In my understanding, the phrase "X is prone to Y" is used to exclusively express the idea that Y is something that frequently happens to X, or (near-equivalently) that Y is particularly likely to happen to X. For example, if you suggested someone was "accident-prone", that would imply that they frequently suffer from accidents.
Google ("Google’s English dictionary is provided by Oxford Languages.") gives the following sample statement for 'prone': "farmed fish are prone to disease". Under the above usage of prone, this would imply that farmed fish are likely to suffer from disease, with emphasis on the 'likely' rather than the 'suffer', i.e. these fish become diseased at a higher rate. (The supplied definition is "Likely or liable to suffer from, do, or experience something unpleasant or regrettable.")
It is clear to me that this sense of "likely" is (at least one) accepted / correct usage for 'prone'.
However, I also see 'prone' being used with a slightly different meaning, whereby "X is prone to Y" is used to express that Y has a particularly devastating effect on X, without regard to its likelihood or frequency of occurrence.
In the above example with the fish, this would be interpreted as "farmed fish are not necessarily more likely to become diseased, but if they do happen to become diseased, they tend to have worse outcomes".
In some cases, as with this fish example, this causes ambiguity because both of the two interpretations are sensible but have distinct meanings. In the case of this fish example, both interpretations can even be true at the same time.
As another example "Racing team X's car is prone to crashes":
- It's considered likely to crash, or does crash frequently. (Perhaps the driver is untalented or the car has an unstable/faulty design), OR:
- A crash would be especially disastrous. (Perhaps the car is made from less-durable components, or built in a way that repairs are challenging or expensive).
In some situations, the context implies that the second 'severity' meaning is the only valid interpretation, something like the following:
"Buildings made of papier-mâché are prone to earthquakes".
Of course, constructing a building from fragile materials doesn't summon earthquakes to the area. From the context it is clear that the intended meaning is that if an earthquake were to happen then the consequences would be dire, not that the building is more likely to be hit by an earthquake.
However, even if I understand this intention after reading the sentence, it still strikes me as incorrect, or at least clumsy. The possibility of this being a valid usage of 'prone' seems to invite confusion in many contexts, yet I see it being used for this 'severity' meaning fairly frequently. (This is merely something I have recently noticed in my own anecdotal experience of informal or conversational English, I'm not trying to claim the word is being used this way universally or as a wider trend).
So there would be a few possible resolutions, feel free to propose anything else you can think of:
- The "papier-mâché" sentence is an incorrect usage of the word 'prone'. The word only refers to the concept of likelihood/frequency. The sentence could be fixed by instead saying "Buildings made of papier-mâché are prone to being damaged in earthquakes", which then correctly refers to the likelihood of the building being damaged given that an earthquake has happened to it.
- Both meanings are valid/correct, but stylistically the word should not be used unless the meaning is obvious from the context.
- Both meanings are valid/correct, the concept of likelihood/frequency should be taken as the 'default' meaning in cases of ambiguity.
- Both meanings are valid/correct, in cases of ambiguity the reader should assume both meanings apply.
- Something else?