Just to be clear, there aren’t multiple clearinghouses for US equities. There’s one, the DTCC. They handle effectively all of the US equities trades’s settlemen

Just to be clear, there aren’t multiple clearinghouses for US equities. There’s one, the DTCC. They handle effectively all of the US equities trades’s settlement cycle. If there’s any kind of doubt that they don’t have enough collateral and they need to take losses, it could cause a lot of chaos. Initiating the kind of “stop all trading” you refer to, is a lot more complicated than it sounds. You would get a sort of a stock market equivalent of a bank run. On the flip side, (1) they’re known to ask people to post more money than necessary, because they like to err on the side of caution, and (2) if they ask you for money, you just post it, or you risk being kicked off from trading US equities for a long time.