Japanese stocks fell 7.5 percent, posting the biggest daily decline since October 2008, and bond yields rose on Monday 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 on Monday, 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 automakers, 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.

Investors are selling aggressively as they want to remove risks. The market will sell until they have completed their selling needs, but I really can't tell the extent of a possible sales as this is not the normal market condition, said Hiroshi Arano, advisor at Mizuho Asset Management in Tokyo.

The market will be very cautious about buying Japanese shares even though the value of Japanese shares are attractive after falls. The market will see the condition of the actual state of the Japanese economy over the next two to three months, before making any concrete decision, Arano said.

CONSTRUCTION FIRMS IN DEMAND, TECHNOLOGY DUMPED

The benchmark Nikkei index <.N225> fell 6.2 percent to 9,620.49, and slumped to a four-month intraday low at one point, with technology companies such as Kyocera Corp <6971.T> and Canon Inc <7751.T> among the biggest drags on the market.

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

Domestic investment trusts and funds are dumping everything today. Sell orders for tens of billions of yen were detected, said an equities trader at a Japanese domestic institutional investor, who declined to be quoted by name.

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 percent 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 <9501.T>, 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 percent.

It will be very difficult for Japan to stop its reliance on nuclear energy, and such energy is necessary for stable economic growth of the country, so if the post-incident inspection is negative, it will impact economy in the long run, Shinichi Ichikawa, chief market strategist for Credit Suisse in Tokyo.

Construction-related businesses rallied on the back of expectations for demand from rebuilding efforts, with Kajima Corp jumping 22.2 percent to 259 yen <1812.T> and Taiheiyo Cement climbing 21.2 percent to 137 yen <5233.T>.

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.

QUAKE COSTS

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 percent 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 on Monday while keeping interest rates unchanged. Japanese financial markets took the decision in their stride.

(Additional reporting by Yoshiko Mori and Hideyuki Sano and Masatsugu Hisatsune at IFR, Writing by Masayuki Kitano; Editing by Kevin Plumberg)

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