Almost all shear-waves from local earthquakes recorded on closely-spaced three-component seismometer networks deployed near the North Anatolian Fault, Turkey, in two experiments in 1979 and 1980, display shear-wave splitting. The observations are consistent with the presence of EDA (extensive-dilatancy anisotropy), distributions of fluid-filled cracks and microcracks aligned by the regional stress field. Temporal changes in the stress-field, which may occur before an earthquake, may modify the geometry and possibly the orientation of the EDA-microcracks, and lead to corresponding changes in the behaviour of the split shear-waves. A third experiment was undertaken in 1984 to investigate EDA further and to search for possible temporal variations of the polarization of the leading split shearwave and the time delay between split shear-waves. Observations indicate that the polarization alignments, which are parallel to the strike of the parallel vertical EDA-cracks, are unaltered between 1979 and 1984, implying that the direction of the regional stress field has not changed significantly. Temporal changes in the stress field are more likely to cause changes in the crack density and/or aspect ratio, which would result in a corresponding change in time delay between the split shear-waves. We examine observations of time delay in relation to their propagation path with respect to the crack geometry since i t is then possible to separate the effects of changes in crack density and changes in aspect ratio. With this procedure, a small temporal variation of time delays is found between 1979 and 1984, consistent with a decrease in crack density, and consequently a relaxation of stress, in this time period. No evidence was found for any observable variation of time delay over a six month observation span in 1984. We suggest that analysis of repeated shear-wave VSPs offers a technique for monitoring stress changes before earthquakes.

In this paper changes in focal mechanisms, parameters of wave spectra, and stress drops for the Ms=5.0 foreshock and Ms=6.0 mainshock in February 2001 in Yajiang County, Sichuan, and seismicity in epicentral region are studied. Comparison of focal mechanisms for the Yajiang earthquakes with distribution patterns of aftershocks, the nodal plane I, striking in the direction of NEN, of the Yajiang M=5.0 event is chosen as the faulting plane; the nodal plane II, striking in the direction of WNW, of the M=6.0 event as the faulting plane. The strikes of the two faulting planes are nearly perpendicular to each other. The level of stress drops in the epicentral region before the occurrence of the M=6.0 earthquake increases, which is consistent with increase of seismicity in the epicentral region. The rate decay of the Yajiang earthquake sequence, changes in wave spectra for foreshocks and aftershocks, and focal mechanisms are complex.

Based on the single scattering model of coda power spectrum analysis, digital waveform data of 50 events recorded by the real-time processing system of the Chengdu telemetry network are analyzed to estimate the Qc values of earth medium beneath the Chengdu telemetry network for several specified frequencies. It is found that the Qc shows the frequency dependency in the form of Qc = Q0f" in the range of 1.0 to 20.0Hz. Estimated Q0 ranges from 60.83 to 178.05, and n is found to be 0.713 to 1.159. The average value of Q0 and n are 117 and 0.978 respectively. This result indicates the strong frequency dependency of the attenuation of coda waves beneath the Chengdu telemetry network. Comparing with the results obtained in other regions of the world, it is found that Q0 -~ value and its change with frequency are similar to those in regions with strong tectonic activity. Key words: Chengdu telemetered network coda Q frequency dependency tectonic activityCoda wave, seismic energy after direct P and S waves, can be interpreted as backscattering waves from the numerous heterogeneity distributed randomly in the earth (Aid, 1969), and a single scattering model was proposed to interpret the origin of coda wave (Aid and Chouet, 1975). According to this model, the coda wave is a superposition of the waves scattered backward direction for once from the scatters distributed within the earth, and a method of determining the coda Q value from the time decay of coda waves was proposed. Now this method has widely applied to seismic records obtained in many areas of the world. Several important results on the frequency dependency of Qc value and its regional variation are obtained ( e.g. Rautian and Khalturin, 1978; Tsujiura, 1978; Aki, 1980; Sato,1986;Matsumoto and Hasegawa, 1989). These results show that the dependency of coda Q can be expressed approximately by the n-th power of frequency in the form of Qc = Q0f n. Matsumoto and Hasegawa(1989) estimated the regional variation of the Qc value beneath the northeastern part of Honshu, Japan. The result showed that the spatial distribution of Qc has a similar pattern to the S wave velocity structure, and Qc values in the regions near the active volcanoes show a strong frequency dependency. In the present study, based on the single scattering model, we estimate coda Q values at several frequency bands and their frequency dependency *

CLC number: P315.3+3 Document code: A Based on waveform data, several methods to determine focal mechanisms of small earthquakes were developed since 1980. Kisslinger (1980) and Julian, Foulger (1996) proposed an approach to determine solution by using amplitude ratio of P and S wave. Schwartz (1995) devised a method to determine solutions by the use of polarity data and amplitudes of seismogram envelopes. Amplitudes of short period seismic waves propagating in an inhomogeneous medium are sensitive to the variation in velocity and Q structure. Nakamura, et al (1999) took medium inhomogeneity into account in determining focal mechanisms of small earthquakes using waveform data. If the locations of small earthquakes are concentrated in a small region, we can assume that the raypaths from the events to a given station are almost the same. So P and S wave attenuations are independent of event locations. In this case it is convenient to determine focal mechanisms of these events by using short period P and S wave data Q. Focal mechanism solutions of small earthquakes in 5 regions, i.e., Rongchang, Mabian-Muchuan, Ya'an, Baoxing and Mianzhu, which are covered by the Chengdu Telemetered Network, are obtained by analyzing the P polarity and short body wave amplitude data recorded in the network since 1992. According to the method proposed by Gephart and Forsyth (1984), based on well determined focal mechanism solutions in 15 sub-zones of Sichuan and Yunnan area, three principal stress tensors cry, Crz, and crj, instead of averages of P, B, and T axis of the solutions, are determined to represent the regional stress field distribution. Method Determination of focal mechanismFor P or S wave, the spectrum amplitude U!j(co) from i-th event observed atj-th station can be written as Uij (co) = m i ( co)Ri/C~ (co) / rij (1)where mi(co) is seismic moment, Ri/is source radiation factor, r,) is source distance, co is angular frequency. Here only *

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