was no big deal. However, to people in socially conservative areas, it was as if an earthquake had struck 8.0 on the Richter scale.

Fault line is a rift usually associated with earthquakes, and it's a metapher in this case as someone already pointed out. Key words here is "social landscape" which means people who belong to different socio-economic strata that making up our society. Some are liberal, some are conservative, some are educated thereby earning higher income, whereas some are in opposite camp. Some are Protestants, some are Catholics, some are recent immigrants, some are old establishment (in America). Those different people who hold different value and viewpoints are the composite of the "social landscape" that may or may not interact with each other. Again landscape is another anology which cannot be mistaken for actual region or area, like rural Kansas as opposed to Manhattan residents (of course that is part of the game as people of different area tend to see things differently).

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Note added at 1 hr 54 mins (2004-07-22 04:50:27 GMT)
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Fault lines refer to those difference or \"gap\".