The S&P 500 fell sharply on Friday, with losses accelerating in afternoon trading. The decline took the benchmark index SPX, -2.19% below its 200-day moving average, a closely watched technical level that is often used to gauge the long-term momentum in an asset’s price. The S&P fell 2.8% to 2,589.17, dropping below the moving average’s level of 2,593.7. The S&P closed below that level for the first time since June 2016 on Monday, but it subsequently rebounded above it. If the S&P closes below it again, that could spur deeper selling ahead. The Dow Jones Industrial AverageDJIA, -2.34% lost 3.1% and the Nasdaq Composite Index COMP, -2.28% was off 2.8%. The day’s losses were driven by concerns over escalating trade tensions between the U.S. and China.