  The relaxation dynamics of aftershocks after large volatility shocks are
investigated based on two high-frequency data sets of the Shanghai Stock
Exchange Composite (SSEC) index. Compared with previous relevant work, we have
defined main financial shocks based on large volatilities rather than large
crashes. We find that the occurrence rate of aftershocks with the magnitude
exceeding a given threshold for both daily volatility (constructed using
1-minute data) and minutely volatility (using intra-minute data) decays as a
power law. The power-law relaxation exponent increases with the volatility
threshold and is significantly greater than 1. Taking financial volatility as
the counterpart of seismic activity, the power-law relaxation in financial
volatility deviates remarkably from the Omori law in Geophysics.
