Jiun-Yee Yen

Determination of shallow water depth using optical satellite images

The Penghu archipelago comprises 64 basaltic volcanic isles lying on the Taiwan Strait between mainland China and Taiwan. The water around and within these isles is shallow and poses considerable difficulty in echo sounding detection for bathymetry. Most existing bathymetry data around such areas are in water depths of greater than 5 m. Therefore, when the water depth is less than 5 m the data tend to be over-extrapolated. In this study, a remote sensing method provides a more effective approach to recording shallow water depths compared to traditional soundings using multitemporal images collected by optical/near-infrared sensors from SPOT satellites. This method employs optical energy reflections to obtain the water depth. In this study, we made several improvements wherein a relative atmosphere correction technique was used to calibrate two images within a similar atmospheric condition. We then compared the satellite images acquired from different dates to obtain the local water attenuation coefficient of sunlight. Finally, we developed a means to estimate the water attenuation coefficient and bottom reflectance which will satisfy the two parameters across the study area. Our results show a high-resolution map of shallow bathymetry for the Penghu archipelago and revealed a maximum depth of about 20 m. This study provides an efficient approach for shallow bathymetry retrieval. Many detailed features revealed by this approach may contribute to further geological research and developments in harbour and coastal engineering.

Deformation and "deformation quiescence" prior to the Chi-Chi earthquake evidenced by DInSAR and groundwater records during 1995-2002 in Central Taiwan

Deformation of Central Taiwan between 1997 and 2002 was investigated using radar interferometry to construct the deformation rate time series before, during, and after the Chi-Chi earthquake. The deformation rate time series was compared to the time series of groundwater level in the monitoring wells of Yuanlin and Tzushan in order to investigate the relation between the geodetic observations and the groundwater levels. The deformation pattern of Central Taiwan generally forms a half-elliptical pattern, with the eastern half of the elliptical shape cut away by the north-south trending Chelungpu fault. Along a selected profile of 10 km near the Chelungpu Fault of Central Taiwan, the differential range change was measured and recalculated into deformation rates. The year-to-year deformation rates near the Chi-Chi earthquake ranged between 1.5 and 3 cm/year in most years except for the years between 1999 and 2000, which showed more than 5 cm/year deformation rates along the profile. Prior to the high deformation rate shown during the coseismic image pair, the deformation rate was low, as shown on the other image pairs. This variation in deformation rate coincides with the recorded trend in the level of groundwater in two monitoring wells in the Yuanlin area. The low deformation rate prior to the Chi-Chi earthquake may—partially—be explained by this lowering in groundwater level.

Deformation field over Western Taiwan Island using satellite InSAR

Ongoing crustal deformations in the active arc-continent collision of Taiwan generate large amount of deformation and therefore pose potential seismic hazards as well as risk to infrastructure on the island. In this paper we used radar image pairs with long time span and large area of coverage in order to investigate deformation over the western Taiwan in large spatial and temporal scale. Pre-seismic, co-seismic, post-seismic, and inter-seismic deformation patterns are spatio-temporally analyzed. These areas of interest are located in the Western Foothills of Taiwan orogeny, which the deformation front of the mountain building is propagating toward. In order to better approach the problem, we classify the crustal deformation into intense and gentle events reflecting different deformation styles. Three case studies of deformation events using DinSAR images will be reported. They are: the co-seismic deformation associated with the Chi-Chi earthquake, uplift of the Tainan area, active deformation of Chinshu fault. Correlation of location and time of these deformations is analyzed to investigate the possible precursory to a major earthquake and better understand dynamic rupture mechanisms.

Temporal and spatial pattern of the 1997 Manyi Earthquake using differential InSAR

Since the beginning of indentation of the Indian Plate toward the Eurasia Plate at round 50 Ma, the convergence of these two continental plates had been changing the surface of our earth. The Indian plate is colliding with Eurasian plate at about 58mm/yr (Bilham et. al 1998); the high strain rate caused by this rapid convergence provides the source to the construction of the Himalaya and the Tibetan plateau. The 1997 Manyi earthquake (Mw=7.5) is one of the largest earthquake occurred in the Tibetan plateau, followed by another even greater seismic events 2001 Kokoxli earthquake (Mw=7.9) in north western part of Qinhai Province. Although the epicenters of these two events are 500 km apart, both events occurred along the Kunlun fault that is trending east-westward and spatially spanned about 1,000 km. The region along the Kunlun fault in northern Tibet serves as a natural laboratory for studying earthquakes and earthquake sequences. By InSAR with ERS radar images, we establish the time series of, to the first order, the deformation rate in the vicinity of the epicenter of 1997 Manyi earthquake. We generate interferograms associated with coseismic deformation to improve our understanding the rupture style and its underlying mechanism. Preliminary investigation of the interferogram revealed that there may be at least two surface ruptures produced during the earthquake.