Taipei Basin Research Articles

Observations Using the Taipei Basin Broadband Downhole Seismic Network: The 26 December 2006, Pingtung Earthquake Doublet, Taiwan

Observations Using the Taipei Basin Broadband Downhole Seismic Network: The 26 December 2006, Pingtung Earthquake Doublet, Taiwan

Thirty-year land elevation change from subsidence to uplift following the termination of groundwater pumping and its geological implications in the Metropolitan Taipei Basin, Northern Taiwan

Several levelling routes in the metropolitan Taipei Basin have been repeatedly conducted during the past decades, mainly in order to monitor the anthropogenic ground subsidence due to massive pumping of groundwater. We analysed the levelling data released from government and investigated the rate of ground level change from 1975 to 2003, which postdate the massive groundwater exploitation in Taipei area. Based on the contour maps created from the levelling data of 406 benchmarks, the overall subsidence rate in the Taipei Basin gradually decreased since 1975, and around 1989 the basin switched to slight uplift throughout a large part of the basin. Three mechanisms are proposed to be responsible for the observed land elevation changes, including shallow soil compaction, deformation within aquifer, and tectonic subsidence. The trend of the ground level change in 1975–2003 essentially demonstrated the effects of natural recharge to previously depleted aquifers, and is explained by the hydro-mechanical coupling of aquifer materials, i.e., elastic rebound, to the rising piezometric level. The rate of shallow soil compaction is estimated about 1–8 mm/yr throughout the basin according primarily to the shallow clay thickness. Asymmetric tectonic subsidence related to the Shanchiao Fault was estimated to be 1.75 mm/yr and 0.9 mm/yr in the western part and the central part of the basin, respectively. By subtracting the components of the soil compaction and tectonic subsidence from the surface land elevation change, the rebound of aquifer strata was estimated to be about 6.7 cm and 16 cm in western margin and Central Taipei, respectively. The amount of rebound is approximately 10% in magnitude comparing to the amount of previous anthropogenic subsidence due to massive groundwater pumping, totally about 2 m.

Enhancement of Afternoon Thunderstorm Activity by Urbanization in a Valley: Taipei

AbstractLocated in northern Taiwan, Taipei is a metropolis surrounded by hills and mountains that form a basin in which two river valleys funnel the surface airflow of this basin to the open sea. Because of the southwest monsoon, summer is a dry season in northern Taiwan but is the season of maximum rainfall in the Taipei basin. This unusual summer rainfall maximum in Taipei is largely produced by afternoon/evening thunderstorms—in particular, on the downwind side and slopes of mountains south of the city. The population in the city of Taipei and the county in which this city is located has more than tripled during the past four decades while land use for building and surface construction increased by a factor of 3. This urbanization may contribute to an increase of 1.5°C in daily mean temperature, a decrease of 1°C in daily temperature range, an increase of more than 67% in the frequency of afternoon/evening thunderstorms, and an increase of 77% in rainfall generated by thunderstorms. These findings may explain the reduction in the water supply deficit to the Taipei metropolitan area and the ground subsidence of the Taipei basin caused by the excessive use of groundwater. Results of this study also provide important information for urban planning and pollution control and for management of the increasing traffic hazards caused by the enhanced thunderstorm activity and rainfall.

Design lateral forces of buildings with and without base isolation system between MOI‐2005 and NEHRP‐2003

The new provision for seismic design of fixed‐base and seismically isolated buildings in Taiwan was put into practice on July 1, 2005. The purpose of this paper is to compare the design forces for seismic design of buildings equipped with and without (fixed‐base buildings) isolation systems in Taiwan. It is shown from the study that for site classes II, III and the Taipei Basin, the design force for isolated buildings is obviously larger than that for fixed‐base buildings. To lower the design force of isolated buildings, It is suggested that the inelastic‐deformation capability of the structure above the isolation system be slightly considered.

