The S&P 500 SPX, -1.11% fell 7% shortly after the opening bell Thursday, triggering a so-called circuit breaker that halts stock-market trading for 15 minutes. The market should resume trading at around 9:50 a.m. Eastern Time. If the benchmark index were to fall 13% on the day once trading resumes, it would trigger another 15-minute halt. Trading wouldn't halt if the decline occurred after 3:25 p.m. Eastern. A 20% drop in the S&P 500 would trigger what's known as a level three circuit breaker, which would stop trading for the remainder of the day. The Dow Jones Industrial Average DJIA, -0.87% was down 7.2%, at last check, while the Nasdaq Composite Index COMP, -1.07% tumbled 7% before the circuit breaker was triggered.