Japanese stocks fell 7.5 per cent, posting the biggest daily decline since October 2008, and bond yields rose today as investors expected the earthquake and tsunami that devastated the country's northeast to take an economic toll and require heavy government borrowing.

The drop on record-high trading volume among the Tokyo Stock Exchange's biggest companies was compounded by fears about the long-term impact on power supplies after the earthquake damaged a nuclear generator. After an explosion today, authorities were working desperately to avert a plant meltdown.

Fears of more aftershocks and further repercussions from the damaged nuclear power plant would probably continue to keep investors on edge and weigh on the market this week.

The yen also slid against the dollar on hedge fund selling after the Bank of Japan announced a total of 15 trillion yen in fund injections to keep money markets stable.

The Japanese government bond yield curve steepened, with super-long debt retreating as market players anticipated the potential fiscal costs of future rebuilding efforts, even as short- to medium-term JGBs rallied.

Japanese carmakers, electronics firms and oil refiners saw their share prices drop by double digit percentages at one point after having to shutter key factories after Friday's earthquake and tsunami, which are feared to have killed more than 10,000 people and severely damage infrastructure.

The benchmark Nikkei index fell 6.2 per cent to 9,620.49, and slumped to a four-month intraday low at one point, with technology companies such as Kyocera and Canon among the biggest drags on the market.

The broader TOPIX index closed down 7.5 per cent, the largest daily decline since October 2008 in the wake of the Lehman Brothers failure, to 846.96 .

The Nikkei's fall comes after several months of solid performance, in which Tokyo shares outperformed their emerging Asian peers, and overseas investors flocked to Japanese equities, buying roughly 3 trillion yen since November.

As of Thursday, the day before the earthquake struck, the Nikkei had climbed 13 per cent in the five months starting in November, far outpacing a 1.7 percent gain in MSCI's index of Asia-Pacific shares outside Japan.

Shares of Tokyo Electric Power , Japan's biggest utility that owns a nuclear plant that may be close to meltdown, were a big focus for the market. TEPCO ended ask-only at 1,621 yen, down 23.6 per cent.

Construction-related businesses rallied on the back of expectations for demand from rebuilding efforts, with Kajima jumping 22.2 per cent to 259 yen and Taiheiyo Cement climbing 21.2 per cent to 137 yen .

In the month after the earthquake in Kobe in January 1995, construction stocks outperformed the broader market consistently for a year after the disaster, Citi research showed.

In the bond market, the five-year/20-year JGB yield spread widened by 11.5 basis points to a three-month high of 159.5 basis points at one point, indicating investors were selling longer-term maturities.

Super-long JGBs, 20 to 30-year bonds, were hit by concerns about the fiscal impact from the damages caused by the earthquake and as market players braced for the possibility that Japanese life insurers may sell the segment of the yield curve to fund payments to policy holders.

Underscoring concerns about Japan's fiscal health, Japanese sovereign credit default swaps stood at 91 basis points, about 10 basis points wider on the day and near the peaks close to 100 basis points reached last year during the euro zone crisis.

Short-term to 10-year JGBs rallied, supported by the BOJ's decision to supply ample liquidity to the money market.

The yen rallied broadly in early Asia but quickly pared gains in volatile trade as nervous traders waited to see what action Japanese officials will take to keep markets calm after Friday's quake and tsunami.

The dollar initially extended its losses after having slid more than one percent against the yen on Friday, touching a four-month low around 80.60 yen . The dollar later rebounded sharply in thin trade and last stood at 82.13 yen, up 0.4 per cent on the day.

The BOJ decided to expand the size of its asset-buying scheme by 5 trillion yen to 10 trillion yen at its policy meeting today while keeping interest rates unchanged. Japanese financial markets took the decision in their stride.

Reuters