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.
This is the 2nd report in which I will report on the main shock and early aftershocks. I will also present two near source accelerograms.
The Taiwan Rapid Earthquake Information Release System (RTD) automatically determined the parameters for the main shock 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 the primary tool used for routine earthquake location. 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: 23.85 120.81 7 ML=7.3 results in ~30 min.
USGS: 23.78 121.09 5 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 from the S13 network, and is given in Figure 1 below:
This focal mechanism differs significantly from the USGS moment tensor solution. However, one of the nodal plane agrees:
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 later), we suggest that faulting occurred along this nodal plane.
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.
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 of the earthquake. The RTD system waits until the signal amplitudes have decayed sufficiently that no large amplitudes will be expected.
As the seismic waves from the main shock 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.
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).