Chien-Fu Wu

CWB Free-Field Strong-Motion Data from the 21 September Chi-Chi, Taiwan, Earthquake

The Chi-Chi earthquake occurred at 17:47 on 20 September 1999 and is the largest earthquake ( M W 7.6) to have occurred on land in Taiwan in the twentieth century. This earthquake caused considerable damage and was named the “921 Chi-Chi Great Earthquake” by the Taiwan government, as the local date was 21 September. Because an extensive strong-motion instrumentation program in Taiwan was completed by the Central Weather Bureau (CWB) in 1996, over 30,000 digital strong-motion records have been obtained from the Chi-Chi earthquake and thousands of its aftershocks. These records form the largest set of strong-motion data recorded from a major earthquake since strong-motion seismology studies began in the 1930s. This data set is important to seismology and earthquake engineering because it includes over 60 recording sites within 20 km of the fault ruptures, which provides a five-fold increase of such near-field records available for the entire world. A prepublication CD was made available in mid-December 1999, and the data on it have been used by many authors in dozens of articles published so far. Since then, we examined about 10,000 strong-motion records and conducted a first-order quality assurance procedure for all the records obtained on 20 September 1999, including the mainshock and hundreds of the early aftershocks. We performed extensive data processing for quality assurance and selected a total of 663 strong-motion data files from 441 accelerographs to construct strong-motion records up to 4-min long for the mainshock whenever possible. In this article, we present a brief description of the processed acceleration data from the Chi-Chi earthquake. The data set (about 500 megabytes) and an extensive 562-page report (documenting the data processing and results of the processed data) are archived on the attached CD in this Special Issue so that users can quickly access this valuable data set for their research. Manuscript received 7 April 2001.

Review: Progress in Rotational Ground-Motion Observations from Explosions and Local Earthquakes in Taiwan

Rotational motions generated by large earthquakes in the far field have been successfully measured, and observations agree well with the classical elasticity theory. However, recent rotational measurements in the near field of earthquakes in Japan and in Taiwan indicate that rotational ground motions are 10 to 100 times larger than expected from the classical elasticity theory. The near-field strong-motion records of the 1999 Mw 7:6 Chi-Chi, Taiwan, earthquake suggest that the ground motions along the 100 km rupture are complex. Some rather arbitrary baseline corrections are necessary in order to obtain reasonable displacement values from double integra- tion of the acceleration data. Because rotational motions can contaminate acceleration observations due to the induced perturbation of the Earths gravitational field, we started a modest program to observe rotational ground motions in Taiwan. Three papers have reported the rotational observations in Taiwan: (1) at the HGSD station (Liu et al., 2009), (2) at the N3 site from two TAiwan Integrated GEodynamics Research (TAIGER) explosions (Lin et al., 2009), and (3) at the Taiwan campus of the National Chung-Cheng University (NCCU )( Wuet al., 2009). In addition, Langston et al. (2009) reported the results of analyzing the TAIGER explosion data. As noted by several authors before, we found a linear relationship between peak rotational rate (PRR in mrad=sec) and peak ground acceleration (PGA in m=sec 2 ) from local earthquakes in Taiwan, PRR 0:002 1:301 PGA, with a correlation coefficient of 0.988.

Revised ML Determination for Crustal Earthquakes in Taiwan

The local magnitude scale M L is an empirically derived scale anchored to a zero magnitude reference event. Richter defined the amplitude of ground shaking at an epicentral distance of 100 km for a zero magnitude earthquake in southern California. The crustal attenuation characteristics of Taiwan result in a relatively high ML when compared to moment magnitude, Mw. We therefore define a revised ML scale for crustal earthquakes in the Taiwan region that is more consistent with Mw. Using observed peak ground shaking and Mw, we determine a new log A0 curve of ground shaking versus hypocentral distance, R, for a zero magnitude earthquake, and station correction factors. The log A0 curve determined in this study is, log A (R) 0.332 1.568 • log(R) 0.280. 0 Using this new log A0 curve and station corrections, the new ML is more consistent with Mw, with a 0.2 magnitude unit uncertainty. The new log A0 curve has a value of 2.80 at the distance of 100 km compared to the anchor point of log A0 3 at the same distance as defined by Richter for southern California. This means that the current ML estimates in Taiwan (which use Richters definition) average 0.2 magnitude units larger than their Mw. The station correction factors also determined are large, from 0.40 to 0.52 magnitude units. The use of station corrections in routine seismic network operation in Taiwan will improve magnitude estimates. This is particularly important for smaller events when recording stations may be pre- dominantly on either hard rock or soft soil sites, which could lead to under- or overestimates of the magnitude by up to half a magnitude unit.

Data Files from “CWB Free-Field Strong-Motion Data from the 21 September Chi-Chi, Taiwan, Earthquake”

The Central Weather Bureau (CWB) of Taiwan completed a deployment of 1200 modern digital strong-motion instruments in 1996 at free-field sites and in buildings and bridges. Consequently, a very extensive set of strong-motion records were obtained for the M W = 7.6 Chi-Chi earthquake in 1999, including over 60 near-field records within 20 km of the fault ruptures. For documentation purposes, we included all relevant data files on the attached CD-ROM from our article, “CWB Free-Field Strong-Motion Data from the 21 September Chi-Chi, Taiwan, Earthquake” in this issue. We presented the data in four different ways to make the data more user friendly: (1) the original recorded data by accelerograph type; (2) the processed data in ASCII text format; (3) the processed data in SUDS format; and (4) the processed …