**3.1 Yushu earthquake area background and data acquisition**

The Yushu earthquake occurred in the Garzê-Yushu Fault Zone. The fault strike runs in a northwest direction for a length of nearly 500 km, and has a fracture width from 50 to several hundred meters. From analysis of the plate tectonics, it can be concluded that the source of this earthquake was in the Qinghai-Tibetan Plateau, located in the north of the collision zone in the Himalayas, which was formed by the subduction of the Indian plate toward the Eurasian continent. Because of this plate subduction, lateral sliding of the internal blocks of the Qinghai-Tibetan Plateau occurred, which caused the northward shift of the plateau and its internal blocks and finally, the formation of strike-slip fault systems with different scales at the edge of the blocks. Zhang et al. (2010) inverted the moment tensor solution using wave-form data from global stations. From this solution and the background of the fault tectonics, it can be concluded that the fault with a trend of 119° and a dip of 83° was the earthquake rupture. The breaking process was determined based on teleseismic data from the 35 global stations. Two active regions on the fault surface were identified. One was located near the micro-epicenter, and the other was located to the southeast at a distance of 10 to 30 km. The latter had the greater slip, 2.4 m, and was a near vertical sinistral strike-slip fault.

The study uses SAR data including RADARSAT-2 wide-mode data and ALOS PALSAR repeat-pass data. The RADARSAT-2 wide-mode data was acquired on April 21, 2010, with a spatial resolution of 40 meters and an incident angle of 21 degrees. ALOS PALSAR data, including two pre-earthquake and post-earthquake scenes, were acquired on January 15, 2010, and April 17, 2010, respectively. Table 1 shows the PALSAR data parameters for repeat-pass SAR interferometry.


Table 1. PALSAR parameters for SAR interferometry

#### **3.2 The method of extracting earthquake geological characteristics and surface deformation information from SAR data**

Ground-fissuring phenomena are often a reflection of different lithological characteristics. SAR image brightness and texture structure can reflect the degree of fissuring. In addition, radar waves are sensitive to the linear structure (Guo, 1996, 1997), so using SAR imagery can help interpret tectonic information.

Interferometric SAR is an important means of extracting surface deformation because it can measure it precisely in three-dimensional space, including small deformations of the surface, and can achieve high spatial resolution observation of surface changes in large areas. Interferometric SAR images of the same area at different times by SAR sensor were obtained at different time SAR complex images. Then we process SAR images acquired at different times to obtain an interferogram. SAR interferograms show electromagnetic wave

Earth Observation for Earthquake Disaster Monitoring and Assessment 303

1997), based on the SAR image interpretation and existing research results of active tectonic plates (Deng, 2007), four main faults of this area have been interpreted as follows: the main faults I and IV are oriented in a northwest-southeast direction; fault IV developed in the limestone areas of the map; and faults II and III are distributed in an east-west direction. According to the structural composition of the faults and existing active tectonics results, the

Using Doris InSAR data processing software and SRTM3 DEM data with 90 m resolution, the two-pass differential interferometry method was used to process the ALOS PALSAR data. We then get the seismic deformation interference phase image shown in

The radar interferogram clearly shows the spatial distribution of the surface deformation field caused by the Yushu earthquake. The coseismic deformation field within the image is about 82 km long and about 40 km wide along the fault. From the distribution of the interferometric fringes caused by the Yushu seismic deformation field, we can see that the distribution of the coseismic deformation is centered on the Garzê-Yushu fault zone, which is the triggered fault (Figure 4, the main fault I), and is parallel to this fault. From the distribution pattern of the interferometric fringes, we can see that the direction and density of the interferometric phase change are different for the two sides of the fault. From the southernmost point A to the fault direction, the interferometric fringe phase indicates an increasing trend from south to north. To the north of the fault, the interferometric fringe phase shows a decreasing trend from north to south. From the whole interferometric phase distribution, the change in the line of sight is left-lateral, revealing significant seismogenic fault sinistral strike-slip properties. It corresponds with the result of wide swath SAR image

The seismogenic fault is in a northwest-southeast direction. Along the seismogenic fault zone, there are two major areas with large surface deformation, shown as ① in Figure 5(b) and ② in Figure 5(c). Position ① corresponds with the instrumental epicenter calculated by the National Earthquake Network, and ② corresponds with the macroscopic epicenter. From enlarged views of the interferogram of the instrumental epicenter area in Figure 5(b) and macroscopic epicenter regions in Figure 5(c), we can also see that the radar interferometric fringes change intensely around the instrumental epicenter, while the central region of the macroscopic epicenter has an apparent decorrelation due to the large surface deformation. Both of the two regional seismic fault slip dislocations are relatively large, but that of the latter region, which is close to the city of Yushu, will inevitably lead to stronger tremors for the city of Yushu and the surrounding area, where rupture has been the predominant cause of enormous casualties

