Geometry and Physical Properties of the Chelungpu Fault, Taiwan, and Their Effect on Fault Rupture

Rupture of the Chelungpu fault during the September 21, 1999, 7.6 Mwearthquake in Taiwan caused a 90-Jr,m-long surface rupture with variable displacement along strike. Analysis of core from two holes drilled through the fault zone, combined with geologic mapping and detailed investigation from three outcrops, define the fault geometry and physical properties of the Chelungpu fault in its northern and southern regions. In the northern region, the fault dips 45-60° east parallel to bedding and consists of a narrow (1-20 cm) core of dark-gray, sheared clay gouge at the base of a 30-50 m zone of increased fracture density that is confined asymmetrically to the hanging wall. Microstructural analysis of the fault gouge indicates the presence of extremely narrow clay zones (50-300 μm thick) that are interpreted as the fault rupture surfaces. Few shear indicators are observed outside of the fault gouge, which implies that slip was localized in the gouge in the northern region. Slip localization along a bed-parallel surface resulted in less high-frequency ground motion and larger displacements during the earthquake than in the southern region. Observations from the southern region indicate that the fault dips 20-30° at the surface and consists of a wide (20- 70 m-thick) zone of sheared, foliated shale with numerous gouge zones. A footwall-ramp geometry juxtaposes 2000-3000 m of flat-lying Quaternary Toukoshan Formation in the footwall with Pliocene and Miocene, east-dipping siltstone and muds tone in the hanging wall. The wide, diffuse fault zone contributed to the lower displacement and higher frequency ground motion in the southern region during the 1999 earthquake. The structure in the northern region is the result of the fault being a very young (ka) fault segment in the hanging wall of an older segment of the Chelungpu fault, buried in the Taichung basin. The fault in the southern region is located on an older (~1 Ma) fault trace. The contrasting fault properties in the different regions are responsible for the variability in strong-motion and displacement observed during the 1999 earthquake

Joint insolation and ice sheet/CO2 forcing on northern china precipitation during pliocene warmth

We demonstrate that the precipitation record on the Chinese Loess Plateau during the middle Piacenzian (3.264–3.025 Ma) has strong 20-kyr precessional cycles, challenging past knowledge about East Asian monsoon variations at the orbital timescales

Geometric Evolution of the Chelungpu Fault, Taiwan: The Mechanics of Shallow Frontal Ramps and Fault Imbrication

The September 21, 1999 Mw=7.6 earthquake in Taiwan ruptured along the north–south-trending, east-dipping Chelungpu thrust fault that bounds the western foothills of the Taiwan fold-and-thrust belt. The near-surface (deep) fault geometry changes from a simple wedge above a footwall ramp in the southern region, to a ramp–flat, bed-parallel geometry in the northern Chelungpu region. Rupture characteristics also varied between the northern and southern regions; higher frequency accelerations but lower velocity shaking was experienced in the southern region compared with the northern region. The hanging-wall flat, bed-parallel geometry of the northern Chelungpu region is the result of recent (~50 ka) fault migration ~1 km eastward away from the Sanyi fault trace. By contrast, the southern part of the Chelungpu fault is confined to its original trace that appears to have been active for much of the Quaternary. We show that this ‘out-of-sequence’ fault migration into the hanging wall in the northern region occurred due to the mechanics of the fault geometry at the leading edge of the thrust sheet where the shallow emergent thrust sheet is elongated due to the near-surface, bed-parallel flat. A simple mechanical model shows how resisting forces in the wedge-shaped hanging wall, which overrides a thick conglomerate sequence in the northern region, exceeds the tectonic load driving the slip of the thrust at the base of the wedge. In response, the fault forms a hinterland imbricate that shortens the hanging wall thrust sheet, reduces the volume of the emergent hanging-wall wedge, and thus reduces the resisting force. The southern region has a simple-wedge geometry that produces resisting forces in the hanging wall, to depths of 4000 m, that are less than or equal to the tectonic driving forces, allowing the thrust sheet to move without internal deformation or imbrication. This investigation of the Chelungpu fault indicates that hindward migration of an individual thrust fault can occur simultaneously with foreland-progression of the fold-and-thrust belt, given a shallow (\u3c4000 \u3em) footwall flat within the emergent thrust

Fault Structure Control on Fault Slip and Ground Motion During the 1999 Rupture of the Chelungpu Fault, Taiwan

The Chelungpu fault, Taiwan, ruptured in a Mw 7.6 earthquake on 21 September 1999, producing a 90-km-long surface rupture. Analysis of core from two holes drilled through the fault zone, combined with geologic mapping and detailed investigation from three outcrops, define the fault geometry and physical properties of the Chelungpu fault in its northern and southern regions. In the northern region the fault dips 45°–60° east, parallel to bedding in both the hanging wall and footwall, and consists of a narrow (1–20 cm) core of dark gray, sheared clay gouge. The gouge is located at the base of a 30- to 50-m zone of increased fracture density confined asymmetrically to the hanging wall. Microstructural analysis of the fault gouge indicates the presence of extremely narrow clay zones (50–300 µm thick) that are interpreted as the fault rupture surfaces. Few shear indicators are observed outside of the fault gouge, implying that slip was localized within the gouge zone. Slip localization along a bed-parallel surface resulted in a narrow gouge zone that produced less high-frequency ground motion and larger displacements (average 8 m) during the earthquake than in the southern region. Displacement in the southern region averaged only 2 m, but ground shaking consisted of large amounts of high-frequency ground motion. The fault in the southern region dips 20°–30° at the surface and consists of a wide (20–70 m thick) zone of sheared, foliated shale with numerous gouge zones. These data demonstrate a potential correlation between fault structure (i.e., gouge width, geometry) and earthquake characteristics such as displacement and ground motion (i.e., acceleration)

