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A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology
Geodesy and Geodynamics xxx (2017) 1e5
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Geodesy and Geodynamics journal homepages: www.keaipublishing.com/en/journals/geog; http://www.jgg09.com/jweb_ddcl_en/EN/volumn/home.shtml
A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology Jiangtao Qiu a, b, *, Xuejun Qiao a a b
Institute of Seismology, China Earthquake Administration, Wuhan 430071, China The Second Monitoring and Application Center, China Earthquake Administration, Xi'an 710054, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 December 2016 Accepted 21 April 2017 Available online xxx
On October 17th, 2016, a Ms6.2 earthquake occurred in Zaduo County of Qinghai Province, China. The aim of this study is to use synthetic aperture radar (SAR) technology aboard the Sentinel-1A satellite to obtain high-resolution co-seismic surface displacement data and then to confirm the geometric parameters of the fault and slip distribution model. To this end, linear and non-linear inversion algorithms based on an elastic half-space dislocation model were used. The results showed that a distributed slip model can explain the surface deformation field measured by InSAR very well. The surface deformation field caused by the earthquake was an oval-shaped region of subsidence with a maximum displacement of 5 cm along the line of sight of the radar waves. This earthquake was mainly the result of a normal-slip fault process with 72 N strike and 65 dip. The slip was mainly concentrated at depths of 9e15 km. The maximum slip was 0.17 m, located at a depth of 12 km. The moment magnitude given by inversion was Mw5.9. This was basically in agreement with the moment magnitudes and surface magnitudes measured by USGS and CENC. © 2017 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Zaduo earthquake InSAR Slip distribution inversion Seismogenic fault
1. Introduction According to measurements from China's national seismological network, a Ms6.2 earthquake occurred at 15:14 h (Beijing time) on October 17th, 2016 in Zaduo County, Yushu Prefecture, Qinghai (32.81 N, 94.93 E). In this article, this event is referred to as the Zaduo earthquake. After the earthquake, the China Earthquake Networks Center, Harvard University's Global Centroid-MomentTensor (CMT) Project and the United States Geological Survey (USGS), separately used far-field wave information to calculate the location of the focus and the focal mechanism solution of the earthquake (Table 1, Fig. 1). Table 1 shows that CENC and USGS
* Corresponding author. Institute of Seismology, China Earthquake Administration, Wuhan 430071, China. E-mail address: [email protected] (J. Qiu). Peer review under responsibility of Institute of Seismology, China Earthquake Administration.
Production and Hosting by Elsevier on behalf of KeAi
recorded the earthquake with a magnitude of Mw5.9 and GCMT recorded it with a magnitude Mw6.0. Although there were significant differences in the calculated rake, all of them agreed that the fault had the characteristics of a normal fault. The epicenter was located near the Zaduo fault zone, which is an Early-Middle Pleistocene sinistral reverse fault. The earthquake occurred in a sparsely populated alpine region, making geological field surveys and geophysical data acquisition very difficult. Interferometric Synthetic Aperture Radar (InSAR) technology was first applied to seismological deformation in 1993 and since then it has been widely accepted and used in the field. InSAR technology was especially useful in this case because there was little vegetation cover in the QinghaieTibet Plateau and this resulted in good coherence in the co-seismic deformation field over long periods of time. Many strong seismic events have occurred in the QinghaieTibet hinterland since the Mani earthquake of 1997. Both international and domestic researchers have studied the region extensively. InSAR technology has been used to analyze the following earthquakes: 1997, Mani, Ms7.9 [1e3]; 2001, Kunlun Mountain, Ms8.1 [4]; 2004, Zhongba, Ms6.7 [5]; 2005, Zhongba, Ms6.5; 2008, Dangxiong, Ms6.6 [6,7]; Gaize, Ms6.9 [8]; Wuqia, Mw6.7 [9]; and Yutian, Ms7.3 [10]; and 2010, Yushu, Ms7.1 [11].
