Emergency Geohazard Mapping for Wenchuan Earthquake Disaster in China
Jian Guo Liu(1) and Chuan Tang(2)
(1) Imperial College London, Prince Consort Road, SW7 2AZ, United Kingdom
(2) Chengdu University of Technology, 1 Dongsan Road, 610059, Chengdu, China
A devastating Mw 8.0 earthquake occurred on 12th May 2008 in Wenchuan country in the west edge of Chengdu basin in Sichuan Province, China followed by thousands strong aftershocks lasting more than a month extending from the major epicentre to the north-east direction along the Longmenshan fault zone extended in Gansu and Shanxi provinces in more than 300 km. The earthquake induced wide spread and large scale geohazards in this mountainous region; massive landslides and mudflows destroyed residential areas and infrastructures and blocked rivers threatening dams and cities.
Soon after the earthquake, JAXA released large quantity of ALOS PRISM, AVNIR-2 and PALSAR imagery data. This paper reports the application of these data for geohazards mapping and management in terms of urgent action and recovery planning. The multiple resolution and steering view angle abilities of ALOS sensor systems enable speedy collection of large quantity of badly needed imagery data covering the earthquake disaster region. Image interpretation indicates that simple standard false colour composites of 10 m resolution AVNIR-2 images are the most effective and efficient way to accurately locate and map the major landslides, mudflows and river blockages as these hazards are characterised by massive destruction of vegetation and altering land surface. For assessment of building and infrastructure damages, 2.5 m resolution panchromatic images of PRISM have advantage of revealing the details of structures. The earthquakes were accompanied by bad weather of heavy rain falls and thick clouds; crippling the observation capacity of optical sensors. In this case, the all weather capacity of PALSAR provides the only available observation. However, the interpretation of SAR images for geohazards phenomena is not as easy as optical images because of geometric distortion, speckles and other factors. In the area where InSAR fringe pairs are available, coherence images can be an effective technique to pinpoint disturbed land surface relating to geohazards. A hybrid optical and SAR complementary image integration mapping procedure is in development in conjunction with field investigation for regional geohazards management. The updated results will be reported in the full paper in due course.