In the complex medium system of the sea area, the overlying seawater 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, comprising a 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 a typical free field based on the input forms of P and SV waves. By comparing the seismic response analysis results of four groups of calculation models, the effects of overlying seawater and soft soil on the peak acceleration and acceleration response spectrum are studied. The results show that when an 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 seawater can significantly reduce the ground motion. When the P wave is used, the effect of overlying seawater and soft soil on the peak acceleration and response spectrum of the 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 seawater and the surface soft soil layer have little effect on the peak acceleration of ground motion below the surface.

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.

Based on the advantages of the chaos particle swarm optimization algorithm and the generalized inversion technology, this article estimates the source parameters and site effects of the Wenchuan earthquake. We used 440 sets of strong‐motion records obtained from 43 aftershocks, and the area covered by the records was divided into subregions A and B. Initial separation of source, path, and site from the seismic spectra of subregions A and B using generalized inversion technique and then the source‐site optimization model is established using chaotic particle swarm technology. From path‐corrected records, we obtained absolute site effects for 33 stations and equivalent source parameters for 43 earthquakes. We made the following conclusions: (1) The moment magnitude $Mw$ was lower than the local magnitude $ML$determined by China Earthquake Network Center. The self‐similarity of the Wenchuan earthquake was confirmed. The stress drop averaged 2.31 MPa, and it was independent of the magnitude size and focal depth. (2) In the frequency 1–10 Hz, the quality factor values in subregions A and B are $110.9f0.6$ and $116.1f1.2$⁠. The decay rate of the crustal medium in the western region of the west Sichuan plateau is significant compared to the eastern part. (3) Bedrock stations 51MXT and L2007 have site effects within a certain frequency. The effect of slope topography on site predominant frequency is not apparent, and the site effects increase with the increase in elevation. The shape of the site amplification curve is more similar in the middle‐ and low‐frequency bands, and different attenuation phenomena will appear in the high‐frequency band.