Meanwhile, on the other side of the world, China is doing everything in the textbook to ignite a "bubble."
I dislike that usually undefined term, which carries a lot of normative baggage. But there are a set of steps that governments often take unwittingly and are later criticized for. China's doing them on purpose. And these steps quite often precede large market declines.
Short sales ban: Financial Times: "opened a probe into market manipulation" ... "The investigation is likely to focus on short selling." The usual witch hunt, with Chinese characteristics. Owen Lamont has a splendid paper on what often follows short-sales bans. The weekend before TARP and Lehman, the US instituted a short-sales ban on bank stocks, just in case there was someone out there who did not know banks were in trouble and they should sell now. Europe instituted a CDS selling ban in the first PIGS crisis...
Lending to encourage highly leveraged speculation: Wall Street Journal: "Under the planned move, China’s central bank will indirectly help investors borrow to buy shares in a market that had already seen a rapid buildup in debt from so-called margin financing." Procyclical credit supply is named by just about every account of a "bubble" followed by a crash.
Prices depend on supply and demand. As well as increasing demand, limit supply: "A halt to new stock listings."
And more. Quartz offers "A complete list of the Chinese government’s stock-market stimulus (that we know about)" including "People’s Bank of China will “provide liquidity assistance” to China Securities Finance Corp., a company owned by the stock regulator. The company will use the money to lend to brokerages, which could then make loans to investors to buy stocks."
This scenario often ends badly.
The only thing I can think of that can actually stop a crash is for the central bank to directly print money to buy stocks. And not just a little bit. A pre-announced and limited quantity won't work. The US QE took billions to alter bond prices a few basis points at most. One has to commit to a price floor and a "do what it takes" amount of money, no matter how large or inflationary. I don't know of it ever being tried. It will be interesting to see if China goes that far. They could hide the fact with extensive bailouts of people "borrowing" to buy stocks, or otherwise cover losses or promise to cover losses.
Of course, the right strategy is to leave it alone. The whole point of stocks is that they go down on occasion, without runs, without defaults, and without financial distress. Unless the people and institutions holding them are highly leveraged. Didn't we just learn this lesson?