INFLUENCE OF INTERFEROMETRIC BASELINE ON MEASUREMENTS OF SEISMIC DEFORMATION: A CASE STUDY ON THE 1997 MANI,TIBET M7.7 EARTHQUAKE

The interferometric baseline is a vital parameter in the InSAR technique,which determines the correlation between two repeat-pass images and imposes direct effect on the accuracy and reliability of the mapping result. If the baseline is not accurately estimated,the residual phases from the orbit and topography will be left in the expected phase of deformation leading to errors of the final result. In this work,we analyze the influences of the baseline on the reference phase and simulated topography phase,and present several methods of interferometric baseline estimation. Then we study the mapping process of the coseismic and post-seismic deformation of the 1997 Mani,Tibet M7.7 earthquake based on the 8-sence ERS2-SAR data and InSAR.Our attention is focused on comparison of interferograms under varied conditions for baseline estimations,such as rough orbit data,precise orbit data,frequency of interferometric fringes and control points on the ground. The result shows that when the baseline is estimated by rough orbit data,the yielded differential interferograms contain considerable phases of orbit residuals which make fringes dense and deformation enlarged. Thus we must use the precise orbit data for baseline estimation. Sometimes,however,the influence of the orbit cannot be removed completely even if we employ precise orbit data. In this case we should make further corrections,including removing superfluous fringes based on interferometric fringes frequency and baseline correction using the control points on the ground. With these improvements,the resultant coseismic displacement along the fault of the Mani earthquake is 4.5m. The post-seismic deformation by this event is concentrated in a narrow 10～20km-long zone around the fault. The accumulated fault slip 508 days after the main shock reaches at least 5.6m,which continues to grow with time. These analysis results are consistent with the field observations,meaning the improvement method presented in this paper is effective.