Creep deformation in the Longitudinal Valley, eastern Taiwan, detected by SAR interferometry

The Longitudinal Valley runs straightly from north to south in eastern Taiwan. The Longitudinal Valley Fault (LVF), an east-dipping reverse fault, lies in the Longitudinal Valley. The LFV is a plate boundary between the Eurasian and the Philippine Sea plates, and a surface thrust creep of the LFV is detected at some places by geodetic observations. We mapped crustal deformation field along the Longitudinal Valley using SAR interferometry for detailed investigation of creeping segments. Interferograms of the Longitudinal Valley area were generated from European Space Agency's ERS-1 and ERS-2 satellite data. We used SAR data obtained from ascending orbit. Ascending data may be more sensitive to detect the creep deformation of the LVF than descending data, because the eastern side of the LVF thrusts over the western side. However, this area is so heavily vegetated that high coherence area is limited on the valley floor and good coherence interfergrams are limited to short time span (within 1 year) and small perpendicular baseline pairs. Therefore we made a stacking image from three highest coherence interferograms spanning 1995-1996. The stacking image shows that a deformation zone lies on the Longitudinal Valley between 23° N and 23.5° N and no displacement offset is detected at the northern and southern end of the valley. Line of sight (LOS) deformation velocity offsets between the blocks on the eastern and western sides of the LVF are about 20 mm/year, which is consistent with other geodetic data. Profiles of LOS displacement show the width of deformation zone varies from 100m to a few kilometers. This means that creeping/locking depth of the LVF varies along the fault, and we will be able to estimate the depth using InSAR data.