I remember learning about this in elementary school but I’ve since forgotten it.
For instance, how do you get earthquakes like those that happened in New Madrid, Missouri (USA) in 1811-1812 if it’s nowhere near a plate boundary?
With those earthquakes, they called the area the “New Madrid Fault Line” and I don’t even know how it can be a fault line without having plates touching. Are there other types of “faults” (or some other name for a weakness in the plates) that exist within a plate without it actually being a boundary between two plates? And if so, what is actually occurring to cause the earthquakes? Like is it shifting the same way a plate boundary would or doing something entirely different? And can those other types of earthquakes be just as devastating as a plate boundary earthquake? I know at least one of the New Madrid earthquakes was an 8.6 magnitude but I don’t know if that was an anomaly or if the other type(s) of earthquakes are often just as strong but just not as common.
In: Planetary Science
While the plates touching each other is one source of shear force, there are others. The plates can be compressed, causing a load throughout the plate. If there is a void or weak spot in the plate, that load might cause a slip there.
You can have a fault even if it is not at the location where two plates are touching: you only need a crack where rocks on either side move relative to one another. A fault is only a plate boundary if it separates two large, well-defined slabs of Earth’s crust*. In my own area, far from any plate boundary, some rock exposures show small faults that only extend for a few hundred feet.
Even away from plate boundaries there are some stresses that can deform crust. In other words, the crust can still be stretched or compressed a bit. In the case of the New Madrid Seismic Zone, the faults are left over from a failed continental rift. About 630 million years ago, the continent that would later become North America started to pull apart but stopped. This left some faults that could later be reactivated.
*One side note here, is even at plate boundaries, you often see a few major faults and many smaller faults.
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