Study On Variation Of The Earthquake Ground And Sub-ground Motion

The variation of ground motion with depth is an important consideration in characterizing ground motion for embedded structures. In this paper, a study on the earthquake ground and underground motion by analyzing the surface and downhole data from Hosokura Mine Array of Japan and CSMIP Geotechnical Arrays of U.S.A. is conducted. The analysis is developed in three aspects ?amplitude, frequency and duration, accordingly, the contents of this paper can be divided into three parts. In the end of each part, a summary is given.According to the earthquake database of this research, the earthquake is classified into three levels-M<4.5, 4.56.0. Based on the Hosokura Mine Array and CSMIP Geotechnical Arrays data. Some meaningful conclusion can then be drawn.1. The amplitude aspect. In this part, PGA, PGV and PGD are studied, the main results are as follows:1) In general, PGA decrease with depth and can reach maximum at surface; The PGA ratio with depth of downhole/surface is affected by magnitude and ground PGA value, the bigger the magnitude and PGA value, the smaller the PGA decrement with depth; The decrement with depth is much more significant in near-surface than that in deep downhole.2) The variation of PGV and PGD with depth is similar to that of PGA. Besides, the decrement with depth of PGV is smaller than that of PGA. and PGD smaller than PGV.2. The frequency aspect. In this part, response spectrum and Fourier amplitude spectrum are studied, the main results are as follows:1) Response spectrum analysis:a) Absolute acceleration response spectrum (Sa) : (1) In general, response spectrum decrease with depth, for M<4.5, The shape and the predominant period of each response spectrum in different depths are similar: the variation of spectra with depth focus on near-surface level; the normalized spectra are similar to each other, except the predominant perk value in middle depth of some earthquakes is higher than other depth.(2) for M>6.0, The spectra of different depth get closer and closer with period increasing; the normalized spectra of downhole are get higher thanthat of surface in long period.(3) for 4.56.0.b) Relative velocity response spectrum (Sv) and Relative displacementresponse spectrum (Sd) :The variation of velocity spectrum with depth is similar to that of acceleration spectrum. But the variation of displacement spectrum with depth is a little different to that of acceleration and velocity spectrum. That is, in strong earthquake, the variation of displacement spectrum and normalized spectrum in different depth is very small.2) Fourier amplitude spectrum analysis:(1)The Fourier amplitude spectra decrease with the increment of depth; and the components of Fourier spectrum become complex in near-surface.(2)The Fourier amplitude decrease faster in shallow soil level, and smaller in deep soil level.(3)Compared with the deepest bedrock Fourier spectrum, spectral components of higher than 5 Hz decrease with depth decrease.(4)For the "soil/rock" site, the up and down spectra in bedrock are almost the same.3. The Duration aspect. In this part, strong motion duration (Trifunac-Brady) is studied, the main results are as follows:In general, For soil site, during strong earthquake, the 90% relative energy duration increases with depth. The duration of the ground motion on surface is about 1/4 of that at depth of 100m. That means the energy distribution of strong motion in deep underground is relatively smoother than that on surface. Also, the intensity variation of strong motion in deep underground is relatively smoother than that on surface.