To Occur, To Happen, To Take Place
intransitive verb, godan verb
You know that 起きる means "to wake up", but did you learn its secondary meaning, "to occur"? Well, this word means only to occur. Just imagine an incident occurring involving a uranium core (こる) to remember that this means to occur. After all, uranium core incidents don't wake up, do they?
おこる
The reading is the same as what you learned with the kanji and the same as 起きる too, meaning you should be able to put together the reading on your own.
まさかこんなことが起こるなんて、思ってもみなかったよ。
I never expected something like this would happen.
どうしてこんなことが起こったのか、よく考えてみてください。
Please try thinking through why something like this happened.
それは数百年に一度起こるか起こらないかの大地震でした。
It was a huge earthquake, the kind that occurs only once every few hundred years.