| Current extensive seismicity in southern Sichuan Basin is ascribed to the reactivation of unstable and severely stressed pre-existing faults,as a result of prolonged fluid injection for salt mining and shale gas development,respectively.However,the structural framework,inducing mechanism and dynamic connection to fluid injection in the region remain poorly understood.Improvement of our understanding of the structural architecture,seismic event locations,focal mechanisms,and mechanism of induced seismicity beneath the southern Sichuan basin,depends on accomplishing geologically precise and efficient seismological investigations,which this thesis undertakes.The studied area of interest is situated within latitude 28.0°N28.6°N and longitude 104.5°E-105.2°E in the southern region of the Sichuan Basin,and covers the Changning salt mine and Xingwen shale gas block.We exploited the robust database of catalog travel time data from local and regional array of seismic networks,made up of 10 and 86 seismic stations respectively,as well as differential times of waveforms of the local array;and applied multiple seismological evaluation techniques to accomplish three separate studies.First,we utilize high quality phase data from 36,314 earthquakes jointly recorded by our local array and a regional seismic network,and employ a multidisciplinary approach involving joint inversion-based Vp/Vs consistency-constrained doubledifference seismic tomography(TomoDDMC),integrated structural characterization and outcrop analysis to accurately relocate seismicity,generate high-resolution 3D velocity models and unravel the structural control on induced seismicity in Sichuan Basin.Earthquake relocations reveal shallow hypocenters for the Ms>5.0 earthquakes and two distinct seismogenic zones corresponding to the salt mine and shale gas regions,with general seismicity forming widespread lineaments that are remarkably similar to the fault and fracture trends interpreted on reflection seismic and outcrops,respectively,some of which extend to the basement.Our 3-D crustal velocity analyses show seismicity beneath the Changing salt mining area is associated with relatively low Vp/Vs anomalies,while most of small earthquakes within the Xingwen shale gas block are associated with relatively high Vp/Vs anomalies,indicating the earthquakes in these two areas are caused by different mechanisms.A high Vp/Vs anomalous zone exists beneath the Changning salt mine up to 4 km,which correlates with zones lacking coseismic surface deformations,thereby acts as an effective upward terminator of rupture.In addition,top of the crystalline Sichuan basement is characterized by the 6.5 km/s Vp contour,which is new for earthquake tomographic studies in the region.Combined with outcrop analysis,we are able to construct a structural framework for induced seismicity in southern Sichuan basin,which unravels some structural control on induced seismicity.We recommend reevaluation of the susceptibility of Sichuan basement to strong earthquakes,given the newly characterized down to basement relocated earthquake lineaments,which imply potential for stress build up for seismic reactivation.Second,we have determined accurate source characteristics,resolved the rupture ambiguities and analyzed the main factors required to induce the Changning Ms>5.0 seismic sequence.This was achieved by executing advanced earthquake seismic tomography,in situ cluster Vp/Vs ratio estimation,stress inversion,excess pore fluid pressure estimation and Coulomb failure stress evaluation.Velocity tomography clearly reveals low Vs expression corresponding to high Vp/Vs anomaly at the source region of the Ms6.0 main shock.The earthquake relocation and focal mechanism results show that the sequence ruptured formerly unmapped oblique pre-existing faults of varying dips in a cascading downward manner of occurrence,which is revealed by the sequential Coulomb stress transfer following the Changning Ms6.0 mainshock with initiation point above 2 km depth.The raised Vp/Vs ratio and shallow rupture point are attributed to combined inducing mechanism involving stress perturbations resulting from diffusion of injected fluids and differential reservoir compaction from salt extraction,coupled with the presence of highly fractured cap rocks enriched in quartz and silica content,which are favorable ingredients for seismic slip at shallow depths.Pore pressure prediction reveals the source region of the Changning Ms6.0 main shock to be marked by high pore pressure.This high pore pressure zone that coincides with high Vp/Vs anomaly is inferred as premier evidence of the out of zone injection previously reported as major water loss,which is a potential environmental hazard given the likelihood of occurrence of a gigantic sink hole in the future if not checked.Finally,exact earthquake locations and precise high-resolution velocity structure within the Xingwen shale gas block were determined by utilizing differential times of cross-correlated waveforms of 11,677 earthquakes recorded by our local array with the joint inversion-based Vp/Vs model consistency constrained double difference seismic tomography,which has been proven to greatly improve earthquake relocation and crustal velocity results.Our analysis yielded outstanding unprecedented results.The resultant 3-D high-resolution velocity structure is marked by extensive lateral heterogeneity with anomalous velocity zones that coincide with prevalent structures in the region.Generally,seismicity is characterized by low Vp/Vs expressions,which correspond to moderately high to high velocity values suggesting them to be structurally controlled.However,a few high Vp/Vs zones that correspond to zones of significantly low Vs anomalies exist,which are inferred as zones of pronounced influence of overpressured hydraulic fracturing fluids for shale gas production.We determined excess pore fluid pressure through integrated focal mechanism and stress inversions,and seismicity associated with it is attributed to hydraulic fracturing within the southeastern zone of shale gas block.This study also provides pioneer upper and lower limits of induced seismicity,which correlates with mudstone and dolomite formations respectively,as well as correspond to low velocity and high Vp/Vs anomalies respectively.Accurately relocated seismicity beneath the shale gas block are remarkably spatially distributed as lineaments of closely knit seismic events,most of which are interpreted as illuminated sub-seismic fault traces,except a few lineaments that align with previously characterized faults.Depth-wise analysis reveals clear concentration of relocated earthquake lineaments within 2 km to 5 km interval,which covers the Silurian Longmaxi target shale formation targeted for gas production.These spatial and depth-wise improved lineament relocations were employed to develop updated fault framework models.Structural characteristics illuminated by the accurately relocated epicenters and hypocenters provided essential guide to improved understanding of the structural architecture beneath the seismically active Xingwen shale gas zone,and continuous monitoring will no doubt result in illumination of more fault and fractures,thereby providing insight into the nature of preexisting faults within this seismogenic environment. |
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