The relationship between Vs, Vp, density and depth based on PS-logging data at K-NET and KiK-net sites

Abstract

\n P-wave velocity (Vp) is an important parameter for constructing seismic velocity models of the subsurface structures by using microtremors and earthquake ground motions or any other geophysical exploration data. In order to reflect the ground survey information in Japan to the Vp structure, we investigated the relationships among Vs, Vp and depth by using PS-logging data at all K-NET and KiK-net sites. Vp values are concentrated at around 500 and 1500\xa0m\u2009s–1 when Vs is lower than 1000\xa0m\u2009s–1, where these concentrated areas show two distinctive characteristics of unsaturated and saturated soil, respectively. Many Vp values in the layer shallower than 4\xa0m are around 500\xa0m\u2009s–1, which suggests the dominance of unsaturated soil, while many Vp values in the layer deeper than 4\xa0m are larger than 1500\xa0m\u2009s–1, which suggests the dominance of saturated soil there. We also investigated those relationships for different soil types at K-NET sites. Although each soil type has its own depth range, all soil types show similar relationships among Vs, Vp and depth. Then, considering the depth profile of Vp, we divided the data set into two by the depth, which is shallower or deeper than 4\xa0m and calculated the geometrical mean of Vp and the geometrical standard deviation in every Vs bins of 200\xa0m\u2009s–1. Finally, we obtained the regression curves for the average and standard deviation of Vp estimated from Vs to get the Vp conversion functions from Vs, which can be applied to a wide Vs range. We also obtained the regression curves for two data sets with Vp lower and higher than 1200\xa0m\u2009s–1. These regression curves can be applied when the groundwater level is known. In addition, we obtained the regression curves for density from Vs or Vp. An example of the application for those relationships in the velocity inversion is shown.