Application Study Of Microtremor Survey On Thick Overburden In Loess Area

Many strong earthquakes have occurred in Loess Plateau, China. Fieldinvestigations showed that some very serious earthquake geotechnical disasters werecaused at regions where intensities were over VIII during six strong earthquakes.Moreover, some small scale disasters associated with loess vulnerability wereobserved at the regions with intensity VI. The overburden thickness and its influenceon seismic ground motion is one of the most important factors inducing seriousearthquake geotechnical disasters. Consequently, in order to provide the anti-seismicdesign of the structures with more reasonable and effective parameters, it is of greatsignificance to survey overburden thickness for quantitative study on the law ofinfluence on seismic ground motion parameters.In order to develop a new method obtaining the distubition of overburdenthickness in the loess areas, a convenient, economic and environmental microtremorsurvey has been carried out. The research regions are the Lanzhou city and theTianshui city, which are the representational valley cities. The geologic structure,terrain and topography, hydrologic conditions data are collected in these two citiesand field complementary boreholes are finished, obtaining geological records ofboreholes and shear wave velocity distribution. As a result, reasonable observationplans are made, and then three instruments are used to carry out continuous andhigh-density microtremor survey in Lanzhou and Tianshui respectively. The stablewaves are selected to analyze the fast Fourier transfer (FFT) and thehorizontal-to-vertical spectral ratio (H/V) after filtering and smoothening, discussingthe time and space stability of microtremor data, dealing with the vibrationcharacteristics of three directions, gaining the predominant frequency of sites. Finally,taking the geological records of boreholes and site conditions into account, twomethods including the quarter-wave length and statistical relationship between theoverburden thickness and predominant frequency are applied to inverse overburdenthickness. Some main conclusions and new findings are shown as follows. 1. The results of microtremor waves show that most waves are stable except fewobservation points that locate in the noisy downtowns. The authentic data collectedcan be used to analyze the characteristic of surface soil and rock masses.2. The horizontal amplitudes of spectrum of the fast Fourier transfer in Lanzhouand Tianshui are controlled by valley terrain and topography, the predominantamplitude of the north-south direction is slightly less than or equal to the west-eastone. The predominant amplitude of the vertical direction is rarely related to thehorizontal one, which is mainly controlled by artificial vibration sources outside.3. The long-period microtremor in Lanzhou and Tianshui city does not vary overtime, but the predominant range during day time is wider than night. The predominantrange in Lanzhou during day time is equal to the one in Tianshui as well as duringnight, which can be inferred that the long-period microtremor has the space stability.4. Analysis results of the spectral ratio of H/V of high-density microtremor dataand the inversion of overburden thickness reveal that the inversion result ofoverburden thickness is coincident with boreholes in two cities. It demonstrates thatmicrotremor survey is available and effective for obtaining overburden thickness atthe loess areas in west of China.5. The serviceabilities of the quarter-wave length and the statistical relationshipbetween the overburden thickness and predominant frequency are assessed at thedifferent sites. The results show that the former is fit for the third and fourth terracewith single stratum; high accuracy formula, simplifying the influence of shear wavevelocity, is drawn from the latter, and it is suitable for the whole studied area.