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Tsai M.-C.,National Central University | Tsai M.-C.,Seismological Center | Yu S.-B.,Academia Sinica, Taiwan | Hsu Y.-J.,Academia Sinica, Taiwan | And 2 more authors.
Tectonophysics | Year: 2012

We derive a velocity field using GPS data between 1993 and 2007 in the Chiayi-Tainan area located in the deformation front of the Taiwan mountain belt. The crustal motion with respect to Penghu shows large velocities of about 33-44. mm/yr in the west-northwest to west directions in the Western Foothills and the velocities decrease westward to 0-5. mm/yr in the coastal area. Significant uplift rates of 5-20. mm/yr are observed at sites to the east of the Jiuchiunken-Muchiliao-Liuchia Fault (JMLF) system. We use a block model, a buried dislocation model, and a two-dimensional fault model to invert for fault geometries and slip rates on major frontal thrust faults. Modeling results from a block model show the inferred long-term slip rate of 42. mm/yr in the direction of 280° and the maximum back-slip rate of 38. mm/yr on a 23° east dipping fault extending to 13. km at depth. On the other hand, the buried dislocation model results in a horizontal décollement at a depth of 8. km with a uniform slip rate of 41.6. mm/yr. If we connect the top edge of décollement to the surface trace of JMLF as a potential future rupture, a 22° east-dipping fault is required. Results from both block model and buried dislocation model suggest the JMLF is nearly fully locked. The results of two-dimensional fault models show the frontal thrust faults have slip rates of less than 2. mm/yr at shallow depths and the inferred décollement is sub-horizontal (5°-7°) at a depth of 10. km with slip rates of 44-46. mm/yr. Results of various approaches show general agreement on fault geometries and slip rates and reveal that the frontal thrust fault system has a high potential for large earthquakes. © 2012 Elsevier B.V.

Li H.-C.,National Central University | Chang C.-H.,Seismological Center | Chen C.-C.,National Central University
Pure and Applied Geophysics | Year: 2016

Seismicity has been identified as an example of a natural, nonlinear system for which the distribution of frequency and event size follow a power law called the “Gutenberg–Richter (G-R) law.” The parameters of the G-R law, namely b- and a-values, have been widely used in many studies about seismic hazards, earthquake forecasting models, and other related topics. However, the plausibility of the power law model and applicability of parameters were mainly verified by statistical error σ of the b-value, the effectiveness of which is still doubtful. In this research, we used a newly defined p value developed by Clausetet al. (Power-Law Distributions in Empirical Data, SIAM Rev. 51, 661–703, 2009) instead of the statistical error σ of the b-value and verified its effectiveness as a plausibility index of the power-law model. Furthermore, we also verified the effectiveness of K–S statistics as a goodness-of-fit test in estimating the crucial parameter (Formula presented.) of the power-law model. © 2015, Springer Basel.

Rau R.-J.,National Cheng Kung University | Lee J.-C.,Academia Sinica, Taiwan | Ching K.-E.,National Cheng Kung University | Lee Y.-H.,National Chung Cheng University | And 2 more authors.
Tectonophysics | Year: 2012

We integrated a regional tomography model, 20-year seismicity and earthquake focal mechanisms of the 4 March 2010 M L 6.4 Jiashian earthquake source region to delineate the seismogenic structure and mechanics of a relatively rare damaging inland earthquake that occurred in the southwestern fold-and-thrust belt of Taiwan. The main shock of the Jiashian earthquake occurred at a depth of 23km beneath the slate belt of the southern Central Range with a sinistral thrust mechanism. Based on seismic tomography and seismicity, the Jiashian earthquake sequence occurred in the Chishan transfer fault zone, which we interpret to be located near the transition zone between the subducted Eurasian plate to the south and the arc-continent Taiwan collision to the north. The distribution of the Jiashian aftershocks define a WNW-ESE striking fault plane that dips 30-40° NNE which is consistent with the optimal fault plane of the GPS-derived coseismic slip model. Focal mechanisms and inverted stress results of the earthquake sequence indicate both thrust and strike-slip motions with the maximum compressive stress at nearly 90° to the regional compressive stress and thus sub-parallel to the strike of the Taiwan orogen. We propose that the 2010 Jiashian earthquake resulted from rupturing of a buried oblique fault within the Chishan transfer fault zone at the subduction-collision transition zone in southern Taiwan. The orogen-parallel P-axis of the 2010 Jiashian earthquake represents collision-related lateral compression, although a local stress deviation is also possible. © 2011 Elsevier B.V.

