The suggestion is not entirely frivolous.

The Tohoku Pacific earthquake and tsunami have destroyed supply capacity but will boost demand beyond the short term as a massive reconstruction programme begins. A rise in demand relative to supply implies a fall in Japan's saving surplus and upward pressure on real interest rates. Higher real yields, in turn, push the yen higher relative to its long-run equilibrium value. Unless this long-run equilibrium simultaneously falls (e.g. because of a permanent rise in the risk premium investors demand to hold Japanese assets), this suggests a stronger yen.

Higher real interest rates and a stronger currency are the means by which capital is attracted to finance reconstruction. This capital inflow is the counterpart of the fall in the saving (i.e. current account) surplus.

The upward pressure on the yen could be neutralised by a sufficiently large loosening of monetary policy. The Bank of Japan, however, is unable to reduce interest rates (its target for the uncollateralized call rate is 0-0.1%) and has yet to expand QE significantly. (The additional ¥5 trillion of asset purchases announced this week will be spread over the 14 months to June 2012, implying a monthly rate of less than $5 billion at the current exchange rate.)

The extent of any rise in the yen will depend on the policy response but a target of ¥67 against the dollar has some "technical" attractions:

• The yen strengthened from ¥98.6 before the Great Hanshin earthquake of 17 January 1995 to an intraday low of ¥79.75 on 19 April, a decline of 19.1%. The yen closed at ¥82.9 on 10 March, the day before the Tohoku Pacific earthquake. A 19.1% fall from this level targets ¥67.1.

• The yen's behaviour echoes that of the S&P 500 index in early 2009. The S&P rallied from temporary support at 805 before breaking this level in a final plunge to an intraday low of 666. The yen, similarly, rallied from a low of ¥80.4 in late October but has now breached this support.

• The yen weakened from ¥102.0 to ¥123.8 between January 2005 and June 2007, a rise of ¥21.8. A Fibonacci 2.618 multiple of ¥21.8 is ¥57.0. A ¥57.0 fall from the June 2007 high targets ¥66.8.