Chelungpu Fault Research Articles

Seismic slip record in carbonate-bearing fault zones: An insight from high-velocity friction experiments on siderite gouge

Pseudotachylyte formed by frictional melting has been the only unequivocal evidence of past seismogenic fault slip. We report from high-velocity friction experiments on siderite-bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte. Experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite, calcite, and quartz, initially at room temperature, under normal stresses of 0.6–1.3 MPa and at seismic slip rates of 1.3–2.0 m/s. In all cases, gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and CO 2 gas, as confirmed by CO 2 measurement, X-ray diffraction analyses, and transmission electron microscopy. The weakening was caused by the low frictional strength of ultrafine decomposition products at seismic slip rates. Magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude. Those changes can be recognized in natural fault zones, and black gouge in the Chelungpu fault zone in Taiwan is perhaps such an example. Thus our results suggest that thermal decomposition in shallow crustal faults can be an important co-seismic process not only for dynamic fault weakening, but also for leaving seismic slip records.

Low-Angle Reverse Faulting During Two Earthquakes on the Northern Part of the Chelungpu Fault, Deduced From the Fengyuan Trench, Central Taiwan

A trench and two boreholes located at the western margin of a young terrace, east of Fengyuan city, show the style and frequency of surface faulting on the northern part of the Chelungpu fault, central Taiwan. Both the 1999 earthquake fault and penultimate earthquake event are characterized by low-angle reverse faulting with overturning of younger strata. The penultimate event occurred between ca. 1400 yr BP (1340 - 1170 cal yr BP) and 2000 yr BP (2120 - 1800 cal yr BP). The age of the lower part of the terrace gravel is ca. 4000 yr BP; thus the two faulting events occurred during the past ca. 4000 years. The amount of horizontal shortening and vertical displacement since the penultimate event are significantly larger (3.7 - 3.9 m

Thermal properties across the Chelungpu fault zone and evaluations of positive thermal anomaly on the slip zones: Are these residuals of heat from faulting?

Thermal properties of the fault zone materials are of fundamental importance for estimate of frictional heat generation during earthquake. The properties across the Chelungpu fault zone activated by 1999 Chi‐Chi earthquake, Taiwan, are measured using the drilled core penetrating the fault zone at around 1100 m depth. The fault zone contains four distinct fracture zones, each of which includes thin slip zones. Thermal conductivity (K) lies between 1.0 and 3.0 Wm−1K−1 and shows the lowest value at the slip zones. Thermal diffusivity (α) varies between 0.8 to 2.0 × 10−6 m2s−1, and is relatively low at the slip zones. Density (ρ) varies between 2200 to 2800 kgm−3 and shows the lowest values at a particular slip zone (1110 m depth). Specific heat (c) is calculated by using above data resulting in the values from 300 to 1000 Jkg−1K−1, and lowest values for slip zones. Using these data and spectral gamma ray logs, reported positive thermal anomalies at the slip zones are re‐examined whether they are regarded as residual heat from friction by faulting.

Low total and inorganic carbon contents within the Taiwan Chelungpu fault system

We presented the total and inorganic carbon contents of core samples recovered from the Taiwan Chelungpu fault system, which slipped at the 1999 Chi-Chi earthquake, and reported lower contents of inorganic carbon within the black gouge zone in FZB1136 (fault zone at depth 1136 m in Hole B) and in the black-material disks in FZB1194 and FZB1243.

Core Slabbing and Nannofossil Analysis on the Chelungpu Fault Zone, Taichung, Taiwan

Core Slabbing and Nannofossil Analysis on the Chelungpu Fault Zone, Taichung, Taiwan

Revealing Surface Deformation of the 1999 Chi-Chi Earthquake Using High-Density Cadastral Control Points in the Taichung Area, Central Taiwan

