Seismic Response Of A Two-Layer Field Under Oblique Incident Seismic Waves

Seismic waves from bedrock usually propagate to the surface in an inclined manner and cause ground motion,and the site effects caused by different types of seismic waves are also different.Therefore,in the study of site seismic response,it is more in line with engineering practice to regard seismic waves as oblique incidence than to assume vertical incidence.It is of great significance to study the site response under the action of oblique incident seismic waves to reveal the relationship between the incident angle of seismic waves and the site response and to guide the anti-seismic design of geotechnical engineering.In order to obtain the theoretical value of the seismic response of the site under the action of oblique incidence seismic waves,the site stratum is regarded as an elastic half-space double-layer site structure composed of a finite thickness of low-speed overburden and an infinite thickness of bedrock,and a multi-level reflected wave site model under the action of oblique incidence seismic waves is established.The reflection characteristics of P-wave and SV-wave at the free surface and elastic interface of oblique incidence seismic waves and the propagation path and propagation time of each level of seismic waves generated in the site are discussed respectively.The superposition principle is used to synthesize these seismic waves reaching the ground in frequency domain to obtain the ground motion time history of the site.In order to improve the calculation efficiency,the corresponding calculation program is written by matlab software,and the reliability of the algorithm and program is verified by an example.The influence of seismic wave type,incident angle and Poisson ’s ratio on ground motion is studied.The results show that:(1)The method of calculating site ground motion based on frequency domain can deal with the problem of complex reflection coefficient(real reflection coefficient is a special case),and its calculation accuracy can be controlled by incidence series.(2)Under the action of oblique incident seismic waves,the ground motion trajectory shows the rotation characteristics similar to the surface wave,and its vibration direction changes with time.(3)Due to the total reflection of SV wave,the variation of ground motion caused by SV wave with incident angle is more complex than that of P wave.When the incident angle is close to the critical angle,the seismic response caused by SV wave is the strongest,that is,the site has a significant near-critical effect under the action of SV wave.(4)Whether the incident wave is seismic P wave or seismic SV wave,the predominant period of horizontal and vertical ground motion has the same rule:the predominant period of horizontal ground motion is close to the predominant period caused by vertical incident SV wave,and the predominant period of vertical ground motion is close to the predominant period caused by vertical incident P wave.(5)Under the action of oblique incidence seismic P wave,the greater the Poisson’s ratio of bedrock,the weaker the horizontal vibration and the stronger the vertical vibration;under the action of oblique incident seismic SV wave,the greater the Poisson’s ratio of bedrock,the smaller the critical angle,and the stronger the horizontal vibration when the critical effect occurs,while the relationship between vertical vibration and Poisson’s ratio is not obvious.