Google has made a change to their search algo in Japan to lower the rankings of low quality sites in the search results.

The Japanese Google Webmaster Blog made a post about improving search quality in Japanese search.

As a part of this, we made improvements to the quality evaluation method of the website this week. This update will lower the ranking of lower quality sites that focus on having your page displayed above the search results rather than providing useful and reliable information to users. As a result, high-quality sites with original and useful content will be displayed higher.

This change is intended for measures against low-quality sites displayed in Japanese search.

One thing that is unusual about this change is that this is an algo change that only affects search in Japanese, which Google confirmed.

Google primarily makes all their algo changes worldwide and impacting all languages, so there their must have been something unique about a spam issue in Japan that either wasn’t impacting other languages or was an issue with how rankings work in Japanese.  They have done previous Japan specific changes though, such as when they targeted a Japanese link network a year ago.

They could possibly be using it to test an algo change, but that seems unlikely since Google is always testing algo changes without making any announcements.

They also add they will continue to improve the search quality in Japan, which leads me to believe there is something specific about Japanese search results that are impacting resulting in quality issues.