| It is generally believed that the front of the Sichuan-Yunnan rhombic block is composed of the nearly north-south Xiao Jiang Fault Zone and the north-west Yunnan Southeast Arc Tectonic Zone.The region is characterized by high frequency and intensity of seismicity,which is the region with the strong seismicity in the Chinese mainland and provides sufficient seismic data for seismological research.History has witnessed the 73/4 magnitude earthquake on 1788,August 2 in Ziniupo,Dongchuan.The 8 magnitude earthquake that hit Songming Yanglin in 1833 on September 6.And The Tonghai 7.8 magnitude earthquake on January 5,1970,which caused huge losses.In recent years,some disastrous earthquakes,such as the Lancang-Gengma earthquake on November 6,1988 and the Lijiang earthquake on February 3,1966,have occurred.These fully show that the current Sichuan-Yunnan region is still a high level of seismic activity.Therefore,it is necessary to delineate the b-value distribution of seismicity in the front of the Sichuan-Yunnan block,to determine the low b-value anomaly area and to determine the potential source boundary to provide scientific basis for earthquake prevention and disaster reduction.How to combine the abundant historical earthquake catalogue,active tectonics and paleo seismic data to establish the relationship with b-value spatial distribution image is the problem to be studied in this paper.The purpose of this paper is to study the relationship between the seismic spatial distribution pattern of the front part of the Sichuan-Yunnan block and the seismotectonic background in the study area.After deleting the foreshock and aftershock and determining the minimum integrity magnitude of the foreshock and aftershock in the front part of the Sichuan-Yunnan block,the b-value of the study area is obtained by using the maximum likelihood,the suitable spatial scanning scheme is selected to draw the b-value spatial distribution image.Secondly,the relationship between the anomaly of the b-value spatial distribution pattern and strong earthquakes is analyzed by taking the M7.6 Lancang-gengma earthquake on November 6,1988 and the M7.0 Lijiang earthquake on February 3,1996 as the research objects.Finally,the relationship between the distribution of the b-value and the main fault distribution in the study area is analyzed.The dominant distribution of b-value in the front part of the Sichuan-Yunnan block is between 0.5 and 1.3,and the distribution range of b-value is wide.The average b-value of the study area is 0.853.The distribution of strong earthquakes in the region with low anomaly of b-value is relatively dense,and the distribution of b-value near the epicenter is relatively low,and the distribution of b-value in the front of Sichuan-Yunnan block is obviously banded the distribution of b-value in different sections of the fault zone and the fault zone is very uneven.The southern segment of the Lancing River Fault,the eastern,western and central branches of the WuLiangshan Fault Zone,and the southern segment of the Dien Bien Phu Fault lie in the low b-value zone of the front of the study area.At present,it is in a relatively high stress state,and strong earthquakes are likely to occur in the future.Based on the seismic catalog data of the forward area of Sichuan-Yunnan rhombic block,the relationship between the abnormal characteristics of b-value spatial distribution image and the strong earthquakes,and the distribution of main faults in this area is analyzed and studied.According to the low b-value state of the fault zone,it is concluded that the southern segment of the Lancang River Fault,the WuLiangshan Fault Zone,and the southern segment of the Dien Bien Phu Fault zone are likely to have strong earthquakes in the future.These results are valuable for seismic risk analysis and earthquakes prediction in the forward area of Sichuan-Yunnan rhombic block. |
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