# Illuminating the Spatio-Temporal Evolution of the 2008–2009 Qaidam Earthquake Sequence with the Joint Use of Insar Time Series and Teleseismic Data

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## Abstract

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## 1. Introduction

_{w}5.4 and a M

_{w}5.6 aftershock one and two days, respectively, after the main-shock, as many as 34 M

_{w}> 4 aftershocks within a month (USGS), and time-dependent after-slip of similar pattern to the co-seismic slip [48,49,50,51]. The two earthquakes represent particular examples of two ∼${M}_{w}$ 6 earthquakes that occurred nearby in space and time, highlighting the need to constrain both along-depth and along-strike segmentation of thrusting events to better quantify seismic hazards [46]. Their segmentations and geometries also need to be placed in the context of the regional tectonic as the deformation style of the northeastern part of the Tibetan plateau is at the heart of the debates about its dynamic evolution. In a first class of deformation models, Burchfiel et al., 1989 [52] first postulated a basement décollement fault in the middle crust of the Qaidam basin that connect to the Qilian Shan-Nan Shan thrust belts in the north, absorbing progressively the left-lateral slip on the Altyn-Tagh Fault (Figure 1). Subsequently, Metivier et al., 1998 [53], Meyer et al., 1998 [54] and Tapponnier et al., 2001 [55] suggested that the Qaidam basin may have been trapped of the surrounding reliefs in the middle-late Miocene as a result of the northward stepwise jumping of the intra-continental deformation. This first class of model proposes that the oblique convergence between the India-Eurasia collision is absorbed by crustal thickening along the major Kunlun or Qilian Shan suture zones, where strike-slip and thrust faults root at depth to extrude laterally and partition vertically the deformation causing mountain uplift on localised ranges (Figure S1a). Conversely, Dupont et al., 2004 [56], Yin et al., 2008a [57], and Bush et al., 2016 [58] suggested a plausible and simultaneous early Eocene onset of the Kunlun and Qilian ranges. This second class of model suggests that the strain from the India-Asia collision was rapidly transmitted to the northern edge of the modern Tibetan Plateau via preexisting and complex zones of weaknesses created during pre-Cenozoic [57,59,60] or via homogeneous deformation driven by mantle or crustal flow [61,62,63] (Figure S1b). Those model contradictions leave thus open the debate on the geometry of the NQT at depth and its possible relations with the Qilian ranges, to the north [55,59].

## 2. Materials and Methods

#### 2.1. Seismic Back-Projection Source Imaging

#### 2.1.1. Data and Station Clustering

#### 2.1.2. Muti-Array Back-Projection Method

#### 2.2. Kinematic Fault Inversion

_{w}> 6, 2008 and 2009 earthquakes that explain the surface displacements and the seismological observations with minimum prior constraints on fault geometry and fully accounting for data uncertainties. We optimise both near-field and far-field data with a Bayesian bootstrapping algorithm implemented in the open source Grond earthquake inversion framework [4,81,82] and part of the Pyrocko software (https://pyrocko.org/grond/docs/current/method/index.html).

#### 2.2.1. Teleseismic Data

#### 2.2.2. Insar Time Series (Ts) Data

#### 2.2.3. Fault Inference Method

#### 2.2.4. Fault Inference of the 2008 Earthquake

#### 2.2.5. Fault Inference of the 2009 Earthquake

## 3. Results

#### 3.1. Back-Projection Imaging of the 2008–2009 Sequence

#### 3.2. 2008 Fault Characteristics Estimations

#### 3.3. 2009 Fault Characteristics Estimations

_{w}6.3 earthquake based on InSAR and far-field teleseismic data (Figure 5), the optimisation converges towards a central segment of $10.0\pm 0.5$ km long and of $1.6\pm 0.5$ km wide with a top edge centre located at $2.7\pm 1.2$ km depth. The eastern segment is constrained with a length of $4.7\pm 0.5$ km, a width of $1.8\pm 0.5$ km, and a depth of $3.1\pm 1.5$ km, while the third western segment is of $9.0\pm 1$ km length, $2.5\pm 1$ km width, and at $4.5\pm 0.5$ km depth. All three segments have narrow widths and are, respectively, dipping with $71\pm {2}^{\circ}$, $57\pm {3}^{\circ}$, and $73\pm {3}^{\circ}$ high-angles to the south. Slip on the fault segments amounts to $2.0\pm 0.4$ m, $2.2\pm 0.3$ m, and $1.9\pm 0.5$ m. A summary of all posterior PDFs is available in the Figure 8, while waveform fits for the 2009 earthquake north-dipping solution for five random stations are shown in Figure S9b.

