REPORT #2 on the Chi-Chi (Taiwan) Earthquake

Mainshock and early aftershocks; Near source accelerograms;

(as of 12 p.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

INTRODUCTION

This is the 2nd report in which I will report on the mainshock and early aftershocks. I will also present two near source accelerograms.

PARAMETERS FOR THE MAINSHOCK

The Taiwan Rapid Earthquake Information Release System (RTD) automatically determined the parameters for the mainshock in about 100 seconds after the earthquake's origin time. I received the information in an e-mail several minutes later.

CWB also operates a digital short-period telemetered seismic network (S13) which is the primary tool used for routine earthquake monitoring. The following table compares these two results, as well as that from the moment tensor solution by USGS using global digital stations around the world.

  • RTD: 23.87 N, 120.75 E, 10 km, ML=7.3, results in 102 sec.
  • S13: 23.85 N, 120.81 E, 7 km, ML=7.3, results in ~30 min.
  • USGS: 23.78 N, 121.09 E, 5 km, Mw=7.6, results in ~3 hours.

    The epicenter, focal depth, and local magnitude values are essentially the same from the RTD or the S13. The latitude and focal depth agree well with the USGS solution, but the longitude differs for about 25 km. However, we may be talking about two different things: first P-arrivals give the initial point of rupture, whereas the moment tensor solution gives the point of the largest energy release.

    Amplitudes from the short-period instruments are saturated, but RTD's ML value agrees well with the Mw value from the moment tensor solution.

    Focal mechanism was determined from first P-motions data of the S13 network, and is given in Figure 1 below:

    chi-chi2fig1

    Figure 1. Focal mechanism of the mainshock as determined from first P-motions.

    This focal mechanism differs significantly from the USGS moment tensor solution. However, one of the nodal planes agrees:

  • S13: Strike = 216 degrees, Dip = 67 degrees
  • USGS: Strike = 202 degrees, Dip = 63 degrees

    Since this nodal plane agrees with the general strike of the fault in the area and the trend of the early aftershocks (see Figure 2 below), we suggest that faulting occurred along this nodal plane.

    Early Aftershocks as Determined by the RTD System

    The following lists the results from the Taiwan Rapid Earthquake Information Release System (RTD) as received in e-mails up to the afternoon of September 22. Results are probably not complete as the e-mails system from Taiwan has not yet back to normal.

    Date

    Origin Time

    Latitude

    Longitude

    Depth (Km)

    ML

    Proc. Time

    9/20/99

    17:47:15.89

    23.87 N

    120.75 E

    10.0

    7.3

    102 sec.

    9/20/99

    17:57:17.16

    23.95 N

    121.03 E

    6.9

    6.1

    57 sec.

    9/20/99

    18:03:42.64

    23.80 N

    120.85 E

    5.0

    6.5

    75 sec.

    9/20/99

    18:11:27.71

    24.03 N

    120.98 E

    5.2

    5.2

    56 sec.

    9/20/99

    18:16:21.01

    23.86 N

    121.03 E

    13.5

    6.9

    85 sec.

    9/20/99

    18:21:31.57

    23.99 N

    121.07 E

    8.9

    5.1

    50 sec.

    9/20/99

    18:32:55.60

    23.82 N

    121.01 E

    2.9

    5.2

    62 sec.

    9/20/99

    19:28:43.48

    23.88 N

    120.97 E

    2.4

    4.9

    53 sec.

    9/20/99

    19:40:33.27

    23.57 N

    120.88 E

    2.5

    5.4

    55 sec.

    9/20/99

    19:57:52.19

    24.17 N

    120.75 E

    6.9

    5.9

    76 sec.

    9/20/99

    20:02:15.36

    24.01 N

    120.67 E

    2.5

    5.7

    69 sec.

    9/21/99

    01:37:09.01

    23.73 N

    120.75 E

    2.5

    5.2

    55 sec.

    9/21/99

    02:24:46.48

    23.93 N

    121.00 E

    8.4

    4.9

    56 sec.

    9/21/99

    03:31:49.58

    23.99 N

    121.02 E

    3.1

    5.1

    58 sec.

    9/21/99

    06:36:27.73

    24.02 N

    120.99 E

    6.9

    4.7

    48 sec.

    9/21/99

    07:06:03.24

    23.79 N

    121.42 E

    13.1

    5.2

    53 sec.

    9/21/99

    07:46:04.79

    24.07 N

    121.12 E

    3.7

    5.2

    57 sec.

    9/21/99

    08:03:18.37

    23.65 N

    120.64 E

    11.0

    5.0

    50 sec.

    9/21/99

    09:10:17.66

    23.85 N

    120.84 E

    1.0

    4.7

    50 sec.

    9/21/99

    09:41:50.65

    23.92 N

    121.09 E

    24.1

    4.7

    49 sec.

    9/21/99

    09:56:04.62

    23.81 N

    120.95 E

    1.0

    4.5

    55 sec.

    9/21/99

    11:07:42.03

    23.67 N

    120.84 E

    5.9

    5.1

    46 sec.

