Afterslip and viscoelastic relaxation following the 1999 M 7.4 İzmit earthquake from GPS measurements
Afterslip and viscoelastic relaxation following the 1999 M 7.4 İzmit earthquake from GPS measurements
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DOI：
10.1111/j.1365-246x.2009.04228.x
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发表时间：
2009-09
影响因子：
2.8
通讯作者：
Lifeng Wang;Rongjiang Wang;F. Roth;B. Enescu;S. Hainzl;S. Ergintav
中科院分区：
文献类型：
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作者：
Lifeng Wang;Rongjiang Wang;F. Roth;B. Enescu;S. Hainzl;S. Ergintav
1999年伊兹米特地震后，利用全球定位系统进行了密集监测，这为了解走滑断层的震后行为和脆性上地壳以下的流变学提供了机会。有两个数据集：在伊兹米特地震后的头300天内测量的位移和2003年至2005年之间的速度测量。利用反演方法和正演模拟，我们分别研究了两种机制：（1）同震破裂面上和下的后滑和（2）下地壳和上地幔中的粘弹性应力松弛，用麦克斯韦或标准线性固体（SLS）流变学描述。反演结果表明，伊兹米特地震发生后的前几个月，地震后的明显余震活动主要发生在30 km以下的浅层，且余震活动量随时间衰减;此后，余震活动的空间分布发生了很大变化，且明显余震活动的位置更深。对于粘弹性松弛，一个模型与弹性上地壳和麦克斯韦粘弹性下地壳覆盖麦克斯韦地幔（E-M-M）适合的数据在第一个300天更好地在远场比在近场。然而，所观察到的远场，300天的位移和长期（2003-2005年）的位移，这可能是由粘弹性松弛为主，不能用恒定粘度的麦克斯韦流变模型描述：有效粘度随时间增加。因此，我们建立了一个精细的流变学模型：弹性上地壳和SLS下地壳覆盖麦克斯韦粘弹性地幔（E-SLS-M）。我们的最佳解得出下地壳的粘度为102 × 1018 Pa·s，弛豫强度为2/3，麦克斯韦地幔的粘度为7 × 1019 Pa·s。最后，我们解释的数据，使用复合模型，由首选的E-SLS-M模型和后滑模型得到的残余位移校正后的粘弹性松弛。对于早期阶段，残余位移主要可以解释为一个浅的后滑，其大小随时间衰减，其空间分布是稳定的，而对于后期阶段的残余位移需要可以忽略的后滑。这表明，深部震源引起的震后后期变形几乎完全可以用E-SLS-M模型来解释。复合模型一般可以解释整个时空空间中的数据。
SUMMARY Intensive global positioning system (GPS) monitoring after the 1999 Izmit earthquake provides an opportunity to understand the postseismic behaviour of a strike-slip fault and the rheology below the brittle upper crust. Two data sets are available: displacements measured during the first 300 days after the Izmit earthquake and velocity measurements between 2003 and 2005. Using an inversion method and a forward modelling, respectively, we investigate two mechanisms: (1) afterslip on and below the coseismic rupture plane and (2) viscoelastic stress relaxation in the lower crust and upper mantle described by a Maxwell or a standard linear solid (SLS) rheology. The inversion results show that the first several months following the Izmit earthquake were dominated by afterslip at depths shallower than 30 km and the slip amount decayed with time; after that, apparent afterslip has a very different spatial distribution and is located much deeper. For viscoelastic relaxation, a model with an elastic upper crust and a Maxwell viscoelastic lower crust overlying a Maxwell mantle (E-M-M) fits the data measured in the first 300 days better in the far field than in the near field. However, the observed far-field, 300-day displacement and the long-term (2003–2005) displacement, which might be dominated by viscoelastic relaxation, cannot be described by a Maxwell rheological model with constant viscosity: the effective viscosity increases over time. Therefore, we have built a refined rheological model: an elastic upper crust and an SLS lower crust overlying a Maxwell viscoelastic mantle (E-SLS-M). Our best solution yields a viscosity for the lower crust of ∼2 × 1018 Pa s, a relaxation strength of 2/3 and a viscosity for the Maxwell mantle of 7 × 1019 Pa s. Finally, we explain the data using a composite model, consisting of the preferred E-SLS-M model and the afterslip model obtained from the residual displacement after correcting for viscoelastic relaxation. For the early time period, the residual displacements can be mainly explained by a shallow afterslip whose magnitude decays with time and whose spatial distribution is stable, whereas the residual displacements for the later time period require negligible afterslip. It indicates that the postseismic deformation in the later time period induced by a deep source can be almost entirely explained by the E-SLS-M model. The composite model can generally explain the data in the entire spatial and temporal space.