Stress State And Seismic Hazard In The Dayi Seismic Gap,the Longmenshan Fault Zone,Sichuan Province,China

The southeastern trend escaping movement of the Bayankala block leads to frequent strong earthquakes in its eastern boundary and adjacent areas.For example,the 2008 Ms 8.0 Wenchuan earthquake and 2013 Ms 7.0 Lushan earthquake struck the Longmenshan fault zone on the eastern boundary of the Bayankala block.After the Wenchuan earthquake,the Baoxing-Xiaojin area was inferred to be an MW6.7～7.3 seismic gap,which was then verified by the occurrence of the Lushan earthquake.Note that the Lushan earthquake did not completely release the energy accumulated in the seismic gap.There is still a 40～60 km seismic gap between the ruptures of the Wenchuan earthquake and Lushan earthquake,that is,the Dayi seismic gap.However,it is still debating on the possibility of future moderate-size earthquakes in the Dayi seismic gap.A maj or concern is whether high stresses have been built up in this seismic gap.The stress status can be obtained through in-situ stress measurements that were unavailable before this thesis project.This work aims to derive the in situ stress state of the Dayi seismic gap through in-situ stress measurements,and to evaluate the seismic hzard in this area.The faults with potential strong seismic hazard in the Dayi seismic gap are:the Wenchuan-Maowen Fault,Yingxiu-Beichuan Fault,Dachuan-Shuangshi Fault,and Dayi Fault.On the basis of comparing the existing large active faults in the neighboring areas and integrating the geometrical characteristics of the faults in the study area,two boreholes with depths of 1000 m and 500 m were constructed in the foot and hanging walls of the Dachuan-Shuangshi fault zone,respectively.We carried out three hydrofracturing in-situ stress measurements in 2019,2020 and 2021.Results show that the stresses of the Dachuan-Shuangshi fault zone are relatively high.In this work we also discuss the correlation between near-surface shallow stress and deep stress based on integrated geological structures,seismic b value,crustal velocity structure,GPS deformation monitoring and geo-temperature data,etc.The results of this study advance the understanding of the seismic hzard of the Dachuan-Shuangshi fault zone as follows.1.The two boreholes in the Dayi seismic gap have indicated SH values ranging in 10.60～62.74 MPa,Sh values of 6.75～35.36 MPa.The relative magnitude of the three stresses is SH>Sh>Sv,which is in a favor of thrust faulting earthquakes.The stress status is consistent with the stress field obtained by focal mechanism analyses,which means that the shallow stress state agrees well with the deep stress state.2.The two boreholes exhibit SH direction of N10°E～N15°E at the shallow depths,and change to N52°W～N70°W with the increasing depth,which is consistent with the regional stress field.The SH direction is mainly NW～NWW,which implies that the background stress state may be controlled by the southeast compression of the QinghaiTibet Plateau.3.In situ stress measurements and seismic b value suggest that the deep and shallow areas are both in a high stress state,which coincides well with fault coupling properties inverted from GPS observations and cross-fault leveling.It is therefore inferred that the deep stress is strongly coupled with shallow stress.Moreover,the velocity structure and geo-temperature measurements show that the crustal structure above 18 km may be a brittle layer,and the deep rock mass structure is typical of high stress accumulation and strong coupling of deep and shallow stress.4.There are two possible reasons for the lack of earthquakes in the Dayi seismic gap:one is the existence of a "local melt zone" at depth;the second is the release of high stress accumulation in the form of strong earthquakes.If the "local melt zone"exists,then it will be a strong evidence to support the high stress:if there is a deep slip layer,then the strong extrusion from the Qinghai-Tibet Plateau to the east will transfer to the north and south ends of the seismic hollow and the east side of the seismic hollow,and the high ground stress will still exist in these parts.In addition,since the seismic hollow zone is less than 14 km(Liang et al.,2018)or 18 km(near Dachuan town)for brittle layers,then the shallow brittle layer may be the seismogenic layer,while the deep low-velocity layer may be the stress transfer site.5.Analyses of in situ stress state and fault stability have revealed that the Dayi seismic gap still has the potential hazards of moderate to large earthquakes.We have estimated potential future earthquakes with magnitude of Mw 6.9.Although this study still cannot determine the exact time of future strong earthquakes,it helps to better understand the relationship between high stress and seismic hzard in this region.6.The response model of the locking portions of the Longmenshan fault indicates that corresponding high stress in the deep and shallow can be helpful for evaluating the occurrence of strong earthquakes,although the seismic hzard of shallow low stress remains uncertain.The deep high stress of the Dachuan-Shuangshi fault zone corresponds to the shallow high stress,which is interpreted to impose the potential hazard of strong earthquakes in the Dayi seismic gap.