A sharp drop in South Korean stocks cut the combined market capitalisation of the KOSPI and KOSDAQ by more than 389 trillion won in a day. About 228 trillion won was wiped out from Samsung Electronics and SK Hynix alone, deepening the overall decline in market value.

As of 10:13 a.m. on Oct. 15, the KOSPI's market capitalisation stood at 4.60 quadrillion won, down 356.2 trillion won, or 7.18 percent, from the previous session's close of 4.96 quadrillion won. KOSDAQ market capitalisation also fell 33.3 trillion won, or 7.16 percent, from the previous day. Combined, the two markets lost 389.4 trillion won in a single day.

The sharp falls in Samsung Electronics and SK Hynix, the No. 1 and No. 2 stocks by market value, had a large impact. At the same time, Samsung Electronics' market capitalisation was tallied at 1.24 quadrillion won, down about 114 trillion won from the previous day, while SK Hynix fell about 114 trillion won to 922.6 trillion won.

The two stocks alone accounted for about 228 trillion won in lost market value, or 64 percent of the KOSPI's total decline.

SK Hynix's market value in particular fell below 1 quadrillion won during intraday trading. The last time its market capitalisation finished a session below 1 quadrillion won was July 30.

Losses in share prices are also widening. Samsung Electronics traded at 212,500 won, down 8.41 percent from the previous session at the same time. It fell as low as 212,000 won intraday, the lowest level since July 30.

SK Hynix tumbled 10.99 percent to 1,263,000 won. Its intraday low was 1,261,000 won, the lowest since July 29.

Investor sentiment around semiconductor stocks deteriorated rapidly as concerns overlapped over a possible expansion in output by Chinese memory maker ChangXin Memory Technologies (CXMT) and worries about profitability from investment in artificial intelligence (AI) infrastructure. Weakness in tech and semiconductor shares in the U.S. stock market also added pressure on South Korean semiconductor shares.

As AI infrastructure investment grows rapidly, concerns are also emerging about whether companies can recoup investment costs as actual profits.

Market research firm Gartner forecast additional AI infrastructure investment at $6 trillion for 2027 to 2030. It estimated that cumulative sales of $8.5 trillion would be needed by 2030 to cover investment and financing costs and achieve a 12 percent return on invested capital. It projected related revenue generated from AI data centres over the same period at up to $3.6 trillion.

The KOSPI also slid to the 5,500 level intraday. As of 10:13 a.m., the index stood at 5,572.24, down 436.57 points, or 7.27 percent, from the previous session. It fell to as low as 5,534.24 during intraday trading, dropping to the 5,500 level for the first time since July 30.

Foreign investors were net sellers of 2.14 trillion won in the main bourse, extending their selling streak to 8 straight sessions. Institutions and retail investors, in contrast, were net buyers of 1.24 trillion won and 712.4 billion won, respectively. The KOSDAQ index also fell 7.32 percent to 763.58.