The 1999 Chi-Chi earthquake, M w 7.6, ruptured a major thrust fault along the western foothills of the Central Mountain Range of Taiwan. Here, we use cadastral control points to detect horizontal displacement in Taichung, central Taiwan. The cadastral control points were used to identify the coordinates of buildings and acreage. The coordinates of each point are measured by total station and connected to Global Positioning System (gps) control points. The density of these control points is nearly 36 points/km2 and the accuracy of each point is within less than 2–3 cm. The government of Taiwan measured cadastral control points before and after the Chi-Chi earthquake in the Taichung area (northern end of the Chelungpu fault); 1269 control points were measured over a 35-km2 area with most control points being located on the hanging wall of the Chelungpu fault. On the footwall, the displacement is about 1.1–1.2 m toward 123° to 128°, which is consistent with gps data. At the hanging wall, the displacement direction concentrates at 326° to 330°. In the eastern section, the displacement is from 7.5 to 8 m, which is consistent with gps and strong- motion data, but in the western part displacement changes from 8 m to 4–6 m and then increases again to 8 m along the 326° trending. We conclude that the changes in displacements were controlled by the geometry of the fault plane. Online material: Horizontal displacement of cadastral control points.

A study of the human-fatality rate in near-fault regions using the Victim Attribute Database

After the 1999 Chi-Chi earthquake, the Taiwanese government immediately issued new guidelines prohibiting the construction of structures for human occupancy within the Chelungpu fault zone. However, these guidelines were not based upon an in-depth hazard analysis of the near-fault regions. The positions of more than 80% of the 2,492 victims of the Chi-Chi earthquake were found by our research team. A Victim Attribute Database has been compiled that includes the GPS coordinates of the positioned victims as well as other attribute data associated with the victims. The human-fatality rates in the near-fault regions have been analyzed with regard to distances from the Chelungpu fault, the hanging-wall and footwall areas, as well as building type. The severity at the human-fatality rates in the near-fault regions is inversely proportional to distances from the causative fault, i.e., the closer the distance, the higher the human-fatality rate observed. The human-fatality rate for victims who lived in closer proximity to the hanging-wall areas is also significantly higher than those who lived in closer proximity to the footwall areas, especially in areas on either side of the fault and within 1,000 m of the fault surface trace. In terms of different building types, factors that include the capacity of the buildings to resistant strong shaking and the level of strong ground-motion greatly affected the human-fatality rates in the hanging-wall and footwall areas. Therefore simply prohibiting the construction of buildings within the active fault zone would be an insufficient method of reducing the number of potential victims; a nationwide effort should be undertaken to upgrade the capacity of buildings to resist strong shaking.

Energy release and heat generation during the 1999 Ms7.6 Chi‐Chi, Taiwan, earthquake

On 20 September 1999, the Ms7.6 Chi‐Chi earthquake ruptured the Chelungpu fault in central Taiwan. Integrating observed and inversed results of source parameters, the fracture energy, Eg, and frictional energy, Ef, on the fault and its northern and southern segments are estimated. Together with given values of strain energy, ΔE, and seismic radiation energy, Es, the seismic efficiency, i.e., η = Es/ΔE, and the radiation efficiency, i.e., ηR = Es/(Es + Eg), are evaluated. The average fracture energy per unit area, G, is also calculated from Eg. The frictional heat caused by dynamic frictional stress is calculated from Ef. Results show a marked difference in source properties between the two segments. The average frictional and ambient stress levels on the two segments are estimated. The total energy budget of and heat generated by the earthquake are elucidated based on a two‐dimensional faulting model with frictional heat. Both observed and calculated results suggest the possible existence of fluids, which produced suprahydrostatic gradients, on the fault during faulting. Lubrication and thermal fluid pressurization might play a significant role on rupture.

Evidence of frictional melting from disk‐shaped black material, discovered within the Taiwan Chelungpu fault system

The Taiwan Chelungpu‐fault Drilling Project penetrated three fault zones as the Chelungpu fault system, which slipped during the 1999 Chi‐Chi earthquake, discovering disk‐shaped black material (BM disk) within the middle and lower fault zones in Hole B. The microscopic features of the BM disks indicated that they were pseudotachylytes, and they showed high magnetic susceptibility, possibly the result of intense shearing or high temperature conditions. Inorganic carbon content of the BM disks was low, possibly because of thermal decomposition of carbonate minerals. The high temperatures might be related to frictional heating during the earthquake, implying that the BM disks were produced under intense shearing with frictional heating that reached melting temperature. Because the disks, which provide the only evidence of melting, pre‐date the 1999 earthquake, we concluded that frictional melting did not occur during the earthquake.

