A sell-off of tech stocks that began earlier this month and gained steam with disappointing earnings from leading tech companies drove US markets down by hundreds of billions of dollars Friday. In total, the decline has knocked nearly $1tn in value off the tech-heavy Nasdaq exchange in one week and erased about $2tn in a month.

The S&P 500, the index of the 500 most valuable companies in the US, fell by more than 1% Friday, losing hundreds of billions of dollars in market value. It was on track to erase its gain for October, even though it set its all-time high during the same month. The weakness for technology stocks dragged the Nasdaq composite down even further, more than 3%, also erasing hundreds of billions in value.…