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The Crustal Dynamics and Its Geological Explanation of the Three-Dimensional Co-Seismic Deformation Field for the 2021 Maduo M_{S}7.4 Earthquake Based on GNSS and InSAR

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## Abstract

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_{S}7.4 earthquake obtained by the method proposed in this paper, was compared with that obtained from the only InSAR measurements obtained using a multi-satellite and multi-technology approach. The results showed the difference in root-mean-square errors (RMSE) of the integration and GNSS displacement was 0.98 cm, 5.64 cm, and 1.37 cm in the east–west, north–south and vertical direction respectively, which was better than the RMSE of the method using only InSAR and GNSS displacement, which was 5.2 cm and 12.2 cm in the east–west, north–south, and no vertical direction. With the geological field survey and aftershocks relocation, the results showed good agreement with the strike and the position of the surface rupture. The maximum slip displacement was about 4 m, which was consistent with the result of the empirical statistical formula. It was firstly found that the pre-existing fault controlled the vertical deformation on the south side of the west end of the main surface rupture caused by the Maduo M

_{S}7.4 earthquake, which provided the direct evidence for the theoretical hypothesis that large earthquakes could not only produce surface rupture on seismogenic faults, but also trigger pre-existing faults or new faults to produce surface rupture or weak deformation in areas far from seismogenic faults. An adaptive method was proposed in GNSS and InSAR integration, which could take into account the correlation distance and the efficiency of homogeneous point selection. Meanwhile, deformation information of the decoherent region could be recovered without interpolation of the GNSS displacement. This series of findings formed an essential supplement to the field surface rupture survey and provided a novel idea for the combination of the various spatial measurement technologies to improve the seismic deformation monitoring.

## 1. Introduction

_{S}7.4 earthquake in Qinghai, China, which occurred in the Bayan Har block as an example, we collected more than 100 GNSS stations in the surrounding area of interest, and the InSAR co-seismic one-dimensional deformation. Based on the elasticity theory [35], we established the linear equation between the strain and displacement pixel by pixel, in which the GNSS stations within the correlation distance were automatically searched, and the homogeneous InSAR pixels involved in the solution were adaptively adjusted. We obtained an accurate co-seismic three-dimensional deformation field of the area of interest, including the deformation of decoherence region, by the variance component estimation (VCE) weighting [36,37,38,39,40,41,42]. Furthermore, we evaluated the accuracy of the calculated three-dimensional displacement field and analyzed the deformation characteristics in the east–west, north–south, and vertical directions, respectively. The crustal dynamics and its geological explanation were analyzed by combining the solution with the field geological survey and relocation.

## 2. Study Area and Data Processing

#### 2.1. Study Area

_{S}7.4 earthquake occurred in Maduo County, Luozhou, Qinghai, China on 22 May 2021, whose epicenter located at 34.59° N, 98.34° E, and the source depth was 17 km (http://www.ceic.ac.cn/history, accessed on 12 July 2021), which was the largest earthquake after the 2008 Wenchuan M

_{W}7.9 earthquake in China. The Maduo M

_{S}7.4 earthquake occurred on the Bayan Har block of the Qinghai–Tibet Plateau secondary tectonic block, 70 km south of the Toso Lake section of the East Kunlun Fault. The Bayan Har block had a very complex tectonic background, and was surrounded by the Ganzi–Yushu–Xianshuihe Fault to the south, the Longmenshan Fault to the east, the East Kunlun Fault to the north, the Maergaichaka Fault and the Altyn Fault to the west. The southern side of the Bayan Har block was pushed by the Indian plate to the northeast and was blocked by the Tarim Basin, which caused material flow and formed the uplift deformation on the Qinghai–Tibet Plateau. Due to the blockage of the eastern Erdos block and the Sichuan Basin, the Qinghai–Tibet Plateau as a whole continued escaping to the southeast, and the geotectonic movements were active and easily induced large earthquakes in the block. In recent years, many strong earthquakes have occurred on the Bayan Har block, such as the 2001 Kunlun Mountain Pass West M8.1 earthquake, the 2008 Wenchuan M8.0 earthquake, the 2010 Yushu M7.1 earthquake, the 2013 Lushan M7.0 earthquake, and the 2017 Jiuzhaigou M 7.0 earthquake, which indicated that the block was in a strong active state. It was necessary to pay more attention to geological hazards in this area. The 2021 Maduo M

_{S}7.4 earthquake occurred on a secondary fault that was approximately parallel to the East Kunlun Fault [43]. However, the seismic hazard of this secondary fault had been lacking special attention. Figure 1 shows the tectonic unit and the location of the 2021 Maduo M

_{S}7.4 earthquake and the used data in the area of interest.

