InSAR Observations of Ground Displacements for the 2010 Magnitude 6.9 Yushu Earthquake, China

John Elliott, Zhenhong Li, Richard Walters, Barry Parsons

COMET researchers are investigating the ground deformation associated with the recent Yushu earthquake using satellite radar measurements. They intend to model the distribution of subsurface slip in order to improve our understanding of faulting and seismic hazard

Background

On the 13th April 2010, a magnitude 6.9 earthquake struck the relatively remote region of Western China, in the county of Yushu at the eastern end of the Tibetan Plateau at about 4000 m elevation. Despite the low population density, approximately 2200 are thought to have died in this event, with over 10,000 injured. The earthquake occurred on the south-east striking Yushu-Ganzi Fault (Figure 1). This fault is part of a series of major left-lateral faults in the region (including the Xianshuihe Fault to the north and the Jiali Fault to the south) that accommodate the eastward movement of crustal material due to the northward convergence of India with Eurasia.

Figure 1: Fault map of Tibet laid over LANDSAT satellite imagery for the Tibetan Plateau and western China. The strike-slip earthquake is located on the Yushu-Ganzi fault, a SE striking left-lateral strike-slip fault. The focal mechanism for this event is taken from the USGS Body wave solution and indicates near pure strike-slip faulting. Fault locations are taken from a compilation by Taylor and Yin, 2009.

InSAR Observations of Ground Deformation

Interferometric Synthetic Aperture Radar (InSAR) is a satellite based technique which can provide measurements of ground motion in an earthquake. These observations are represented in an interferogram as fringes which mark contours of ground motion in the line-of-sight of the satellite. This enables a much more precise identification of the fault location and extents of slip for the earthquake rupture when compared to the seismological solutions.

Figure 2 is an interferogram formed from a pair of SAR images from the Japanese ALOS satellite. It shows the observed ground deformation in the direction of the satellite (39 degrees from the vertical and looking east) due to the earthquake with a peak motion of 42 cm away from the satellite (red) on the south side of the fault and 32 cm towards the satellite on the north side (blue). The fault rupture strikes NE-SW and is about 70 km long, with a clear discontinuity for 30 km for the eastern half of the rupture indicating the the fault slip reached the surface here. Figure 3 shows the elevation and LANDSAT satellite imagery.

Figure 4 is an interferogram derived from radar images from the European Space Agency’s ENVISAT satellite. It covers the western part of the fault and shows a deformation pattern consistent with that of the ALOS interferogram.

Figure 2: (top) Unwrapped ALOS ascending interferogram covering the fault rupture overlain on hillshaded topography. Peak deformation away from the satellite is 42 cm, and towards the satellite is 32 cm. (bottom) Interferogram wrapped to 11.8 cm contours of deformation. Coloured fringes indicate surface deformation in the satellite line-of-sight. Cycles of colour from blue to red indicate the ground has moved away from the satellite. SAR data from the Japanese Space Agency (JAXA) – see GEO Supersite.

Figure 3: (top) Digital elevation model showing a clear valley cutting through the landscape in a NW-SE direction. (bottom) Landsat Satellite Image (Bands 742).

Figure 4: (top) Unwrapped ENVISAT ascending interferogram covering the western part of the fault rupture overlain on hillshaded topography. Peak deformation away from the satellite is 18 cm, and towards the satellite is 3 cm. (bottom) Interferogram wrapped to 2.8 cm contours of deformation. Coloured fringes indicate surface deformation in the satellite line-of-sight. Cycles of colour from blue to red indicate the ground has moved away from the satellite. SAR data from the European Space Agency (ESA) – see GEO Supersite.

Ground Deformation Profiles

Figure 5 illustrates profiles of line-of sight displacements resolved into the fault parallel direction assuming all the motion is in the horizontal plane parallel to the fault. The profiles are derived from the ALOS interferogram in Figure 2. A clear offset is seen in the eastern portion of the rupture, with displacements equivalent to left-lateral motion of 1.7m. The profile in the central part of the rupture is smooth indicating that the slip remains buried. The most westerly profile suggests possible surface ruptures of up to 50cm.

Figure 5: Fault parallel component of ground deformation profiles taken perpendicular to the strike of the inferred fault trace and assuming all motion is in the  horizontal plane. The ground deformation is greatest for the eastern profile (C) with a clear left-lateral offset of ~1.5m.

GPS velocities relative to Eurasia

GPS measurements across the plateau (Gan et al, 2007) relative to a fixed Eurasian plate (Figure 6) show the northward convergence of India occurring at a rate of 40 mm/yr. Most of this motion is taken up on the main Thrust Fault between India and the Himalayas. However, at the eastern edge of the Tibetan Plateau, crustal material is moving eastwards and the motion is accommodated on a series of left-lateral strike-slip faults (listed north-to-south as the Kunlun, Xianshuihe Yushu and Jiali Faults).

Figure 6: Topographic map of the Tibetan Plateau with GPS vectors marking velocities relative to a fixed Eurasia PS data from Gan et al, 2007).The northward motion of the India plate occurs at approximately 40 mm/yr and the collision for at least the last 25 million years, resulting in the creation of a high 5km plateau over many thousands of kilometres. The line X-X’ indicates the location of the GPS profile shown in Figure 7.

Figure 7: Fault parallel GPS velocities (blue points-left axis) and topographic (grey shade for minimum, mean and maximum elevations-right axis) profiles across the series of strike-slip faults at the western edge of the plateau. The line of profile is donated as X-X’ in Figure 6.

Previous Earthquake Deaths in China and Eastern Asia

China and the region of Eastern Asia has suffered some of the most fatal earthquakes in the last century (Figure 8 and Table 1). The most deadly earthquake was the 1976 earthquake in Tangshan with a reported 255,500 deaths although the actual figure may be 2-3 times larger. More recently the 2008 Sichuan earthquake occurred a few hundred kilometres south-east of this year’s Yushu earthquake resulting in almost 90,000 deaths.

Figure 8: Distribution and scale of earthquake related deaths for 1900-2008 with fatalities greater than 1,000. The area of the circles is proportional to the number of deaths (with the number of thousands killed labelled in the centre) and the colour represents the earthquake magnitude. Earthquakes with a large number of fatalities are clustered around the western, southern and eastern margin of the plateau where large magnitude 8 thrust events occur and are coincident with large population densities. Inset map shows the earthquakes denoted by white circles over topography.

Table 1: Chinese earthquakes with 1,000+ related deaths for the last century to present from the USGS compilation, listed in order by the number of deaths. The total number of casualties listed for these 18 earthquakes is ~0.65 million deaths.The distribution of earthquakes is plotted in Figure 8.