Compositions and flux of soil gas in Liu-Huang-Ku hydrothermal area, northern Taiwan

Soil CO 2 flux has been considered a useful proxy to remotely monitor volcanic activity in a hydrothermal area within a safe distance. The Liu-Huang-Ku (LHK) area, which is close to the Taipei basin and many existing active fumaroles and hot springs, was chosen for the first systematic soil gas study in a hydrothermal area of Taiwan in 2004 and 2006. The soil CO 2 flux was measured by the closed-chamber method with a non-dispersive infrared detector. Soil CO 2 emission rate was estimated 19.8 ± 0.2 t day − 1 and 582 t km − 2 day − 1 in 2004, and 22.4 ± 0.2 t day − 1 and 659 t km − 2 day − 1 in 2006, respectively. These values are close to those reported for other active high CO 2 flux hydrothermal areas of the world. It implies that magmatic gases are actively degassing from subsurface in northern Taiwan. Helium and carbon isotopic data reveal that soil gas in LHK is mainly derived from the mantle and is mixed with a small amount of crustal and air components. The abundance of soil CO 2 shows a positive correlation with the 3He/ 4He ratios and total sulfur contents, which are considered as magmatic-sensitive indicators for the fumarolic samples. Therefore, soil CO 2 could be used as a good proxy for future monitoring of magmatic activity in this area.

CULTURAL RESOURCE MANAGEMENT OF TAIPEI'S INDIGENOUS FOLK TEMPLES

Culturally preserving the historical, cultual, and architectural legacy of Chinese folk temples such as Lung-shan Ssu, Pao-an Kung and Hsing-t'ien Kung remains the provenance of the masses of devoted followers of these temples. But government, scholars, government tourist agencies and non-government organizations (NGOs) can also assist in their preservation. Indigenous folk temples in Taipei represent the religious and historical conflation of influences from the great tradition (Redfield 1953) of the literati/gentry of Chinese settlers to the island and influences from the little tradition of the common people. The Han settlers who came to Taiwan did not come to an uninhabitated area. Austronesian-speaking peoples inhabited the land, such as the pingpu peoples who inhabited the Taipei basin, and eventually intermarried with the Han population and assisted them in their tasks of construction. In the rigors of early settlement to the island of Taiwan, pioneers depended on ling or magical influence and the efficacy of local gods brought with them from the mainland to protect them. Having lost their lineage roots in immigration, folk temples became the source of social solidarity among the Taiwanese settlers and a source of conflict between them, while such folk temples were associated with a Chinese mainland place of origin. As the settlers increased, the literati/gentry immigrated to the island bringing concepts of jen or humanity and propriety in religious rites and rituals that were absorbed by the folk temples of the common people. Lung-shan Ssu, a Buddhist temple or monastery, and Pao-an Kung, a Taoist temple, were built during the Ch'ing dynasty, yet having survived the rigors of Japanese occupation and having developed syncretic forms of their religion to survive, remain popular today. A very popular 20th century folk temple is Hsing-t'ien Kung which represents the aspirations of Taiwanese in a burgeoning industrializing society, such aspirations being reflected in the syncretic origins and practices of the temple.

Lahars in and around the Taipei basin: Implications for the activity of the Shanchiao fault

In the last decade, more than 21 deep geological cores have been drilled in the Taipei basin to obtain a firmer grasp of its basic geology and engineering properties prior to the construction of new infrastructure. Thirteen of those cores contain lahar deposits, with the number of layers varying from one to three and the thickness of each layer varying from several to over 100 m. Based on their occurrence, petrology and geochemistry, it has been determined that the deposits originated from the southern slope of the Tatun Volcano Group (TVG). K–Ar age dating has shown that the lower layer of lahars was deposited less than 0.4 Ma, and this is clearly correlated to outcrops in the Kauntu, Chengtzeliao and Shihtzutao areas. These findings may well suggest that the Taipei basin has been formed in last 0.4 Ma and that the Shanchiao normal fault commenced its activity within this period. The surface trace and the activity of the Shanchiao normal fault have also been inferred and subsequently defined from stratigraphic data derived from these cores.

Prehistoric earthquakes along the Shanchiao fault, Taipei Basin, northern Taiwan

Stratigraphic relations recorded in boreholes drilled along the Shanchiao fault, a normal fault that bounds the western margin of metropolitan Taipei, reveal evidence of early–mid Holocene subsidence and active extension of the Taipei Basin. Paleoseismic data suggest that the maximum displacement for each subsidence event ranges from 2.3 to 4.5 m. Despite large uncertainties, the large offsets imply earthquake magnitudes that range in size from 6.9 to 7.1. Our results suggest that large magnitude earthquakes along the Shanchiao fault are one of the major seismic hazards in the metropolitan Taipei area.

