While most investors have been focused on the rapid decline in the stock market, which had its worst day since the crash of 1987 on Thursday, a different drama is going on behind the scenes on Wall Street — potentially with bigger implications for the economy.

The swift, global spread of the coronavirus has chipped away at one of the cornerstones of the financial system: the vast market for bonds, where companies and governments go to borrow money to fund operations. Wall Street banks, as well as sophisticated investors such as pension funds and hedge funds, trade these bonds, including the government debt that is considered among the safest assets in the world: United States Treasuries.

But in recent days, as fears of a pandemic have escalated, the market for Treasuries has experienced a liquidity problem — meaning that the buying and selling of this kind of debt at reasonable prices has suddenly gotten a lot tougher. That, in turn, made already nervous investors even more frantic on Thursday, when the S&P 500 index plunged so far and so fast that trading was temporarily halted. Stocks closed down 9.5 percent, the worst performance since Black Monday more than three decades ago.

The drama in the Treasury market prompted the Federal Reserve Bank of New York to step in on Thursday, saying that it would quickly offer banks $1.5 trillion in funding, via short-term loans, to keep the bond markets functioning smoothly.