## 4. Discussion

#### 4.1. Benefits of InSAR Time Series (TS) for Fault Inference

^{2}, while the correlation lengths are about 0.5–2 km.

#### 4.2. Back-Projection Imaging for Moderate-Size Earthquakes

#### 4.3. The 2008–2009 Qaidam Earthquake Sequence

_{w}5.3 earthquakes and the ∼12 km deep October 2004 thrust ∼M

_{w}5.5 earthquake (according to gCMT, Table S1) may belong to the same flower structure that ruptured in 2003 (Figure 10). Static stress transfer within this flower structure may have triggered the 2004 sequence of earthquakes one year after the 2003 earthquake.

## 5. Conclusions

## Supplementary Materials

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Seismotectonic setting of the Olongbulak ranges in the northeastern part of the Tibetan Plateau superimposed on a false colour Landsat 8 satellite image (U.S. Geological Survey, https://earthexplorer.usgs.gov/). The uplifted Paleogene, Cretaceous and Jurassic deposits form a distinctive yellow band within the Olongbulak pop-up, between dark Jurassic to Cambrian bedrock. Mapped faults from our own analysis are shown as black lines. The locations and focal mechanisms of the three ${M}_{w}>5.2$ earthquakes of the 2008–2009 Qaidam sequence are shown with their mechanisms as lower-hemisphere stereo-projections [64] (2008 and 2009 in green and blue, respectively). Grey transparent circles show the historical seismicity from 2003 to 2018 from the U.S. Geological Survey catalogue (M > 4.0) and a regional Chinese catalogue (M > 2.0, http://data.earthquake.cn).

**Figure 2.**(

**a**) Parametric time series (TS) decomposition for four pixels of the tracks T319, T047 and T455 marked by crosses in (

**b**). (

**b**) Line-of-sight (LOS) surface displacement maps wrapped between −20 mm and 20 mm associated with the 2008 (

**b**) and 2009 (

_{1}**b**) co-seismic surface displacements, and the corresponding afterslip surface displacements over a 4 month interval (

_{3}**b**,

_{2}**b**) extracted from the parametric decomposition of tracks T319 (top), T047 (middle), and T455 (bottom). Black lines show mapped fault traces of the NQT and Xietie–Shan thrust (XT).

_{4}**Figure 3.**Co-seismic Differential interferograms and stacks from track 319 (top), 047 (middle), 455 (bottom). (

**a**) 2008 coseismic Differential interferograms. (

**b**) 2009 coseismic Differential interferograms. (

**c**) Stack of 2008 coseismic Differential interferograms. (

**d**) Stack of 2009 coseismic Differential interferograms.

**Figure 4.**Seismic back-projection for the 10 November 2008 and the 28 August 2009 Haixi earthquakes. (

**a**) P wave beampower (filled red curve) and maximum semblance from both LF and HF emissions (white filled curve) as a function of time for the 2008 (left) and 2009 (right) earthquakes. (

**b**) Back-projected stack amplitudes shown for given time intervals and grid depths for the two earthquakes (green: 2008, blue: 2009) with warm colours being associated with higher semblance. Semblance peaks are numbered 1, 2, and, 3.

**Figure 5.**Posterior models for the 10 November 2008 and 28 August 2009 earthquakes and their post-seismic deformation from optimisations obtained with fixed dip directions. (

**a**) Best-fitting posterior geometries in map view for the 2008 north-dipping co-seismic (coral red), the 2008 north-dipping post-seismic (orange), the 2008 south-dipping co-seismic (pink), the 2008 south-dipping post-seismic (magenta), and for the three segments of the 2009 co-seismic (dark blue, cyan, blue) fault inferences. (

**b**) As for top figure, but along the N22° E profile perpendicular to the Olongbulak Shan marked A-A’ in (

**a**) and with interpreted fault geometry in the middle/upper crust. Based on the coplanarity of the 2008 co- and post-seismic slip, we interpret that the 2008 earthquake ruptured a 32° north-dipping plane at 12 km depth rooting below the Olongbulak Shan and that the after-slip was mainly down-dip of the rupture plane. The 2009 earthquake broke three distinct 55–75° high-angle south-dipping back-thrust segments of the Olongbulak pop-up structure.

