TAIPEI, Oct 31 (Reuters) - Production of Apple Inc's (AAPL.O) iPhones could slump by as much as 30% at one of the world's biggest factories next month due to tightening COVID-19 curbs in China, a person with direct knowledge of the matter said on Monday.Manufacturer Foxconn, formally Hon Hai Precision Industry Co Ltd (2317.TW), is working to boost production at another factory in Shenzhen city to make up for the shortfall, said the person, declining to be identified as the information was private.Its main Zhengzhou plant in central China, which employs about 200,000 people, has been rocked by discontent over stringent measures to curb the spread of COVID-19, with several workers fleeing the site over the weekend.Foxconn is Apple's biggest iPhone maker, producing 70% of iPhone shipments globally, which in turn makes up 45% of the Taiwanese firm's revenue, analysts at Taipei-based Fubon Research said this month.STRICT COVID-19 MEASURESUnder China's ultra-strict zero-COVID-19 policies, localities must act swiftly to quell outbreaks, with measures including full-scale lockdowns."