Paleo-environmental evolution as revealed by analysis of organic carbon and nitrogen: A case of coastal Taipei Basin in Northern Taiwan

In an attempt to discriminate different sources of organic matter and reconstruct the paleo-environment of the Taipei Basin, northern Taiwan over the past 250,000 years, organic carbon and nitrogen content (C/N ratios) and isotope composition (δ13C and δ15N) are measured for sediment samples collected from core Wu-ku drilled at the depocenter in western Taipei Basin. The high δ13C values (up to -17.0‰) and high C/N ratio (up to 14.9 and 13.0, respectively) indicate that this area underwent a period of relative aridity during MIS 2 and mid MIS 6. Another two periods of high and variable δ15N values (up to 15.5 and 13.5‰, respectively) in MIS 1 and MIS 7 represent different nutrient condition in the basin. In MIS 1, high δ15N variability along with increasing TOC may reflect an unstable trend of aquatic productivity related to the dynamic mixing ratios of seawater and fresh water. The high δ15N values observed in the sediments of MIS 7 could be explained by the employment of a 15N-enriched nitrate from denitrification as the main source of nitrogen for primary producers.

The continuous field measurements of soluble aerosol compositions at the Taipei Aerosol Supersite, Taiwan

The characteristics of ambient aerosols, affected by solar radiation, relative humidity, wind speed, wind direction, and gas–aerosol interaction, changed rapidly at different spatial and temporal scales. In Taipei Basin, dense traffic emissions and sufficient solar radiation for typical summer days favored the formation of secondary aerosols. In winter, the air quality in Taipei Basin was usually affected by the Asian continental outflows due to the long-range transport of pollutants carried by the winter monsoon. The conventional filter-based method needs a long time for collecting aerosols and analyzing compositions, which cannot provide high time-resolution data to investigate aerosol sources, atmospheric transformation processes, and health effects. In this work, the in situ ion chromatograph (IC) system was developed to provide 15-min time-resolution data of nine soluble inorganic species (Cl −, NO 2 −, NO 3 −, SO 4 2−, Na +, NH 4 +, K +, Mg 2+ and Ca 2+). Over 89% of all particles larger than approximately 0.056 μm were collected by the in situ IC system. The in situ IC system is estimated to have a limit of detection lower than 0.3 μg m −3 for the various ambient ionic components. Depending on the hourly measurements, the pollutant events with high aerosol concentrations in Taipei Basin were associated with the local traffic emission in rush hour, the accumulation of pollutants in the stagnant atmosphere, the emission of industrial pollutants from the nearby factories, the photochemical secondary aerosol formation, and the long-range transport of pollutants from Asian outflows.

Chemical compositions and radiative properties of dust and anthropogenic air masses study in Taipei Basin, Taiwan, during spring of 2004

Asia is one of the major sources of not only mineral dust but also anthropogenic aerosols. Continental air masses associated with the East Asian winter monsoon always contain high contents of mineral dust and anthropogenic species and transported southeastward to Taiwan, which have significant influences on global atmospheric radiation transfer directly by scattering and absorbing solar radiation in each spring. However, few measurements for the long-range transported aerosol and its optical properties were announced in this area, between the Western Pacific and the southeastern coast of Mainland China. The overall objective of this work is to quantify the optical characteristics of different aerosol types in the Eastern Asian. In order to achieve this objective, meteorological parameters, concentrations of PM 10 and its soluble species, and optical property of atmospheric scattering coefficients were measured continuously with 1 h time-resolved from 11 February to 7 April 2004 in Taipei Basin (25°00′N, 121°32′E). In this work, the dramatic changes of meteorological parameters such as temperature and winds were used to determine the influenced period of each air mass. Continental, strong continental, marine, and stagnant air masses defined by the back-trajectory analysis and local meteorology were further characterized as long-range transport pollution, dust, clean marine, and local pollution aerosols, respectively, according to the diagnostic ratios. The aerosol mass scattering efficiency of continental pollution, dust, clean marine, and local pollution aerosols were ranged from 1.3 to 1.6, 0.7 to 1.0, 1.4 and 1.4 to 2.3 m 2 g −1, respectively. Overall, there are two distinct populations of aerosol mass scattering efficiencies, one for an aerosol chemical composition dominated by dust (<1.0 m 2 g −1) and the other for an aerosol chemical composition dominated by anthropogenic pollutants (1.3–2.3 m 2 g −1), which were similar to the previous measurements with high degree of temporal resolution.

