To rapidly assess the potential for damage of an earthquake for purposes of earthquake early warning in Taiwan, we used the peak displacement and velocity amplitudes of the first 3 sec of the P wave. The vertical-component records, highpass filtered at 0.075 Hz, are used. We found that the peak initial-displacement amplitude (Pd) correlates well with the peak ground-motion displacement (PGD) and the peak ground-motion velocity (PGV) at the same site. When Pd Ͼ 0.5 cm, the event is most likely damaging. If Pd is combined with the period parameter s c , determined in an earlier study, then s c ‫ן‬ Pd provides an even more robust parameter for assessing the potential for damage.

[1] The 2003 Chengkung earthquake (Mw 6.8) provided diagnostic evidence for a source model showing the deformation process of the seismogenic Chihshang fault in eastern Taiwan. The aftershocks show a fault-bend at a depth of 18 km. Coseismic ground displacements recorded by strong-motion records allow us to deduce instant rupturing of this event. Our resulting model shows a fault length of $33 km and dip-slip dominant rupture on fault-plane deeper than 18 km. Estimated coseismic displacements constrain two fault planes: one at 5 -18 km depth dipping 60°E and 18-36 km depth dipping 45°E. The uppermost fault-plane of the Chihshang Fault (0 -5 km) did not break immediately after the main shock; however, it may have a major role in after-slip and even interseismic ground deformation. The Taiyuan basin developed in the hanging wall is a geomorphic feature consistent with and adequately explained by coseismic ground displacements.

No abstract

Since 1990, digital strong-motion accelerographs and global positioning system (GPS) instruments have been widely deployed in the Taiwan region (Shin et al. 2003;Yu et al. 2001). The 1999 Chi-Chi, Mw 7.6 earthquake and the 2003 Chengkung, Mw 6.8 earthquake were well recorded by both digital accelerographs and GPS instruments. These data offer a good opportunity to determine coseismic displacements from strong-motion records and to compare the results with those derived from GPS measurements. As noted by Boore (2001), a double integration of the acceleration data often leads to unreasonable results, and baseline corrections are therefore required in most cases before the integration. Based on the works of Iwan et al. (1985) and Boore (2001), we developed an improved method for baseline correction and validated it using an extensive set of data from shake-table tests of a known "step" displacement on 249 accelerographs. Our baseline correction method recovered about 97% of the actual displacement from the shaketable data. We then applied this baseline correction method to compute coseismic displacements from the strong-motion data of the Chi-Chi and Chengkung earthquakes. Our results agree favorably with the coseismic displacements determined by the GPS measurements at nearby sites. The ratio of seismic to geodetic displacement varies from 0.78 to 1.41, with an average of about 1.05.

Based on the strong-motion data set from the 1999 Chi-Chi, Taiwan, earthquake and a shaking damage statistics database, we investigated the correlations between strong ground motions and earthquake damage (fatalities and building collapses) through a regression analysis. As a result, the current earthquake intensity scale I t is placed on a more reliable instrumental basis. This is necessary for the realtime seismic monitoring operation in Taiwan where programs for earthquake rapid reporting (RRS) and earthquake early warning (EWS) are actively pursued. It is found that the earthquake damage statistics give a much closer correlation with the peak ground velocity (PGV) than with the peak ground acceleration (PGA). The empirical relationship between PGV and the intensity I t determined in this study can be expressed as I ‫ס‬ 2.14 ‫ן‬ log (PGV) ‫ם‬ 1.89. t 1 0 This PGV-based intensity is particularly useful in real-time applications for damage prediction and assessment, as the damage impact of high PGV is much more important for mid-rise and high-rise buildings that are characteristic of a modern society. For smaller earthquakes (M Ͻ5), the PGV-intensity correlation also out-performs the PGA-intensity correlation, as large sharp PGA spikes are often observed for rather small nondamaging events at close-in distances. However, as the lower level intensity is conventionally defined through human feelings, for even smaller events (M Ͻ3) humans are more sensitive to PGA than to PGV. Since the RRS and EWS operations are mainly dealing with large and damaging earthquakes, the above PGV-based empirical relationship should prove to be more appropriate in these real-time operations.