Huobi (15.26 kBTC) and BTC-China (1.17 kBTC) were the most affected; both had about 1/6 of the volume of last Tuesday's (Jan/28). For the non-Chinese exchanges, that ratio was around 1/3.
The decrease at Chinese exchanges is easy to understand, since Jan/31 is the Chinese new Year, and AFAIK people have several days of vacations before and/of after it. But the fall at the non-Chinese sites is puzzling, since they are usually very active on Fridays.
This "experiment" may show how much China infuences other exchanges. Except for MtGOX, the prices in all exchanges are obviously tied by arbitrage. So perhaps (less volume in China) --> (less price variation) --> (less volume at all exchanges). Or perhaps arbitrage trading is a large part of the volume; then (less volume in China) --> (less price variation in China) --> (less arbitrage trade).
OKCoin fell "only" to 1/3 compared to Tuesday. That is another hint that its clientele includes a significant non-Chinese component (the people who keeep trading while the Chinese are all asleep).
We all know about BTC-China's rogue robot that traded ~40,000 BTC in 2.5 hours, 40 BTC at a time, around Jan/30 18:00 UTC (Jan/31 02:00am in China) . But there are other weird things going on at that site. For example, its order book now has an offer to sell ~100 BTC @ 4872.38 CNY, and a bid for ~175 BTC @ 4872.37 CNY -- and those two guys just sit there, nose-to-nose, without moving a penny.
Like some other exchanges, BTC-China also has a stream of small trades within that narrow spread, moslty below 40 BTC. I suspect that it is a robot trading with itself, to cover up the lack of real transactions and/or to inflate their volume.
Once in a while, there is a transaction that changes the price: arbitrage, perhaps?
The crypto-coin community urgently needs to define professional ethics standards for exchanges, and set up an international entity that will audit them and rate them for compliance.