High magnetic susceptibility of fault gouge within Taiwan Chelungpu fault: Nondestructive continuous measurements of physical and chemical properties in fault rocks recovered from Hole B, TCDP

The Taiwan Chelungpu‐fault Drilling Project (TCDP) was undertaken in 2002 to investigate the faulting mechanism of the 1999 Taiwan Chi‐Chi earthquake. Hole B penetrated the Chelungpu fault, and recovered core samples from between 948.42 m and 1352.60 m depth. Three zones, marked 1136mFZ, 1194mFZ and 1243mFZ, were recognized in the core samples as active fault‐zones within the Chelungpu fault. Multi‐Sensor Core Logger measurements revealed lower densities and higher magnetic susceptibilities within the black gouge zones in all three fault zones. Even though the fault zone that slipped during the 1999 earthquake has not been identified, higher magnetic susceptibilities indicate that frictional heating has taken place in the Chelungpu fault.

In situ measurement of the hydraulic diffusivity of the active Chelungpu Fault, Taiwan

Hydraulic diffusivity controls fluid pressure and hence affects effective normal stress during rupture. Models suggest a particularly spectacular example of fluid pressurization during the Mw = 7.6 1999 Chichi earthquake when pressurization may have reduced high‐frequency shaking in the regions of large slip if the fault was sufficiently sealed. We investigate in situ hydraulic diffusivity which is the key parameter in such models through a cross‐hole experiment. We find a diffusivity of D = (7 ± 1) × 10−5 m2/s, which is a low value compatible with pressurization of the Chelungpu fault during the earthquake. In most poroelastic media, the hydraulic storativity S lies between 10−7 and 10−5, so that the transmissivity T along the fault zone is comprised between 10−11 m2/s and 10−9 m2/s. The corresponding permeability (10−18–10−16 m2) is at most one hundred times larger than the value obtained on core samples from the host rock. The fault zone is overpressurized by 0.06 to 6 MPa, which is between 0.2% and 20% of the lithostatic pressure.

Deformation and „deformation quiescence” prior to the Chi-Chi earthquake evidenced by DInSAR and groundwater records during 1995–2002 in Central Taiwan

Abstract Deformation of Central Taiwan between 1997 and 2002 was investigated using radar interferometry to construct the deformation rate time series before, during, and after the Chi-Chi earthquake. The deformation rate time series was compared to the time series of groundwater level in the monitoring wells of Yuanlin and Tzushan in order to investigate the relation between the geodetic observations and the groundwater levels. The deformation pattern of Central Taiwan generally forms a half-elliptical pattern, with the eastern half of the elliptical shape cut away by the north-south trending Chelungpu fault. Along a selected profile of 10 km near the Chelungpu Fault of Central Taiwan, the differential range change was measured and recalculated into deformation rates. The year-to-year deformation rates near the Chi-Chi earthquake ranged between 1.5 and 3 cm/year in most years except for the years between 1999 and 2000, which showed more than 5 cm/year deformation rates along the profile. Prior to the high deformation rate shown during the coseismic image pair, the deformation rate was low, as shown on the other image pairs. This variation in deformation rate coincides with the recorded trend in the level of groundwater in two monitoring wells in the Yuanlin area. The low deformation rate prior to the Chi-Chi earthquake may—partially—be explained by this lowering in groundwater level.

Heat signature on the Chelungpu fault associated with the 1999 Chi‐Chi, Taiwan earthquake

We have made observations of a heat signature that is associated with the frictional heat generated at the time of faulting for a large earthquake. Temperature measurements in a borehole that intersects the Chelungpu fault at a depth of about 1100 m, show a small increase near the fault even six years after the earthquake. The temperature signature has a symmetric shape with a width of about 40 m and is centered on the fault that slipped about 5 m during the 1999 Chi‐Chi, Taiwan earthquake. The small amplitude of 0.06°C for the observed temperature anomaly indicates a very low level of friction that generated heat at the time of the earthquake.

The moment method used to infer stress from fault/slip data in sigma space: invalidity and modification

The moment method has recently been used to infer stress in sigma space from fault/slip data. However, if these data are distributed along a hyperplane having a smaller dimension than that of the space minus one, due to limited fault/slip population or biased sampling of it, the best solution of stress vector is not in most cases, as expected, the eigenvector of the datum matrix relating to the smallest eigenvalue. The solution lies within the subspace composed of the eigenvectors relating to the small eigenvalues, for which some auxiliary constraints need to be included. Shear sense constraint alone is adopted, and incorporated by way of grid search, which gives rise to a range of accepted stress vectors in the subspace. Examples from the Chelungpu fault, Taiwan, illustrate the feasibility of the proposed scheme.