    9/21/99

    11:39:34.87

    23.63 N

    120.72 E

    12.4

    4.9

    47 sec.

    9/21/99

    14:29:21.04

    24.08 N

    121.26 E

    13.3

    4.5

    22 sec.

    9/21/99

    14:32:14.13

    24.07 N

    121.38 E

    21.4

    5.0

    58 sec.

    9/21/99

    14:40:04.27

    24.32 N

    120.83 E

    2.5

    5.0

    62 sec.

    9/21/99

    15:28:10.85

    23.61 N

    120.85 E

    2.5

    5.1

    49 sec.

    9/21/99

    15:47:22.70

    23.80 N

    120.96 E

    6.5

    4.6

    51 sec.

    9/21/99

    17:38:36.77

    23.81 N

    121.32 E

    13.7

    5.2

    53 sec.

    9/21/99

    18:18:37.47

    24.17 N

    121.01 E

    2.5

    5.0

    53 sec.

    9/21/99

    18:58:40.11

    23.73 N

    120.78 E

    1.6

    4.7

    50 sec.

    9/21/99

    22:17:01.25

    23.90 N

    121.14 E

    1.7

    5.4

    21 sec.

    9/21/99

    22:17:03.78

    23.95 N

    121.39 E

    9.9

    5.1

    52 sec.

    9/21/99

    23:53:01.04

    23.87 N

    120.97 E

    5.8

    4.4

    55 sec.

    9/22/99

    00:14:40.91

    23.84 N

    121.04 E

    7.5

    6.8

    107 sec.

    9/22/99

    00:25:46.69

    23.73 N

    121.00 E

    19.6

    5.1

    47 sec.

    9/22/99

    00:31:58.06

    24.11 N

    121.35 E

    5.0

    4.2

    23 sec.

    9/22/99

    00:49:44.22

    23.75 N

    121.02 E

    6.3

    6.2

    76 sec.

    9/22/99

    02:19:33.00

    23.78 N

    121.41 E

    11.6

    5.2

    57 sec.

    9/22/99

    03:08:59.30

    23.65 N

    120.82 E

    1.7

    4.9

    49 sec.

    9/22/99

    03:47:48.58

    23.83 N

    121.02 E

    8.5

    4.6

    61 sec.

    9/22/99

    06:08:05.41

    24.05 N

    121.22 E

    5.0

    4.2

    18 sec.

    9/22/99

    06:10:03.24

    23.62 N

    120.82 E

    1.0

    4.7

    47 sec.

    9/22/99

    09:15:56.62

    23.96 N

    121.21 E

    13.8

    4.4

    20 sec.

    9/22/99

    12:35:48.35

    23.65 N

    120.69 E

    1.0

    5.0

    50 sec.

    9/22/99

    13:11:26.36

    24.28 N

    121.16 E

    5.6

    4.9

    22 sec.

    9/22/99

    13:30:16.50

    23.63 N

    120.83 E

    11.7

    4.6

    50 sec.

    -------------------------------------------------------------

    An epicenter map of the mainshock and the early aftershocks as listed above is given in Figure 2 below with the location of the free-field digital accelerographs in Taiwan. Some of the events listed above and plotted next are probably not aftershocks, but usual background earthquakes.

    chi-chi2fig2

    Figure 2. The mainshock and early aftershocks are shown with the accelerograph sites shown in the background.

    Performance of the RTD System

    The last column in the table above is the Proc Time, or processing time that the RTD took to obtain the hypocenter and magnitude. It is the elapsed time from the earthquake's origin time to the time the results are sent to the e-mail server. This Proc Time varies from about 20 seconds to 100 seconds and depends on many factors, most important of all is the size and location of the earthquake with respect to the stations. The RTD system waits until the signal amplitudes have decayed sufficiently that no large amplitudes will be expected.

    As the seismic waves from the mainshock were so violent that many phone lines used in the telemetry were out shortly after the P-wave arrived. Consequently, the RTD system suffered from the lost of some stations. This indicates that a realtime seismic system performs no better than that of its weakest link. Fortunately, with sufficient number of stations, the RTD system performed reasonably well under this handicapped situation.

    Accelerograms from Two Near Source Free-Field Stations

    As reported in Report #1, 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).

    Now I have obtained the actual digital records from CWB. They are shown in Figure 3 and Figure 4 below.

    Station TCU078 is located 8 km SE from the epicenter and Station TCU129 is located 10 km NW from the epicenter. Please note that no time correction has been applied yet, and further analysis will be made in the future.

    In both cases, amplitudes of the EW-component are larger than that in the NS-component. Please note that these two accelerograms are plotted in different scales.

    chi-chi2fig3

    Figure 3. Accelerogram from Station TCU078 at epicentral distance of 8 km.

    chi-chi2fig4

    Figure 4. Accelerogram from Station TCU129 at epicentral distance of 10 km.

    The URL of this page is http://www.diggles.com/chi-chi/chi-chi2.html
    Date created: 09/23/1999
    Last modified: 01/15/2002

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    c/o U.S. Geological Survey, MS-977, 345 Middlefield Road, Menlo Park, CA 94025. (650) 329-4781 email to Willie

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