Factors controlling temporal and spatial variations of atmospheric deposition of 7Be and 210Pb in northern Taiwan

Fluxes of 7Be and 210Pb monitored at two contrasting sites in northern Taiwan were studied along with meteorological data to elucidate factors controlling their temporal and spatial variations. The 9 year time series of both nuclides at Nankang in the Taipei Basin show dependence on wet precipitation and follow an annual cycle regulated by typhoons, monsoons, and mei‐yü. Superimposed on the annual cycle are interannual variation caused by El Niño–Southern Oscillation and intraseasonal oscillations due to the passage or invasion of fronts, cold surges, dust storms, and the Pacific high‐pressure system. The observed 7Be/210Pb ratios are indicative of the source regions of the fallout nuclides, with higher ratios from high‐altitude rain and lower ratios from dry fallout and low‐altitude rain. The 2 year time series at Yangminshan shows that 7Be and 210Pb fluxes at the mountainous site are in phase with those at Nankang. However, nuclide fluxes at Yangminshan are 4–5 times those at Nankang. After combining results from this and other ancillary studies at Yangminshan, we suggest that (1) in applying fallout nuclides to study the Earth's surface processes, the assumption of constant flux should be made judicially according to the timescales involved, and that (2) the application of fallout nuclides to study soil erosion in hilly areas must consider not only control by wet and dry precipitation but also by direct contact and interaction of clouds with soils enhancing nuclide fluxes.

Source identifications of PM10 aerosols depending on hourly measurements of soluble components characterization among different events in Taipei Basin during spring season of 2004

Continuous measurements of hourly PM10 soluble ions were performed by the in situ IC technology in order to assess the impact of Asian outflows on local air quality. The intensive aerosol observation was carried in Taipei from 11 February 22:00 to 7 April 19:00, 2004. Concentrations of the water-soluble ions (Cl −, NO 2 - , NO 3 - , SO 4 2 - , Na +, NH 4 + , K +, and Ca 2+) were measured in a total of 3300 samples. The characteristics of air pollutant events in Taipei Basin were classified as frontal dust, dust, northeast monsoon, south wind and sea/land breeze according to the hourly meteorology and air pollutant concentrations. Factor analysis was conducted based on hourly data for 13 variables to find the group of variables with similar behavior. According to the source characteristics of high loading species, the possible sources of PM10 aerosols in each group were identified. Three to four factors were identified for each event. The total variances of frontal dust, dust, northeast trade, south wind, and sea/land breeze events were explained about 85%, 86%, 76%, 77%, and 80%, respectively, indicating that the identified factors were satisfactory.

A Summertime Severe Weather Event Occurring in the Taipei Basin

A Summertime Severe Weather Event Occurring in the Taipei Basin

Seismicity increase after the construction of the world's tallest building: An active blind fault beneath the Taipei 101

Seismic activity began to slightly increase during the construction but rose sharply upon the completion of the tallest building in the world, the Taipei 101, which stands at 508 m in the Taipei basin, where local seismicity had historically very low. Besides an increase in both seismic energy and the number of micro‐earthquakes, two felt earthquakes astonishingly occurred beneath the completed building. The focal mechanism of the larger felt earthquake and its aftershocks are indicative of an active blind normal fault just beneath the building. Estimations of the vertical loading of the Taipei 101 show that local stress on its foundation increased at least 4.7 bars, making it seem likely that the increased seismicity was a direct product of the loading of the mega‐structure. Further investigations in unison with continuous seismic monitoring must be conducted because the safety of the high‐rise building in the Taipei basin be comprehensively assessed.

Distribution of the East China Sea continental shelf basins and depths of magnetic sources

