Portable Broadband Seismic Array Obseration: Shear-Wave Splitting And Crustal Stress Field In The Capital Circle

The broadband seismic array observations provide a new approach for high-resolution seismic imaging of the continental lithosphere. Seismic array system has been used extensively in the continental dynamics investigation, and it makes possible to perform a comprehensive seismic study in a key place on the Earth.The shear-wave splitting induced by the medium cracks is a hot spot investigated in seismology and many results have been published in recent years. Now it has been verified that in the upper crust the cracks exist widely and the polarization direction of the fast shear wave is parallel with the maximal principle compressive stress. All of these constitute a basis for investigating the crustal stress according to the shear-wave splitting.The Capital Circle is not only an economic, political and cultural center of China, but also an area suffering from many earthquake hazards in the history. The Capital Circle, in view of its high seismic risk in the near future, has been confirmed to be an earnestly monitored region.In 2002, to explore the tectonic environment dealing with the long-term earthquake prediction of the Capital Circle, the Continental Lithosphere Seismic Array Center, Institute of Geology under China Earthquake Administration, carried out a passive seismic array experiment in this region. The permanent station network and our portable seismic array constitute a dense network covering the Capital Circle.During this experiment, besides a large number of teleseismic waveform data, many local events were also recorded. This makes it possible to build up our comprehensive database consisting of local events recorded by the movable and permanent stations. In this study the shear wave splitting in these local-event data is investigated in terms of polarization analysis to obtain the principle compressive stress field of the crust beneath the Capital Circle. Our main results can besummarized as follows:1. The stress field of the crust beneath the Capital Circle can be separated into regional background stress field towards the NE direction and local stress field towards the NW direction controlled by the Zhangjiakou-Penglai faults.2. The maximal principal compressive stress nearby Beijing and Tianjin is towards NE60~70, while that around Tangshan and its eastern side is close to the EW direction.3. The maximal principal stress of the local stress fields is consistently toward 120~130, which is roughly parallel with the strike of the Zhangjiakou-Penglai faults. This indicates that t the stress field of the crust beneath the Capital Circle is controlled mainly by the Zhangjiakou-Penglai faults4. The results about the source mechanism solutions indicate that about 43% of the P-axes is towards the NW direction. This observation is consistent with our results obtained from the data of the shear wave splitting.To interpret our observations mentioned above, the numerical modeling of the crustal stress field was made using the 2-D finite element method, including: 1) the effect of different medium parameters of faults on the regional stress field; 2) the effect of uniform slip of faults on the stress field; 3) the effect of uneven slip of faults on the stress field. The following results are given by our numerical modeling:1 .The regional stress field is influenced to a large degree by the faults, and the existence of faults is the most important factor leading to the local stress fields, which would be related closely to genesis of earthquakes;2.The amount of the regional stress field is influenced greatly by the medium differences between the inside and outside of a fault. But the direction of the maximal principle stress has not very much relation with the medium parameters. The direction of the maximal principle stress in the local stress field is controlled mainly by faults.3.When a fault generates a uniform slip, the stress concentrates on the both ends of the fault, and the amount and direction of the maximal principle stress are not changed obviously in the central part of the fault. However, when th...