_{S}7.4 earthquake had been researched by various means [45,46,47,48,49,50,51,52,53], which showed that this earthquake was a special and complex left-lateral slip earthquake. Unlike the previous recognition that the main surface rupture was located at the epicenter, no obvious surface rupture had been found near the epicenter according to the field investigation; the obvious surface rupture was far away from the epicenter, and many fissures and extrusion bulges in very different directions were near the surface rupture [45,54], which showed the earthquake was a complex event. As the most intuitive manifestation of an earthquake, the accurate measurement of the three-dimensional deformation contributes to a better understanding of crustal dynamics and seismic triggering mechanisms.

#### 2.2. GNSS Data Estimated the Three-Dimensional Deformation

#### 2.3. InSAR Data Estimated the One-Dimensional Deformation

_{S}7.4 earthquake, as well as the precise orbit files at the corresponding time. The specific information is shown in Table 1.

_{S}7.4 earthquake. The results were expressed as DInSAR_LOS_des, DInSAR_LOS_as, MAI_LOS_des, MAI_LOS_as, MAI_AZI_des, MAI_AZI_as, POT_LOS_des, POT_ LOS_as, POT_AZI_des, and POT_AZI_as. For example, DInSAR_LOS_des represented the descending LOS deformation obtained by the DInSAR technique, and MAI_AZI_as represented the ascending azimuthal deformation obtained by the MAI technique. The specific co-seismic deformation results from different techniques are shown in Figure 3.

## 3. Methodology

#### 3.1. Multi-Source Integrating the Three-Dimensional Deformation

#### 3.2. Adaptive Selection of the Relevant Points

#### 3.3. Integrating Data Based on the VCE Weighting

^{2}. That is, the end-of-iteration condition was satisfied ${\sigma}_{\mathrm{max}}^{2}-{\sigma}_{\mathrm{min}}^{2}<0.0001$. ${\sigma}_{\mathrm{max}}^{2}$ denoted the maximum value of the unit weight variances for the various types of observations, and ${\sigma}_{\mathrm{min}}^{2}$ denoted the minimum value.

## 4. Results and Analysis

_{S}7.4 earthquake indicated [44] that no obvious main surface rupture had occurred near the epicenter (shown in Figure 4d–f), while there was secondary crack development in a wide range (mainly seismic fractures). Away from the epicenter, obvious main surface ruptures had been found along the eastern and western sides of the earthquake fault. Continuing eastward along the fault, no main surface rupture was found but some small secondary cracks (mainly seismic fractures), until the main surface rupture appeared at the northeast and southwest bifurcation phenomenon. However, at the western end of the main surface rupture, the rupture strike also shifted suddenly to a southern deflection.

_{S}8.1 earthquake [63]. According to the field investigation, the continuity and large surface rupture were found here, including a series of the alternate tensile fissure and squeeze drums, as well as en echelon arrays of shear rupture [48,53]. The above energy release means could also be reasonably explained in the vertical direction. Figure 4c showed that the deformation in the vertical component was alternate subsidence (the maximum reached 0.3 m) and uplift (the maximum reached 0.68 m), which was consistent with the results of the vertical displacement being about 1 m by the unmanned aerial vehicle (UAV) measurement [45]. Where the main surface rupture started from the epicenter to the east, obvious subsidence (Figure 4c) and the small horizontal displacement (Figure 4a) formed some left-stepping tensional cracks; those were consistent with the phenomenon of the small pull-apart basin found in the field [48].

_{S}7.4 earthquake were from the epicenter to the east and to the west and had obvious broom-shaped bifurcations at both ends of the rupture. This phenomenon was a typical manifestation of energy release and conformed with the principles of seismology. However, it was a relatively rare deformation feature within the Bayan Har block.

## 5. Discussion

#### 5.1. The Integrating Mutil-Source Data

#### 5.2. The Geodynamic Characteristic

_{S}7.4 earthquake.

_{S}7.4 earthquake caused at least three different strike ruptures. As field investigation from Li et al. [45] and Pan et al. [54] showed, the surface rupture caused by this earthquake had obvious space segmentation. XIAO et al. [79] used the Monte Carlo method and the steepest descent method (SDM) to study the slip distribution of this earthquake and divided the fault into three segments, which also verified the viewpoint of this paper. The result was consistent with the focal mechanism and rupture process research from Deng et al. [80], who believed that the seismogenic fault plane caused by the strong earthquake was not a single plane structure. The above results indicated that the underground structure of the earthquake was complex [81].

_{S}8.1 earthquake [63], located on the northern boundary of the Bayan Har block, was somewhat similar to that of the Maduo earthquake; it also showed strike deflection at the west end of the rupture. However, the rupture process of the 2010 Yushu M

_{S}7.1 earthquake [83], which occurred on the southern boundary of the Bayan Har block, was a typical unilateral rupture, and the deformation characteristic was significantly different from that of the Maduo earthquake in the vertical direction, which was uplift and subsidence, but with no obvious alternation on both sides along the surface rupture of the Yushu earthquake.

