Slope stability monitoring from microseismic field using polarization methodology
Numerical simulation of seismoacoustic emission (SAE) associated with fracturing in zones of shear stress concentration shows that SAE signals are polarized along the stress direction. The proposed polarization methodology for monitoring of slope stability makes use of three-component recording of the microseismic field on a slope in order to pick the signals of slope processes by filtering and polarization analysis. Slope activity is indicated by rather strong roughly horizontal polarization of the respective portion of the field in the direction of slope dip. The methodology was tested in microseismic observations on a landslide slope in the Northern Tien-Shan (Kyrgyzstan).
Geophysical Investigations Of Seismically Induced Surface Effects: Case Study Of A Landslide In The Suusamyr Valley, Kyrgyzstan
In summer 1998, a geophysical survey including seismic profilesand electrical tomography has been carried out in the Suusamyr valley, Kyrgyzstan. The scope wasto investigate surface effects induced by the Ms = 7.3 Suusamyr earthquake, the 19th of August, 1992. Inthis paper, special attention is paid to the case study of a debris slide triggered by the earthquake.Seismic data are analysed by P-wave refraction technique and by surface wave inversion. Electrical tomographicprofiles are processed by 2D-inversion.Using geotechnical and geological information, P-velocity modelsand resistivity sections are interpreted in terms of geological materials, in order to build a geological3D model. On the basis of the latter, we carried out static finite element computations as well as staticand pseudo-static calculations with Janbu’s method. Newmark displacement was computed, considering or notthe influence of the shallow soft deposits. The results are compared to the real displacementobserved in the field and conclusions are drawn about the mechanism of the landslide.
A scaling law for the spectra of large earthquakes
Abstract An ω-squared model suggested by Hanks and McGuire has had impressive success in predicting measures of strong ground motions over a broad range in moment magnitude. There remains the question of how the ground motion spectra should scale for earthquakes large enough to rupture the entire width of a seismogenic zone, i.e., earthquakes for which the usual similarity principles do not apply. The scaling law proposed in this paper takes as its fundamental postulate that the high-frequency level of the acceleration spectrum is proportional to the square root of the rupture area for all earthquakes, not just those for which the similarity principles apply. The spectrum corresponding to the proposed scaling law is controlled by two corner frequencies, one inversely proportional to the rupture length and the other inversely proportional to the rupture width. The proposed law agrees with the Hanks-McGuire model below the value of moment corresponding to rupture of the whole width of the seismogenic zone. A stochastic kinematic model is devised for the purpose of making ground motion predictions corresponding to the proposed law. The spectrum of the stochastic kinematic model can be written in terms of the product of two factors, one controlled by the corner frequency related to rupture length and the other by the corner frequency related to rupture width. Above their respective corner frequencies both factors are proportional to frequency raised to a power. The two exponents are chosen in order to make the spectrum correspond to the proposed law, but it is noteworthy that both exponents can easily be justified on other grounds. The exponent in the factor controlled by the corner frequency related to rupture width corresponds to the square root singularity in slip velocity exhibited by Kostrov9s solution for a constant stress relaxation propagating at constant velocity. The exponent in the factor controlled by the corner frequency related to rupture length has the value implied by the well-established proportionality between average dislocation and rupture length, on the assumption that dislocation is a random function of position along the fault. Predictions of peak horizontal acceleration, velocity, and response spectra corresponding to the proposed law are made with the stochastic kinematic model and also using random vibration theory. The two sets of predictions show good agreement. The predictions are consistent with strong-motion data for shallow earthquakes in western North America, within the scatter of the data and the uncertainty of the parameters of the proposed law. The predictions differ by more than a factor of two from a simple extrapolation of the Hanks-McGuire model only at moment magnitudes more than one unit greater than the critical magnitude corresponding to rupture of the entire width of the seismogenic zone. It is likely that few if any such earthquakes ever occur.