The first-quarter earnings look to be another impressive one. The number of profit-warning announcements over the past three months is near a decade low. Analysts expect S&P 500 earnings to rise 12% from a year ago, and that may prove conservative.

So why isn’t the US economy accelerating?  According to WSJ analysis, most growth of corp. earnings came from overseas, as US companies take advantage of faster economic growth in emerging markets by investing abroad.

Let’s call this dark side of strong corporate earnings.

Growth may firm up later this year. But the relative weakness with each quarter raises a question: Why isn’t the U.S. economy accelerating, especially with the corporate sector in such good shape?

Perhaps because corporate earnings aren’t being driven by strength in the U.S., but to some degree by its weakness. Companies have shifted production and sourcing overseas to cut costs and to tap demand in faster expanding markets. Capital spending in emerging markets like China and India has more than doubled in the past decade, and surpassed developed markets last year for the first time, HSBC estimates.

That is fostering growth abroad but undercutting prospects in the U.S. The nation’s factors of production, such as factories and equipment, or capital stock, actually shrank in 2009 for the first time in postwar history. Business investment did rebound last year, but the 15% increase, which should have generated 380,000 jobs on average a month, created only 78,000, notes UniCredit economist Harm Bandholz. The rebound appears largely to have been maintenance-driven, and investment has leveled off since.

The shifting of investment overseas and erosion of the nation’s capital stock are no small matter. They imply a lower potential growth rate for the U.S. and higher structural unemployment. As that reality dawns, the corporate sector’s strength may start to lose its luster.