Which event triggered the onset of the Great Depression of the 1930s?

This is straightforward, and all you need to do is a tiny bit of work to find the answer. While there were a lot of reasons why the Depression happened having to do with structural issues in the world economy, the "trigger" was clearly the great stock market collapse that started on the New York market ("Wall Street"). Stocks fell drastically in several consecutive days, bankrupting many individuals who had bought large amounts of stock on margin, ie when the price fell they owed more on the stock than it was then worth. That started a whole cascade of events, leading to the world-wide Depression. It was not inevitable, we have since seen larger stock market collapses without depression but with recessions, but that is because the Federal Reserve, the US government, other major world governments, and the largest banks have learned at least some lessons about responding to an economic crisis such as that one.