ALBERT

Description: The purpose of this study is to derive attenuation relationships of the peak ground acceleration (PGA) and peak ground velocity (PGV) in Ilan, Taiwan, including factors of fault type and site effect. A total of 2852 accelerograms recorded from 92 shallow earthquakes with M (sub w) magnitude ranging from 4.0 to 7.7 are used to develop the regional attenuation relationships. Two models are used: Model 1 uses records from 65 strong-motion sites. The results will be especially useful for the Central Weather Bureau (CWB) early warning system to quickly assess and report the PGA and PGV maps for effective emergency-response operations. Model 2 uses records from the 46 strong-motion sites with available V (sub S30) to incorporate a site-effect term, aiming to reduce the standard deviation of the predicted ground motion for engineering applications. First, the results show that the fault-type amplification factor decreases faster with increasing magnitude for the strike-slip fault type than the reverse or normal fault types. In addition, from the intraevent residuals as function of distance of horizontal PGA for reverse earthquakes, we found clusters of higher PGA values at a distance of 110-130 km from events in southwestern areas. This was probably due to Moho reflection along paths crossing the Central Mountain Range. Second, the PGV intraevent residual is more closely correlated with V (sub S30) than that of the PGA because the latter is not a simple function of V (sub S30) . Finally, by comparing the standard deviations of the total residuals between the observed and predicted values before and after incorporating the fault-type and site-effect (V (sub S30) ) terms, the change of standard deviation for PGA is 2.3%. In contrast, the PGV standard deviation is significantly decreased by about 11.6%. Evidently, the Model 2 attenuation relationships, especially for PGV, would be better suited for engineering applications.

Description: The 1999 Chi-Chi, Taiwan, earthquake caused many fatalities and much economic loss for people living in the central part of the island. It also provided many valuable lessons for mitigating future earthquake loss. In this article we have analyzed the human-fatality data from the earthquake in terms of spatial distribution and age dependence to reach the following conclusions. First, rupturing of the Chelungpu fault definitively influenced the spatial distribution of fatalities. Ground ruptures caused by unusually large thrust and left-lateral displacement of the east-dipping fault resulted in almost total destruction of structures on the hanging-wall block along the 100-km-long fault zone. Patterns of damaging ground motion were highly asymmetric about the fault trace. High ground accelerations above 400 gal resulted in high fatality rates up to 1.112% in the sparsely populated rural areas east of the fault. Fortunately, the densely populated urban areas west of the fault suffered substantially lower fatality rates below 0.002% due to low ground accelerations significantly below 400 gal. Secondly, clear age dependence of the human-fatality rate was found from demographic data of the two hardest-hit Nantou and Taichung counties. Results for both counties define almost identical functions that shows people older than age 40 are increasingly more vulnerable with increasing age to life loss in earthquakes. These two conclusions can be applied to make a reliable estimation of the total human fatalities in areas of high seismic intensity either before a large earthquake by performing scenario studies, or shortly after a real earthquake by a system of rapid intensity mapping. Finally, empirical time functions of the cumulative numbers of people found killed, injured, and missing during the first hours following the Kobe, Japan, and Chi-Chi, Taiwan, earthquakes both show that search and rescue operations were critical in the first 48 hr.

Description: Taiwan has a relatively complete catalog of earthquakes since the first seismograph was installed in 1897. However, due to changes in seismographic characteristics, network coverage, and observational practice, the definition and procedure for magnitude determination were different during different time periods. Recognizing a complete catalog of earthquakes with consistent magnitudes is essential for delineating seismicity patterns and assessing seismic hazards for Taiwan; efforts have been made to convert the original magnitudes of earthquakes in Taiwan based on various magnitude scales to a common M (sub L) or M (sub S) magnitude scale. Unfortunately, the M (sub L) or M (sub S) magnitude scales chosen for previous studies all are subject to a fundamental limitation of saturation toward large earthquakes. Besides, these studies are nearly two decades old. In order to avoid this limitation and to follow the current trend, we have chosen in this study to convert original magnitudes of various scales to a common M (sub w) scale. In this study we used two independent methods for magnitude conversion. In the first method we converted the original magnitudes to M (sub w) through empirical relations between these magnitudes and M (sub w) . This magnitude is called the old M (sub w) . In the second method we used the best-fitting a- and b-values to convert the original magnitudes to M (sub w) . This magnitude is called the new M (sub w) . The converted M (sub w) magnitudes from both methods have resulted in significant improvements over the original magnitudes and are in good agreement with each other. Nevertheless, by examining the log (sub 10) N versus M (sub w) plots, we found better linearity and tighter overlap among different time periods for the new M (sub w) than for the old M (sub w) . Thus, we chose the new M (sub w) as the unified magnitude for the catalog of Taiwan earthquakes. Finally, a list of 899 earthquakes from 1900 to 2006 with M (sub w) 〉 or =5.5 is presented (Formula see the earthquake lists available in the electronic supplement to this article). The list is considered complete for the whole period from 1900 to 2006.

Description: Lateral variations of seismic attenuation are investigated using data from a linear seismic array deployed across southern Taiwan. The attenuation parameter t (super *) is obtained by fitting the amplitude spectra of P and S waves with a theoretical spectrum using an omega (super 2) model. The observed t (super *) data are then plotted against the travel time, back azimuth, and focal depth, respectively, to explore the spatial variations of t (super *) for P and S waves. Significant lateral variations of seismic attenuation are found between the eastern mountainous and western plain areas of Taiwan. Large attenuation contrast with depth in the east indicates that an anomalously high-attenuation zone is located in a shallower area north of the array in southeastern Taiwan. A 2D raytracing method is applied to show that the probable depth of the anomalous zone is at about 15-20 km. This anomalous attenuation zone coincides with an area marked by low-P and low-S velocities as well as a total absence of seismicity. The area is also marked by other prominent manifestations of active collision between the Eurasian and Philippine Sea plates, for example, high mountain ranges, rapid uplift, and high heat flows.

