Ching Weei Lin

Earthquake-triggered increase in sediment delivery from an active mountain belt

In tectonically active mountain belts, earthquake-triggered landslides deliver large amounts of sediment to rivers. We quantify the geomorphic impact of the 1999 Mw 7.6 Chi-Chi earthquake in Taiwan, which triggered >20,000 landslides. Coseismic weakening of substrate material caused increased landsliding during subsequent typhoons. Most coseismic landslides remained confined to hillslopes. Downslope transport of sediment into the channel network occurred during later storms. The sequential processes have led to a factor-of-four increase in unit sediment concentration in rivers draining the epicentral area and increased the magnitude and frequency of hyperpycnal sediment delivery to the ocean. Four years after the earthquake, rates of hillslope mass wasting remain elevated in the epicentral area.

Tectonic evolution of accretionary prism in the arc-continent collision terrane of Taiwan

Abstract The thick sedimentary and meta-sedimentary rocks west of the Eocene-Paleozoic metamorphic basement of Taiwan represent an accretionary prism developed between the Eurasian continent and the Philippine Sea plate. The accretionary prism consists of a subduction wedge in the east and a collision prism in the west. The deep-marine subduction wedge developed during the eastward subduction of the South China Sea oceanic crust since the Early Miocene. In the Central Range, a regional unconformity with mylonite structure occurred between the Miocene deep-marine slates-turbidites and the Paleozoic-Eocene metamorphic basement. The unconformity marks the tectonostratigraphic break between the overlying subduction wedge and the underlying underthrust Eurasian continent. The subduction wedge extends from the western Central Range southwards through the Hengchun Peninsula to the offshore Hengchun Ridge. Subduction of the South China Sea oceanic crust further led to the oblique arc-continent collision starting about 6.5 Ma in northern Taiwan. During the collision, the shallow-marine passive margin and foreland sequences were progressively incorporated to the collision prism by a series of west-vergent thrusts in the Hsuehshan Range and the Western Foothills. The Kaoping Slope west of the subduction wedge of the Hengchun Ridge represents the modern collision prism in the active arc-continent collision zone. The collision prism is juxtaposed against the subduction wedge to the east along the Lishan-Laonung-Hengchun fault, which extends offshore to the 30-km-wide fault zone between the Kaoping Slope and the Hengchun Ridge. Before the onset of the arc-continent collision, the Lishan-Laonung-Hengchun fault developed along the northern part of the proto-Manila trench and acted as the thrust front located to the west of the subduction wedge. At present, the thrust front has migrated southwestward to the west of the collision prism. The arc-continent collision propagated southwards and yielded the time-transgressive deformations from north to south to result a south tapering configuration of Taiwan. This paper is the first to recognize the consistent occurrence of the subduction wedge and collision prism onshore and offshore Taiwan. This allows reconstruction of the tectonic evolution of the accretionary prism during the subduction and collision tectonics of Taiwan.

Impact of Chi-Chi earthquake on the occurrence of landslides and debris flows: Example from the Chenyulan River watershed, Nantou, Taiwan

Abstract The Chenyulan River watershed in central Taiwan was chosen for evaluating the impact of the Chi-Chi earthquake on the occurrence of landslides, and for exploring the initial conditions triggering debris flows. Interpretations of aerial photographs and SPOT images as well as field investigations were used to identify landslide and debris flow occurrences. Precipitation data were then utilized to investigate critical conditions leading to the debris flow. Comparison of five SPOT images taken between June 1996 (before the July 1996 Typhoon Herb) and September 1999 (before the Chi-Chi earthquake) shows that the landslide area increased from 7.1×106 to 7.5×106 m2 during that time. However, by January 2000 (after the Chi-Chi earthquake), the landslide area almost tripled to 20.8×106 m2. Measurement of images taken in March and August 2001 reveals that the landslide area had further expanded to 24.2×106 and 27.5×106 m2, respectively. Significant differences before and after the earthquake are also noticed in (1) the intensity and amount of precipitation required for triggering debris flow, (2) the size of contributing drainage basin in which debris flows occurred, and (3) the frequency of debris flows. After the Chi-Chi earthquake, maximum hourly rainfall intensity and critical accumulated precipitation necessary to initiate debris flow reduced to as low as 1/3 of the pre-earthquake figures. Prior to the Chi-Chi earthquake, most debris flows occurred in gullies with slopes greater than 10°, and in drainage basins larger than 0.1 km2. Conversely, after the earthquake, debris flows were observed even in gullies with effective drainage area smaller than 0.03 km2. Furthermore, before the earthquake, the debris flow recurrence time in the study area was greater than 5 years, whereas six debris flow events have been observed in the 2 years since the earthquake.

