Shallow Velocity and Q Structure of Tianjin Basin from P, PP, PPP and Psed

Earthquake engineer consider that shallow structure is a major issue on earthquake ground motions. Conventionally, the S velocity of 30 meters is consulted to determine the ground standard. On the other hand, there are suggestions from some researchers that the whole unconsolidated sediments should be considered because large intrinsic attenuation may be a natural consequence of nonlinear, strain-dependent degradation of the shear modulus. (Vucetic, 1994 ). At the same time, almost all the unconsolidated sediments are the nature of basin morphology. The research about shallow detailed velocity and Q structure from 30 m to 400 m is not sufficient. Recently, Langston provided some results that Qp is remarked large that do not like conventional seismological opinions, in the Mississippi Embayment (Langston, 2005). Qp could be effectively determined by Psed which is the robust phase in seisgrams. Psed wave is the trapped wave in unconsolidated sediments. It can be considered the “Whispering Gallery” phase that propagates just below the surface (Walter Mooney, 1980). In the other way, Qp can be estimated by transformed waves and Qs can be evaluated by transformed waves and Rayleigh waves. There are abundance of refraction/reflection experiments in Bohai basin where is more over 30 million peoples settled down. The active fault experiment of China Earthquake Administration in Tianjin is with dense shot points and receiver spaces recorded plenty of refraction seisgrams, from Dec. 2005 to Jan. 2006. We analyzed the P, PP, PPP and Psed phases in the refraction seisgrams and provided the shallow velocity and Q structure in Tianjin basin. We prefer to use the reflectivity method, calculation synthetic seismogram (Wang R, 2007), to model those seismic refraction data. We presented one dimensional detailed velocity model that contain 3 distinct high velocity gradients ranged 20km and analyzed P and the secondary phases to estimate the Q structure of the sedimentary section. Q structure is larger than earlier estimates reported for sedimentary environments, and it implies that strong-motion prediction in the unconsolidated sediments of the Tianjin embayment should be considered in earthquake hazard assessments.