I contacted Prof. Lu. It is based on regression. The relevant info is shown in the attached image which is a screen shot of what he sent.

The details of the calculation of lambda can be found in Appendix A of Lu X , Han B , Hori M , et al. A coarse-grained parallel approach for seismic damage simulations of urban areas based on refined models and GPU/CPU cooperative computing[J]. Advances in Engineering Software, 2014, 70:90-103.