Rupture Dynamics And Strong Ground Motion Simulation Of Destructive Earthquakes In The Sichuan-Yunnan Region

Regional stress states and fault geometries play important roles in real earthquake rupture dynamics.The Sichuan-Yunnan block,located at the eastern margin of the Tibetan Plateau,has variable regional tectonic stess fields and complex boundary fault geometries.At the same time,those boundary faults are tectonically active and have given birth to lots of desctructive earthquakes,thus brought severe damage and serious treat to the seimogenic region.In this work,first,two typical earthquakes that occurred on those boundary faults,i.e.,the 1970 Tonghai earthquake and the 1833 Songming earthquake,were selected to perform dynamic rupture and wave propagation simulations using the curved grid finite-difference method(CG-FDM),to investigate the rupture processes of these two earthquakes and provide scientific support for regional earthquake risk analysis and damage assessment.Then,8 other historical earthquakes with magnitude greater than or equal to 7.5 were selected for dynamic rupture simulation,to investigate the rupture process on the corresponding fault plane.In the simulation of 1970 Tonghai earthquake,a nonplanar Qujiang fault(QF)model with topography was adopted and embedded in heterogeneous media.Regional stress orientation with an interval of 5° were tested,and various fault geometry models with different fault surface traces and fault dips were discussed.We also provided explanations for the unbroken northwestern segment of the QF and seismic intensity anomaly in the Tonghai basin during the 1970 Tonghai event.In the end,we presented several future potential earthquake scenarios occurring on the QF at three different nucleation locations.Our simulation results suggested that the maximum principal stress azimuth around the Tonghai area is about N25°W.And the QF is not likely to have a Stepover at Eshan or Wujie.Morever,the QF is most likely a complex dipping fault with southeastern segment dips to the northeast,whereas the northwestern segment dips to the southwest.Our simulations also revealed that multiple explanations,including a regional stress rotation,an increase in the cohesion force on the northwestern QF segment,as well as fault segment absence,could account for the unbroken northwestern segment of the QF.Furthermore,the seismic intensity anomaly in the Tonghai basin can be explained by a low-velocity structure and reproduced in numerical modeling.The results of future earthquake scenarios demonstrated that earthquakes nucleating at Eshan and Wujie could rupture the entire QF not only in the vertical dipping fault model but also in the complex dipping fault model,which could pose severe seismic risks to nearby regions.Morever,when the nucleation point was located at Quxi,although the rupture can propagate through the whole fault plane in the vertical dipping fault model,it could be constrained to the initial fault segment of the QF in the complex dipping fault model.Even so,cautions should still be exercised in the Quxi area because this scenario produces a maximum intensity of VIII.As for the Songming earthquake,we used a nonuniform velocity model and two nonplanar west Xiaojiang fault models(i.e.,the continuous fault model and the Qingshuihai stepover fault model)to simulate the spontaneous rupture process and wave propagation.Our dynamic rupture model results showed that the obtained moment magnitude,fault rupture length,as well as the fault surface dislocations are in good agreement with the observations.Among these two simulation results,the fault suface dislocations of the continueous fault model matched well with the field investigation data,especially in those measuring points with large amount of dislocation,such as P2,P6 and P8.However,the comparison looks bad at those measuring points with small amont of dislocation,such as P3.The results from Qingshuihai stepover fault model demonstrated that the dislocations matched very well not only in the measuring points with large amont of dislocation but also the points with small amount of dislocation.Both fault model presented free surface induced supershear rupture on the south segment of the west Xiaojiang fault,which can be explained by the phase conversion of SV to P-diffracted waves.In addition,neither of the rupture of these two models can be propagated to the Yangzonghai-Chengjiang fault segment of the west Xiaojiang fault,which explains well that no surface rupture at this part can be observed after the Songming earthquake.The wave propagation simulatations from the above two rupture models illustrated that the seismic enengy released by the Songming earthquake is mainly concentrated in the horizontal direction,and the damage south of the epicenter is slightly severe than that of the area north of the epicenter,which is consistent with the damage pattern deduced from written records.The intensity distributions obtained from these two simulations are both hourglass-shaped,while the deduced intensity is generally spindle-shaped.The reasons for this difference may be as follows:first,the simplification of model parameters,such as fault geometry and stress fields;second,attenuation of seimic waves is not considered in CG-FDM;third,the simulation fails to include the sedimentary velocity structure of the shallow crust.In the rupture process simulation of the destructive earthquakes occurred in the eastern and northern boundary faults of the Sichuan-Yunnan block,we first constructed the seismogenic fault geometry model and velocity structure model for each earthquake.Then we chose proper parameters for the initial stress and fault plane friction.And finally we performed the simulations and presented the rupture process as well as the fault plane final slip distribution of each earthquake.Moreover,we also compared the simulated rupture length and fault dislocation with observed data.The dynamic rupture simulation results revealed that the rupture processes of 7 earthquakes are controlled by the fault geometry,such as the 1500 Yiliang earthquake,the 1536 Mianning earthquake,the 1786 Kangding earthquake and the 1854 Ganzi earthquake.The rupture of these earthquakes would dramatically slow down or die out once the rupture front encounted the bending part fault segment.From the simulation results we can also find that 6 earthquake scenarios present free surface induced supershear ruptures.After the rupture propagated away from the nucleation patch about 40 km,the supershear rupture emerged at the free surface,then propagated through the fault plane.These supershear ruptures also can be explained by the stress loading due to phase conversion of SV to P-diffracted waves.Besides,the simplification of our fault geometry model may also contribute to the occurring of supershear rupture.Last but not least,we find that the maximum principal stress orientations we used in our simulations are counterclockwisely distributed from south to north,which is in agreement with the real stress orientations.