Without more history of the tree's past, the solution to it's wounds origin may remain a mystery..............simply too many variables with minimal history! In my opinion, which is merely given on the provided pictures, I would say that you could rule out frost cracks...................this particular crack is too wide...............the level of decay is too advanced. Frost cracks typically and I stress typically open under constriction in the colder months and more-less close in the summer. Being that part of the wound faces west, sunscald could have been a factor in the tree's more juvenile years, when the bark was more thin and smooth. Judging by the evident 'rams-horning' (curling of wound-wood), the damage occured many years ago. Typically the insulative value of thick, furrowed bark makes trees more resistent to such temperature fluxuations. The sad part of an arborists life, is that all (well almost all) forms of damage are irreversible. Once it's done, it must be dealt with. I wouldn't rule out a bad mechanical wound that allowed a point of entry for decay, such a ripped off limb or possibly a poor (meaning aweful) flush-cut.
I would suggest that you call in a local expert with the credentials to add validity to his/her assessment. Find out the structural integrity and the overall vigor of the tree in question. Seeing how you have already identified a problem, you are aware of its potential hazard. Even if the city/town is responsible for the care and removal of the tree, it will most likely stand there until you show them the problem and hastle them into solving it!
