Landslides and catchment sedimentation in the Ta-Chia River influenced by the 1999 Taiwan Chi-Chi earthquake

In the Ta-Chia River catchment, large numbers of landslides were induced by the 1999 Chi-Chi earthquake (M-w = 7.6), during which about 1.5 x 10(6) m(3) of earth was driven from broken slopes. The impact of this earthquake not only increased fracturing of the bedrocks but also changed the river morphology of the Western Foothills area. The main purpose of this study is to investigate the correlation of catchment sedimentation and landslides before and after the 1999 Chi-Chi earthquake. The study comprises two major parts: the preparation of databases of landslide inventory and catchment sedimentation and analyses of the correlation of catchment sedimentation and landslides. A conceptual model is developed to investigate the control factors of catchment sedimentation. Multi-variable analysis was applied to study the impact of landslides, triggered by the typhoons Herb (1996), Toraji (2001) and Mindulle (2004), on sediment production and sediment transportation

Stable isotopic characteristic of Taiwan's precipitation: A case study of western Pacific monsoon region

The stable oxygen and hydrogen isotopic features of precipitation in Taiwan, an island located at the western Pacific monsoon area, are presented from nearly 3,500 samples collected during the past decade for 20 stations. Results demonstrate that moisture sources from diverse air masses with different isotopic signals are the main parameter in controlling the precipitation's isotope characteristics. The air mass from polar continental (Pc) region contributes the precipitation with high deuterium excess values (up to 23%.) and relatively enriched isotope compositions (e.g., -3.2 parts per thousand for delta(18)O) during the winter with prevailing northeasterly monsoon. By contrast, air masses from equatorial maritime (Em) and tropical maritime (Tm) supply the precipitation with low deuterium excess values (as low as about 7 parts per thousand) and more depleted isotope values (e.g., -8.9 parts per thousand and -6.0 parts per thousand for delta(18)O of Tm and Em, respectively) during the summer with prevailing southwesterly monsoon. Thus seasonal differences in terms of delta(18)O, delta D, and deuterium excess values are primarily influenced by the interactions among various precipitation sources. While these various air masses travel through Taiwan, secondary evaporation effects further modify the isotope characteristics of the inland precipitation, such as raindrop evaporation (reduces the deuterium excess of winter precipitation) and Moisture recycling (increases the deuterium excess of summer precipitation). The semi-quantitative estimations in terms of evaluation for changes in the deuterium excess suggest that the raindrop evaporation fractions for winter precipitation range 7% to 15% and the proportions of recycling moisture in summer precipitation are less than 5%. Additionally, the isotopic altitude gradient in terms of delta(18)O for summer precipitation is -0.22 parts per thousand/100 m, greater than -0.17 parts per thousand/100 m of winter precipitation. The greater isotopic gradient in summer can be attributed to a higher temperature vs. altitude gradient relative to winter. The observed spatial and seasonal stable isotopic characteristics in Taiwan's precipitation not only contribute valuable information for regional monsoon research crossing the continent-ocean interface of East Asia, but also can serve as very useful database for local water resources management. (C) 2009 Elsevier B.V. All rights reserved

Post-collisional collapse in the wake of migrating arc-continent collision in the Ilan Basin, Taiwan

The Ilan Basin of northern Taiwan forms the western limit of the Okinawa Trough, where the trough meets the compressional ranges of central Taiwan. Apatite fission-track ages of 1.2 ± 0.5 Ma and 3.5 ± 0.5 Ma, measured north and south of the basin, respectively, indicate faster exhumation rates in the Hsüehshan Range to the north (>1.6 mm/yr) than in the Backbone Range to the south (0.7 mm/yr). Reconstructed subsidence rates along the northern basin margin are also faster than in the south (6–7 compared with 3–5 mm/yr). Global positioning system (GPS) and active seismological data indicate motion of the southern basin margin to the east and southeast. We propose that the Ilan Basin is being formed as a result of extension of northern Taiwan, largely controlled by a major southeast-dipping fault, modeled at ∼30° dip, and mapped as a continuation of the Lishan Fault, a major thrust structure in the Central Ranges. Flexural rigidity of the lithosphere under the basin is low, with elastic thickness ∼3 km. A southwest-migrating collision between the Luzon Arc and southern China, accompanied by subduction polarity reversal in the Ryukyu Trench, has allowed crustal blocks that were previously held in compression between the Eurasian and Philippine Sea plates to move trenchward as they reach the northern end of the collision zone. Subduction polarity reversal permits rapid extension and formation of the Ilan Basin and presumably, at least, the western Okinawa Trough, as a direct consequence of arc-continent collision, not because of independent trench rollback forces. This conceptual model suggests that migrating arc-continent collision causes the rapid formation of deep marginal basins that are then filled by detritus from the adjacent orogen, and that these should be common features in the geologic record