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本文利用远震波形探讨了2008年5月12日汶川地震的震源过程。我们将震源分成西南段和东北段,并通过反演的波形残差分析推测了东北段可能的倾角。滑动分布的结果表明:震中附近表现为逆冲运动,而从震中东北方向110km附近至断层的东北端主要表现为走滑运动。推断的最大滑移量和地震矩分别高达11.0m和1.2×1021Nm(Mw8.0)。此外,我们假定了一个三维地球结构,采用谱元法,通过宽频带地震波的模拟验证了该震源模型。在20~150s的频率范围内,无论是体波还是面波,模拟得到的合成波形和观测到的波形都具有较好的吻合度。本模型中,东北段高倾角的走滑机制所对应的面波拟合残差在60~500s范围内,该残差总体小于其他东北段为逆冲或低倾角走滑机制的模型。使用本文模型计算得到的库仑应力分布表明:西南段的破裂使得东北段的库仑应力发生大幅度的增加。本文认为2008年汶川地震的震源断裂至少包括两个主要的分段:一段为低倾角,以走滑运动为主;另一段为高倾角,以逆冲运动为主。另外,我们还认为汶川主震震源过程中,破裂在这两个分段中发生转换。
In this paper, the teleseismic waveforms are used to discuss the source process of Wenchuan earthquake on May 12, 2008. We divide the source into the southwestern segment and the northeast segment, and infer the possible dip of the northeast segment by the waveform residual analysis of inversion. The results of the slip distribution show that there is a thrusting motion near the epicenter, and the strike-slip movement mainly occurs from the northeast of the epicenter to the northeast end of the fault at a distance of about 110km. Inferred maximum slip and seismic moment are as high as 11.0m and 1.2 × 1021Nm respectively (Mw8.0). In addition, we assumed a three-dimensional earth structure using the spectral element method to validate the source model by simulating broadband seismic waves. In the frequency range of 20 ~ 150s, both the body wave and the surface wave, the simulated synthetic waveform and observed waveforms have good agreement. In this model, the surface wave fitting residuals corresponding to the high-dip strike-slip mechanism in the northeastern segment are within the range of 60-500 s, which is generally smaller than that of other northeastern segments that are thrust or low-slip strike-slip mechanisms. The Coulomb stress distribution calculated by this model shows that the Coulomb stress in the northeastern segment increases greatly due to the rupture in the southwest section. This paper argues that the source faults of the 2008 Wenchuan earthquake include at least two major sections: one is low dipole and the other is high dip, which is dominated by thrusting. In addition, we also believe that during the Wenchuan earthquake, the rupture occurred in the two sub-sections.