TOKYO — Japan’s central bank today pumped a record $184 billion into money markets and took other measures to protect a teetering economy, as the Tokyo stock market nose-dived following the devastating earthquake and tsunami.

The benchmark Nikkei 225 stock average slid 6.2 percent in its first day of trading since the 8.9-magnitude quake centered on northeastern Japan struck Friday, triggering enormous waves that swamped towns and killed thousands.

Escalating concerns about the financial and economic fallout — plus the risk of meltdown at damaged nuclear power reactors — triggered a plunge that hit all sectors of the stock market. The broader Topix index lost 7.5 percent.

“Japan will be poorer, for this disaster,” said Peter Morici, a business professor at the University of Maryland. “Rebuilding will run down Japan’s financial wealth.”

The Bank of Japan moved quickly to try to keep financial markets calm. By flooding the banking system with cash, it hopes banks will continue lending money and meet the likely surge in demand for post-earthquake funds.

Also, the central bank’s nine-member policy board voted to ease monetary policy. It will expand the size of an existing program to buy assets — such as government and corporate bonds — by 5 trillion yen to 40 trillion yen, $486.4 billion.