Y. T. Chen

Source process of the 1990 Gonghe, China, earthquake and tectonic stress field in the northeastern Qinghai-Xizang (Tibetan) plateau

TheMs=6.9 Gonghe, China, earthquake of April 26, 1990 is the largest earthquake to have been documented historically as well as recorded instrumentally in the northeastern Qinghai-Xizang (Tibetan) plateau. The source process of this earthquake and the tectonic stress field in the northeastern Qinghai-Xizang plateau are investigated using geodetic and seismic data. The leveling data are used to invert the focal mechanism, the shape of the slipped region and the slip distribution on the fault plane. It is obtained through inversion of the leveling data that this earthquake was caused by a mainly reverse dip-slipping buried fault with strike 102°, dip 46° to SSW, rake 86° and a seismic moment of 9,4×1018 Nm. The stress drop, strain and energy released for this earthquake are estimated to be 4.9 MPa, 7.4×10−5 and 7.0×1014 J, respectively. The slip distributes in a region slightly deep from NWW to SEE, with two nuclei, i.e., knots with highly concentrated slip, located in a shallower depth in the NWW and a deeper depth in the SEE, respectively.Broadband body waves data recorded by the China Digital Seismograph Network (CDSN) for the Gonghe earthquake are used to retrieve the source process of the earthquakes. It is found through moment-tensor inversion that theMs=6.9 main shock is a complex rupture process dominated by shear faulting with scalar seismic moment of the best double-couple of 9.4×1018 Nm, which is identical to the seismic moment determined from leveling data. The moment rate tensor functions reveal that this earthquake consists of three consecutive events. The first event, with a scalar seismic moment of 4.7×1018 Nm, occurred between 0–12 s, and has a focal mechanism similar to that inverted from leveling data. The second event, with a smaller seismic moment of 2.1×1018 Nm, occurred between 12–31 s, and has a variable focal mechanism. The third event, with a sealar seismic moment of 2.5×1018 Nm, occurred between 31–41 s, and has a focal mechanism similar to that inverted from leveling data. The strike of the 1990 Gonghe earthquake, and the significantly reverse dip-slip with minor left-lateral strike-slip motion suggest that the pressure axis of the tectonic stress field in the northeastern Qinghai-Xizang plateau is close to horizontal and oriented NNE to SSW, consistent with the relative collision motion between the Indian and Eurasian plates. The predominant thrust mechanism and the complexity in the tempo-spatial rupture process of the Gonghe earthquake, as revealed by the geodetic and seismic data, is generally consistent with the overall distribution of isoseismals, aftershock seismicity and the geometry of intersecting faults structure in the Gonghe basin of the northeastern Qinghai-Xizang plateau.

Inversion of near-source-broadband accelerograms for the earthquake source-time function☆

Abstract A regularization method is applied to retrieve the far-field source-time function of some small and moderate aftershocks of the April 18, 1985, Luquan, Yunnan Province, China, Ms = 6.1 earthquake. Digital broadband accelerograms recorded at stations usually within 10 km of the epicentral distance are used in the inversion. To isolate the source effect from the effects of transmission path, recording site, anelastic attenuation and instrument response, the acceleration record of a smaller aftershock with the same hypocenter location and focal mechanism is treated as an empirical Greens function and incorporated in the inversion. The far-field source-time functions inverted independently from three components at all available stations of the deployed temporary accelerograph network are in good agreement. The results obtained show that the far-field source-time function of smaller aftershocks (ML ≤ 3.0) usually presents a simple spike-like pulse with a short rise time of approximately 0.1 s, while that of larger aftershocks (ML ≈ 4.0 or greater) has not only a longer rise time (0.3 s or longer), but also is fairly complex and consists of several prominent stages, exhibiting the heterogeneity of rupture process at the earthquake source.

Spontaneous growth and autonomous contraction of a two-dimensional earthquake fault

Abstract We numerically study a model of an earthquake as a spontaneous rupture on a fault. We assume that a two-dimensional antiplane shear crack initiates at a point in an infinite, homogeneous and isotropic elastic medium and subsequently propagates with variable velocity under the influence of nonuniform stress drop on the crack and nonuniform cohesive resistance at the crack edges. To begin with, we analyze the dynamical rupture process immediately after nucleation. We determine the subsequent extension of each edge by solving an ordinary differential equation of the first order, as well as the dynamical stress in the regions between each edge and the nearest wave front. Application of this procedure to both edges of the crack in an alternating manner yields the complete history of extension of the crack. We use the critical stress-intensity fracture criterion to determine the conditions on propagation and arrest of the crack. To verify the accuracy and the stability of our numerical technique we compare it with some special cases in which analytical solutions are available. The comparison indicates that the numerical results are in good agreement with the analytical ones obtained previously by Knopoff and Chatterjee (1982) and Chatterjee and Knopoff (1983). We apply this technique to the computation of the dynamical extension of the crack for several representative cases. Among them are: 1. (1) uniform stress drop but nonuniform cohesion (a barrier model), 2. (2) nonuniform stress drop but uniform cohesion (an asperity model) and 3. (3) nonuniform stress drop and nonuniform cohesion (a combination of both the barrier and the asperity models). The numerical results indicate that the heterogeneities of both the stress drop and the cohesion are the main factors which control the growth, cessation and healing of the crack, and that the complexities in the seismic radiation are caused by the complex healing process as well as by the complex rupture propagation. In contrast to the asperity model, the barrier model is characterized by abrupt changes in the slope of the source-time function and the theoretical seismogram. The collision of the stress pulses from each pair of barriers is the potential source of the crack fission. In the general model, the fission process occurs in a complex sequence, and the effects of this process have to be taken into account in the interpretation of observations.

Definition of Seismic Moment at a Discontinuity Interface

When considering a seismic source operating along a discontinuity interface, seismic moment has to be redefined considering the jump of rigidity. The traditional definition of seismic moment may lead to underestimation of the slip or rupture length of an earthquake. Redefined seismic moment can be represented as M 0 = μ * Δ uA , in which μ * , the effective rigidity, can be represented by the average rigidity ({bar{{mu}}}) and the difference of rigidity across the interface Δμ via ({mu}^{*}={bar{{mu}}}{ }{1-({Delta}{mu}{/}2{bar{{mu}}})^{2}}) .

Decomposition of seismic moment tensors for underground nuclear explosions

Generally the decomposition of a seismic moment tensor is not unique. However, to favorably view the characteristics of a certain seismic source, one must decompose a seismic moment tensor into parts according to assumptions about the properties of the seismic source. Different from natural earthquakes in which the shear dislocation component plays a predominant role in the source process, and the seismic moment tensor can be separated into an isotropic component, a double couple, and a compensated linear vector dipole (CLVD), underground nuclear explosions have three major components in their source process, i.e., the explosion, the tensional spalling, and the tectonic strain release associated with the explosion. In such a situation the conventional moment tensor decomposition for earthquakes is not convenient to estimate the yield of the explosion and to characterize the tectonic strain release. In this paper, an alternative decomposition scheme is proposed to deal with the moment tensor of underground nuclear explosions, which might benefit the approach to study the tectonic strain release induced by underground nuclear detonations.

A theorem for the direct estimation of seismic source spectra

Abstract In this note we prove a mathematical theorem which is of help to the direct estimation of seismic source spectra using a single channel seismogram. For the situation that the propagation operator may be approximated as frequency independent, and the attenuation/scattering and the site effect may be attributed to the quality factor, it is possible to seperate the effect of the source term and the attenuation term even for a single channel seismogram. In signal processing technique, such an approach may be expressed in the scheme of the estimation of Wigner-distribution.