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A new characteristic parameter Sdτ is proposed to improve the performance of magnitude estimation in earthquake early warning (EEW). Sdτ is the product of summation of absolute displacement multiplied by the maximum predominant period (τmaxP) for the first arriving seconds of a seismic wave. About 30,725 underground records at borehole stations for 3645 earthquakes with magnitude between 4.0 and 9.0 from the Japanese KiK-net were used to compare the magnitude proxy performance based on the proposed Sdτ with that based on either τmaxP or peak displacement Pd. The comparison results show that for a magnitude between 4.0 and 7.3, Sdτ has a better correlation with magnitude and higher estimated accuracy than either τmaxP or Pd. Hypocentral distance is not required when using Sdτ, but it can be used to further improve the accuracy of magnitude estimate. These results confirm that Sdτ can significantly improve the accuracy and timeliness of continuous magnitude estimation in an EEW system.

In the complex medium system of sea area, the overlying sea water and the surface soft soil have a significant impact on the seafloor ground motion, which brings great seismic risk to the safety of offshore engineering structures. In this paper, four sets of typical free field models are constructed and established, which are land model, land model with surface soft soil, sea model and sea model with surface soft soil. The dynamic finite difference method is used to carry out two-dimensional seismic response analysis of typical free field based on the input forms about P and SV wave. By comparing the seismic response analysis results of four groups of calculation models, the effects of overlying seawater and soft soil on peak acceleration and acceleration response spectrum are studied. The results show that when SV wave is input, the peak acceleration and response spectrum of the surface of soft soil on the surface and the seabed surface can be amplified, while the overlying sea water can significantly reduce the ground motion. When P wave is used, the effect of overlying seawater and soft soil on peak acceleration and response spectrum of surface and seabed can be ignored. The peak acceleration decreases first and then increases from the bottom to the surface, and the difference of peak acceleration calculated by four free field models is not obvious. The results show that the overlying sea water and the surface soft soil layer have little effect on the peak acceleration of ground motion below the surface.