Geometric evolution of the Chelungpu fault, Taiwan: the mechanics of shallow frontal ramps and fault imbrication

The September 21, 1999 M w=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 (<4000 m 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 (<4000 m) footwall flat within the emergent thrust.

Resistivity Structures of the Chelungpu Fault in the Taichung Area, Taiwan

We conducted magnetotelluric prospecting in the Taichung area to investigate subsurface resistivity structures of the Chelungpu fault and the resistivity of rock formations. The results indicate that the Chelungpu fault is a complex fault system consisting of two major fault zones, several fracture zones, and back thrust. The two major fault zones, the basal and the Chi-Chi fault zone are about 800 m apart on the ground and converge to a narrow band at a depth of 3000 m. The fault zones are not smooth, composed of ramps and platforms with an average eastward dipping angle of 35° - 37° within the depth of 3000 m. In the shallower region, the basal fault zone has developed along the boundary of the Toukoshan Formation (resistivity: 200 - 400 Ω−m) at the footwall and the Neogene formations on the hanging wall, where the Cholan Formation, the Chinshiu Shale, and the Kueichulai Formation have respective resistivity mainly in the ranges: 40 - 100, 8 - 60, and 50 - 150 Ω−m. While the Chi-Chi fault zone has developed along the weak layers of the Cholan Formation where resistivity is lower than the unsheared block. At the TCDP site, the use of MT soundings, prior to TCDP drilling, predicted the position of the Chi-Chi fault zone to be at a depth between 1100 m and 1250 m. Drilling subsequently confirmed this result, indicating MT sounding to be a reliable approach in revealing subsurface structures.

Coseismic Deformation of Chi-Chi Earthquake as Detected by Differential Synthetic Aperture Radar Interferometry and GPS Data

A rupture in the Chelungpu fault caused an Mw 7.6 earthquake on 21 September 1999 near Chi-Chi in central Taiwan. This earthquake was the most destructive experienced in Taiwan for the past century along this fault. In this study, we examined the earthquake-induced surface deformation pattern using differential synthetic aperture radar interferometry (D-InSAR) combined with global positioning system (GPS) data regarding the footwall of the Chelungpu fault. Six synthetic aperture radar (SAR) scenes, approximately 100×100 km each, recorded by the European Remote Sensing Satellite 2 (ERS-2), spanning the rupture area, were selected for study. The data were used to generate a high-resolution, wide-area map of displacements in flat or semi-flat areas. Interferograms show radar line contours indicating line-of-sight (LOS) changes corresponding to surface displacements caused by earthquake ruptures. These results were compared to synthetic interferograms generated from GPS data. Displacements shown by GPS data were interpolated onto wide-area maps and transformed to coincide with the radar LOS direction. The resulting coseismic displacement contour map showed a lobed pattern consistent with the precise GPS based displacement field. Highly accurate vertical displacement was determined using D-InSAR data using the coordinate transform method, while GPS data was effective in showing the horizontal component. Thus, this study confirmed the effectiveness of the D-InSAR method for determining the coseismic deformation caused by the Chi-Chi earthquake at the footwall of the Chelungpu fault.

Surface Waves in the Western Taiwan Coastal Plain from an Aftershock of the 1999 Chi-Chi, Taiwan, Earthquake

