Stocks were flying Thursday, but their initial reaction to the Federal Reserve's interest-rate decision on Wednesday featured a surprising calm, all the more striking given that it was the Fed's first rate increase in more than three years.

The Federal Open Market Committee voted unanimously to raise the federal funds range to 3.75% to 4%, representing a shift from its previous stance, or regime.

Historically, such regime changes from the Fed have triggered sizable stock market swings. Before yesterday, the Fed had made seven regime changes (a shift in direction from its most recent move) since 2000.

On average, the first rate-regime change in a Fed cycle has moved the S&P 500 nearly 2% on an absolute basis. Yesterday, the index fell just 0.45%-one of the smallest reactions in decades.

The S&P 500 dropped in the 90 minutes following the announcement, down roughly 1.2% at its intraday low, before largely recovering into the close.

By Thursday, investors seemed to reassess yet again, with the index up 1.1% at 2 p.m. ET on Thursday.

Wednesday's decision came as little surprise to most traders, more than 90% of whom expected a quarter-point rate increase in September, according to CME Group's FedWatch data.

Projections from the Fed signaled that one more 25-basis-point rate increase is likely before the end of the year. But exactly when that increase arrives remains an open question.

Traders now see a 50.9% chance of a second 25-basis-point rate increase in October, up from 48.7% a day earlier and 27.2% a week ago.