The Mw 7.9, 12 May 2008 Sichuan earthquake rupture measured by sub-pixel correlation of ALOS PALSAR amplitude images
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB 2010
Received: 9 October 2008
Accepted: 23 May 2009
Published: 26 January 2011
PALSAR L-band spaceborne Synthetic Aperture Radar (SAR) amplitude images are used to map the Sichuan earthquake rupture (China, Mw 7.9, 12 May 2008) and to identify the faults activated by the earthquake. A sub-pixel correlation method is used to retrieve the coseismic displacement field projected into the line of sight of the satellite and the horizontal along-track direction, and to map the surface rupture. The earthquake broke ∼270 km of the Beichuan fault and ∼70 km of the Guanxian fault, with a complex thrust-dextral slip mechanism. Along the southwestern part of the rupture, slip seems to be partitioned into a dextral-dominant component on the Beichuan fault and a thrust-dominant component on the Guanxian fault. Dextral slip may also be dominant at the northeastern tip of the Beichuan ruptured fault. Coseismic surface displacements reach on average 3 to 4 m in both measured directions. The SAR rupture mapping has proven complementary to field studies extending the zone of co-seismic displacements and identifying other possible co-seismic rupture strands.
2. Data Analysis
A SAR system sends radar pulses to the ground and measures the amplitude and the phase of the backscattered signal. Each radar return is sampled at the base of an azimuth/slant range grid so that each echo corresponds to a geographic location, after geometric transformation. If between two radar acquisitions the position of a target changes we can precisely measure its corresponding offset by subpixel correlation analysis (e.g., Michel et al., 1999; Barbot et al., 2008). We use PALSAR amplitude images acquired before and after the Sichuan earthquake and measure the offset field generated by the earthquake in the range and azimuth directions by using the subpixel correlation routine implemented in the GAMMA software (Werner et al., 2005). We calculate offsets based on a 64 pixel window size on PALSAR fine beam single polarisation mode (FBS) data acquired during six parallel overlapping ascending tracks (Table 1).
PALSAR data used in this study.
Pixel size range/azimuth (m)
Figures 2 and 3 present maps of the range and azimuth offsets. We interpret steep offset gradients as the surface expressions of the earthquake rupture. The clearest offsets indicate that the rupture broke a 270 km-long major segment of the Beichuan Fault and a 70 km-long segment of the Guanxian Fault, both trending ∼N55E. It seems that another 10 km-long segment ruptured perpendicularly to the Beichuan and Guanxian Faults at the southwestern end of the Guanxian inferred rupture (grey segment in Fig. 2). Its signature is markedly visible on both offset maps. Our SAR rupture map is confirmed by field observations made by Dong et al. (2008) and Xu et al. (2009) and complements them in inaccessible areas. Further interpreting the SAR offset results, other ruptured fault segments might be evidenced although their signatures are less clear due to noise. These segments are typically inside the Longmen Shan, along the Wenchuan Fault and along the northeastern part of the Guanxian fault (black dotted lines in Figs. 2 and 3).
4. Discussion and Conclusions
Figures 2 and 3 display a complex rupture pattern that propagated on different fault segments. Results in Fig. 4 show the offset values in the range and azimuth direction. Taking into account the rupture strike, the satellite acquisition geometry (inset of Fig. 4) and field observations we can try to constrain the interpretation of the fault slip direction during the Sichuan earthquake based on range and azimuth offset values of Fig. 4. Assuming that the faults ruptured with a reverse dextral mechanism (Dong et al., 2008; Xu et al., 2009), we can state that positive offsets in both range and azimuth represent dextral-dominant slip while negative values in both directions correspond to thrust-dominant slip. With this in mind, we can infer the spatial partitioning of reverse and strike-slip faulting along the Beichuan and Guanxian faults. We can argue that the Beichuan fault ruptured with a complex thrust-dextral mechanism while the Guanxian fault seems to have ruptured as an almost pure thrust. Dextral-reverse slip on segment a of the Beichuan fault becomes partitioned into a dextral dominant component on segment b of the Beichuan fault and thrust-dominant component on segment e of the Guanxian fault (Fig. 4). From segments c to d of the Beichuan fault, slip seems to evolve from dextral-reverse to dextral-dominant. This is not totally in agreement with field data, which show mostly thrust on the Guanxian fault, though equivalent reverse and dextral motion on the Beichuan fault.
In conclusion, sub pixel correlation of ALOS PALSAR amplitude images allowed us to map and measure the co-seismic rupture of the Sichuan earthquake. Our results complement field observations and thus allow a broader view of the rupture extension and segmentation.
The data used in this study were provided by the Japanese Space Agency (JAXA) through the CIEST agreement (Cellule d’Intervention et d’Expertise Scientifique et Technique) and the International Charter on Space and Major Disasters. M de Michele and D. Raucoules are funded by the research programs of BRGM.
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