Significant surface waves were recorded in the western coastal plain (WCP) of Taiwan during the 1999 Chi-Chi, Taiwan, earthquake and its series of aftershocks. We study in detail the surface waves produced by one aftershock (20 September 1999, 18hr 03m 41.16sec, M 6.2) in this paper. We take the Chelungpu- Chukou fault to be the eastern edge of the WCP because it marks a distinct lateral contrast in seismic wave velocities in the upper few kilometers of the surface. For many records from stations within the WCP, body waves and surface waves separate well in both the time domain and the period domain. Long-period (e.g., 2 sec) ground motions in the plain are dominated by surface waves. Significant prograde Rayleigh wave particle motions were observed in the WCP. The observed peak ground velocities are about 3-5 times larger than standard predictions in the central and western part of the plain. Observed response spectra at 3 sec, 4 sec, and 5 sec at the center of the plain can be 15 times larger than standard predictions and 10 times larger than the predictions of Joyner (2000) based on surface wave data from the Los Angeles basin. The strong surface waves were probably generated at the boundary of the WCP and then propagated toward the west, largely along radial directions relative to the epicenter. The geometry of the boundary may have had a slight effect on propagation directions of surface waves. Group velocities of funda- mental mode Rayleigh and Love waves are estimated using the multiple filter analysis (MFA) technique and are refined with phase matched filtering (PMF). Group velocities of fundamental mode surface waves range from about 0.7 km/sec to 1.5 km/sec for the phases at periods from 3 sec to 10 sec. One important observation from this study is that the strongest surface waves were recorded in the center of the plain. The specific location of the strongest motions depends largely on the period of surface waves rather than on specific site conditions or plain structures. Accordingly, we conjecture that surface waves could be generated in a wide area close to boundaries of low-velocity sedimentary wave guides. In the case studied in this article the area can be as wide as 30 km (from the Chelungpu fault to the center of the plain). Surface waves converted by P and S waves at different locations would overlap each other and add constructively along their propagation paths. As a result, the surface waves would get stronger and stronger. Beyond a certain distance to the boundary, no more surface waves would be generated. Consequently, no more local surface waves would be superimposed into the invasive surface waves, and the surface waves would tend to decay in amplitude with distance.

Surface deformation related to kink-folding above an active fault: Evidence from geomorphic features and co-seismic slips

Abstract By means of a geomorphological study using 40 m digital elevation model, structural reconstruction by field data, and related geodetic results, we reevaluate the geomorphic features in the Hsinshe area, where multi-step terraces have been previously documented and supposedly provide more information of the neotectonic history. Our results show that two kink-shaped scarps can be discriminated from other terrace risers, which are probably formed by folding instead of faulting as suggested by previous researchers. Two E–W-reconstructed hanging wall cross sections in the north and south show that the fault plane of Chelungpu fault flattens to the east and sinks to the south, creating a monoclinal hanging wall resembling a quarter hemisphere in terms of a 3D model. We propose a kinetic model to explain the surface monocline fold: it is developed when the fault plane presents a fault-bend, such as the case along the Chelungpu fault at Hsinshe. The co-seismic deformation from the 1999 Chi-Chi earthquake demonstrates that the model is probably valid. Furthermore, the higher terrace showing larger deformation indicates the influences of active structures on terraces have continuously acted for a long time. The westward tilting of the hanging wall terraces result from the action of the Chelungpu fault and its related structural geometry. Based on the optical-stimulated-luminescence age of 55.0±12.6 ka of the highest terrace, we can compute the vertical rates across the main thrust and fold scarp in the hanging wall as 7.0 and 2.5 m/kyr, respectively.

Neotectonic architecture of Taiwan and its implications for future large earthquakes

The disastrous effects of the 1999 Chi‐Chi earthquake in Taiwan demonstrated an urgent need for better knowledge of the island's potential earthquake sources. Toward this end, we have prepared a neotectonic map of Taiwan. The map and related cross sections are based upon structural and geomorphic expression of active faults and folds both in the field and on shaded relief maps prepared from a 40‐m resolution digital elevation model, augmented by geodetic and seismologic data. The active tandem suturing and tandem disengagement of a volcanic arc and a continental sliver to and from the Eurasian continental margin have created two neotectonic belts in Taiwan. In the southern part of the orogen both belts are in the final stage of consuming oceanic crust. Collision and suturing occur in the middle part of both belts, and postcollisional collapse and extension dominate the island's northern and northeastern flanks. Both belts consist of several distinct neotectonic domains. Seven domains (Kaoping, Chiayi, Taichung, Miaoli, Hsinchu, Ilan, and Taipei) constitute the western belt, and four domains (Lutao‐Lanyu, Taitung, Hualien, and Ryukyu) make up the eastern belt. Each domain is defined by a distinct suite of active structures. For example, the Chelungpu fault (source of the 1999 earthquake) and its western neighbor, the Changhua fault, are the principal components of the Taichung Domain, whereas both its neighboring domains, the Chiayi and Miaoli Domains, are dominated by major blind faults. In most of the domains the size of the principal active fault is large enough to produce future earthquakes with magnitudes in the mid‐7 values.