Kou-g Chen

A Model for the High-Cut Process of Strong-Motion Accelerations in Terms of Distance, Magnitude, and Site Condition: An Example from the SMART 1 Array, Lotung, Taiwan

Parameters controlling the high-cut process of strong-motion accelerations are analyzed using the recordings collected from the SMART 1 array in Lotung, Taiwan. A regression model in terms of distance, earthquake magnitude, and site condition is generally found to be suitable to describe the parameters under investigation. Results show that the high-cut process is controlled by both the site and source effects, which might bring resolution to the long-standing controversy over the origin of the cutoff frequency. In this note, it becomes almost certain that distance is the least significant parameter controlling the high-cut process.

Three-dimensional elastic wave velocity structure of the Hualien region of Taiwan: Evidence of active crustal exhumation

The Hualien region of Taiwan is located at a complex transition of the boundary between the Eurasian and Philippine Sea Plates. To the southwest, the mountains of Taiwan are uplifting rapidly as a consequence of an ongoing arc-continent collision, while to the east the oceanic Philippine Sea Plate is subducting northward beneath Eurasia. We investigated the structure and dynamics of this region by analyzing seismograms of local earthquakes recorded during a deployment of the Portable Array for Numerical Data Acquisition II network. P and S wave velocity structures deduced from travel time tomography analysis show that the collisional suture to the south of Hualien is characterized by a narrow (< 10 km width), near vertically dipping zone of low velocities that extends to depths in excess of 20 km. Velocities in the Eastern Central Range west of the suture zone are significantly higher and define a feature 10–15 km wide that appears to be continuous from the near surface to depths as great as 40 km. Farther to the west beneath the Western Central Range, the velocities again decrease. Focal mechanisms of local earthquakes show that while thrust faulting is the predominate mode of deformation throughout the region, normal faulting occurs as well beneath the Eastern Central Range. Thus the rapid uplift of the mountains of Taiwan may be a result not only of compressional shortening but also of an excess of positive buoyancy. We suggest that the higher velocities and extensional mechanisms in the Eastern Central Range are caused by the ongoing exhumation of previously subducted continental crust, while the lower velocities to the west reflect continued underthrusting of the crust beneath the Eastern Central Range.

The vibrational dephasing and relaxation of CH and CD stretches on diamond surfaces: An anomaly

The temperature dependence of infrared absorption spectra of CH and CD on diamond nanocrystal surfaces has been investigated. Phase relaxation was closely examined by analyzing frequency shifts and line broadening in the spectra. Based on the model of Persson and Ryberg [Phys. Rev. B 40, 10 273 (1989)], coupling phonons responsible for the pure dephasing process were found to resonate at ω0≊1200 cm−1 for the CH stretch. By including both the phase and energy relaxation in the linewidth analysis and assuming that energy relaxes via three‐phonon emission, we estimate a pure dephasing time of T*2≊340 ps at room temperature. This value is one order of magnitude larger than the energy relaxation time, T1≊19 ps, measured by Chin et al. [Europhys. Lett. 30, 399 (1995)] on a C(111) single crystal surface. We interpret the anomalous observation to be the result of the high frequency of the coupling phonons. For the CD stretches, however, severe line broadening due to exceedingly rapid energy relaxation disallows a...

An Observation of Rupture Pulses of the 20 September 1999 Chi-Chi, Taiwan, Earthquake from Near-Field Seismograms

The ground-velocity recordings of the 20 September 1999, Chi-Chi, Taiwan earthquake recorded at stations near the ruptured fault trace show a simple, large-amplitude, and long-period pulse following the S wave, which is closely associated with the surface faulting and the rupture process of thrust faulting. The conspicuous pulse on the ground-velocity seismogram following the S -wave arrival, called the S 1 phase, is interpreted as the superposition of the rupture pulses that nucleate at an asperity near and underneath the station and propagate up-dip and laterally along the fault plane toward the surface stations. The arrival times of the S 1 phase and the onsets of the permanent displacement at stations near and along the ruptured fault trace increase with hypocentral distance, suggesting that the rupture of the Chi-Chi earthquake might have initiated at the hypocenter of the mainshock and propagated both upward and laterally from south to north. On the basis of the travel-time differences between the S 1 phase and the direct S wave at the stations near and along the ruptured fault trace, the rupture velocities varied from 2.28 to 2.69 km/sec, with an average rupture velocity of about 2.49 km/sec. The rupture velocities decreased from south to north.

