Seismic wave velocity in the Tehran seismic network region



The velocity data of seismic waves in a region has an important role in locating occurred events. Information such as velocity ratio of VP/VS and velocity curves for Pg, Pn, Sg, and Sn phases directly affect the locating results. It is well known that the VP/VS ratio can be used to predict lithology, and that S-waves are sensitive to microcracks aligned by the present day stress field. Also, the variation of VP/VS could be used as a precursor of earthquakes.
To obtain seismic wave velocity, a travel time curve can be constructed by arranging observed records of ground motion due to the explosion or earthquake as a function of distance. On the other hand, the (local) slope of the travel time curves contain important information about the horizontal slowness, and thus about the wave speed, and also the zero offset time contains information about the layer thickness. It is clear that travel time curves cannot be expressed by a single formula, but only in so-called parametric form. In other words, we are encountering additional complications in the estimation of wave velocity, but the average estimation by using both travel times and arrival times can be useful to assess the velocity macro-variations in the study area. Thus, a significant body of research is based on the arrival times of first arriving. It should be noted that some uncertainties such as picking and location errors can cause a substantial scatter in the arrival time and travel time data, respectively. This is significantly unavoidable and unpredictable for small signal to noise ratio in the data (for instance when there is a small earthquake). Applying cross correlation techniques to phase picking and using a modern relative location method, apparently, are better ways to solve these problems.
In this research, the average VP/VS ratio of 1.727 is obtained using the arrival and travel time data of P- and S-waves. Comparison of obtained VP/VS value with other researches shows that there is not a significant difference. Also, local velocity curves for Pg, Pn, Sg, and Sn phases are obtained in the study area by using the data base 1996 through 2006. The slopes of these curves give crustal P and S velocities of 6.128±0.051 and 3.524±0.12 kms-1, and Moho P and S velocities of 8.083±0.082 and 4.752±0.08 kms-1 , respectively. This research shows that the Pn and Sn phases in the study area up to about 175 km are the first arrivals in comparison with Pg and Sg phases respectively.