http://dx.doi.org/10.1016/j.geog.2017.04.008 1674-9847/© 2017 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: J. Qiu, X. Qiao, A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.04.008
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J. Qiu, X. Qiao / Geodesy and Geodynamics xxx (2017) 1e5
Table 1 Focal parameters of the October 17th, 2016 Zaduo earthquake given by different organizations. Organization
CENC GCMT USGS
Longitude ( E)
94.93 94.95 94.88
Latitude ( N)
32.81 32.84 32.90
Focal depth (km)
9.0 19.0 35.0
Nodal plane I
Nodal plane II
Magnitude (Mw)
Strike ( )
Dip ( )
Slip angle ( )
Strike ( )
Dip ( )
Slip angle ( )
45 64 20
67 55 51
64 28 99
174 171 214
35 68 40
136 142 79
5.9 6.0 5.9
seismic surface deformation field for the Zaduo earthquake, as shown in Fig. 2. 2.2. InSAR co-seismic deformation results
Fig. 1. Regional geological setting around the epicenter of the October 17th, 2016 Zaduo Earthquake. The black frame in Fig. 1 represents the limits of the InSAR data. The inset in the upper-right corner is the research zone. The topographic data comes from the shuttle radar topography mission (SRTM) 90 m resolution.
The co-seismic interferometric deformation field (Fig. 2) shows that interference fringes are not correlated with terrain. Therefore, interference from atmospheric phase delay can be eliminated. In addition, there was no error caused by satellite orbit in the interference fringes. Therefore, the deformation field in the interferogram is reliable. Fig. 2 shows that the ascending orbit image from Sentinel-1A completely contains the co-seismic deformation field for this earthquake. The co-seismic deformation field caused by this earthquake was an elliptical subsidence region. As the phase was continuous throughout the interferogram, the deformation characteristics were clear and distinct. The largest subsidence was around 5 cm. The size of the deformation zone was 7 km from north to south, and 8.3 km from east to west. The distribution of interference fringes initially indicated that the earthquake was mainly due to normal fault activity. 3. InSAR co-seismic deformation slip inversion 3.1. OKADA elastic dislocation model inversion
In this study, wideband interferometry SLC IW L1.1 products from the Sentinel-1A satellite were used. High-resolution coseismic surface deformation fields were obtained to study the Zaduo earthquake. These are used in the inversion analysis of this earthquake's fault parameters and slip distribution for an in depth understanding of the characteristics of the region's tectonic activities and focal mechanisms. 2. Acquisition of InSAR co-seismic deformation field 2.1. SAR data and processing methods This research used VV co-polarized C-band images taken before and after the Zaduo earthquake by the Sentinel-1A satellite in ascending orbit (Fig. 1). The images were taken at an interval of 24 days, on September 29th, 2016, and October 23rd, 2016 with a spatial baseline length of 79 m. Swiss GAMMA commercial software was used for differential interference processing of the two SAR images using the two-pass method [12e14]. A 90 m resolution DEM from NASA's SRTM was used to eliminate terrain effects. Satellite orbit information was obtained from precise orbit determination (POD) orbital regression released by the European Space Agency with a position error of 10 cm or less. During InSAR processing, a 2:10 range to azimuth ratio was used for co-registration. A repeated Goldstein wave filtering method was used to increase the signal to noise ratio in the interferogram. During phase unwrapping, the Minimum Cost Flow (MCF) method was used to set the coherence threshold to 0.2 in order to obtain a phaseunwrapped image. After removal of orbital error and flattening, the result was the geocoded, line-of-sight, high-resolution, co-
The Okada model [15] is a function that relates underground fault parameters to surface deformation data. It is mainly used to simulate observed deformation fields from interferograms to estimate fault parameters. To efficiently obtain the characteristics of the co-seismic slip distribution for the Zaduo Ms6.2 earthquake, the co-seismic deformation field was first downsampled using the uniform grid sampling method. As a result, a total of 7737 downsampled data points were obtained. According to the locations of these sample points, the actual angles of incidence and orbital azimuth of the satellite were calculated. Next, a non-linear inversion of the geometric parameters of the fault (longitude, latitude, strike,
Fig. 2. InSAR, line-of-sight, and co-seismic deformation field for the 2016 Ms6.2 Zaduo earthquake. The solid arrow represents the satellite's ground track. The hollow arrow represents the satellite's line-of-sight direction. Negative values represent movement away from the satellite.