Estimates of Stress Drop of the Chi-Chi, Taiwan, Earthquake of 20 September 1999 from Near-Field Seismograms

The apparent stress and stress drop of the Chi-Chi, Taiwan, earthquake are estimated from near-field seismograms. The estimated apparent stress and stress drop for the southern part of the fault are about 100 bars lower than those for the northern part. The estimated ratio E s/ M also suggests that there is a higher dynamic stress drop in the northern part than in the southern one. This indicates the transformation of a higher percentage of strain energy into the seismic-wave energy in the northern part than in the southern part. Based on a parameter proposed by Ramon Zuniga (1993), we propose that the stress model of frictional overshoot can interpret the rupture of the Chelungpu fault, on which the Chi-Chi, Taiwan, earthquake occurred. Manuscript received 31 July 2000.

Estimates of source parameters for the 1999 Chi-Chi, Taiwan, earthquake based on Brune's source model

The general features of the rupture of the 1999 Chi-Chi, Taiwan, earthquake ( M s 7.6) can be explained by the displacement waveforms derived from the accelerograms recorded at short distances from the fault traces. Applying Brunes model, we have determined important source parameters, such as rise time, stress drop, offset, and particle velocity. Generally, the earthquake is characterized as having had two distinct fault segments. The southern segment, dominated by thrust motion, started from the focus on a fault plane raking at 78° and extended about 30 km to the north. The northern segment, dominated by thrust with significant strike-slip motion, began next to the end of the southern segment on a fault plane raking at 53° and extended northward for 25 km. Slips in the southern segment were followed by a small dislocation (∼1 m), while those in the northern segment were followed by a much larger dislocation (∼9 m). The average slip velocity was distributed at 34-49 cm/sec, along the southern segment, and an unusual slip velocity exceeding 2 m/sec was observed along the northern segment. Furthermore, the southern segment experienced a rise time of 1.8 sec and a stress drop of 65 bars, in contrast to a rise time longer than 4 sec and a stress drop larger than 300 bars registered to the north. Our results also indicate that, along the southern segment, the rupture propagated northward at an average velocity of 2.84 km/sec, but along the northern segment, the rate declined to less than 2 km/sec. The difference in the source parameters between these two segments suggests that the rupturing associated with the Chi-Chi earthquake may have encountered a resistive patch and changed course in the middle part of the fault. After crushing that resistance, the long rise time and high stress drop probably caused substantially slower motion and larger slip along the northern segment. Manuscript received 10 November 2000.

Characteristics of strong ground motion across a thrust fault tip from the September 21, 1999, Chi‐Chi, Taiwan earthquake

Near fault tip strong motion records from the northern part of the major earthquake (Mw = 7.6), namely the Chi-Chi earthquake on September 21, 1999 in central Taiwan demonstrated systematic differences on the hanging wall and footwall, and simulated by the finite element method. The extraordinary ground motion differences on either side of the northern fault tip can be explained by a 2-dimension kinematic source model with fault rupture breaking to surface. In this study, the earthquake faulting was considered as bilateral from the center of a low angle thrust fault which is 30 km in length with a dip angle of 31°. Based on waveform modeling, the source rupture velocity, rise-time and dislocation of 2.0 km/sec, 5 sec and 6 meters, respectively are suggested. The results of this study show that on the northern part of the Chi-Chi earthquake fault there was lower rupture velocity and longer rise-time of the fault slip than that previously reported. Furthermore, the effects of surface breaking from the fault movement contributed large ground deformations near the fault tip and, consequently, induced extensive damage.

