And finally, in the sixth chapter, we present our mapping and investigation results of the Longitudinal Valley Shift near the southern end of the valley. The latter three chapters provide important information on the evolution of the Longitudinal Valley fault suture.
Abstract
Introduction
Neotectonic belts and domains of Taiwan
The western belt reflects the anchoring of the Central Range (a continental shelf) on the Eurasian continental shelf. GPS geodesy indicates that the rate of closure of the southern half of the valley is about 40 mm/year [Yu et al., 1997].
The Taiwan orogen as accreting and disarticulating twin sutures
Further north, the similarity of the basement rocks beneath the Ryukyu volcanoes to the rocks of the Central Range [Kizaki, 1986]. Along the eastern neotectonic belt, the suture of the volcanic arc in the Central Range begins near Taitung in southeastern Taiwan.
Implications for future earthquake sources
The western margin of the Ryukyu subduction zone might be expected to generate a surprising variety of large earthquakes given the abundance of right-lateral faults and normal faults there. Finally, rapid uplift of the Central Range may not imply loading of seismic sources within the Central lithosphere.
Concluding Remarks
The blind thrust faults of the western neotectonic belt (in the Chiayi and Miaoli domains) can reasonably be expected to produce earthquakes at least as large as those generated in 1999 by the intermediate Chelungpu thrust. The normal fault on the west side of the Taipei Basin half graben should produce earthquakes less frequently than normal faults near the Lanyang Plain because the stress rates are so much lower throughout the Taipei Basin.
Acknowledgments
Sheu (1992), Marine geology in the arc continent collision zone off southeastern Taiwan: implications for the late Neogene evolution of the coastal zone, Mar. Hesselbo (2003), Cenozoic stratigraphy and subsidence history of the South China Sea margin in the Taiwan region, Basin Res.
Abstract
Introduction
Fluvial surfaces in the northern part of the Peinanshan were clearly skewed to an anticline (in red) and a syncline (in blue). However, we do not claim to have made a complete description of Taiwan's active structures.
Neotectonic overview of Taiwan
Clastic sedimentary rocks of the shallow marine continental shelf make up the western half of the island. The basement rocks and deep marine sedimentary rocks of the fore-arc ridge form the high mountain pillar of the island.
Neotectonic domains of Taiwan
We find little evidence for the activity of the Central Range thrust within the Hualien domain. The termination of seismicity at the western margin of the Wadati-Benioff zone is abrupt. Two more right-lateral faults are present in the eastern part of the Hoping Basin.
Docking of the two terranes begins in the southernmost (Kaoping) domain, and withdrawal occurs in the northern (Ilan and Taipei) domains.
Discussion and earthquake scenarios
The discreteness of the faults and their transition zones may be a reflection of this basic principle. The length of the fault within the Hualien domain yields a hypothetical largest event of Mw 7.2. Rupture of only the onland part of the Chaochou fault could also produce a Mw 7.2 earthquake.
Another important structure in the Kaoping domain is a strike-slip fault along the western margin of the Pingtung Plain.
Conclusions
This is complicated by the fact that the 1951 earthquake rupture extended into this creeping sector [Hsu, 1962; Shyu et al., 2005b]. In conclusion, then, the current state of structural, stratigraphic, rheological, and geodetic knowledge prevents us from providing meaningful mean return time calculations for seismic rupture of major faults within any domain.
Acknowledgments
Liu (1998), A geomorphological study of river terraces in Miaoli Hills (in Chinese with English summary), Geogr. 1974), subsurface geological study of the Miaoli area, Taiwan, Pet. Chang (2000b), The Chi-Chi earthquake fault and structural analysis of the area south of Choshuihsi, central Taiwan (in Chinese with English summary), Spec. 1998), Depositional System of the Maanshan Formation, Hengchun Peninsula (in Chinese), MS 1965), Subsurface Geology of the Hsinchuang Structure in the Taipei Basin, Pet. 1986), a geomorphological study of active faults in Taiwan - especially op.
Liu (1989), Fault creep on the central segment of the Longitudinal Valley fault, eastern Taiwan, Proc.
Abstract
Introduction
The 1951 earthquakes were the result of failure of faults along the Longitudinal Valley Suture (LVS), the eastern of the two sutures. More detailed maps of the active structures of the Longitudinal Valley have recently been produced [e.g. Shyu et al b]. The main question we are trying to address is whether the 1951 events were caused by the main structures of the Longitudinal Valley.
Furthermore, some of the information we obtained during the interviews was vague, uncertain or even contradictory.
Tectonic background of the Longitudinal Valley
The geomorphic manifestation of this shift is clear along most of the valley, but is rather complex, especially along the southern two-thirds. Another important active structure along the southern two-thirds of the valley is a reverse fault that dips westward beneath the eastern flank of the Central Range (Figure 3.2). The most prominent feature of this is the Wuhe Tableland near Rueisuei in the middle of the valley [Shyu et al., 2002;.
Shaded relief map showing active structures in the middle part of the Longitudinal Valley, between the Central and Coast Ranges.
The November 1951 earthquake series
The analyzes by Taiwan Weather Bureau [1952] and Hsu [1980] show that the former was the larger of the two, while the catalog by Lee et al. They also found that the magnitude of the second shock was greater than the first shock (Mw 7.0 vs. Mw 6.2). Moreover, they inferred focal mechanisms using the first motions reported by the Taiwan Weather Bureau [1952] and Hsu [1962] on surface rupture maps.