Please cite this article in press as: J. Qiu, X. Qiao, A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.04.008
J. Qiu, X. Qiao / Geodesy and Geodynamics xxx (2017) 1e5
dip, slip angle, depth, the fault length, fault width, and amount of slip) was performed using Okada's uniform elastic half-space dislocation model. In order to fit any remaining error from the satellite's orbit, six orbital parameters were added to estimate the orbital error linearly. The iterative Levemberg-Marquardt least squares optimization algorithm was used to solve for the nine geometric parameters and the six orbital parameters in order to provide constraints for the fault dislocation parameters in the distributed slip inversion later on. Through inversion, the geometric parameters of the fault as given in Table 2 were obtained. This table shows that the strike of the fault that caused this earthquake is roughly north-east-east (NEE). Its strike is 72 N and dip is 65 . It is located 5 km below the surface. These data are similar to those provided by CENC for nodal plane 1 of the focal mechanism solution. According to the formula for calculating the earthquake's magnitude, the calculated seismic moment for this earthquake was 9.81 1017N m, and the moment magnitude was Mw6.0. Table 2 contains the seismogenic fault parameters obtained by the inversion of Okada's dislocation model. They can be used to develop and fit the theoretical and residual displacements of the Zaduo earthquake (Fig. 3). Fig. 3 shows that the near field fault deformation fits rather well, but the far field deformation does not. The overall RMS misfit in the fitted residual displacement is 1.8 cm. 3.2. Co-seismic distributed slip inversion While the Okada uniform dislocation model described above can effectively simulate the near-field displacement of the Zaduo earthquake, it cannot simulate far-field displacement very well. To address this, the present study used the geometric parameters of the seismogenic fault obtained using Okada's rectangular dislocation model, combined with the SDM process developed by Wang et al. [16], to obtain detailed information about slip distribution on the fault plane. The principle is to use the gradient descent method to calculate the inversion. The relationship between the InSAR data and the model is:
d ¼ Gm þ ε
(1)
3
where d is the line-of-sight value observed by InSAR, G is Green's Matrix, m is the amount of slip of each sub-fault on the fault plane in the strike direction and in the dip direction, and ε is the error in the observed value. The deformation map obtained from InSAR data, as well as the fault parameters obtained in Table 2 confirm the single fault model with a 30 km strike and a 20 km dip. The fault plane is divided into 30 20 sub-faults, each 1 km 1 km. The dimensions of each subfault is 1 km in the direction of the strike and 1 km in the direction of the dip. The ultimate goal of the inversion was to optimize the model to fit the observations. Therefore, the fault dislocation model required certain additional smoothing conditions.
2
2 FðsÞ ¼ d Gs þ a2 Hs /min
(2)
where the smoothing factor a is a trade-off between the fit of the data and the roughness of the inversion results. The smoothing factor is generally determined by plotting a degree of fit versus roughness curve. H is the Laplace operator, s is the amount of slip in the underground fault plane, d is the value observed at the surface. In the inversion process, the CRUST1.0 model was used to determine the regional crust stratification structure. Fig. 4 is the Zaduo earthquake slip distribution obtained using InSAR data and hierarchical dislocation model inversions. Now, the distributed slip inversion more closely fits both near-field and farfield observed deformation than the uniform slip inversion results. The 1.6 cm error in the residual displacement is also less than the error from the uniform slip model, which was 1.8 cm. Fig. 5 shows the co-seismic slip distribution results. It shows that the rupture zone of the Zaduo earthquake was around 17 km long along the direction of strike with the slip distribution was mainly concentrated at a depth range between 9 km and 15 km along the direction of dip. It was a typical shallow tectonic earthquake. The southwest portion of the rupture was mainly normal slip. The northeast section transitioned to sinistral strike slip. The maximum slip was 0.17 m, located at a depth of 12 km along the direction of dip. Fault dislocation did not reach the surface. The moment magnitude derived from the slip distribution was Mw5.9. This was basically in agreement with the moment magnitudes and wave magnitudes measured by USGS and CENC and was slightly less than the uniform slip inversion result.
Table 2 Geometrical parameters of the seismogenic fault obtained by inversion of Okada's dislocation model. Length (km)
Width (km)
Depth (km)
Strike ( )
Dip ( )
Slip Angle ( )
Longitude ( E)
Latitude ( N)
Magnitude (Mw)
6.9
2.5
4.9
72
65
60
94.855
32.868
6.0
Fig. 3. Okada's rectangular dislocation model fitted interferogram (a) and residual (b).
Please cite this article in press as: J. Qiu, X. Qiao, A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.04.008
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J. Qiu, X. Qiao / Geodesy and Geodynamics xxx (2017) 1e5
Fig. 4. Distributed slip model fitting interferogram (a) and residual (b).