PALSAR operates in the L-band, and a color change cycle in the interferogram represents 11.8 cm in the line of sight. According to the interferometric fringes analysis, on the north of the fault, the maximum sinking displacement in the line of sight is 11.8×3=35.4 cm. Since the surface near the epicenter was damaged during the earthquake, the coherence of the corresponding region in the two radar images is very low and cannot form effective interferometric fringes. Therefore, it is reasonable to conclude that one fringe remains on

**3.3.2 Yushu earthquake area InSAR deformation extraction analysis result** 

main fault I is a strike-slip sinistral fracture.

Figure 5.

interpretation.

and economic losses.

transmission path length variation from the SAR antenna to the target in two images. Electromagnetic wave transmission path length changes are generally subject to the following three factors: satellite position changes, surface changes, and atmospheric changes. Product by the satellite position changes is terrain interferometry phase, which produced by surface changes is the atmospheric phase. Generally speaking, the SAR interference phase can be expressed as type (5).

$$
\Phi\_{\rm IFG} = \mathcal{W} \left\{ \Phi\_{\rm drop} + \Phi\_{\rm def\phi} + \Phi\_{\rm atm} + \Phi\_{\rm noise} \right\} \tag{5}
$$

where �� � is a phase winding operator, and deformation interference phase in type (5) ���� represents monitoring surface deformation ability of SAR interferometry. In order to obtain the deformation interference phase information, the atmosphere in the mathematical model of SAR interference measurement will be classified as noise signal, which directly considers the SAR interferometry phase as the terrain interferometry phase and deformation interference phase (Rosen, 2000), and then the removal of the terrain interferometry phase can obtain surface deformation information.

#### **3.3 Multi-mode SAR data and the Yushu earthquake area evaluation results 3.3.1 Yushu earthquake area lithology and SAR image fraction analysis results**

To further analyze the regional earthquake geology, wide-swath RADARSAT-2 SAR data were acquired on April 21, 2010, with HH polarization, a spatial resolution of 40 m, and an incident angle of 21°. Combined with geological data, the study area can be divided into A to E regions (Figure 4).Because radar waves are sensitive to the linear structure (Guo, 1996,

Fig. 4. Geological analysis from RADARSAT-2 HH polarization wide-swath SAR imagery. (from Guo et al., 2010b)

transmission path length variation from the SAR antenna to the target in two images. Electromagnetic wave transmission path length changes are generally subject to the following three factors: satellite position changes, surface changes, and atmospheric changes. Product by the satellite position changes is terrain interferometry phase, which produced by surface changes is the atmospheric phase. Generally speaking, the SAR

*IFG W topo defo atm noise* (5)

where �� � is a phase winding operator, and deformation interference phase in type (5) ���� represents monitoring surface deformation ability of SAR interferometry. In order to obtain the deformation interference phase information, the atmosphere in the mathematical model of SAR interference measurement will be classified as noise signal, which directly considers the SAR interferometry phase as the terrain interferometry phase and deformation interference phase (Rosen, 2000), and then the removal of the terrain interferometry phase

To further analyze the regional earthquake geology, wide-swath RADARSAT-2 SAR data were acquired on April 21, 2010, with HH polarization, a spatial resolution of 40 m, and an incident angle of 21°. Combined with geological data, the study area can be divided into A to E regions (Figure 4).Because radar waves are sensitive to the linear structure (Guo, 1996,

Fig. 4. Geological analysis from RADARSAT-2 HH polarization wide-swath SAR imagery.

**3.3 Multi-mode SAR data and the Yushu earthquake area evaluation results 3.3.1 Yushu earthquake area lithology and SAR image fraction analysis results** 

interference phase can be expressed as type (5).

can obtain surface deformation information.

(from Guo et al., 2010b)

1997), based on the SAR image interpretation and existing research results of active tectonic plates (Deng, 2007), four main faults of this area have been interpreted as follows: the main faults I and IV are oriented in a northwest-southeast direction; fault IV developed in the limestone areas of the map; and faults II and III are distributed in an east-west direction. According to the structural composition of the faults and existing active tectonics results, the main fault I is a strike-slip sinistral fracture.