Chen Y.-N.,National Taiwan University | Gung Y.,National Taiwan University | You S.-H.,National Taiwan University | Hung S.-H.,National Taiwan University | And 5 more authors.
Geophysical Research Letters | Year: 2011

Taking advantage of a unique opportunity provided by a dense array of coastal short-period seismic stations and the diverse bathymetry around Taiwan, we examine how the long-range coherent ambient noises are influenced by surrounding ocean settings using the cross-correlation functions (CCFs) between pairs of stations. The effective energy of the CCFs derived from three components of short-period seismometer data falls within the frequency range of the short period secondary microseism (SPSM). The spatial variations mapped from the amplitude asymmetry of CCFs and source migration images evidently demonstrate that the SPSM strengths are closely linked to the drastic changes in offshore ocean characteristics and result in much stronger SPSM in the shallow and narrow Taiwan Strait than in deep open seas of eastern Taiwan. The temporal variations of the CCF strengths exhibit very good correlations with the wind speeds and wave heights, explicitly indicating the observed SPSM is dominated by local sources generated from wind-driven ocean waves around offshore Taiwan. Copyright © 2011 by the American Geophysical Union.

You S.-H.,National Taiwan University | Gung Y.,National Taiwan University | Chiao L.-Y.,National Taiwan University | Chen Y.-N.,National Taiwan University | And 3 more authors.
Bulletin of the Seismological Society of America | Year: 2010

Using the ambient noise cross-correlation technique, we derived empirical Green's functions (EGFs) of short-period Rayleigh waves from the continuous vertical component of data recorded by 34 seismic stations in northern Taiwan. We measured the group velocity of the EGFs in the frequency range of 0.2-0.5 Hz. The derived EGFs and group velocity measurements were carefully examined, and only quality results were used for the tomographic inversion. We implemented a wavelet-based multiscale inversion technique to construct the group velocity map for 0.35-Hz Rayleigh waves in northern Taiwan. The resulting model shows excellent correlation to the surface topography and geological units. To compare with models derived from traditional body-wave tomography, we calculated the group velocity maps and their fit to the noise-derived EGFs for two recent body-wave models. The comparison indicates that body-wave velocities, particularly VS, at shallow depth in the west coast plain are overestimated in two recent body-wave-derived models, and/or a much higher VP=VS ratio may exist for this area. Our results suggest that the noise-derived short-period EGFs may provide important constraints on shallow Earth structure, in complement to that provided by body waves.

Hsiao N.-C.,Seismological Center | Wu Y.-M.,National Taiwan University | Zhao L.,Academia Sinica, Taiwan | Chen D.-Y.,Seismological Center | And 4 more authors.
Soil Dynamics and Earthquake Engineering | Year: 2011

A new prototype earthquake early warning (EEW) system is being developed and tested using a real-time seismographic network currently in operation in Taiwan. This system is based on the Earthworm environment which carries out integrated analysis of real-time broadband, strong-motion and short-period signals. The peak amplitude of displacement in the three seconds after the P arrival, dubbed Pd, is used for the magnitude determination. Incoming signals are processed in real time. When a large earthquake occurs, P-wave arrival times and Pd will be estimated for location and magnitude determinations for EEW purpose. In a test of 54 felt earthquakes, this system can report earthquake information in 18.8±4.1 s after the earthquake occurrence with an average difference in epicenter locations of 6.3±5.7 km, and an average difference in depths of 7.9±6.6 km from catalogues. The magnitudes approach a 1:1 relationship to the reported magnitudes with a standard deviation of 0.51. Therefore, this system can provide early warning before the arrival of S-wave for metropolitan areas located 70 km away from the epicenter. This new system is still under development and being improved, with the hope of replacing the current operational EEW system in the future. © 2010 Elsevier Ltd.

Wang J.-H.,Academia Sinica, Taiwan | Chen K.-C.,Academia Sinica, Taiwan | Huang W.-G.,Academia Sinica, Taiwan | Chang K.-H.,Academia Sinica, Taiwan | And 2 more authors.
Terrestrial, Atmospheric and Oceanic Sciences | Year: 2014