A Simple Algorithm for Local Earthquake Location Using 3D VP and VS Models: Test Examples in the Central United States and in Central Eastern Taiwan

Traditional local-earthquake location using a horizontally layered homogeneous velocity model is limited in its resolution and reliability due to the existence of frequently overlooked 3D complexity of the real Earth. During traditional 3D seismic tomography, simultaneous earthquake relocation using the resultant 3D velocity model has produced reliable earthquake locations; however, only a small subset of events are typically used and thus relocated in the inversion. The rest of the events in a catalog must then be relocated using the 3D models. The repeated calculation of travel times across 3D V P and V S models is also not efficient and not practical for a routine network earthquake location when the very time-consuming exact 3D raytracing is used. Because high-resolution earthquake data are now available from many modern seismic networks, representative high-resolution 3D V P and V S models for a region can be better determined. By taking advantage of recently available high-speed computer technology and large disk space, we implemented a simple algorithm to efficiently locate every local earthquake using the best available regional 3D V P and V S models. Once the V P and V S information for all cubic cells in a 3D grid model are determined, P and S travel times from each grid point to all seismic stations can be calculated and stored on disk files for later usage. During the iteration process for earthquake location, travel times from a trial hypocenter to all recording stations can be determined simply by a linear interpolation from those of the adjacent eight grid points available in the previously stored disk files without the need for raytracing. The iterations continue until the hypocenter adjustments at the end of the last iteration are below the given criteria and the travel-time residual, or the difference between the observed and the calculated travel times, is a minimum. Therefore, any local earthquake can be efficiently and reliably located using the available 3D velocity models. This simple location program has been applied to relocate earthquakes in the New Madrid Seismic Zone (nmsz) of the central United States and in the central eastern Taiwan region. Preliminary results in both regions reveal that earthquake hypocenters can be efficiently relocated in spite of the very significant lateral structural variations. Tests with data from Taiwan further demonstrate that the resolution of seismic tomography and the relocated seismicity is sensitive to relative distribution of seismic-network stations and background seismicity. Thus, this single-event location program can be applied to relocate all earthquakes in a seismic-network catalog and, more importantly, to allow routine earthquake location for any seismic network using the available 3D velocity models.

Interaction of atomic hydrogen with a Ge(111) surface: low-energy electron diffraction and surface Raman studies

Abstract We report the preparation and characterization of a sufficiently ordered Ge(111)-1×1:H surface by prolonged hydrogenation of Ge(111)-c(2×8) at elevated temperatures. For both annealed and sputtered/annealed c(2×8) surfaces, a (1×1) pattern with distinct primary-order spots was observed by low-energy electron diffraction (LEED) after extensive hydrogenation treatment. We demonstrated that a surface Raman spectroscopic method based on polarization effects can be used successfully to characterize such a prepared Ge(111)-1×1:H surface, which is flat enough to yield a single prominent peak of the monohydride GeH stretch. The possible mechanism for surface smoothing by atomic hydrogen is also discussed. The smoothness of this surface makes various spectroscopic characterization methods feasible.

Aspects of characteristics of near-fault ground motions of the 1999 Chi-Chi (Taiwan) earthquake

Abstract In this work, we first study the peak ground accelerations (PGA), the peak ground velocities (PGV) and spectra of acceleration waveforms, based on a coordinate system defined on the focal plane of the earthquake, at nine near‐fault seismic stations along the fault trace. Results show that except for a station, near which there is a remarkable change of the fault trace, the near‐fault PGA value decreases and the PGV value increases from south to north along the fault. Although there exist variety and complexity in near‐fault acceleration spectra, some substantial conclusions can still be retrieved. The source and site effects are two major factors in controlling the variation in near‐fault acceleration spectra along the fault. The site effect acts mainly on the high‐frequency spectra, while the source effect on low‐frequency ones. For the three components, the value of the predominant frequency is, on the average, higher in the hanging wall than in the foot wall.