Social networks play a vital role in rural China. Households are not isolated; they are connected via a network through various cooperative, sharing, and gift exchange behaviors. In the absence of well-functioning formal institutions, such social networks often serve as the informal contracts. This paper develops a model of risk sharing in the wake of an earthquake, and examined the determinants of the self-enforcing conditions. Based on the theoretical result, we further provide empirical evidence of the labor sharing behavior of 1420 rural household before and after the 2008 Sichuan earthquake. We found that the households exposed to higher intensity of earthquakes significantly reduced their effort in investing in and maintaining the labor sharing network. We further examined gift-exchange, another important social interactions that links various households in rural China as a robustness check. We found that people significantly reduce their gift giving behavior in the aftermath of the disaster: one degree increase in earthquake intensity is associated a reduction in gift frequency for one 0.85 to 1 times per year in 2009, and 0.26-0.41 times per year in 2011, respectively. The potential channel is that the unexpected natural disaster introduced a shift in time discount factor, and people were "living like there's no tomorrow" (\cite{filipski2015living}), reduced the reliance on informal social networks, and cut down social investment for the future. While other channels proposed in the model, such as income, income correlation coefficient, risk aversion rate, and network size are not significant.