The generalized fractal dimensions are measured for M ≥ 3 shallow earthquakes with focal depths of ≤ 40 km in the Taipei Metropolitan Area (from 121.3 to 121.9°E and 24.8 to 25.3°N) over the 1973 - 2010 period based on spatial distribution (using epicentral and hypocentral distances between two events, r) and time sequence (using the inter-event time between two events, t). The multifractal measures are estimated from log-log plots of Cq(r) versus r and Cq(t) versus t, where Cq(r) and Cq(t) are the generalized correlation integral, respectively, of r and t at positive q. For the spatial distribution, Cq(r) is calculated based on the epicentral distance (i.e., the 2D measure) and hypocentral distance (i.e., the 3D measure). Under both 2D and 3D measures, the log-log plot of Cq(r) versus r shows a linear distribution when log(ro) ≤ log(r) ≤ log(rub) and roll-over when r > rub. For all cases log(ro) is 0.3, and log(r ub) are 1.7 and 1.4 for the 2D and 3D measures, respectively. D q, which is the slope of the linear portion, monotonically decreases with increasing q, thus indicating that the epicentral and hypocentral distributions of earthquakes are multifractal. The values of Dq are lower than 2 and 3, respectively, for the 2D and 3D measures. For the time sequence of the events in study, Cq(t) is calculated based on the inter-event time between two events. The log-log plot of Cq(t) versus t does not seem able to show a linear relationship in a large range of log(r) or r and the value of Dq cannot be evaluated, thus suggesting that the time sequence of M ≥ 3 shallow earthquakes in the Taipei Metropolitan Area (TMA) is not multifractal.

Wang J.-H.,Academia Sinica, Taiwan | Chen K.-C.,Academia Sinica, Taiwan | Leu P.-L.,Seismological Center | Chang C.-H.,Seismological Center
Journal of Seismology | Year: 2016

Seismic observations exhibit the presence of abnormal b-values prior to numerous earthquakes. The time interval from the appearance of abnormal b-values to the occurrence of mainshock is called the precursor time. There are two kinds of precursor times in use: the first one denoted by T is the time interval from the moment when the b-value starts to increase from the normal one to the abnormal one to the occurrence time of the forthcoming mainshock, and the second one denoted by Tp is the time interval from the moment when the abnormal b-value reaches the peak one to the occurrence time of the forthcoming mainshock. Let T* be the waiting time from the moment when the abnormal b-value returned to the normal one to the occurrence time of the forthcoming mainshock. The precursor time, T (usually in days), has been found to be related to the magnitude, M, of the mainshock expected in a linear form as log(T) = q + rM where q and r are the coefficient and slope, respectively. In this study, the values of T, Tp, and T* of 45 earthquakes with 3 ≤ M ≤ 9 occurred in various tectonic regions are compiled from or measured from the temporal variations in b-values given in numerous source materials. The relationships of T and Tp, respectively, versus M are inferred from compiled data. The difference between the values of T and Tp decreases with increasing M. In addition, the plots of T*/T versus M, T* versus T, and T* versus T-T* will be made and related equations between two quantities will be inferred from given data. © 2016, Springer Science+Business Media Dordrecht.

Wang J.-H.,Academia Sinica, Taiwan | Chen K.-C.,Academia Sinica, Taiwan | Lee S.-J.,Academia Sinica, Taiwan | Huang W.-G.,Academia Sinica, Taiwan | Leu P.-L.,Seismological Center
Terrestrial, Atmospheric and Oceanic Sciences | Year: 2012

The M ≥ 3 earthquakes which occurred in the Taipei Metropolitan Area from 1973 through 2010 are used to study the memory effect of earthquake sequences in the area by applying a fluctuation analysis technique in the natural time domain. The earthquakes can be divided into two groups: the first for shallow events with focal depths ranging 0 - 40 km and the second with focal depths deeper than 60 km. For both shallow and deep earthquakes, three magnitude ranges, i.e., M ≥ 3, M ≥ 3.5, and M ≥ 4, are taken into account. The calculations are also made for the events in a smaller area. Calculated results show that the exponents of the scaling law of fluctuation versus window length for all earthquakes sequences in consideration are not larger than 0.5, thus suggesting that the M ≥ 3 and M ≥ 3.5 earthquakes in the TMA are short-term corrected. On the other hand, the M ≥ 4 earthquakes are weakly corrected.

Chen K.-C.,Academia Sinica, Taiwan | Wang J.-H.,Academia Sinica, Taiwan | Kim K.-H.,Pusan National University | Leu P.-L.,Seismological Center
Terrestrial, Atmospheric and Oceanic Sciences | Year: 2014

Strong-motion seismograms from the 11 February, 2014 Tatunshan earthquake were recorded at stations around the source area. These recordings were used to analyze the strong-motion characteristics in the area. The largest peak ground acceleration (PGA) values of 100.7, 93.4, and 66.6 cm sec-2 in the vertical, EW, and NS directions, respectively, were recorded at station TAP056, about 4.9 km to the northwest of the epicenter. The PGA decays fast with distance, indicating high attenuation in the Tatun volcanic area. The PGA ratio of vertical to horizontal ground motions decreases with increasing epicenter distance. The PGA values in the EW component are larger than those in the NS component. This might be associated with the focal mechanism of the earthquake. The spectral accelerations decrease rapidly with increasing period. © 2014, Chinese Geoscience Union. All rights reserved.

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