Research On Dynamic Stress Triggering At Chinese North-South Seismic Belt In Recent Years

Seismic stress triggering points to the influence induced by one earthquake to the nearby and remote seismic activity, including static stress triggering and dynamic stress triggering. Scientists have studied static stress triggering for a long time and have got lots of achievement. On the other hand, researching on dynamic stress triggering was scarce. The time we indeed researched on seismic dynamic stress triggering was after the Landers earthquake of 1992, USA. As dynamic stress triggering has superiority in explaining remote triggering, so it is developed rapidly in recent years. China locates between the Pacific Ocean seismic zone and the Asia-Europe seismic zone, so Chinese mainland and its periphery has more strong shocks. Although Chinese seismologists study seismic dynamic stress triggering later, it is necessary to study seismic dynamic stress triggering in China. In order to explore Chinese seismic dynamic stress triggering, we take Chinese North-South seismic belt as an example in this article.First, we summarized the development of seismic dynamic stress triggering at home and abroad systematically, including the existed evidence of dynamic stress triggering, current situation, triggering mechanism and so on. From Landers earthquake in 1992, seismologists began to seek evidence of dynamic triggering on a global scale. The main existent evidence at present includes: seek triggered earthquake swarm and single earthquake in seismic frequency map, find new triggered earthquake by digital filter, co-seism change and stress step recorded by strain instrument. The triggering mechanism includes coulomb failure model, rate-state model and pore pressure expansion etc. The reasons why the study on dynamic stress are so less in China mainly maybe rely on the complexity of the computation. After Sumatra 8.7 magnitude earthquake in 2004, some seismologists start to study dynamic stress triggering in China.The main methods of computing dynamic are reflectivity method and the discrete wavenumber decomposition method. It involves lots of mathematics functions, including Bessel function, Green function, Reflectivity function and Bouchon slope function etc. The physical process related is also complicated: break time scattering, Green function convolution and different in frequency area etc. According to some predecessors'formulae, we deduced seismic dynamic stress calculating formula and rewritted part FORTRAN procedure that Cotton provided. In order to calculate coulomb stress on triggered fault, we connected FORTRAN procedure with MATLAB. At the same time, we realized the MATLAB procedure visualization interface for calculating and drawing. The calculation of Seismic dynamic stress comprised two steps: first, calculate six component of stress with FROTRAN procedure; second, calculate dynamic stress on triggered fault and its curve.With the mentioned method and procedure of calculating seismic dynamic stress above, we researched the triggered situation by some strong earthquakes in Chinese mainland. As the North-South seismic belt is a representative one in Chinese mainland, so we take it as the researching zone, and study mainly from two aspects. First, calculate stress tensor and coulomb stress in triggered area, including M_S8.0 earthquake of 2000 in Sumatra triggered M_S5.8 earthquake of Jingtai in Gansu and M_S6.5 earthquake of Burma, M_S7.9 earthquake of 2003 in border of China,Russia and Mongolia triggered M_S6.1 earthquake of Minle-Shandan and M_S5.2 earthquake of Minxian in Gansu, M_S8.7 earthquake of 2004 in Sumatra triggered M_S5.1 earthquake of Shuangbai and M_S5.1 earthquake of Simao in Yunnan. The results show that the dynamic stress peak value on triggered fault produced by the several strong shocks all exceeds to triggering threshold value. Second, the small earthquakes were strengthened abruptly in triggered area. We discussed the triggering effect on the North-South seismic belt by using the change of some earthquakes frequency since 1990. At the same time, we take west Kunlun mountains pass M_S8.1earthquake as an example to analyse triggering effect in Sichuan and Gansu provinces. All in all, the earthquake activity is triggered easily in the North-South seismic belt, but the earthquakes in different area have different triggering effect in the North-South seismic belt, it is influenced geologic structure.The research on seismic dynamic stress is therefore complicated, because it is affected by so many factors: the main earthquake parameters and structure in triggered area, there still have the time delay problem at the same time. The main earthquake parameters have big effect when we calculate stress tensor in the triggered point. Change fault three key elements(rake, dip angle and slide angle),breaking time and rising time have great effect on the final result and trend of the dynamic stress. The change of the main seismic moment and the size of the seismic fault only affect calculated result, but whose trend does not change. The change of most main earthquakes'parameters can affect the final result, so we must get much more accurate parameters when calculating the dynamic stress changing produced by earthquakes. Besides, tectonic setting also has great effect on the earthquake dynamic triggering. Studying indicates that most triggered earthquakes happened in geothermal or volcanic area and extension blocks. We also find a little of triggered earthquakes in nobbing blocks. Strike-silp and normal fault earthquakes are more easily triggerred dynamic stress. Seismic activity will be triggered not long before strong shocks surface wave arrived triggered area, but lots of triggered earthquakes have time delay, the time may change from several days to several weeks.In short, in my thesis, I have improved the calculating method and procedure of dynamic stress, calculated dynamic coulomb stress change, which triggered by some earthquakes in the North-South seismic belt. At the same time, we studied effect of earthquake dynamic stress triggering induced by main earthquakes'parameters and tectonic characteristic of triggered area, and also,we discussed the problem of dynamic triggering time delay . At last, we found several problems that are worth to study in future.