I shared this article earlier, but I did not realize the importance of the passage shown in the screenshot until just now.
It appears that, for some reason, the gas supply did not shut off automatically. But that raises another question: Why was it not—or could it not be—shut off manually?
In Japan, people are often reminded to close the main gas shutoff valve—元栓（もとせん, motosen）—when leaving home, especially in an emergency. Was there a manual shutoff valve for the mall’s gas supply? If so, could it have been closed after the earthquake? Could it really be that no one thought to do so, or was manual shutoff somehow impossible?
-- Kirk