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We analysed the within-earthquake correlation of ground motion using the strong-motion records accumulated by the TSMIP (Taiwan Strong Motion Instrumentation Program) network in Taiwan during 1993-2009. Two ground-motion prediction equations, which were recently developed for peak ground acceleration (PGA) in the region and based on moment and local magnitude and hypocentral distance, were used for the calculation and analysis of ground-motion residuals. We also used the database containing shear-wave velocity data averaged for the top 30 m of the soil column (Vs30) for the TSMIP stations. We showed that the within-earthquake correlation may vary significantly depending on site classes, gross geological features of the area, and magnitude of earthquakes, records of which dominate the analysed dataset. On the one hand, there is a prominent correspondence between the within-earthquake correlation of PGA residuals and spatial correlation of Vs30 values, which was estimated for particular geological structures (e.g., sedimentary filled basins and large plain areas). On the other hand, the high level of ground-motion correlation (or significant non-random component of residuals) may be caused by the joint influence of soft surface soil and thick sediments and by the path or azimuthal effects. The point-source approximation of extended fault and neglected hanging- and foot-wall effects may also result in non-random residuals. The application of empirical correction factors, which consider the magnitude of earthquakes, source-to-site distance and Vs30 value for given stations, allows for the effective reduction in the level of within-earthquake correlation, as well as the within-earthquake standard deviation. The results of the analysis may be used in practical estimates of seismic hazard, damage and loss for spatially distributed structures (portfolios, lifelines) in Taiwan, as well in other regions with similar geological characteristics. [ABSTRACT FROM AUTHOR]

Abstract: The frequency-dependent amplification for rock (NEHRP-class B) sites was studied using earthquake ground-motion database collected in Taiwan during implementation of the Taiwan Strong Motion Instrumentation Program. The database used includes several hundred records from earthquakes of M L 4.0–7.3 occurred between 1993 and 2004. The characteristics of amplification were evaluated using the well-known technique of horizontal-to-vertical Fourier spectral ratio (H/V) of the S-wave phase [Lermo J, Chavez-Garcia FJ. Site effect evaluation using spectral ratios with only one station. Bull Seism Soc Am 1993;83:1574–94]. The study allows us to analyze peculiarities of rock sites amplification in Northern and Eastern Taiwan. It was suggested to divide the NEHRP-class B site amplification into four types based on frequency of maximum amplification and the shape of amplification function. The applicability of the technique was also checked for a few stiff and soft soil sites (NEHRP-classes D and E). [Copyright &y& Elsevier]

We analyze the ability of different spectral models to describe the frequency content of ground motion during the 1999 Chi–Chi earthquake (MW=7.6, Taiwan) and two large (ML=6.8) aftershocks. The spectral models evaluated include the one-corner model of Brune applied with various key parameters (seismic moment and stress drop), and the two-corner-frequency models proposed for eastern North America [Bull. Seismol. Soc. Am. 83 (1993) 1778] and California [Bull. Seismol. Soc. Am. 90 (2000) 255]. The ground-motion spectra predicted by these spectral models for hypothetical very hard rock site were compared with the Chi–Chi earthquake data obtained on rock (class B) and soft rock or very dense soil (class C) sites. The approach also allows us evaluating the generalized empirical amplification function for class B and C sites in the region.It has been found that, the amplitude spectra of recorded ground acceleration (the mainshock and aftershocks) for frequencies larger than 0.3–0.4 Hz agree with the modelled two-corner-frequency spectra calculated using the model proposed for California. The single-corner-frequency model also provides a good agreement with the observations when using so-called ‘short-period seismic moment’ [Phys. Earth Planet. Interiors 37 (1985) 108] instead of the reported values obtained from long-period waves. The key parameters used in the single-corner model coincide with parameters of subsources evaluated for the complicated mainshock source. Therefore, it is possible to confirm the suggestion that the short-period seismic waves, at least for the thrust earthquakes, are generated mainly from the fracture of small-scale heterogeneities. The use of two-corner-frequency source model for earthquake spectrum that is based on long-period seismic moment value is equivalent, for frequencies larger than 0.3–0.4 Hz, to the use of single-corner-frequency model that is based on the parameters of major subsource. [Copyright &y& Elsevier]