Abstract
Inferring the geometry and evolution of an earthquake sequence is crucial to understand how fault systems are segmented and interact. However, structural geological models are often poorly constrained in remote areas and fault inference is an ill-posed problem with a reliability that depends on many factors. Here, we investigate the geometry of the Mw 6.3 2008 and 2009 Qaidam earthquakes, in northeast Tibet, by combining InSAR time series and teleseismic data. We conduct a multi-array back-projection analysis from broadband teleseismic data and process three overlapping Envisat tracks covering the two earthquakes to extract the spatio-temporal evolution of seismic ruptures. We then integrate both geodetic and seismological data into a self-consistent kinematic model of the earthquake sequence. Our results constrain the depth and along-strike segmentation of the thrust-faulting sequence. The 2008 earthquake ruptured a ∼32° north-dipping fault that roots under the Olongbulak pop-up structure at ∼12 km depth and fault slip evolved post-seismically in a downdip direction. The 2009 earthquake ruptured three south-dipping high-angle thrusts and propagated from ∼9 km depth to the surface and bilaterally along the south-dipping segmented 55–75° high-angle faults of the Olonbulak pop-up structure that displace basin deformed sedimentary sequences above Paleozoic bedrock. Our analysis reveals that the inclusion of the post-seismic afterslip into modelling is beneficial in the determination of fault geometry, while teleseismic back-projection appears to be a robust tool for identifying rupture segmentation for moderate-sized earthquakes. These findings support the hypothesis that the Qilian Shan is expanding southward along a low-angle décollement that partitions the oblique convergence along multiple flower and pop-up structures.
Highlights
Inferring earthquake slip distributions is useful for understanding the spatial and temporal evolution of crustal strain, in order to investigate how faults are segmented and interact, and to study the relationship of an earthquake with the structural tectonics of an area
HF is emitted during an intermediate time step (3–6 s), in an area that roughly matches a gap in the geodetic displacement data (Figure 2b) and it may be associated with the jump of the rupture across fault-segments and the activation of a third fault segment
The analysis shows that, as for the solution constrained with the stack of co-seismic interferograms (Figure 7), the fault parameters inference constrained with Differential Interferometric Synthetic Aperture Radar (DInSAR) are biased towards a solution that averages the co-seismic and the post-seismic solutions derived from time series (TS) data (Figure 6)
Summary
Inferring earthquake slip distributions is useful for understanding the spatial and temporal evolution of crustal strain, in order to investigate how faults are segmented and interact, and to study the relationship of an earthquake with the structural tectonics of an area. Near-field surface data provide constraints on the fault location and mechanism, the amount of slip, and the extension of the rupture Another way to limit biases in the inversion is to account for data and model parameter uncertainties due to the data noise and the imperfect knowledge of the medium through data weighting e.g., [1,14,15,16], and to rigorously propagate errors through optimisation algorithms, such as with un-regularised Bayesian optimisation frameworks e.g., [2,9,13,17,18,19,20]
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