_{S}7.4 earthquake [86]. The focal mechanism data for the Maduo earthquake showed two nodal faults trending nearly EW and NS, which was in accordance with the field investigations [75,76,77,78,79,80,81,82,83,84,85,86,87]. The Riedel shear model primarily controlled the earthquake surface ruptures of a strike-slip earthquake and dominated the formation and evolution of strike-slip faults from a wider perspective.

_{S}7.4 earthquake was a typical left-lateral strike-slip earthquake; the revealed kinematic characteristics of the seismogenic fault showed that there was a relatively obvious dip-slip component at the ends of the rupture.

## 6. Conclusions

- (1)
- The accuracy obtained by integrating GNSS and InSAR increased by 81.2% and 53.8% in the east–west direction and north–south direction, respectively. Additional constraints were helpful to obtain a high accuracy regional three-dimensional deformation field, such as increasing the GNSS stations, considering deformation correlation, removing heterogeneous points around the reference points, and the variance component estimation weighting.
- (2)
- The spatial position and the left-lateral strike-slip motion of the earthquake were consistent with those of the Jiangcuo section of the NWW strike of the Kunlun Mountain Pass–Jiangcuo fault in the south of the East Kunlun Fault. The mechanism of the left-lateral left-stepping was caused by the Riedel shear, which dominated the formation and evolution of strike-slip faults.
- (3)
- The maximum slip of the earthquake was about 4 m, which was difficult to investigate in the geological field survey due to the lack of reliable markers, and the slip was consistent with the empirical statistical results.
- (4)
- The ruptures started from the epicenter to both ends and formed bifurcations, which were rarely found in the same type of previous earthquakes that occurred in the Qinghai–Tibet Plateau.
- (5)
- The pre-existing fault controlled the vertical deformation at the north side of the near east–west surface rupture, which only dominated the east–west strike-slip. The three-dimensional deformation was consistent with the surface rupture trace. This provided direct evidence for the theoretical hypothesis that large earthquakes can not only produce a surface rupture on seismogenic faults, but also trigger pre-existing faults or new faults to produce surface ruptures or weak deformation in areas far from seismogenic faults.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**The tectonic location of the 2021 Maduo M

_{S}7.4 earthquake and distribution of the used GNSS station in the study area. (

**a**) is the tectonic unit of the Qinghai-Tibet Plateau and (

**b**) is the specific tectonic location and active fault of the black box in (

**a**): the green-yellow font represents the abbreviated name of the secondary tectonic block, the light blue font represents the abbreviated name of the main fault, the cyan line represents the primary block boundary, the blue line represents the secondary tectonic block boundary, and the black line is the active fault divided by Deng et al. [44]. The yellow pentagram represents the epicenter location of the 2021 Maduo MS7.4 earthquake according to the China earthquake networks center (CENC), the green triangle represents the GNSS stations near the area of interest, which shows some stations overlap because of close ranges, the purple dotted frame is the area of interest, and the red line is the surface rupture caused by the earthquake, the light blue square and dot are city and town, respectively. BHB—Bayan Har Block, QLB—Qilian Block, QDB—Qiadam Block, GTB—Qiangtang Block, LSB—Lhasa Block, EKLF—East Konglun Fault, MDF—Maduo Fault, KJF—Kunlongshankou–Jiangcuo Fault, DF—Dari Fault, BHF—Bayan Har Mountain piedmont Fault, GYF—Ganzi-Yushu–Xianshuihe Fault.

**Figure 2.**The co-seismic displacement obtained by GNSS data. The yellow lines are the secondary tectonic block boundary, (

**a**) represents the co-seismic displacement in horizontal direction, the blue and the red arrows represent the different displacement ratio, and (

**b**) represents the co-seismic displacement in the vertical direction. The blue represents the uplift displacement, and the red represents the subsidence displacement: the red circle represents the error ellipse, the purple star represents the epicenter of the Maduo M

_{S}7.4 earthquake, the red curve near the epicenter represents.

**Figure 3.**The LOS and azimuthal deformations obtained by the DInSAR, MAI and POT, the x-axis represents the east longitude (E), unit is the degree; the y-axis represents the north latitude (N), unit is the degree; (

**a**) DInSAR_LOS_as; (

**b**) DInSAR_LOS_des; (

**c**) MAI_LOS_as; (

**d**) MAI_LOS_des; (

**e**) MAI_AZI_as; (

**f**) MAI_AZI_des; (

**g**) POT_LOS_as; (

**h**) POT_AZI_as. For example, DInSAR_LOS_des indicated the descending LOS deformation obtained by DInSAR technique; the black line represents the surface rupture in field, which is agree with the InSAR measurement results.