Description: In this study we recreated peak ground accelerations (PGA) and peak ground velocity (PGV) distributions for Taiwan by applying the attenuation relations of Liu and Tsai (2005) to calculate the PGA and PGV values for 1989 M (sub w) 〉 or =5.0 earthquakes in a catalog of earthquakes from 1900 to 2008 with homogenized magnitude (M (sub w) ) (Chen and Tsai, 2008). We further combined the PGA and PGV values to obtain corresponding modified Mercalli intensity (MMI) values (Wald, Quitoriano, Heaton, Kanamori, et al., 1999) and their spatial distributions and recurrence intervals. We adopted a logarithmic functional form analogous to the Gutenberg-Richter relation for seismicity to represent the annual frequency of seismic intensity parameters: log (sub 10) (N)=a-blog (sub 10) (PGA), log (sub 10) (N)=a-blog (sub 10) (PGV), and log (sub 10) (N)=a-bI. The regions with high PGA and PGV values are often associated with low b values in these equations. As it is well known that the M (sub w) 7.45 Chi-Chi earthquake of 21 September 1999 had produced high PGA values (in excess of 0.9 g) and PGV values (in excess of 300 cm/s), we used these relations to estimate the Poisson probability distributions in Taiwan for MMI〉 or =VIII (i.e., PGA〉 or =485 g) for recurrence intervals of 30, 50, and 100 years. The results show a wide range of differences in the Poisson probability of MMI〉 or =VIII among different areas of Taiwan. For example, for a 50-year interval, this probability at 10 major cities in Taiwan is as follows: Taipei 0.67%, Hsinchu 2.15%, Taichung 5.24%, Chiayi 24.35%, Tainan 1.61%, Kaohsiung 0.04%, Hengchun 4.94%, Ilan 17.67%, Hualien 37.04%, and Taitung 9.82%. These estimates should be of interest to city planners, especially for earthquake preparedness planning.

Description: The purpose of this paper is to study the site effects for Ilan in northeastern Taiwan based on attenuation relationships of spectral acceleration (SA). Over 2660 accelerograms recorded from 92 shallow earthquakes with M (sub w) magnitude ranging from 4.0 to 7.7 are used to develop these regional attenuation relationships. Site effects are investigated using the site residuals obtained from the SA attenuation relationships. The results show the standard deviations of residuals decrease by 1.7%-16.4% in horizontal SA after incorporating the site-effect term V (sub S30) in the attenuation relationship of periods greater than 0.3 s. Clearly, the attenuation relationships are improved by including the site-effect term, especially for the longer periods. In addition, from the contour maps of intraevent site residuals relative to an attenuation equation of our model 2 phase 1 for horizontal SA, we find high amplification factors of short period (0.1-0.2 s), reaching 2.7 in Nanao with hard schist rocks. High amplification factors of intermediate periods (0.3-1.0 s) reach 1.9 and 2.0, respectively, in the city of Ilan and Zhuangwei. High amplification factors of long period (1.0-3.0 s) reach 2.1 in Zhuangwei, where there is the greatest sediment thickness. The results call for special attention to the seismic design of structures in these locations. Furthermore, the intraevent residuals of the four site classes converging the 0.15-0.2 s period range are in agreement with the findings by Aki (1988). Accordingly, site classes D and E have the largest ground-motion amplification factor in the 0.15-0.5 s and 〉0.5 s period ranges, respectively. Finally, the predominant-period contour maps deduced from V (sub S30) , V (sub S75) , and sediment depth in the Ilan Plain fall in the ranges of 0.4-1.0, 1.0-1.5, and 1.5-8.0 s, respectively. This suggests that construction of buildings taller than nine stories in the Zhuangwei area should pay special attention to avoid resonance of seismic excitation.

Description: A total of 336 three-component strong-motion recordings from the M (sub w) 6.35 Niu Dou earthquake of 25 June 1995 at a focal depth of 39.9 km in northern Taiwan are used to study the effects on strong ground motion due to Moho reflection of S waves. The residuals of both horizontal peak ground acceleration (PGA) and peak ground velocity (PGV) recorded from the earthquake are analyzed. The results confirm that many Class E soft soil stations in the Taipei Basin and the Ilan Plain had the expected large amplification of about 1.7 and 1.5 times, respectively, the predicted median PGA values. Surprisingly, a large group of Class C or D dense and stiff soil sites in Taoyuan (TCU007), Lungtan (TCU013), Guanshi (TCU021), Hsinchu (TCU095), and Miaoli (TCU047) areas in northwestern Taiwan had unusually large amplification of about 3.4-8.1 and 1.7-3.3 times the predicted median PGA and PGV values, respectively. They are interpreted in terms of focusing and interference between SmS waves reflected from the horizontal and inclined portions of an east-dipping Moho discontinuity in this area. This interpretation is supported by the close agreement between the expected amplitudes and arrival times of the largest shear waves with the observed data. Our results suggest that when a damaging earthquake occurs near an inclined Moho boundary, the reflected SmS waves can result in significantly amplified ground motions at distances beginning about 50 km. The exact distance range will depend on the thickness of the crust and the dip angle of the Moho boundary.