汶川MS8.0地震逆冲滑动沿断层深度的分布特征及其数值模拟解释

Distribution Characteristics of the Thrust Slip along Fault Depth in the Wenchuan MS8.0 Earthquake and the Interpretation Based on Numerical Simulation

摘要:

汶川MS8.0地震逆冲滑动量沿断层深度的分布有明显的特点，可以分解成三个破裂的滑动量叠加：深度18 km附近的底部破裂滑动、深度11 km附近的中部破裂滑动和两个局部破裂以外的主体滑动。扣除局部破裂滑动量后，沿断层走向从映秀到南坝近200 km范围内，逆冲滑动量沿断层深度的分布形态是一致的。基于滑动反演几何模型的有限元模拟显示，汶川地震逆冲滑动分布的整体性特征可能源自于断层浅部构造及巴彦喀拉块体SE向挤压强度沿断层走向上的一致性。

Abstract:

The Wenchuan MS8.0 earthquake occurred on the Longmenshan fault zone，which is a convergent zone separating the Sichuan basin from the Bayan Har block.In this paper，we analyze the latest slip model inverted from geodetic data.We find that the thrust slip distributions along the fault depth in different segments of the Wenchuan MS8.0 earthquake have common characteristics，although there are apparent complexities and differences in the Yingxiu segment，the Hongkou segment and the Beichuan segment.We decompose the thrust slip into three segments.The total amount of slip in the three segments can be decomposed into three partial slides.The first partial slide is a bottom slide distributed at a starting depth of around 17 or 18 km，with the depth range of the slide being about 2 km.The second partial slide is an intermediate slide distributed at a starting depth of around 11 km，with the depth range of the slide being about 10 km.The third slide is the remaining slide after removing the previous two partial slides from the total slip amount.The first partial slide and the second partial slide may be caused by some local rupture energy，and probably have little to do with the macroscopic mechanical environment.Thus，we take the third slide as the typical thrust slip of the three segments to compare with the other segments.After the above decompositions，the thrust-slip distributions in the Yingxiu segment，the Hongkou segment and the Beichuan segment seem to show a similar shape with the other segments.That means the thrust-slip distributions along the fault depth in the Wenchuan MS8.0 earthquake exhibit almost the same shape and follow the strike of the fault within the 200 km range from Yingxiu to Nanba，which may lead to the consistency.We construct a plane-strain numerical model using a finite-element method to examine the potential strike-slip scenarios.Our finite-element model takes the same geometry as used in the inversion of the slip model，with the exception of using a spline to imitate the seismogenic fault.Faults in the Longmenshan fault zone have steep slope at the shallow part and gentle slope at the deep part，and the dip angles of the faults gradually become smaller with depth. Therefore，a spline with two end points having fixed tangential angles is more realistic and reasonable.The southeast boundary of the model is constrained to move only up and down.Pressure is applied to the northwest boundary of the model to simulate the force exerted by the Bayan Har block on the Longmenshan fault zone.Results show that the slip distribution shapes of the fault model are closely related to the force exerted on the northwest boundary.Under the appropriate force，ruptures on the fault model would appear to have the same slip distribution shape as shown in the Wenchuan MS8.0 earthquake.Thus we conclude that the shape consistency of the thrust-slip distribution is probably due to the consistency of the shallow structures and the macroscopic mechanical environment within the 200 km range following the strike of the fault.The shallow structures of the seismogenic fault within the 200 km range are basically the same，and the force exerted by the Bayan Har block on the seismogenic fault is also essentially the same.