A NUMERICAL RESEARCH OF NONLINEAR BODY-WAVE INTERACTIONS

This paper presents numerical research results of nonlinear body-wave interactions. The body-wave interac- tion is treated as a transient problem with known initial conditions. The development of the flow can be obtained by a time-stepping procedure, in which the velocity potential of the flow at any instant is obtained by utilizing a source density distribution on all boundary surfaces. The Orlanski's method is used to implement the boundary con- dition at the open boundary. The position of intersection points are determined by a direct method. The contents of this paper are: (1) A study of the nonlinear radiation problem of a floating body; (2) A study of nonlinear wave diffraction problem around large structures; (3) Interaction of nonlinear waves with a free floating body; (4) An attempt in generating a numerical wave tank. Pretty good agreement is met between the numerical results and the analytical solution.

Preliminary Results of Tomography from Permanent Stations in the Anhui Airgun Experiment

In order to explore the ability of airgun sources in imaging the crustal structure,we conducted an experiment along the Yangzte River at the section from Ma 'anshan to Anqing using an offshore airgun source( referred as the Anhui Airgun Experiment).During the experiment,the airgun source was fired 2973 times at 20 multiple shot points,while 109 permanent stations and 700 temporary stations( 11 profiles) were used to record the signal. For 20 multiple shot points,we picked up 335 P-wave arrivals at 52 permanent stations. Then we obtained the P-wave velocity structure of the crust beneath the southern tip of the Tancheng-Lujiang( Tan-Lu) fault. The preliminary result of P-wave velocity structure confirms the possibility of 3D body wave imaging using large volume airgun sources. The imaged velocity heterogeneity at a depth of 15 km is correlated to the geological settings. High velocity anomaly beneath the Qinling-Dabie orogenic belt could be explained by the ultrahigh-pressure metamorphic rocks in the deep region,while the low velocity anomaly at the Yangtze River region may correspond to the special background of mineralization.

Research on Stress Drops and the Focal Mechanisms of the Xinyuan- Hejing ML6.8 Earthquake Sequences

Based on the digital waveform data recorded by Xinjiang Digital Seismic Network for the Xinyuan-Hejing M_L6.8 earthquake sequences of June 30,2012,this paper analyzes the stress drops of earthquake sequences and the correlation coefficients of focal mechanisms significant for strong aftershocks.Firstly,the source parameters of the Xinyuan-Hejing M_L6.8 earthquake sequences are obtained by applying the spectrum analysis and the Brunes source model.Then,the correlation coefficients of spectral amplitudes are calculated using the low-frequency spectral amplitude recorded by the same station for the different events.Finally,based on the results of the correlation coefficients of spectral amplitudes,the events with similar focal mechanisms are grouped using the clustering method.The results show that:(1)The stress drop values show a steady trend in the aftershock sequence calm period and the stress drop values show a rise-fall in strong aftershocks.(2)The moving average correlation coefficient of amplitude spectrum begins to spread after the main shock.It shows that the correlation decreases between the main shock and the aftershocks in mechanisms.(3)The results of focal mechanism groups show that the earthquake sequences are mainly strike slips.The stress distribution of the main pressure axis is nearly NS,which is the same as the structural stress field.(4)The magnitude and mechanism show that there is an agreement before the strong aftershock,which shows that the regional stress field is enhanced.