Before revision:  We find that the stock correlations are significantly increased in the periods of two market crashes in 2001 and 2008,  and the systemic risk is higher in the volatile periods than calm periods.
After revision:  We find that the stock correlations are significantly increased in the periods of two market crashes in 2001 and 2008, during which only five eigenvalues significantly deviate from the random correlation matrix, and the systemic risk is higher in the volatile periods than calm periods.