The 1739 M8.0 Pingluo earthquake occurred around the Yinchuan Graben,bounded by the Helan Mountains to the west and the Ordos Block to the east.Seismological observations have shown that surface fault displacement rea...The 1739 M8.0 Pingluo earthquake occurred around the Yinchuan Graben,bounded by the Helan Mountains to the west and the Ordos Block to the east.Seismological observations have shown that surface fault displacement reaches about 2–3 m,mainly by dip-slip motion along the Helanshan Piedmont Fault.However,the documented seismic intensity is distributed predominantly within the basin area,exhibiting a sharp asymmetry across the Helanshan Piedmont Fault.Thus,the general pattern of earthquake faulting is still under debate.We built a three-dimensional elastodynamic finiteelement model to reappraise the fault mechanism.In the model,predictions from synthetic rupture models,based on available observations and the earthquake scaling law,were used as an input with the split-node technique,and the effect of basin sediment on elastic wave propagation was considered.The numerical results show that if an earthquake occurred on the Helanshan Piedmont Fault characterized by a high-angle(70°)normal fault,earthquake shaking,as predicted from the modeled peak ground velocity and peak ground acceleration,has difficulty fitting the observed result,even when the effect of sediment amplification is considered.To better fit the observed shaking pattern,the dip angle of the Helanshan Piedmont Fault must be less than about 35°between the depths of about 8–27 km,where the coseismic slip may reach about 6 m.This result leads us to conclude that the 1739 M8.0 great earthquake likely occurred on a listric normal fault at depth,in agreement with the geometry of the Helanshan Piedmont Fault,as recently evidenced by seismic reflection explorations.This conclusion means that in an intracontinental setting,a reduction in the fault dip angle along the subsurface could increase the width of the fault in the elastic crust,making misalignment between the surface rupture and the isoseismals and resulting in an increase in the upper bound of earthquake magnitude relative to simple high-angle faulting.展开更多
基金Natural Science Foundation of China(No.42120104004)。
文摘The 1739 M8.0 Pingluo earthquake occurred around the Yinchuan Graben,bounded by the Helan Mountains to the west and the Ordos Block to the east.Seismological observations have shown that surface fault displacement reaches about 2–3 m,mainly by dip-slip motion along the Helanshan Piedmont Fault.However,the documented seismic intensity is distributed predominantly within the basin area,exhibiting a sharp asymmetry across the Helanshan Piedmont Fault.Thus,the general pattern of earthquake faulting is still under debate.We built a three-dimensional elastodynamic finiteelement model to reappraise the fault mechanism.In the model,predictions from synthetic rupture models,based on available observations and the earthquake scaling law,were used as an input with the split-node technique,and the effect of basin sediment on elastic wave propagation was considered.The numerical results show that if an earthquake occurred on the Helanshan Piedmont Fault characterized by a high-angle(70°)normal fault,earthquake shaking,as predicted from the modeled peak ground velocity and peak ground acceleration,has difficulty fitting the observed result,even when the effect of sediment amplification is considered.To better fit the observed shaking pattern,the dip angle of the Helanshan Piedmont Fault must be less than about 35°between the depths of about 8–27 km,where the coseismic slip may reach about 6 m.This result leads us to conclude that the 1739 M8.0 great earthquake likely occurred on a listric normal fault at depth,in agreement with the geometry of the Helanshan Piedmont Fault,as recently evidenced by seismic reflection explorations.This conclusion means that in an intracontinental setting,a reduction in the fault dip angle along the subsurface could increase the width of the fault in the elastic crust,making misalignment between the surface rupture and the isoseismals and resulting in an increase in the upper bound of earthquake magnitude relative to simple high-angle faulting.