Geodynamic processes of Taiwan arc–continent collision and comparison with analogs in Timor, Papua New Guinea, Urals and Corsica

Abstract The Taiwan arc–continent collision involves four geodynamic processes: intra-oceanic subduction; initial arc–continent collision; advanced arc–continent collision; and arc collapse/subduction. These processes now occur simultaneously in 19–24°30′N but have operated sequentially southward since the Late Miocene. Although the geological and geophysical features appear to change progressively from north to south across the island, they are distinct within individual tectonic regimes. Using the present scenario as a basis for comparison, it is suggested that Timor is at the initial arc–continent collision stage, while Papua New Guinea and Urals proceeded to the advanced arc–continent collision stage. However, Corsica represents an even more advanced stage than in Taiwan, where a previously accreted arc has collapsed and been totally removed.

The Climatic Signature of Incised River Meanders

Messy Mountain Meandering Predicting the influence of climate on landscapes is sometimes straightforward; for example, river deposits might grow with increased rainfall because erosion rates and sediment transport increase. However, long-term tectonic processes complicate the geomorphic signatures of more gradual climate-related phenomena that reconfigure landscapes. By correlating a decades-long record of typhoon rainfall in Japan with digital elevation models, Stark et al. (p. 1497) show that climate directly influences the extent of river meandering. When expanded to a larger region of the western North Pacific, this analysis revealed a strong climatic imprint on the landscape of humid mountainous areas. The region-wide analysis also revealed that underlying bedrock strength, as opposed to tectonic uplift, acts as a secondary control. Typhoon frequency and bedrock strength influence river meandering in mountain environments. Climate controls landscape evolution, but quantitative signatures of climatic drivers have yet to be found in topography on a broad scale. Here we describe how a topographic signature of typhoon rainfall is recorded in the meandering of incising mountain rivers in the western North Pacific. Spatially averaged river sinuosity generated from digital elevation data peaks in the typhoon-dominated subtropics, where extreme rainfall and flood events are common, and decreases toward the equatorial tropics and mid-latitudes, where such extremes are rare. Once climatic trends are removed, the primary control on sinuosity is rock weakness. Our results indicate that the weakness of bedrock channel walls and their weakening by heavy rainfall together modulate rates of meander propagation and sinuosity development in incising rivers.

Tectonics of short‐lived intra‐arc basins in the arc‐continent collision terrane of the Coastal Range, eastern Taiwan

The Coastal Range in eastern Taiwan was originated from an oblique collision between the Luzon volcanic arc and Asian continent since the late Neogene. In this collision terrane, two intra-arc basins, the Pliocene Chingpu and Pleistocene Chengkung basins, were developed on the eastern part of the Neogene Chimei and Chengkuangao volcanic islands, respectively, prior to their accretion to eastern Taiwan. The tectonic evolution of these Neogene volcanic islands and associated intra-arc basins is reconstructed by stratigraphic and sedimentological analysis, igneous rock geochemistry, and comparison with observations in modern collision zone in the regions off southeastern Taiwan. In the Coastal Range, the intra-arc basin sequences are 1.5–10 km wide and 40 km long, comparable in size to their modern analogues in the active collision zone. The basin axis trends subparallel to the volcanic ridge. In both basins, deepwater flysch overlies shallow marine reef carbonates, which in turn rest on volcanic basement, indicating rapid arc collapse (minimum rate of 1 km/m.y.) soon after the arc-continent collision. The arc collapse occurred earlier in the north (Chimei, between 5.1 and 3.5 Ma) and later in the south (Chengkuangao, between 2.9 and 1.8 Ma), in concert with a southward propagation of the oblique collision. Sedimentation ended about 2 Ma and 1 Ma in the Chingpu and Chengkung basins, respectively, coeval with rotation of the Neogene volcanic islands. This suggests that the rotation inverted the intra-arc basin into thrusting, uplifting, and final emergence. Thus the duration of sedimentation for the intra-arc basins spanned only about 0.8–3.1 m.y. On the basis of land geology, offshore observations, and a clay model experiment simulating oblique arc-continent collision, a model for the intra-arc basin evolution in eastern Taiwan is proposed. During the collision, strike-slip faults would have been developed in the eastern part of volcanic islands to induce transtension movements, thus forming pull-apart, intra-arc basins on the collapsed volcanic island. This mechanism is believed to be responsible for the formation of the Pliocene Chingpu and Pleistocene Chengkung basins as well as the present-day offshore intra-arc basins found on the Lutao and Lanhsu volcanic islands. The two intra-arc basins on Lutao and Lanhsu are predicted to be short lived. As collision continues, these two basins, together with their underlying northern part of the Luzon arc, will be rotated, thrust, and uplifted in the next 1 m.y. and, finally, will become part of the southern extension of the Coastal Range.

