CONCERNS about weaker-than-expected corporate earnings sent the Nikkei 225 average plunging 418.94 points, or 2.3 per cent, to close at 18,074.61 yesterday, its lowest close in eight months. A report by Japan's largest securities company, Nomura Research Institute, that the Nikkei might fall below 16,000 points also contributed to the sharp fall, brokers said. The Nikkei has shed 10.41 per cent since October 10. Yesterday, it just managed to close above the psychologically important 18,000 level as institutional investors intent on erasing paper losses on their equity portfolios aggressively bid up share prices in the last few minutes of trading. Disappointing earnings results from major manufacturers, including Nippon Steel Corp and Mazda Motor Corp on Friday, were weighing the market down, said Graham Biggart, general manager of futures and options trading at Schroder Securities. Hundreds of companies were reporting earnings this month and many were posting results well below initial expectations, he said.