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

Segall 0106695 1999年9月21日台湾集集地震发生在台湾密集GPS网络的中心。 在128个台站测量了同震位移，震级高达12米。 一组显着的瞬态震后变形信号也被记录。 几个连续记录的全球定位系统台站测量到的瞬时信号的垂直和水平分量的幅度都超过10厘米。密集的空间覆盖和非凡的信噪比使得集集地震的同震和震后形变场可以说是有史以来最好的记录。 同震位移的初步分析表明，地震是由东倾逆冲断层引起的，与现场和地震观测结果一致。 然而，GPS数据不能与单一断层面上的滑动拟合。 表面破裂在其北方末端经历了90度的弯曲，并有一个约15公里长的东西走向断裂。 余震震源机制表明在这个东西走向的分支上有走滑运动。然而，GPS数据最适合于在南北向和东西向段上具有斜滑的弯曲倾斜断层。 首选模型解释了数据中98%的方差。然而，残差太大，并且空间相干，不能用测量误差来解释。 车笼埔断层是一个褶皱逆冲系统的一部分，它在沉积物上侵位了较老和较硬的岩石，相应的弹性性质的空间变化可能会使建模产生偏差。 研究人员和他们的台湾合作者提出了一个详尽的分析同震变形帐户的垂直和横向变化的弹性特性，非平面断层几何形状，空间变化的滑动，和可能的影响不规则的表面地形。 尽管同震位移令人兴奋，但震后变形更为显着。 地震后的加速变形已经知道了几十年，但是对这种现象的物理学知之甚少。 余震滑动、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震后瞬时变形。 理论研究比比皆是;问题是缺乏定义数据集。 集集地震的资料是如此的高信噪比，我们希望最终能回答这些长期存在的问题。 具体来说，研究人员提出了一个密集的建模工作，比较粘弹性，孔隙弹性和后滑模型的数据预测。初步结果表明，余震提供了一个合理的拟合主震后的第一个100天的数据，虽然需要更多的分析。 GPS数据的反演将揭示震后滑动的时空演化。 这些结果对碰撞带的构造研究具有重要意义。 许多褶皱冲断带的模型都显示断层在台湾下方合并成一个主要的滑脱构造。 通过确定车笼埔断层在孕震和次孕震深度的几何形状，研究人员将对这一经典碰撞造山带的结构提供重要的见解。 研究人员计划开发模型，既适合地震间的大地测量数据，又能在多个地震周期的整合中产生地质上合理的位移。 这项研究将与台湾科学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.