Abstract The acoustic basement map of the East China Sea, established by the Shanghai Offshore Petroleum Bureau with all available industry seismic data, shows the existence of a 30-km-wide, 10-km-deep basin, that we named the Ho Basin. The Ho Basin belongs to a series of elongated deep basins extending over 600 km east of the Taiwan-Sinzi Ridge and flanked to the East by a ridge named the Longwan Ridge in its northern part. This new system of basin and ridge was probably formed during middle Miocene, sometimes in between rifting episodes occurring in the Taipei Basin and Okinawa Trough. It complements the already defined system of five belts of backarc basins and associated arc volcanic ridges in the East China Sea, which are progressively younger from the Mainland China shoreline (late Cretaceous/early Tertiary) to the Okinawa Trough (Present). In order to determine the crustal thickness beneath the East China Sea continental shelf, we used a power spectrum method to calculate the depth of the top (Zt) and the centroid (Zo) of the magnetic basement by fitting a straight line through the high- and low-wave number portions of the power spectrum, respectively. Then, the depth of the base (Zb) is estimated from Zt and Zo. After optimizing the size of the data squares, we demonstrate that, except for basins more than 10 km deep, Zt corresponds to the basement depths and Zb, the depth of the Curie point, to the Moho depth. As wide-angle reflection and refraction data are scarce in the East China Sea, this method provides a way to characterize the crustal thickness of the East China Sea and to compute the theoretical heat flow values.

Attenuation Relationships of Peak Ground Acceleration and Velocity for Crustal Earthquakes in Taiwan

Strong seismic ground-motion data obtained by the Taiwan Strong Motion Instrumentation Program (tsmip) and Central Mountain Strong Motion Array (cmsma) are used to derive new attenuation relationships for the vertical and horizontal peak ground acceleration (pga) and peak ground velocity (pgv) for crustal earthquakes in Taiwan. More than 7900 three-component accelerograms recorded from 51 crustal earthquakes in Taiwan, with M w magnitudes ranging from 4.0 to 7.1, have been analyzed to study the dependence of peak ground motion parameters on magnitude, distance, regional and local site effects, through attenuation relationships. We first found that, for both pga and pgv, the attenuation relationships decay faster with distance for the vertical component than for the horizontal component. Also, the attenuation relationships decay faster with distance for the vertical pga than for the vertical pgv. We further compared the attenuation relationships for three subregions (chy, iwa, and nto) and the whole Taiwan region (twn). It is found that the chy area has higher ground motion, either in pga or pgv, than the other areas, especially at near-source distances. This is because the chy area is located on a thick, recent alluvial plain. Comparison of our new attenuation relationships with strong-motion data from the 1999 Chi-Chi earthquake ( M w 7.7) and the 2003 Cheng-Kung earthquake ( M w 6.8) shows that the attenuation relationships developed for M w ≤7 can be extrapolated to make reasonable estimates of strong motion from larger M w ≥7 earthquakes. Finally, we analyzed the residuals to investigate variations of pga and pgv with respect to site conditions. The results show that (1) the residual contour maps, especially for the pgv, have high consistency with the regional geology and topography of Taiwan. (2) The pgv residual contours reveal that Taipei Basin, Changhua Plain, Chianan Plain, Pingtung Valley, Ilan Plain, and Taitung Longitudinal Valley have high residual values. Note that most major metropolitan areas all fall in high residual areas. (3) The site classification based on geologic criteria by Lee et al. (2001) can be simplified into three classes, that is, class E for soft soils, a combination of classes C and D for dense and stiff soils, and class B for rocks. These three site classes are mostly distributed, respectively, in the alluvial plains at an elevation less than 50 m, in terraces and hills at an elevation less than 1000 m, and in high mountainous areas.

Strong Ground Motion in the Taipei Basin from the 1999 Chi-Chi, Taiwan, Earthquake

The Taipei basin, located in northwest Taiwan about 160 km from the epicenter of the Chi-Chi earthquake, is a shallow, triangular-shaped basin filled with low-velocity fluvial deposits. There is a strong velocity contrast across the basement interface of about 600 m/sec at a depth of about 600–700 m in the deeper section of the basin, suggesting that ground motion should be amplified at sites in the basin. In this article, the ground-motion recordings are analyzed to determine the effect of the basin both in terms of amplifications expected from a 1D model of the sediments in the basin and in terms of the 3D structure of the basin. Residuals determined for peak acceleration from attenuation curves are more positive (amplified) in the basin (average of 5.3 cm/sec 2 compared to −24.2 cm/sec 2 for those stations outside the basin and between 75 and 110 km from the surface projection of the faulted area, a 40% increase in peak ground acceleration). Residuals for peak velocity are also significantly more positive at stations in the basin (31.8 cm/sec compared to 20.0 cm/sec out). The correlation of peak motion with depth to basement, while minor in peak acceleration, is stronger in the peak velocities. Record sections of ground motion from stations in and around the Taipei basin show that the largest long-period arrival, which is coherent across the region, is strongest on the vertical component and has a period of about 10–12 sec. This phase appears to be a Rayleigh wave, probably associated with rupture at the north end of the Chelungpu fault. Records of strong motion from stations in and near the basin have an additional, higher frequency signal: nearest the deepest point in the basin, the signal is characterized by frequencies of about 0.3 – 0.4 Hz. These frequencies are close to simple predictions using horizontal layers and the velocity structure of the basin. Polarizations of the S wave are mostly coherent across the array, although there are significant differences along the northwest edge that may indicate large strains across that edge of the basin. The length of each record after the main S wave are all longer at basin stations compared to those outside. This increase in duration of ground shaking is probably caused by amplification of ground motion at basin stations, although coda Q (0.67 – 1.30 Hz) is slightly larger inside the basin compared to those at local stations outside the basin. Durations correlate with depth to basement. These motions are in the range that can induce damage in buildings and may have contributed to the structural collapse of multistory buildings in the Taipei basin.

