REPORT #1 on the Chi-Chi (Taiwan) Earthquake

(as of 6 a.m., Sept. 22, 1999)

Prepared by W. H. K. Lee with seismic data kindly provided by Dr. K. W. Kuo, Director of the Seismology Center, Central Weather Bureau (CWB), Taipei, Taiwan and his staff.

CWB, Taipei

A preliminary note was written by Lee on the night of Sept. 20, based on the information received in e-mails from the CWB AutoLocation system. This is the first report based on some seismic waveform data and preliminary results that Lee had received. More specific reports containing tables, graphs, and maps will be prepared later today.

INTRODUCTION

A major earthquake occurred near Chi-Chi in the Nantou county, Taiwan, on 1:47 a.m., Sept. 21, local time, about 90 miles south of Taipei. The death toll has exceeded 1800 and is mounting; thousands of houses collapsed making more than 100,000 people homeless. It is a major disaster. This big quake provided a wealth of modern digital data for seismology and earthquake engineering because an extensive six-year, strong-motion instrumentation program for Taiwan was successfully implemented 3 years ago.

Very good estimate of the hypocenter and magnitude was determined automatically in about 100 seconds (after the earthquake's origin time), and I received this information in an e-mail, several minutes later. Thousands of aftershocks have been recorded.

PGA VALUES NEAR THE EPICENTER

Strong-motion records have been retrieved at two stations in the epicentral area (epicentral distance of 8 and 10 km) with the following PGA values. Strong ground motion lasted over 30 seconds.

  • Station TCU078: 0.17 g (vertical), 0.30 g (NS), and 0.44 g (EW).
  • Station TCU129: 0.34 g (vertical), 0.61 g (NS), and 0.98 g (EW).

    INSTRUMENTATION IN OPERATION

    In 1990, Professor Ta-Liang Teng of the University of Southern California and Professor Yi-Ben Tsai (now at the National Central University of Taiwan) persuaded the local government to fund an extensive seismic instrumentation program for the urban areas of Taiwan with mostly strong-motion instruments. After 6 years and about $40 million US dollars later, the Seismology Center of the Central Weather Bureau completed the installation in 1996 of:

  • About 700 modern digital accelerograhs in free-field sites,
  • About 50 realtime seismic arrays (up to 60 accelerometers) in representative buildings and bridges,
  • A rapid earthquake information release system based on 60+ telemetered digital accelerographs, and
  • A prototype earthquake early warning system in Hualien.

    This program was executed by the CWB's Seismology Center under the direction of Dr. T. C. Shin (now Deputy Director-General of CWB). An advisory committee consists of about 10 seismologists and earthquake engineers assisted CWB in the planning of the program and the design of the instrumentation. Cooperative projects with the U. S. Geological Survey (1991-1995), and with the Southern California Earthquake Center (1991-present) greatly accelerated the implementation and successful operation.

    Taiwan is a small island, about 8% of the area of California or Japan. With the above instrumentation, Taiwan operates the densest digital strong-motion instruments of the world. For comparison, station spacing of the free-field accelerographs in Taiwan is about 3 km in the metropolitan area (vs a 25-km uniform spacing of K-Net in Japan). The basic design of the realtime strong-motion array system for structures has since been implemented by PG&E for the headquarter building in San Francisco, and is now being implemented in the Millikan Library building of Caltech and in a building at UCLA.

    CWB's Rapid Earthquake Information Release System (AutoLocation) began operation in March 5, 1996. Continuously telemetered data from 60+ digital accelerographs were automatically processed in a dual PC-based system (for redundancy) in Taipei, using hardware and software designed and implemented in the early 1990's (Lee et al., 1996; Shin et al., 1996). Early results had been published by Teng et al. (1997), and by Wu et al. (1997).

    REFERENCES

    Lee WHK, Shin TC and Teng TL (1996). Design and implementation of earthquake early warning systems in Taiwan. Proc. 11th World Conf. Earthq. Eng., Paper No. 2133.

    Shin TC, Tsai YB, and Wu YM (1996). Rapid response of large earthquake in Taiwan using a realtime telemetered network of digital accelerographs. Proc. 11th World Conf. Earthq. Eng., Paper No. 2137.

    Teng TL, Wu L, Shin TC, Tsai YB, and Lee WHK (1997). One minute after: strong motion map, effective epicenter, and effective magnitude. Bull. Seism. Soc. Am., vol 87, p 1209-1219.

    Wu YM, Shin TC, Chen CC, Tsai YB, Lee WHK, and Teng TL (1997). Taiwan rapid earthquake information release system. Seism. Res. Letters, vol 68, p 931-943.

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    Date created: 09/22/1999
    Last modified: 01/15/2002

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