4.2. Seismogenic structure analysis
Fig. 5. Zaduo earthquake co-seismic slip distribution based on inversion results.
4. Conclusions and discussion The Zaduo earthquake occurred on October 17th, 2016 in the alpine region of southern Qinghai. The natural conditions of the area made field observations and geophysical data collection work difficult. In the present study data from the Sentinel-1A satellite were used to carry out DInSAR processing to obtain a high quality co-seismic surface deformation field for this earthquake. Based on this, the size and spatial distribution characteristics of the Zaduo earthquake coseismic deformation were analyzed. In addition, using uniform slip distribution and distributed slip distribution modeling, the geometric parameters of the seismogenic fault and detailed slip distribution characteristics were calculated. 4.1. Epicenter location analysis After the earthquake, CENC, GCMT, and USGS used far-field wave data to calculate both the focus location and focal-mechanism solution for this earthquake. However, due to the scarcity of seismological monitoring stations in this part of Asia and the unevenness of the Earth's crust in the QinghaieTibet Plateau, there were large uncertainties in the focal-mechanism and epicenter location calculations using surface waves (Table 1). Using the location of the center of the co-seismic deformation, as shown in Fig. 2, the epicenter of the Zaduo Ms6.2 earthquake rupture can be roughly determined to be 94.83 E, 32.84 N (Fig. 1). This is 10.6 km from the epicenter location calculated by USGS, 11.4 km from the centroid location calculated by GCMT and 9.1 km from the location calculated by CENC.
InSAR co-seismic deformation (Fig. 2) and dislocation inversion results show that there was not significant strike-slip during this earthquake. The dislocation during this earthquake was predominantly dip-slip, so the Zaduo earthquake was mainly a normal rupture process. Deformations were concentrated 9e15 km below the surface. The maximum slip was 0.17 m and the moment magnitude was Mw5.9. This was basically in agreement with the moment magnitudes and surface-wave magnitudes measured by USGS and CENC. The seismogenic fault was determined to be a sinistral normal fault structure with a 72 strike and a 65 dip. This was similar to the characteristics of nodal plane 1 in the focal mechanism solution provided by CENC. From a larger perspective, this earthquake occurred in the middle of the Qiangtang microplate north of the LongmucuoShuanghu suture belt (Fig. 1). This microplate is a long structure with numerous normal faults and associated seismicity [17]. Jiang Zaisen et al.'s velocity field results, calculated using China's crustal movement network from 1999 to 2004, showed that the strain in the Qiangtang microplate was increasing in a roughly east to west direction [18]. This was consistent with a geological background of south to north normal fault activity [19]. This indicates that normal fault seismic mechanisms were the most important characteristic of seismic activity in the Qiangtang microplate. Acknowledgments The Sentinel-1A SAR data are provided by European Space Agency (ESA) through Sentinels Scientific Data Hub and the SDM inversion program provided by Professor Rongjiang Wang. This study was supported by the National Natural Science Foundation of China (No. 41604015). References [1] J. Sun, Y. Shi, Z. Shen, X. Xu, F. Liang, Parameter inversion of the 1997 Mani earthquake from insar co-seismic deformation field based on linear elastic dislocation modeldII. Slip distribution inversion, Chin. J. Geophys. 50 (5) (2007) 1097e1110 (in Chinese). [2] J. Zhang, L. Zhao, Application of InSAR technology the strong earthquake in Mani, Tibet, J. Tsinghua Univ. 42 (6) (2002) 847e850. [3] J.I. Ling-Yun, R.C. Liu, C.S. Yang, Obtaining deformation of Mani earthquake by three-pass D-InSAR technique, Eng. Surv. Mapp. 18 (2) (2009) 5e8. , Y. Klinger, J. Van der Woerd, P. Tapponnier, [4] C. Lasserre, G. Peltzer, F. Crampe Coseismic deformation of the 2001 Mw ¼ 7.8 Kokoxili earthquake in Tibet, measured by synthetic aperture radar interferometry, J. Geophys. Res. Atmos. 110 (B12) (2005) 1e18.
Please cite this article in press as: J. Qiu, X. Qiao, A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.04.008
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Jiangtao Qiu, Postgraduate of Institute of Seismology, China Earthquake Administration (CEA), Engineer of the Second Monitoring and Application Center, CEA. His main research direction is the application of space geodetic technology in seismic monitoring. Email address: [email protected].
