Co-seismic Surface Dilatation and Shear Strain Fields Derived from SPOT-5 Displacements

Repeated earthquake cycles produce topography, fault damage zones, and other geologic structures along faults. These geomorphic and structural features indicate the presence of co-seismic permanent (inelastic) surface deformation, yet a long-standing question in earthquake research is how much of the co-seismic deformation field is elastic versus inelastic. These questions arise in part because it is unclear what measurable co-seismic characteristics, such as off-fault or distributed surface deformation and cracking, represent true unrecoverable deformation. One emerging descriptor of permanent co-seismic deformation is surface strain magnitudes inferred from imaging geodesy observations. In this study, we present the surface strain field of the 2013 Mw7.7 Baluchistan strike-slip earthquake in southern Pakistan. We invert co-seismic displacement fields generated from pixel-tracking of SPOT-5 and WorldView optical imagery for co-seismic surface horizontal strain tensors. We observe that co-seismic strain field is dominated by negative dilatation strains, indicating that the co-seismic fault zone contracted during the earthquake. We show that co-seismic inelastic failure exhibits a relatively consistent width along the rupture that is localized to a zone 100-200 m wide on the hanging wall side. The width of co-seismic permanent deformation does not correlate with variations in off-fault deformation or surface geology. Based on comparisons to other recent earthquakes, we posit that the permanent surface strains reflect inelastic deformation of the faults inner damage zone, and that the width of this zone reflects fault maturity.

重复发生的地震循环沿断层带塑造了地形、断层损伤带及其他地质构造。这些地貌与构造特征印证了同震永久（非弹性）地表变形的存在，但地震研究领域长期存在的一个核心问题是：同震变形场中，弹性变形与非弹性变形的占比分别为多少。该问题的产生，部分源于学界尚未明确：诸如断层外分布式地表变形与破裂等可观测的同震特征，究竟代表何种真正的不可恢复变形。目前，一种用于表征同震永久变形的新兴指标，是通过成像大地测量（imaging geodesy）观测反演得到的地表应变幅值。本研究呈现了巴基斯坦南部2013年Mw7.7级俾路支走滑地震的地表应变场。我们对基于SPOT-5与WorldView光学影像像素追踪得到的同震位移场进行反演，求解同震地表水平应变张量。观测结果显示，同震应变场以负膨胀应变为主，表明同震断层带在地震过程中发生了收缩。研究表明，同震非弹性破裂沿破裂带呈现相对一致的宽度，且集中分布在上盘侧100~200米宽的区域内。同震永久变形的宽度与断层外变形或地表地质的变化并无显著关联。通过与近年其他地震的对比分析，我们提出：永久地表应变反映了断层内部损伤带的非弹性变形，而该损伤带的宽度则体现了断层的成熟度。