Shortening Rate and Holocene Surface Rupture on the Riasi Fault System in the Kashmir Himalaya: Active Thrusting Within the Northwest Himalayan Orogenic Wedge

New mapping demonstrates that active emergent thrust faulting is occurring within the fold-and-thrust belt north of the deformation thrust front in the NW Himalaya. The \u3e60-km-long Riasi fault system is the southeasternmost segment of a seismically active regional fault system that extends more than 200 km stepwise to the southeast from the Balakot-Bagh fault in Pakistan into northwestern India. Two fault strands, the Main Riasi and Frontal Riasi thrusts, dominate the fault system in the study area. The Main Riasi thrust places Precambrian Sirban Formation over folded unconsolidated Quaternary sediments and fluvial terraces. New age data and crosscutting relationships between the Main Riasi thrust and the Quaternary units demonstrate that the Main Riasi thrust accommodated shortening between 100 and 40 ka at rates of 6–7 mm/yr. Deformation shifted to the southern Frontal Riasi thrust splay after ca. 39 ka. Differential uplift of a 14–7 ka terrace yields a range of shortening rates between 3 and 6 mm/yr. Together, shortening across the two strands indicates that a 6–7 mm/yr shortening rate has characterized the Riasi fault system since 100 ka. Geodetic data indicate that an 11–12 mm/yr arc-normal shortening rate characterizes the interseismic strain accumulation across the plate boundary due to India-Tibet convergence. These data combined with rates of other active faults in the Kashmir Himalaya indicate that the Suruin-Mastgarh anticline at the thrust front accounts for the remainder 40%–50% of the convergence not taken up by the Riasi fault system. Active deformation, and therefore earthquake sources, include both internal faults such the Riasi fault system, as well as rupture of the basal décollement (the Main Himalayan thrust) to the thrust front. Limited paleoseismic data from the Riasi fault system, the historical earthquake record of the past 1000 yr, the high strain rates, and partitioning of slip between the Riasi fault system and the thrust front demonstrate that a substantial slip deficit characterizes both structures and highlights the presence of a regionally important seismic gap in the Kashmir Himalaya. Slip deficit, scaling relationships, and a scenario of rupture and slip on the basal décollement (the Main Himalayan thrust) parsed onto either the Riasi fault system or the thrust front, or both, suggests that great earthquakes (Mw \u3e 8) pose an even greater seismic hazard than the Mw 7.6 2005 earthquake on the Balakot-Bagh fault in Pakistan Azad Kashmir

Orbital forcing of Plio-Pleistocene climate variation in a Qaidam Basin lake based on paleomagnetic and evaporite mineralogic analysis

Inland Asian aridification is threatening the global ecological system under continued global warming, requiring a full understanding of its forcing mechanisms. Past geological studies in this region focus mainly on initiation timing and million-year timescale variations in aridification. Few studies address the relationship between aridification and insolation forcing. Here we present a paleomagnetic and evaporite mineralogic study of the 476-m-long Huatugou (HTG) section (ca. 3.9 to 2.1 Ma) from the western Qaidam Basin. The results show that the drying of this part of the basin began by at least 3.9 Ma. Furthermore, we show that the relative content variation of evaporite minerals was dominated by 100-kyr cyclicity during the Pliocene. This is consistent with the result from the Xining area during the late Eocene. These results suggest that mid-latitude lacustrine evaporite minerals in semi-arid to arid regions are particularly sensitive to eccentricity forcing, improving understanding of aridification forcing at orbital timescales

Qaidam Basin and northern Tibetan Plateau as dust sources for the Chinese Loess Plateau and paleoclimatic implications

The Chinese Loess Plateau of central Asia is composed of interbedded loess and paleosol layers, deposited during glacial and interglacial cycles, respectively, during the past similar to 2.5 m.y. Understanding the provenance of loess is fundamental to reconstructing wind patterns during Quaternary glacial periods. We determined and compared U-Pb ages on zircon crystals from Loess Plateau strata and potential source areas. The results indicate that the loess was largely derived from the Qaidam Basin and the northern Tibetan Plateau to the west, both of which exhibit spatially extensive geomorphic landforms indicative of past (interpreted as pre-Holocene) wind erosion and/or deflation by westerly winds. This challenges the current paradigm that the loess of the Chinese Loess Plateau was largely sourced from deserts located to the northwest, as observed in the modern interglacial climate. We propose that during glacial periods, the mean annual positions of the polar jet streams were shifted equatorward, resulting in more southerly tracks for dust-generating storms and suppression of the East Asian monsoon by inhibiting the subtropical jet from shifting northward across the Tibetan Plateau.</p