Please cite this article in press as: J. Qiu, X. Qiao, A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.04.008
Contents lists available at ScienceDirect
Geodesy and Geodynamics journal homepages: www.keaipublishing.com/en/journals/geog; http://www.jgg09.com/jweb_ddcl_en/EN/volumn/home.shtml
A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology Jiangtao Qiu a, b, *, Xuejun Qiao a a b
Institute of Seismology, China Earthquake Administration, Wuhan 430071, China The Second Monitoring and Application Center, China Earthquake Administration, Xi'an 710054, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 December 2016 Accepted 21 April 2017 Available online xxx
On October 17th, 2016, a Ms6.2 earthquake occurred in Zaduo County of Qinghai Province, China. The aim of this study is to use synthetic aperture radar (SAR) technology aboard the Sentinel-1A satellite to obtain high-resolution co-seismic surface displacement data and then to confirm the geometric parameters of the fault and slip distribution model. To this end, linear and non-linear inversion algorithms based on an elastic half-space dislocation model were used. The results showed that a distributed slip model can explain the surface deformation field measured by InSAR very well. The surface deformation field caused by the earthquake was an oval-shaped region of subsidence with a maximum displacement of 5 cm along the line of sight of the radar waves. This earthquake was mainly the result of a normal-slip fault process with 72 N strike and 65 dip. The slip was mainly concentrated at depths of 9e15 km. The maximum slip was 0.17 m, located at a depth of 12 km. The moment magnitude given by inversion was Mw5.9. This was basically in agreement with the moment magnitudes and surface magnitudes measured by USGS and CENC. © 2017 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Zaduo earthquake InSAR Slip distribution inversion Seismogenic fault
1. Introduction According to measurements from China's national seismological network, a Ms6.2 earthquake occurred at 15:14 h (Beijing time) on October 17th, 2016 in Zaduo County, Yushu Prefecture, Qinghai (32.81 N, 94.93 E). In this article, this event is referred to as the Zaduo earthquake. After the earthquake, the China Earthquake Networks Center, Harvard University's Global Centroid-MomentTensor (CMT) Project and the United States Geological Survey (USGS), separately used far-field wave information to calculate the location of the focus and the focal mechanism solution of the earthquake (Table 1, Fig. 1). Table 1 shows that CENC and USGS
* Corresponding author. Institute of Seismology, China Earthquake Administration, Wuhan 430071, China. E-mail address: [email protected] (J. Qiu). Peer review under responsibility of Institute of Seismology, China Earthquake Administration.
Production and Hosting by Elsevier on behalf of KeAi
recorded the earthquake with a magnitude of Mw5.9 and GCMT recorded it with a magnitude Mw6.0. Although there were significant differences in the calculated rake, all of them agreed that the fault had the characteristics of a normal fault. The epicenter was located near the Zaduo fault zone, which is an Early-Middle Pleistocene sinistral reverse fault. The earthquake occurred in a sparsely populated alpine region, making geological field surveys and geophysical data acquisition very difficult. Interferometric Synthetic Aperture Radar (InSAR) technology was first applied to seismological deformation in 1993 and since then it has been widely accepted and used in the field. InSAR technology was especially useful in this case because there was little vegetation cover in the QinghaieTibet Plateau and this resulted in good coherence in the co-seismic deformation field over long periods of time. Many strong seismic events have occurred in the QinghaieTibet hinterland since the Mani earthquake of 1997. Both international and domestic researchers have studied the region extensively. InSAR technology has been used to analyze the following earthquakes: 1997, Mani, Ms7.9 [1e3]; 2001, Kunlun Mountain, Ms8.1 [4]; 2004, Zhongba, Ms6.7 [5]; 2005, Zhongba, Ms6.5; 2008, Dangxiong, Ms6.6 [6,7]; Gaize, Ms6.9 [8]; Wuqia, Mw6.7 [9]; and Yutian, Ms7.3 [10]; and 2010, Yushu, Ms7.1 [11].
