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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.

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.

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.

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.

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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