The Study Of Focal Mechanism Of Small And Moderate Earthquakes In Divided Zones Around Tangshan

The Tangshan earthquake with magnitude M_S=7.8 occurred on 28th July 1976, followed by amajor 7.1 magnitude aftershock some 15 hours later in Luanxian.After 4 months another largeaftershock with magnitude Ms=6.9 happened in Ningho. Up to now the activities of small andmoderate earthquakes are still frequent in Tangshan region. Many seismologists made studies onTangshan sequence, stress field and got abundant productions. In recent years, with thedevelopment of many observation networks, especially digital seismic observation system, massdata of small and moderate earthquakes are accumulated, providing the objective condition forfocal mechanism solution. The research work of this dissertation is developed under suchcircumstances.Based on a method to transform epicenter distribution of small and moderate earthquakes toseismic pattern by calculating seismic density at each grid node (Wang jian, 1999) which dividesthe researched region, grid points test method is used to compute composite plane solution in eachdense zone. The direction of P axis' azimuth angle and characteristics of the solution year by yearare analyzed, then compared in order to describe the characteristics of local stress field.The major content for this paper consists of four parts:In the first part, the geologic tectonic background and small and moderate earthquakes'spatial distribution in Tangshan area are described briefly. The result shows that small andmoderate earthquakes are very frequent, the distribution of epicenters is not uniform in Tangshanarea, which appears irregular strikes and density areas. With a method to transform epicenterdistribution of small and moderate earthquakes to seismic pattern by calculating seismic density ateach grid node (Wang jian, 2004), we deal with seismic patterns using data from 2002 to 2006 todivide tangshan aftershock area.According to the isocline's distribution of these earthquakes, theresearch area can be divided five seismic dense zones,thcu we analyze temporal distributions of ingiven time period in each seismic density area from 1986 to 2006.With the digital seismic data from 2002 to 2006, Grid points test method is used to computecomposite nodal-plane solution of small and moderate earthquakes in each density zone byadopting divided rule in the second part.The result shows that focal mechanism solution of small and moderate earthquakes as well asazimuth angle of P axis have some differences in each zone. Azimuth angle of P axis is NEEdirection in both the first zone where Ningho earthquake (Ms=6.9) occurred and the second whereTangshan earthquake (Ms=7.8) took place, then it turns to East-West direction along thedistribution of small and moderate earthquakes. Finally azimuth angle of P axis turns to NEdirection. The ratios of contradiction signs vary with different zone. It is the highest 0.41 in thesecond zone where Tangshan 7.8 earthquake occurred, whereas it is lower 0.23 in the zone where no strong aftershock happened. The research of composite nodal-plane solution of small andmoderate earthquakes shows that stress field has slight difference and tectonic stress field'scomplexity in Tangshan aftershock region.In the third part, we further analyze the change of composite plane solution year by year ineach dense zone and study the change of azimuth angle of P axis and the ratio of contradictionsign.The result shows that azimuth angle of P axis is East-West direction and the amplitude ofvariation of the ratio of contradiction signs is stable and the dip angle is fairly low in the third andfourth dense zone. Azimuth angle of P axis is NE or NEE direction and the amplitude ofvariation of the ratio of contradiction signs is obvious in the second zone where Tangshan 7.8earthquake occurred and the first zone where Ningho 7.8 earthquake took place. The dip angle isrelatively high in the five zone where ao strong earthquakes happened.In the rough part, five divided small areas are treated as a whole part to analyze the change ofcomposite plane solution during 1993 to 2006. Discussions on the change of contradiction ratiossigns show the reason of vibration is rather complex. As a window exploring the stress field, thechange of contradiction ratios signs should be attentioned to, however ,many questions still needto make a deep study and discuss how to combine it with earthquake forecasting.