http://dx.doi.org/10.1016/j.geog.2017.04.008 1674-9847/© 2017 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: J. Qiu, X. Qiao, A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.04.008
2
J. Qiu, X. Qiao / Geodesy and Geodynamics xxx (2017) 1e5
Table 1 Focal parameters of the October 17th, 2016 Zaduo earthquake given by different organizations. Organization
CENC GCMT USGS
Longitude ( E)
94.93 94.95 94.88
Latitude ( N)
32.81 32.84 32.90
Focal depth (km)
9.0 19.0 35.0
Nodal plane I
Nodal plane II
Magnitude (Mw)
Strike ( )
Dip ( )
Slip angle ( )
Strike ( )
Dip ( )
Slip angle ( )
45 64 20
67 55 51
64 28 99
174 171 214
35 68 40
136 142 79
5.9 6.0 5.9
seismic surface deformation field for the Zaduo earthquake, as shown in Fig. 2. 2.2. InSAR co-seismic deformation results
Fig. 1. Regional geological setting around the epicenter of the October 17th, 2016 Zaduo Earthquake. The black frame in Fig. 1 represents the limits of the InSAR data. The inset in the upper-right corner is the research zone. The topographic data comes from the shuttle radar topography mission (SRTM) 90 m resolution.
The co-seismic interferometric deformation field (Fig. 2) shows that interference fringes are not correlated with terrain. Therefore, interference from atmospheric phase delay can be eliminated. In addition, there was no error caused by satellite orbit in the interference fringes. Therefore, the deformation field in the interferogram is reliable. Fig. 2 shows that the ascending orbit image from Sentinel-1A completely contains the co-seismic deformation field for this earthquake. The co-seismic deformation field caused by this earthquake was an elliptical subsidence region. As the phase was continuous throughout the interferogram, the deformation characteristics were clear and distinct. The largest subsidence was around 5 cm. The size of the deformation zone was 7 km from north to south, and 8.3 km from east to west. The distribution of interference fringes initially indicated that the earthquake was mainly due to normal fault activity. 3. InSAR co-seismic deformation slip inversion 3.1. OKADA elastic dislocation model inversion
In this study, wideband interferometry SLC IW L1.1 products from the Sentinel-1A satellite were used. High-resolution coseismic surface deformation fields were obtained to study the Zaduo earthquake. These are used in the inversion analysis of this earthquake's fault parameters and slip distribution for an in depth understanding of the characteristics of the region's tectonic activities and focal mechanisms. 2. Acquisition of InSAR co-seismic deformation field 2.1. SAR data and processing methods This research used VV co-polarized C-band images taken before and after the Zaduo earthquake by the Sentinel-1A satellite in ascending orbit (Fig. 1). The images were taken at an interval of 24 days, on September 29th, 2016, and October 23rd, 2016 with a spatial baseline length of 79 m. Swiss GAMMA commercial software was used for differential interference processing of the two SAR images using the two-pass method [12e14]. A 90 m resolution DEM from NASA's SRTM was used to eliminate terrain effects. Satellite orbit information was obtained from precise orbit determination (POD) orbital regression released by the European Space Agency with a position error of 10 cm or less. During InSAR processing, a 2:10 range to azimuth ratio was used for co-registration. A repeated Goldstein wave filtering method was used to increase the signal to noise ratio in the interferogram. During phase unwrapping, the Minimum Cost Flow (MCF) method was used to set the coherence threshold to 0.2 in order to obtain a phaseunwrapped image. After removal of orbital error and flattening, the result was the geocoded, line-of-sight, high-resolution, co-
The Okada model [15] is a function that relates underground fault parameters to surface deformation data. It is mainly used to simulate observed deformation fields from interferograms to estimate fault parameters. To efficiently obtain the characteristics of the co-seismic slip distribution for the Zaduo Ms6.2 earthquake, the co-seismic deformation field was first downsampled using the uniform grid sampling method. As a result, a total of 7737 downsampled data points were obtained. According to the locations of these sample points, the actual angles of incidence and orbital azimuth of the satellite were calculated. Next, a non-linear inversion of the geometric parameters of the fault (longitude, latitude, strike,
Fig. 2. InSAR, line-of-sight, and co-seismic deformation field for the 2016 Ms6.2 Zaduo earthquake. The solid arrow represents the satellite's ground track. The hollow arrow represents the satellite's line-of-sight direction. Negative values represent movement away from the satellite.
