Coseismic and Postseismic Deformation from the 1999 Chi-Chi, Taiwan Earthquake

1999年9月21日，台湾集集地震发生在密集的GPS台网中心，震级为Mw = 7.5。在128个站点测量了同震位移，震级最高可达12米。还记录了一组显著的瞬态地震后变形信号。几个连续记录的GPS站在垂直和水平分量中测量了振幅超过10厘米的瞬态信号。密集的空间覆盖和非凡的信噪比使得赤池地震的同震和震后形变场可以说是有史以来最好的。同震位移的初步分析表明，这次地震是由东倾逆冲引起的，与野外和地震观测结果一致。然而，GPS数据不能与单一断层表面的滑动拟合。地表断裂北端发生90度弯曲，东西向断裂约15公里长。余震震源机制表明在这个东西向分支上有走滑运动。GPS资料最适合于南北向和东西向的斜滑弯曲倾斜断层。首选模型解释了数据中98%的方差。然而，残差太大，而且空间上连贯，无法用测量误差来解释。车隆堡断裂是褶皱逆冲系统的一部分，该系统将更古老、更坚硬的岩石置于沉积物之上，相应的弹性性质的空间变化可能会影响建模。研究人员和他们的台湾合作者提出了一个详尽的同震变形分析，考虑了弹性特性的垂直和横向变化，非平面断层几何形状，滑动的空间变化，以及不规则地表地形的可能影响。与同震位移一样令人兴奋的是，震后变形更为显著。几十年前人们就已经知道了地震后加速变形的现象，但是人们对这种现象的物理原理却知之甚少。余震、下地壳和上地幔的粘性流动以及孔隙弹性松弛都被用来解释地震后的瞬态变形。理论研究比比皆是；问题在于缺乏可定义的数据集。赤池地震的数据具有如此高的信噪比，我们希望最终能回答这些长期存在的问题。具体来说，研究人员建议进行密集的建模工作，将粘弹性、孔弹性和余震模型的预测与数据进行比较。初步结果表明，余震与主震后100天的数据相吻合，但还需要进行更多的分析。GPS数据的反演将揭示地震后滑动的时空演变。这些结果将对碰撞带的构造学具有重要意义。大量的褶皱冲断带模型显示，台湾下方的断层合并成一个主滑脱。通过确定车隆堡断层在发震深度和次发震深度的几何形状，研究人员将为这个经典碰撞造山带的结构提供重要的见解。研究人员计划开发既能拟合地震间大地测量数据的模型，又能在多个地震周期中综合产生地质上合理的位移。该研究计划将与台湾中央研究院地球科学研究所的台湾同事广泛合作，支持两个博士项目。

Segall 0106695 The Mw = 7.5, September 21, 1999 Chi-Chi, Taiwan earthquake occurred in the center of a dense GPS network operated by the Taiwanese. Coseismic displacements were measured at 128 stations, with magnitudes of up to 12 meters. A remarkable set of transient postseismic deformation signals were also recorded. Several continuously recording GPS stations measured transient signals with amplitudes of more than 10 cm in both the vertical and horizontal components. The dense spatial coverage and extraordinary signal to noise ratio make the coseismic and postseismic deformation fields of the Chichi earthquake arguably the best ever recorded. Preliminary analysis of the coseismic displacements demonstrate that the earthquake was caused by an east-dipping thrust, consistent with field and seismological observations. However, the GPS data can not be fit with slip on a single fault surface. The surface rupture undergoes a 90 degree bend at its northern end, with a ~15 km long east-west trending break. Aftershock focal mechanisms suggest strike slip motion on this E-W striking branch. The GPS data, however, are best fit with a curved dipping fault with oblique slip on both the N-S trending and E-W trending segments. The preferred model explains 98% of the variance in the data. The residuals, however, are far too large, and spatially coherent to be explained by measurement error. The Chelungpu fault, is part of a fold thrust system, which emplaced older and stiffer rocks over sediments and the corresponding spatial variations in elastic properties may bias the modeling. The investigators and their Taiwanese collaborators propose an exhaustive analysis of the coseismic deformation accounting for vertical and lateral variations in elastic properties, non-planar fault geometry, spatial variations in slip, and possibly effects of irregular surface topography. As exciting as the coseismic displacements are, the postseismic deformations are even more significant. Accelerated post earthquake deformation has been known for decades, however the physics of the phenomenon are poorly understood. Afterslip, viscous flow of the lower crust and upper mantle, and poroelastic relaxation have all been proposed to explain transient postseismic deformation. Theoretical studies abound; the problem has been a lack of defining data sets. The data from the Chichi earthquake is of such high signal to noise ratio that we hope to finally answer these long standing problems. Specifically, the investigators propose an intensive modeling effort to compare predictions from viscoelastic, poroelastic, and afterslip models to the data. Preliminary results suggest that afterslip provides a reasonable fit to the data from the first 100 days after the mainshock, although much more analysis is needed. Inversion of the GPS data will reveal the spatiotemporal evolution of postseismic slip. These results will have important implications for the tectonics of collision zones. Numerous models of fold thrust belts show faults merging into a master decollement beneath Taiwan. By determining the geometry of the Chelungpu fault at seismogenic and subseismogenic depths the investigators will provide important insights into the structure of this classic collisional orogen. The investigators plan to develop models that both fit interseismic geodetic data and yield geologically reasonable displacements when integrated over multiple earthquake cycles. The proposed research will support tow Ph.D projects in an extensive collaboration with Taiwanese colleagues at the Institute of Earth Sciences, Academica Sinica Taiwan.