Image processing of FORMOSAT-2 data for monitoring the South Asia tsunami

We report on the actions of the first daily revisit satellite, FORMOSAT‐2, in the recent Indian Ocean tsunami disaster. Starting from the first images of Banda Aceh and Phuket taken on 28 December 2004, FORMOSAT‐2 used its unique orbit and pointable sensor system to demonstrate the extent to which it is able to respond to emergencies. A total of 137 images throughout the Indian Ocean rim countries were taken within a month. The data were immediately analysed and turned into damage‐assessment maps and other information resources for humanitarian aid. This paper focuses on the image‐processing procedure followed for a fast response to the South Asia tsunami event. The imageodesy technique is used to coregister the level‐2 product of FORMOSAT‐2 image at high accuracy and speed. A novel approach for spectral reservation data fusion has also been proposed. With the advantages of accurate coregistration and reliable spectral property, the colour composites of FORMOSAT‐2 imagery have been used as the principle source of information for our tsunami hazard assessment. The potential of FORMOSAT‐2 for disaster monitoring is discussed. The technique developed in this research will be adapted to produce pan sharpened images as a standard value added product of FORMOSAT‐2.

Stratigraphic variation of transport properties and overpressure development in the Western Foothills, Taiwan

[1]xa0Overpressure, fluid pressure higher than hydrostatic pressure, has developed below the middle Miocene formations in the north central Western Foothills of Taiwan. To study the mechanism by which overpressure is generated and maintained in the Taiwan oil fields, we estimated the fluid pressure history and overpressure distribution by using a one-dimensional basin model incorporating laboratory-approximated hydraulic parameters. Transport properties of outcropping sedimentary rocks were measured at effective pressures of 5 to 200 MPa. All parameters showed apparent stratigraphic variation, decreasing with increasing burial depth. Permeability showed the strongest sensitivity to depth, decreasing by 6 orders of magnitude to 10−20 m2 at the bottom of the basin. A critical sealing layer was not identified in the geologic column. The basin model incorporates overburden loading due to sediment accumulation, aquathermal expansion of water, the dehydration reaction of expandable clay to nonexpandable clay, and oil generation. Predicted overpressure was generated dramatically from 3 Ma, when the accumulation rate increased rapidly as a result of tectonic collisions in the area. If we assume a fluid influx from the bottom of the basin, the predicted overpressure is consistent with the observed overpressure, implying that continuous inflow from depth, possibly along the decollement or normal faults, may be the main cause of overpressure generation in this area. Stratigraphic variation of transport properties, which decrease with depth, also influences overpressure trends in the Western Foothills, where overpressure is generated only in deeper horizons. The clay mineral distribution estimated by a kinetic smectite-illite dehydration model is consistent with the observed mineralogical data.

Detecting and Characterizing Active Thrust Fault and Deep-Seated Landslides in Dense Forest Areas of Southern Taiwan Using Airborne LiDAR DEM

Steep topographic reliefs and heavy vegetation severely limit visibility when examining geological structures and surface deformations in the field or when detecting these features with traditional approaches, such as aerial photography and satellite imagery. However, a light detection and ranging (LiDAR)-derived digital elevation model (DEM), which is directly related to the bare ground surface, is successfully employed to map topographic signatures with an appropriate scale and accuracy and facilitates measurements of fine topographic features. This study demonstrates the efficient use of 1-m-resolution LiDAR for tectonic geomorphology in forested areas and to identify a fault, a deep-seated landslide, and the regional cleavage attitude in southern Taiwan. Integrated approaches that use grayscale slope images, openness with a tint color slope visualization, the three-dimensional (3D) perspective of a red relief image map, and a field investigation are employed to identify the aforementioned features. In this study, the previously inferred Meilongshan Fault is confirmed as a NE–SW-trending, eastern dipping thrust with at least a 750 m-wide deformation zone. The site where future paleoseismological studies should be performed has been identified, and someone needs to work further on this site. Signatures of deep-seated landslides, such as double ridges, trenches, main escarpments, and extension cracks, are successfully differentiated in LiDAR DEM images through the use of different visualization techniques. Systematic parallel and continuous lineaments in the images are interpreted as the regional cleavage attitude of cleavage, and a field investigation confirms this interpretation.

Magnitude‐frequency distributions of boundary shear stress along a rapidly eroding bedrock river

[1]xa0The magnitude-frequency distribution of boundary shear stress frames erosion rates in bedrock rivers, but empirical constraints are rare, particularly for extreme floods. Here we present measurements of mean stress τb and its frequency distribution along a fast-eroding river in Taiwan. We construct rating functions of discharge and hydraulic geometry using high-resolution satellite images of flood patterns, river stage time-series, topographic profiles, and post-flood field surveys. The method allows us to assess the spatiotemporal variation in τb along the channel. The boundary shear stress PDF p(τb) has a steep power-law tail, and includes semiannual floods generating τb ≈ 100–200 Pa and 50-year, 3000 m3 s−1 events driving τb ≥ 300 Pa. All such floods contribute to modification of the coarse alluvial cover and erosion of the bedrock bed. Given the steep tail decay in p(τb), the rapid channel incision probably owes more to the exceptional frequency of moderate shear stresses than to the magnitude of the extremes.