“Investors are seeking to buy 30- and 40-year bonds while there are positive yields left,” said Tadashi Matsukawa, head of fixed-income at PineBridge Investments Japan Co. “They face the possibility that yields across all maturities will fall into negative territory.”

The next catalyst to send yields even lower maybe the Bank of Japan, which could be forced to widen the target range for 10-year bond yields. While the central bank has stated that a range of around 0.2% above and below 0% is acceptable, yields have already pushed into a grey zone at about minus 0.25%.

Governor Haruhiko Kuroda has indicated there is space to lower the target and as recently as this month his deputy Masayoshi Amamiya suggested the band could be widened. The policy board next meets to consider its monetary settings Sept. 18-19.

Foreign investors more than doubled purchases of Japanese debt in July from a month earlier, with bulk of purchases in the two-to-five-year bracket.

But global funds also bought more super-long bonds — those due in 20 years or more — amid speculation the worsening outlook for global growth will spur the BOJ to adjust policy.

Japan’s benchmark 20-year bond yielded just 0.085% on Thursday, down from 0.23% a month ago. The yield on 30-year and 40-year debt has also halved over the same period, to 0.19% and 0.21%, respectively.

The BOJ has sought to steepen the yield curve through modest, incremental reductions and tweaks to its regular bond market operations in recent months.

While any aggressive shift would run the risk of being taken as monetary tightening, triggering unwelcome gains in the yen, widening the trading range for 10-year yields, or simply removing the floor, appear to be options on the table for BOJ.

If this was done together with a small 10 basis points cut to interest rate cuts — which is already priced into money market this year — yields would make another move lower and all maturities could go negative.