DOBSON: a Pentium-based SMP Linux PC Beowulf for distributed-memory high resolution environment modelling

With the increasing computational speed of PC processors, lowering hardware prices, and the increasing accessibility of open source software, PC clusters have become an attractive option for exploring budgetary high performance computation on high resolution environment modelling. This paper documents the implementation and operation of a Beowulf-class PC cluster called DOBSON built in the Atmospheric Modelling Laboratory at National Central University in Taiwan. Based on our current configuration, the DOBSON cluster achieves a 4.4–8.7 times of speedup with 8 and 14 CPUs, respectively, compared with a 1 CPU in simulations of the Antarctic vortex. The potential impacts of running a high resolution environmental modelling system on the DOBSON cluster were examined by case simulations of Typhoon Haiyan with a nested five-domain configuration. These results show that the complicated topographical flow in the Taipei Basin can only be properly resolved when model grid resolutions less than 9 km were used.

An Overview of the 1999 Chi-Chi, Taiwan, Earthquake

The Chi-Chi earthquake was the largest onland earthquake to occur in Taiwan in the twentieth century. It inflicted severe damage in central western Taiwan: the excited strong shaking projected impact at cities as far as 150 km away and destroyed several high-rise buildings in the Taipei basin. Having a very complex source, the Chi-Chi earthquake ruptured the 100-km-long Chelungpu fault in a series of jumping dislocations that did not follow a process commonly assumed for an orderly propagating rupture. Furthermore, the rupture developed over a surface that was by no means planar. Principally a N-S-trending thrust of shallow (30°) dip to the east, the northern end of the ruptured Chelungpu fault turned into a more easterly trending rupture surface with an oblique-slip motion. The recently completed Taiwan Strong-Motion Instrumentation Program (TSMIP) with more than 600 modern digital instruments scored the historically largest and most significant strong-motion data recovery. In the meantime, the upgraded Taiwan Central Weather Bureau Seismic Network (CWBSN), all with digital telemetered stations each having six components in both high- and low-gain operations, electronically issued earthquake information (hypocenter, magnitude, and isoseismal map) within minutes of the mainshock to all pertinent emergency management agencies. This rapid reporting significantly improved timely emergency response and effective dispatching of rescue missions. Based on these local data, as well as on GPS, leveling, and geological ground truth observations of the surface rupture, preliminary results show that the rupture started at the southern part of the Chelungpu fault. Dislocations (or the rupture of asperities) jumped around behind the S -wave front in a rather random spatial distribution by which no rupture propagating velocity can be properly defined. Large ground acceleration, some over 1 g , occurred in the southern part of the Chelungpu fault where the rupture initiated. Toward the northern section of the Chelungpu fault, a decrease in ground acceleration was accompanied with an increase in ground velocity (to as much as 300 cm/sec) and ground displacement (to as much as 8 m). Most of the large motions were confined to the hanging wall (i.e., the eastern block of the thrust), and relatively small ground motions occurred in the footwall. Thus the hanging wall contributed the most to the rupture process. During the later part of the rupture, the N-S-trending Chelungpu fault made an easterly bend and the thrust motion turned into a more oblique-slip motion at the northern end. Meanwhile, the strong dynamic rupturing process triggered two M 6 events, each one having occurred in the vicinity of a known fault: one off the southern end and the other off the northern end of the Chelungpu fault. Manuscript received 12 December 2000.