There are many psychos in Japan. If you had followed recent news, you'd see a guy who raped and killed a schoolgirl in Nara (sending picture of his deeds to the child's mother), school children killing each others with knives or pushing toddlers from the top of a building, teenagers killing their whole family when they are sleeping at night, a housewife who murdered her husband and keep the body in the fridge for months, someone killing the whole neighbouring family with a gun because they were too noisy, etc. ad nauseam. And that's just a sample of the weekly stories that are translated in English on sites like the Mainichi online. How can we be sure that some ultraracist guys wouldn't take advantage of the confusion to start killing foreigners after a tremendous earthquake ?
I voted probably not as that was 1923 when people were not as educated or internationalized as today. Back then there was hardly any interaction with foreigners. In 2005 or later I doubt it. Granted you may have one psycho who might try something like that, but I doubt it.

Getting to your quote, it is not that unusual what you mentioned as the same type of murders and such were going on back in the early 1970's and the years that followed and probably even years before then. I remember stories of kids killing their whole families, mothers killing children and themselves, kids throwing other kids off of buildings, etc. It has, and will always be going on in Japan. There are always a few psychos. Anyone remember the guy who cut off the heads of school children and kept them? Or the woman who cut off the penis of her military American lover with a piano wire because he confessed to her that he was indeed married and that his wife was coming to live with him in a few weeks?