**Figure 6.**(

**a**) Summary of the posterior PDFs for the optimisation of one north-dipping rectangular fault in agreement with the co-seismic (coral red) and post-seismic (orange) surface displacements of the 2008 earthquake. Dashed vertical lines are best-fitting models. (

**b**) Same as (

**a**), but for the optimisation of one south-dipping rectangular fault in agreement with the co-seismic (coral red) and post-seismic (orange) surface displacements of the 2008 earthquake.

**Figure 7.**Posterior models for the 10 November 2008 earthquake obtained from the optimisation with a free dip-angle orientation based on both stack of long-baselines interferograms and teleseismic data. (

**a**) Comparison between data and model from the optimisation. Left: sub-sampled surface displacements for tracks 319 (top), 047 (middle), and 455 (bottom). Middle: modeled displacements associated with the maximum likelihood of the posterior probability distribution. Right: residuals between the forward model and the observations. (

**b**) Sequence plots for selected parameters of the optimisation with a color-scale that varies depending on the misfit from high (blue) to low (red). North-dipping and south-dipping bimodal solutions are explored simultaneously, as sampling in all regions is encouraged by random offsets. The Bayesian bootstrap inversion converges towards an $9.8\pm 1$ km deep plane dipping towards the north with an angle of $32\pm {10}^{\circ}$.

**Figure 8.**Summary of the posterior PDFs for the optimisation of the three rectangular faults (middle, east, west) in agreement with the co-seismic surface displacements of 2009 earthquake (

**a**) and with a stack of co-seismic interferograms of 2009 earthquake (

**b**). Dashed vertical lines are best-fitting models.

**Figure 9.**Comparison between posterior Probability Density Functions (PDFs) of the 2008 earthquake parameters that were obtained from the optimisation of InSAR co-seismic TS data + teleseismic data (blue) and DInSAR interferograms + teleseismic data (coral red). Dashed vertical lines are best-fitting models.

**Figure 10.**Three-dimensional block diagram of the proposed geometry for the North Qaidam thrust system superimposed on a digital elevation model (3× vertically exaggerated), along with the cumulative LOS displacement map from descending track 319 and the 10 November 2008 and 28 August 2009 co-seismic LOS displacements profiles from Figure 2. Insert at the bottom left shows interpreted conservation of motion vectors across the fault-system, where high-angle thrusts and folds vertically partition the horizontal shortening transferred from the South Qilian Shan to the Qaidam basin.

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## Share and Cite

**MDPI and ACS Style**

Daout, S.; Steinberg, A.; Isken, M.P.; Heimann, S.; Sudhaus, H. Illuminating the Spatio-Temporal Evolution of the 2008–2009 Qaidam Earthquake Sequence with the Joint Use of Insar Time Series and Teleseismic Data. *Remote Sens.* **2020**, *12*, 2850.
https://doi.org/10.3390/rs12172850

**AMA Style**

Daout S, Steinberg A, Isken MP, Heimann S, Sudhaus H. Illuminating the Spatio-Temporal Evolution of the 2008–2009 Qaidam Earthquake Sequence with the Joint Use of Insar Time Series and Teleseismic Data. *Remote Sensing*. 2020; 12(17):2850.
https://doi.org/10.3390/rs12172850

**Chicago/Turabian Style**

Daout, Simon, Andreas Steinberg, Marius Paul Isken, Sebastian Heimann, and Henriette Sudhaus. 2020. "Illuminating the Spatio-Temporal Evolution of the 2008–2009 Qaidam Earthquake Sequence with the Joint Use of Insar Time Series and Teleseismic Data" *Remote Sensing* 12, no. 17: 2850.
https://doi.org/10.3390/rs12172850