Time-space evolution characteristics of abrupt variation of wave velocity ratio in the seismogenic process of recent strong earthquakes in Yunnan area

Within a short period of 7 months from July 12, 1995 to February 3, 1996, three earthquakes occurred continually to west Menglian in the border area of China Myanmar ( M =7.3), in Wuding county M =6.5 and in Lijiang ( M =7.0) in Yunnan area. In this paper, the authors have studied the time space evolution characteristics of wave velocity ratio ( γ ) in observed at 5 single stations and the average value of several stations before the recent strong earthquakes. It is discovered that 5～8 years before the earthquake with M =7, source precursors of long medium term, with high (low) drastic variations of wave velocity ratio, appeared within 120 km from the epicenters, and the source precursors of long medium term entered into medium short term stage when the amplitude of wave velocity ratio suddenly increased (decreased) or times of earthquake increase drastically. Three to five years before M =6 earthquakes, source precursors of long medium term and near field precursors of long medium term, with abrupt velocity change of high values, appeared within 40 and 150 km; however, the indicator is not clear in the period of transition to medium short stage. The anomaly of γ within 150 km from the epicenter reaches as much as 3.0, while the maximum value is 2.36 in the area 250 km away, showing the characteristics that the shorter the distance the bigger the abrupt change. Namely, the anomalous amplitudes of source precursors and near field precursors are 20%～60% bigger than that of far field precursors. The reliability of abrupt variation data of γ and its physical mechanism have also been explored in this paper.

Stepwise joint inversion of surface wave dispersion,Rayleigh wave ZH ratio,and receiver function data for 1D crustal shear wave velocity structure

Accurate determination of seismic velocity of the crust is important for understanding regional tectonics and crustal evolution of the Earth. We propose a stepwise joint linearized inversion method using surface wave dispersion, Rayleigh wave ZH ratio （i.e., ellipticity）, and receiver function data to better resolve 1D crustal shear wave velocity （Vs） structure. Surface wave dispersion and Rayleigh wave ZH ratio data are more sensitive to absolute variations of shear wave speed at depths, but their sensi- tivity kernels to shear wave speeds are different and complimentary. However, receiver function data are more sensitive to sharp velocity contrast （e.g., due to the existence of crustal interfaces） and Vp/Vs ratios. The stepwise inversion method takes advantages of the complementary sensitivities of each dataset to better constrain the Vs model in the crust. We firstly invert surface wave dispersion and ZH ratio data to obtain a 1D smooth absolute vs model and then incorporate receiver function data in the joint inver- sion to obtain a finer Vs model with better constraints on interface structures. Through synthetic tests, Monte Carlo error analyses, and application to real data, we demonstrate that the proposed joint inversion method can resolve robust crustal Vs structures and with little initial model dependency.

Interpretation of S-wave data from Tai′anXinzhou DSS profile and its relationship with Xingtai earthquakes

On the basis of S wave information from Tai′an Xinzhou DSS profile and with reference to the results from P wave interpretation, the 2 D structures , including S wave velocity V s, ratio γ between V p and V s; and Poisson′s ratio σ , are calculated; the structural configuration of the profile is presented and the relevant inferences are drawn from the above results. Upwarping mantle districts (V s≈4.30 km/s)and sloping mantle districts (V s≈4.50 km/s) of the profile with velocity difference about 4% at the top of upper mantle are divided according to the differences of V s , γ and σ in different media and structures, also with reference to the information of their neighbouring regions; the existence of Niujiaqiao Dongwang high angle ultra crustal fault zone is reaffirmed; the properties of low and high velocity blocks(zones) including the crust mantle transitionalzone and the boudary indicators of North China rift valley are discussed. A comprehensive study is conducted on the relation of the interpretation results with earthquakes. It is concluded that the mantle upwarps, thermal material upwells through the high angle fault, the primary hypocenter was located at the crust mantle juncture 30.0～33.0 km deep, and additional stress excited the M S=6.8 and M S=7.2 earthquakes at specific locations around 9.0 km below Niujiaqiao Dongwang, the earthquakes took place around the high angle ultra crustal fault and centered in the brittle media and rock strata with low γ and low σ values.

Research on propagation properties of elastic waves in two-phase anisotropic media

With the development of seismic engineering and seismic exploration of energy, the underground media that westudy are more and more complicated. Conventional anisotropy theory or two-phase isotropy theory is difficult todescribe anisotropic media containing fluid, such as fractures containing gas, shales containing water Based onBlot theory about two-phase anisotropy, with the use of elastic plane wave equations, we get Christoffel equations.We calculate and analyze the effects of frequency on phase velocity, attenuation, amplitude ratio and polarizationdirection of elastic waves of two-phase, transversely isotropic media. Results show that frequency affects slow Pwave the greatest among the four kinds of waves, i.e., fast P wave, slow P wave, fast S wave and slow S wave.Fluid phase amplitude to solid phase amplitude ratio of fast P wave, fast S wave and slow S wave approaches unitfor large dissipation coefficients. Polarization analysis shows that polarization direction of fluid phase displacement is different from, not parallel to or reverse to, that of solid phase displacement in two-phase anisotropic media.

Temporal vP/vS variation characteristics in different zones of China's Capital Circle area before and after Wen'an earthquake

On July 4, 2006, an earthquake of MS5.1 took place in Wen’an, Hebei Province, just at the south center of China’s Capital Circle area digital seismograph network. It is the strongest event recorded ever since the network went into operation in 2002. We processed the vast amounts of phase data yielded by the 107 digital seismic stations between 2002~2007 using Wadati method. In order to improve the precision and stability of shear and compressional wave velocities (vP/vS) calculation, we impose a number of restrictions on the computation environment and condition, e.g., the earthquakes are densely concentrated, selected stations are limited in range, the number of stations in- volved in the computation is larger than 5 and linear fitting features high precision and small error. Under these restrictions, the study shows that vP/vS in and around Wen’an and Tangshan underwent a normal-low-normal proc- ess one year before Wen’an earthquake, vP/vS became obviously low and the low ratio lasted for about one year, meanwhile, little variation of vP/vS was seen in Xingtai, northwest of Beijing, southwest of Beijing, Beijing-Tianjin and Beijing; after the quake, the vP/vS returned normal in Wen’an and Tangshan. Error and stability analysis of the calculated result for vP/vS shows it is convincible that anomaly appeared in and around Wen’an and Tangshan be- fore Wen’an earthquake.