Please cite this article in press as: J. Qiu, X. Qiao, A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.04.008
J. Qiu, X. Qiao / Geodesy and Geodynamics xxx (2017) 1e5
dip, slip angle, depth, the fault length, fault width, and amount of slip) was performed using Okada's uniform elastic half-space dislocation model. In order to fit any remaining error from the satellite's orbit, six orbital parameters were added to estimate the orbital error linearly. The iterative Levemberg-Marquardt least squares optimization algorithm was used to solve for the nine geometric parameters and the six orbital parameters in order to provide constraints for the fault dislocation parameters in the distributed slip inversion later on. Through inversion, the geometric parameters of the fault as given in Table 2 were obtained. This table shows that the strike of the fault that caused this earthquake is roughly north-east-east (NEE). Its strike is 72 N and dip is 65 . It is located 5 km below the surface. These data are similar to those provided by CENC for nodal plane 1 of the focal mechanism solution. According to the formula for calculating the earthquake's magnitude, the calculated seismic moment for this earthquake was 9.81 1017N m, and the moment magnitude was Mw6.0. Table 2 contains the seismogenic fault parameters obtained by the inversion of Okada's dislocation model. They can be used to develop and fit the theoretical and residual displacements of the Zaduo earthquake (Fig. 3). Fig. 3 shows that the near field fault deformation fits rather well, but the far field deformation does not. The overall RMS misfit in the fitted residual displacement is 1.8 cm. 3.2. Co-seismic distributed slip inversion While the Okada uniform dislocation model described above can effectively simulate the near-field displacement of the Zaduo earthquake, it cannot simulate far-field displacement very well. To address this, the present study used the geometric parameters of the seismogenic fault obtained using Okada's rectangular dislocation model, combined with the SDM process developed by Wang et al. [16], to obtain detailed information about slip distribution on the fault plane. The principle is to use the gradient descent method to calculate the inversion. The relationship between the InSAR data and the model is:
d ¼ Gm þ ε
(1)
3
where d is the line-of-sight value observed by InSAR, G is Green's Matrix, m is the amount of slip of each sub-fault on the fault plane in the strike direction and in the dip direction, and ε is the error in the observed value. The deformation map obtained from InSAR data, as well as the fault parameters obtained in Table 2 confirm the single fault model with a 30 km strike and a 20 km dip. The fault plane is divided into 30 20 sub-faults, each 1 km 1 km. The dimensions of each subfault is 1 km in the direction of the strike and 1 km in the direction of the dip. The ultimate goal of the inversion was to optimize the model to fit the observations. Therefore, the fault dislocation model required certain additional smoothing conditions.
2
2 FðsÞ ¼ d Gs þ a2 Hs /min
(2)
where the smoothing factor a is a trade-off between the fit of the data and the roughness of the inversion results. The smoothing factor is generally determined by plotting a degree of fit versus roughness curve. H is the Laplace operator, s is the amount of slip in the underground fault plane, d is the value observed at the surface. In the inversion process, the CRUST1.0 model was used to determine the regional crust stratification structure. Fig. 4 is the Zaduo earthquake slip distribution obtained using InSAR data and hierarchical dislocation model inversions. Now, the distributed slip inversion more closely fits both near-field and farfield observed deformation than the uniform slip inversion results. The 1.6 cm error in the residual displacement is also less than the error from the uniform slip model, which was 1.8 cm. Fig. 5 shows the co-seismic slip distribution results. It shows that the rupture zone of the Zaduo earthquake was around 17 km long along the direction of strike with the slip distribution was mainly concentrated at a depth range between 9 km and 15 km along the direction of dip. It was a typical shallow tectonic earthquake. The southwest portion of the rupture was mainly normal slip. The northeast section transitioned to sinistral strike slip. The maximum slip was 0.17 m, located at a depth of 12 km along the direction of dip. Fault dislocation did not reach the surface. The moment magnitude derived from the slip distribution was Mw5.9. This was basically in agreement with the moment magnitudes and wave magnitudes measured by USGS and CENC and was slightly less than the uniform slip inversion result.
Table 2 Geometrical parameters of the seismogenic fault obtained by inversion of Okada's dislocation model. Length (km)
Width (km)
Depth (km)
Strike ( )
Dip ( )
Slip Angle ( )
Longitude ( E)
Latitude ( N)
Magnitude (Mw)
6.9
2.5
4.9
72
65
60
94.855
32.868
6.0
Fig. 3. Okada's rectangular dislocation model fitted interferogram (a) and residual (b).
Please cite this article in press as: J. Qiu, X. Qiao, A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.04.008
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J. Qiu, X. Qiao / Geodesy and Geodynamics xxx (2017) 1e5
Fig. 4. Distributed slip model fitting interferogram (a) and residual (b).