1996], on the other hand, suggested that sinistral reverse slip on the Chihshang fault produced the first of the two earthquakes and that similar slip on the Yuli fault produced the second earthquake.
Re-evaluation of the November 1951 ruptures
It is quite clear that the Chihshang fault occurred on part of the Longitudinal Valley fault. First, the Yuli Fault crossed the floor of the Longitudinal Valley, while the Rueisuei Fault ran along the eastern edge of the valley. We believe that the Yuli Fault, unlike the Chihshang and Rueisuei Faults, is not part of the Longitudinal Valley Fault.
Thus, we believe that the Yuli fault is completely separate from this part of the Longitudinal Valley fault.
Discussion
His idea was that since the Yuli fault ruptured in 1951, it was probably the main fault of the Longitudinal Valley. A schematic E-W tectonic cross-section of the Longitudinal Valley at the latitude of the Wuhe Tableland. The Yuli fault is a separate, discontinuous strike-slip fault developed in the sediments of the valley, structurally separate from the Longitudinal Valley fault system.
The Chihshang and Rueisuei faults clearly represent seismic failure along the Longitudinal Valley fault segments.
Conclusions
Acknowledgments
Angelier (1994), Non-seismic rupture of the Tapo and the Chinyuan area on the southern segment of the Huatung Longitudinal Valley Fault, Eastern Taiwan (in Chinese with English abstract), Program with Abstracts, 1994 Ann. 2003), Surface Rupture Reevaluation of the 1951 Middle Long Valley Earthquake Sequence and Neotectonic Implications (in Chinese with English summary), M.S. Chen (2003), A two-dimensional dislocation model for interseismic deformation of the Taiwan mountain belt, Earth Planet. Jeng (2001), Continuous monitoring of an active fault in a plate attachment zone: a creep gauge study of the Chihshang fault, eastern Taiwan, Tectonophys.
Millennial rate of slip of a longitudinal valley fault from fluvial terraces: Implications for convergence across the active suture of eastern Taiwan.
Abstract
Introduction
Near the center of the Longitudinal Valley, the Hsiukuluan River cuts through the Coast Range and flows eastward to the Philippine Sea. Along this segment of the river (hereafter referred to as Hsiukuluan canyon) are many levels of flood terrace [e.g. In this article we first review the geological setting of the Coast Range and the Longitudinal Valley.
These rates of rise and their spatial relationships provide information on the geometry and long-term slip rate of the Longitudinal Valley fault.
General geologic setting
The other major fault of the Longitudinal Valley is the Central Range fault, which extends along the western flank of the valley [Biq, 1965; Shyu et al., 2005a, 2005d]. The Coastal Range is the shortened and accreted portion of the Luzon Volcanic Arc and its frontal basin [e.g. Huang et al., 1997; Chang et al., 2001]. The Coastal Range consists of the arc volcanic Tuluanshan Formation and the marine turbidite of the Paliwan Formation.
Between the east-dipping Longitudinal Valley fault (LVF) and the west-dipping Central Range fault (CRF) are fluvial valley sediments.
River terraces of Hsiukuluan canyon
Typical exposures of fluvial terrace deposits along western Hsiukuluan Canyon. a) Photograph and (b) sketch of an outcrop at the CYC location (terrace 4). Detailed maps of the fluvial terraces of western Hsiukuluan Canyon showing sampling sites and strata contacts. Terrace ages are calibrated ages (2σ) in cal BP. a) Map of the Tewu and Monkey Hill loop terraces.
These samples appear to have been collected from the overbank deposits and have younger ages than the ages of the terraces.
Incision and uplift rates along Hsiukuluan canyon
It is more difficult to correctly determine the width of the river bed than the slope of the river. This trend differs from that of the incision rates obtained from the ages of the fluvial terraces (in red). The square is the minimum subsidence rate of the longitudinal valley wall obtained from the KKL site [W.-S.
Reconstruction of the subsurface geometry of the Longitudinal Valley Fault near Hsiukuluan Gorge from bedrock dip angles.
Discussion
Such a thrust would cause about 3 mm/year of subsidence of the floor of the longitudinal valley. Thus, the slip rate calculated from the incision rate is only a partial slip rate of the Longitudinal Valley fault. The longitudinal width difference of the Longitudinal Valley fault thus reflects the ongoing maturation of the seam.
The exceptionally high percentage of the Longitudinal Valley fault also implies that the current fault is a fairly new structure.
Conclusions
If the fault ruptures at similar intervals near the Hsiukuluan Gap, the characteristic rupture of the fault should have co-seismic slip greater than 3.5 m. More studies are needed along the central segment of the fault to better understand the strain accumulation and release there.
Acknowledgments
Liu (1996), Sea-level changes in the past several thousand years, Penghu Islands, Taiwan Strait, Quat. Green (2002), Paleozoic tectonic evolution of the Middle Urals in the light of ESRU seismic experiments, J. Xia (1997), Tectonic evolution of accretionary prisms in the arc-continent collision landscape of Taiwan, Tectonophysics.
Ansorge (1998), Crustal structure of the Middle Urals: results of the (ESRU) Europrobe seismic reflection profiling in the Urals experiments, Tectonics.
