Seismicity Analysis Of Faults Based On The GIS Method

The intersection area of the north-south seismic zone and the central orogenic belt in China, includes southern Ningxia,southeastern Gansu and northern Sichuan. On the map of fourth-generation seismic zoning (China Earthquake Administration,2001),this district is located at the adjoining area among the 6 degree of the Ordos area,6 degree of South China and 7 degree of the Qinghai-Tibet area.This region is one of the most active earthquake-happening areas,and also a focused study. The historical earthquakes recorded are as many as 108 times in this area,including 6 events of Ms≥7 earthquake.Since 1970,there are 141 earthquakes which are Ms≥4, including 2 Ms≥7(China Earthquake Network Center and the historical seismic data). The relationship between earthquakes and active faults of this area is very important in seismology and geology.The geological structure of the study area is very complicated,where there are lofty mountains and steep hills everywhere, vegetation coverage is heavy,hence the conventional geological survey is much difficult to carry out.Geographical Information System(GIS) provides a convenient method for the quantitative study of the relationship between earthquakes and faults in such area.This thesis is based on the domestic and foreign GIS software platform.and makes a study on the single fault,dense fracture belts with relation to seismicity.First I build the spatial database of the intersection area of the study area,including faults,earthquakes,remote sensing data of topography,and so on.The fault database is based on 1:200000 digital fault map,added by 1:50 million digital geological map,1:250 million digital geological map,1:500 million digital geological map and China’s major active faults of the study area.earthquake databases includes the Network Center of National real-time seismic catalog (1970-01-01 to 2009-12-31), provincial station seismic records(1990-01-01 to 2008-12-31), historical earthquakes(-1831 to 1969).The remote sensing data include 9 TM/ETM images,4 SRTM 90-meter resolution DEM images and 30 GDEM 30-meter resolution DEM images. The GIS overlay analysis module is used to display all the data and statistical analysis, buffer analysis module for correlation analysis between fault and earthquake,the GIS secondary development module for three-dimensional seismic profile analysis and analysis of the density of the earth’s crust. Then I compare the fault activity of the study area, three-dimensional geometry of the faults, density of faults,to help further study on the intersection region of the north-south seismic belt and the central orogenic system. The main technical means used are as follows:(1). Build the database using GIS method:from the original data pre-process (coordinate projection,geometric correction,error analysis) to mid-results (fault merge,earthquake symbolism and so on),the final database(count the length of faults,dividing the earthquakes based on magnitude,RS data,DEM data).(2).Quantize the parameters of the data:length of faults,buffer width of faults,division of earthquakes based on time and magnitude,count the number of earthquakes,seismicity level division by natural classification).(3).GIS buffer analysis:Academician Sherman(Russian Academy of Sciences)suggested the relationship between lengths of faults(L) and effect widths(M) for the Baikal rift zone,east African rift zone,Altai-Sayan fold area and faults in Europe:m≤0.5*K*L^C.Considering the features of study area,I set the K\C value range,and select the faults of which lengths are above 20km,make statistic analysis of the buffer width,length of faults,number of earthquakes,seismicity values,then conduct comparison of the active faults in this area with prtvious work.(4)Make analysis and statistics of the fracture density of the 1:200000 fault data,divide the high and low value zones,and overlay the earthquakes to find the relation of N-S belt and fault distribution.(5)Use the GIS secondary development method for seismic profile analysis,try to explain the fault seismic profiles and the relationship between different manifestations and faults.The main conclusions are as follows:(1) GIS methods have natural advantages in establishing the spatial database of the study area. They can display faults,earthquakes,topography data,remote sensing image data,magnetic-electrical data,gravity etc based on strictly geographical coordinates,and overlay the geographic map with the specific attributes of the form-one correspondence. Establishing the spatial database of the study area is helpful for data storage, inquiries, processing, mapping and so on.(2).Using the 1:200000 faults as the object,there are 2555 faults,and number of analysis objects (L≥20km) is 309.The longest is the Yaojiaping-Zaojiaowan fault and the shortest is Banzuo fault.Using the GIS buffer method to statistic the number of earthquakes on the per km fault:yields 4 faults of very highly active in seismicity(Huya-Xueshan fault,Zhugenka-Songpan fault,Zhugenka-Suoziguo fault,Senalu-Baihegou fault),the number of earthquakes on per km fault is 25-38,which is obvious higher than others.There are 5 fautls of highly active in seismicity (Gaofengba-Mingjingsi fault,Pingding-Huama fault,Shiyanzi-Puguangsi fault,Baima-Jiachang fault,Yingxiu fault),of which the number of earthquake nn per km fault is 6-10,It also determines 52 medium faults(Kaba-Yangbu fault,Zhaliekuo-Sangbagou-fault,south Hetaoba fault,Maowen fault)and 216 weak faults,and most of the faults in this area are of weak seismicity,for which the number of earthquakes on per km fault is 0-1.3.Besides,there are32 faults without earthquakes.The analysis of active faults in the area using this method finds these faults and 1:200000 fautls have a similar seismicity level distribution.(3)Making statistics of the fault density based on the 1:200 000 digital fault map,suggests the relationship between S-N earthquake belt and fault density, which means that the high fault density zone is in well coincidence with the S-N earthquake belt,which can explain the spatial relationship between the S-N earthquake belt and the faults in the study area. It notes an interesting phenomenon:most M≥4 earthquakes are distributed in the low fault-density zones.From the aspect of tectonic earthquake mechanism,there are more small faults in the high zone,where stress can release along with the small ruptures,and is hard to accumulate the energy of high-magnitude earthquakes.This is the reason why M<4 earthquakes often happened in the high fault-density zonea.The low fault density value zones are the dead zones crossed by serval faults,with few small faults,where strain can be built up to cause major earthquakes.(4)Making the seismic profile projection to the important section of the faults,yields 25 seismic profiles,which indicate the relationship between different seismicity types and active faults,it can be descibed as:①Strike-slip fault:showing linear distribution along the fault on the earth surface, with steep bottom and divergent top,just like "anthoid structure" on the seismic profile.②Reverse fault:showing mostly on the hanging side of fault and segmented distribution on the earth surface, showing average distribution along the hanging wall of the fault on the seismic profile, and on some profiles there is also segmented distribution, shows that hte fault is not a uniform rupture when a reverse faulting happens.