4.2. Seismogenic structure analysis
Fig. 5. Zaduo earthquake co-seismic slip distribution based on inversion results.
4. Conclusions and discussion The Zaduo earthquake occurred on October 17th, 2016 in the alpine region of southern Qinghai. The natural conditions of the area made field observations and geophysical data collection work difficult. In the present study data from the Sentinel-1A satellite were used to carry out DInSAR processing to obtain a high quality co-seismic surface deformation field for this earthquake. Based on this, the size and spatial distribution characteristics of the Zaduo earthquake coseismic deformation were analyzed. In addition, using uniform slip distribution and distributed slip distribution modeling, the geometric parameters of the seismogenic fault and detailed slip distribution characteristics were calculated. 4.1. Epicenter location analysis After the earthquake, CENC, GCMT, and USGS used far-field wave data to calculate both the focus location and focal-mechanism solution for this earthquake. However, due to the scarcity of seismological monitoring stations in this part of Asia and the unevenness of the Earth's crust in the QinghaieTibet Plateau, there were large uncertainties in the focal-mechanism and epicenter location calculations using surface waves (Table 1). Using the location of the center of the co-seismic deformation, as shown in Fig. 2, the epicenter of the Zaduo Ms6.2 earthquake rupture can be roughly determined to be 94.83 E, 32.84 N (Fig. 1). This is 10.6 km from the epicenter location calculated by USGS, 11.4 km from the centroid location calculated by GCMT and 9.1 km from the location calculated by CENC.
InSAR co-seismic deformation (Fig. 2) and dislocation inversion results show that there was not significant strike-slip during this earthquake. The dislocation during this earthquake was predominantly dip-slip, so the Zaduo earthquake was mainly a normal rupture process. Deformations were concentrated 9e15 km below the surface. The maximum slip was 0.17 m and the moment magnitude was Mw5.9. This was basically in agreement with the moment magnitudes and surface-wave magnitudes measured by USGS and CENC. The seismogenic fault was determined to be a sinistral normal fault structure with a 72 strike and a 65 dip. This was similar to the characteristics of nodal plane 1 in the focal mechanism solution provided by CENC. From a larger perspective, this earthquake occurred in the middle of the Qiangtang microplate north of the LongmucuoShuanghu suture belt (Fig. 1). This microplate is a long structure with numerous normal faults and associated seismicity [17]. Jiang Zaisen et al.'s velocity field results, calculated using China's crustal movement network from 1999 to 2004, showed that the strain in the Qiangtang microplate was increasing in a roughly east to west direction [18]. This was consistent with a geological background of south to north normal fault activity [19]. This indicates that normal fault seismic mechanisms were the most important characteristic of seismic activity in the Qiangtang microplate. Acknowledgments The Sentinel-1A SAR data are provided by European Space Agency (ESA) through Sentinels Scientific Data Hub and the SDM inversion program provided by Professor Rongjiang Wang. This study was supported by the National Natural Science Foundation of China (No. 41604015). References [1] J. Sun, Y. Shi, Z. Shen, X. Xu, F. Liang, Parameter inversion of the 1997 Mani earthquake from insar co-seismic deformation field based on linear elastic dislocation modeldII. Slip distribution inversion, Chin. J. Geophys. 50 (5) (2007) 1097e1110 (in Chinese). [2] J. Zhang, L. Zhao, Application of InSAR technology the strong earthquake in Mani, Tibet, J. Tsinghua Univ. 42 (6) (2002) 847e850. [3] J.I. Ling-Yun, R.C. Liu, C.S. Yang, Obtaining deformation of Mani earthquake by three-pass D-InSAR technique, Eng. Surv. Mapp. 18 (2) (2009) 5e8. , Y. Klinger, J. Van der Woerd, P. Tapponnier, [4] C. Lasserre, G. Peltzer, F. Crampe Coseismic deformation of the 2001 Mw ¼ 7.8 Kokoxili earthquake in Tibet, measured by synthetic aperture radar interferometry, J. Geophys. Res. Atmos. 110 (B12) (2005) 1e18.
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Jiangtao Qiu, Postgraduate of Institute of Seismology, China Earthquake Administration (CEA), Engineer of the Second Monitoring and Application Center, CEA. His main research direction is the application of space geodetic technology in seismic monitoring. Email address: [email protected].
Please cite this article in press as: J. Qiu, X. Qiao, A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.04.008