**Figure 4.**The co-seismic three-dimensional deformation field of the 2021 Maduo M

_{S}7.4 earthquake: (

**a**,

**d**) the displacement of the east–west direction; (

**b**,

**e**) the displacement of the north–south direction; (

**c**,

**f**) the displacement of the vertical direction. In Figure 4a–c, the black line is the fitted rupture from the filed investigation [44] and the InSAR deformation field in this study. In Figure 4d–f, the black line is the surface rupture from the fine interpretation from the unmanned aerial vehicle (UAV), the red star represents the epicenter of the Maduo M

_{S}7.4 earthquake.

**Figure 5.**The vertical component displacement of the presumed rupture. (The red line is the presumed rupture that caused the vertical deformation, the purple line is the actual field investigation of the surface rupture, and the black line is the complete rupture according to the actual rupture).

**Figure 6.**The distribution map of the NW–NWW strike pre-existed faults near the 2021 Maduo M

_{S}7.4 earthquake and its vicinity [55,66,67,68,71,72]. (

**a**) is the tectonic unit of the Qinghai-Tibet Plateau. (

**b**) is the distribution map of active faults in and around the Bayan Har Block, (

**c**) is the specific tectonic location and active faults of the white box in (

**b**): BHB—Bayan Har Block, LSB—Lhasa Block, QTB—Qiangtang Block, QDB—Qiadam Block, QLB—Qilian Block. EKLF—East Kunlun Fault, MGF—Maduo-Gande Fault, SGF—South Gande Fault, XCF—Xizangdagou-Changmahe Fault, KJF—Kunlunshankou-Jiangcuo Fault, DF—Dari Fault.

**Figure 7.**Comparison of GNSS corresponding three-dimensional deformation and the integrated three-dimensional deformation by GNSS and InSAR (the x-axis represents the names of the GNSS station for verification, the y-axis represents the displacement of the deformation, the red line represents the integrated displacement by the GNSS and InSAR in three directions, and the blue line represents the displacement of the corresponding GNSS stations in three directions).

**Figure 8.**The surrounding faults and the aftershocks relocation of the 2021 Maduo M

_{S}7.4 earthquake: the yellow pentagram represents the location of the epicenter of the 2021 Maduo M

_{S}7.4 earthquake, the purple dot represents the aftershocks sequence relocation of the 2021 Maduo M

_{S}7.4 earthquake, the green line represents the secondary tectonic block boundary, and the black line represents the faults around the epicenter, the red line represents the surface rupture of the 2021 Maduo M

_{S}7.4 earthquake.

**Figure 9.**The Riedel shear in strike-slip fault zone. (

**a**) An idealized Riedel shear zone is composed of six principal elements, which are X shear and Y shear faults, Riedel (R) and R′ conjugate shears, T tensional fractures, and P shears, which are all oriented at specific angles with respect to the general trend of the shear zone. (

**b**) Mechanism of the right-stepping in strike-slip fault zone. (

**c**) Mechanism of the right-stepping in strike-slip fault zone (modified from Li et al. [85]): The blue arrows represent the compressional tectonics, and the white arrows represent the extensional tectonics. The red lines represent the cracks, the bule lines represent squeeze effect on the plane.

Orbit | Acquisition Date of the Master Image | Acquisition Date of the Slave Image | Imaging Mode | Wavelength /cm | Revisit Period /d |
---|---|---|---|---|---|

Ascending (T99) | 20 May 2021 (Sentinel-1A) | 26 May 2021 (Sentinel-1B) | IW | 5.6 | 6 |

Descending (T106) | 20 May 2021 (Sentinel-1A) | 26 May 2021 (Sentinel-1B) | IW | 5.6 | 6 |

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**MDPI and ACS Style**

Li, X.; Chen, Y.; Wang, X.; Xiong, R.
The Crustal Dynamics and Its Geological Explanation of the Three-Dimensional Co-Seismic Deformation Field for the 2021 Maduo *M*_{S}7.4 Earthquake Based on GNSS and InSAR. *Sensors* **2023**, *23*, 3793.
https://doi.org/10.3390/s23083793

**AMA Style**

Li X, Chen Y, Wang X, Xiong R.
The Crustal Dynamics and Its Geological Explanation of the Three-Dimensional Co-Seismic Deformation Field for the 2021 Maduo *M*_{S}7.4 Earthquake Based on GNSS and InSAR. *Sensors*. 2023; 23(8):3793.
https://doi.org/10.3390/s23083793

**Chicago/Turabian Style**

Li, Xiaobo, Yanling Chen, Xiaoya Wang, and Renwei Xiong.
2023. "The Crustal Dynamics and Its Geological Explanation of the Three-Dimensional Co-Seismic Deformation Field for the 2021 Maduo *M*_{S}7.4 Earthquake Based on GNSS and InSAR" *Sensors* 23, no. 8: 3793.
https://doi.org/10.3390/s23083793