What are fault lines?

A fault line in geology is the point where two plates are touching each other, usually moving in opposite direction, scraping each other in the process. Because the touching surfaces are not smooth, but rugged, the plates build up enormous tension that eventually releases itself in the form of earth quakes. For example the San Andreas Fault line in california which is expected to cause a major earthquake in the near future.
The usage in your example is most likely an analogy to geology.

In Geology, fault lines are the cracks in the earth that represent the interaction between the tectonic plates that make up the crust of the earth.

In philosophy, fault lines are considered to be that period in history when one Civilization can be considered sundered or separated from another. They are theoretical divisions that are very difficult to pin down.

They’re conflicts waiting to happen because of unreconciled differences.

For example, when the British arbitrarily fashioned states (like Iraq) in the Middle East, they drew boundaries that didn’t take ethnic or tribal allegiances into consideration. Even though those states were, theoretically, unified entities, in fact they were only held together by force of autocratic governance. All of the religious and tribal tensions among the various groups created fault lines, weaknesses in the social fabric that would cause the rupture of those tenuous aggregations under stress.

In the sense it’s used here, societal fault lines are the lines of division in societies and political systems that simulate the lines that fractures follow in solids. For example, the Democratic / Republican division in the US Congress, the black / white divide in many municipalities (even today it’s too often a very apparent dividing line), religious schisms, and whatever other “divisions” you can identify that represent splits that – regardless of whether they ‘could be’ bridged or not – are not generally bridged or otherwise relaxed or compromised.

Thorninmud has the answer here. It is also worth mentioning that, previous to the advent of rail and flight many peoples and large cities were relegated to the mouth of a river, as ships were the prime method of transporting large amounts of cargo and the necessities of life.

These “fault lines” may refer to this effect (as a river may draw it’s course from a fault line or provide the division between a people) and the geological division of a mountain range but that two cultures would evolve as separate peoples in the inneccessity of crossing this geologic boundary.

A fault line is where plates in the Earth are touching each other and moving in a different direction. When there is an Earthquake it only goes along the fault line and the fault line plates then slide in the opposite direction of each other and form a crack in the Earth because the plates move in different directions and make the ground lift up or make it go down.

My wife states “fault lines” are the wrinkles in her face.

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