The Relationship Between The Adjustment Process Of Coulomb Stress And Strong Earthquakes In The East Segment Of Northern Qilian Fault

Strong earthquakes are result from fault activities under fault tectonic stress fieldcondition, and the fault activities is closely related with the activities of the tectonicstress fracture. A coseismic offset changed the stress state around the fracture area,leading to the Coulomb stress changes, which can alter the possibility of theoccurrence of earthquakes near the faults. Therefore, it is very important to study theseismic hazard and risk assessment.According to the study of seismic data, first of all, we made a primary researchabout strong earthquakes by Coulomb3.3on the static Coulomb stress changes in theeastern margin of north Qilian Mountain and adjacent areas since1561of9M≥6.0earthquakes. We used cumulative and individual methods respectively to calculate theCoulomb stress changes between foreshocks and aftershocks. Meanwhile, accordingto the b-value scanning method and Dieterich’s rate-state friction law, we analyzed thefuture seismic activities of the studied areas, the results are as follows:1) According to the calculation and analysis of Coulomb stress changes, theresults show that: we studied the Coulomb stress changes after the9strong shocks,when we use cumulative method, it shows that the next events except Menyuanearthquake in1986would occur in the stress triggering areas, the value of changes isbetween0.1bar and4.066bar, the trigger rate reached at a high level of87.5%; For thecalculation of the effect of the earthquake on the succedent earthquake, we found thathalf of them are easily to be triggered; to the left half, however, shows inhibitoryeffect. Whether it will trigger the aftershocks or delay them depend on the relativeposition between source fault and aim fault and the property of fault activity.2) According to the calculation of the stress accumulation rate and cumulativetime, we draw the conclusion that Zhongning M7made Guyuan M7occured70aearlier; Guyuan M7made Zhongwei M73a later. However, by the cumulativeCoulomb stress change made it happen18a earlier, and Zhongwei M7promptedYinchuan M87a earlier; the most important of which is Haiyuan M8promptedGulang M89a earlier; Meanwhile, Gulang M8.0played a depress role in MenyuanM6.4over13a.3) Strong earthquakes tend to have a large adjustment on regional stress field,there occurred two M8earthquakes in the studied areas in1920and1927, whichhave obvious adjustment effect on the distribution of stress in the areas. According tothe calculation of Coulomb stress changes in the areas the two strong shockshappened, Coulomb stress significantly increased in the eastern side of YunwuMountain and western part of Jingtai-Tianzhu-Gulang, where there have a risk oftriggering earthquakes in advance. It is noteworthy that the Tianzhu6.2earthquakeoccured in the western part of the danger zone after former two earthquakes.Considering the Coulomb stress triggering and seismic activity history,futureearthquake risky areas should be strong as well as the eastern side of YunwuMountain and Jinqianghe fault zone in the western part of Jingtai-Tianzhu-Gulang. 4) There are two active screens since1400in the studied area, the active screenfor the first time is in1477-1739, a period of262years, of which the earthquakefrequency of M≥6.0is0.02. The other screen last162years since1852and theearthquake frequency is0.06, indicating that the region may enter the era of strongearthquakes.5) By the b-value scanning method, b value in the studied area is below0.7.Inthe Gulang west-Wuwei-Menyuan, the b value is the minimum and b value inTianzhu-Jingtai-Yongdeng is higher, and the exact value is0.65, showing that thefrequency of earthquake can be higher than other regions, the b value in Zhongweiand its eastern region are relatively lower, however, its still higher than that of thewest of Menyuan, that is to say, its much safer than that of the region.6) As Coulomb stress changes can result in the change of activity, we use theDieterich s rate-state friction law to simulate the stress changes caused by the activityof the temporal evolution. The result showed that, the seismicity of Lenglonglingwest、 Jinqianghe fault-Minle and Yinchuan-Pingluo-Niushoushan are the mostfrequently, the long axis of seismicity is consistent with the fault strike and graduallyweaken from the center to the sides of the seismic activity, the strongest area is inMenyuan-Gulang-Wuwei region. Based on the analysis of the relationship betweenseismic activity and the background seismicity rate, seismicity of the western partabout M≥6is the strongest, followed by eastern, they are above the backgroundseismicity during100years. Due to the Haiyuan earthquake, the central region resultsin the release of energy, Coulomb stress change is negative, and its activity is greatlyreduced in the next period of time, the activity of the central region has been on therise, about600a was to restore the original state; for M≥7, the western and easternregions need400a to restore the background state, the central region can not in thenext800a; for M≥8, the West, Central, East three areas can not reach the backgroundseismicity stage within the next800a. The difference is that in the west and east arethe stress release phase, which is greater than the background seismic activity, but thecentral is just the opposite.