EIC Seismological Note No. 66 Sep. 21, 99

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Preliminary Results of Teleseismic Body Wave Analysis

Source Rupture Process of the central Taiwan earthquake of September 21, 1999

 Outline 
A devastating earthquake (Ms 7.7) struck the central area of
Taiwan before the dawn of September 21, 1999.
QED (quick epicenter determination) information due to USGS is:

Origin Time  17:47:19, Sep. 20, 1999 (UT)
Epicenter    23.8N, 121.1E
Depth        5 km

Data and Analysis
   Vertical components of P-wave data at IRIS stations were
retrieved and converted into ground displacement.  With a fixed
fault mechanism, we inverted the teleseismic data into a series of
subevents using Kikuchi and Kanamori (1991)'s method.  We used the
mechanism solution of Harvard CMT.
   Before the waveform inversion, we adjusted the P-wave arrival time
inspecting the data and relocated the epicenter.

Results
   The results are shown in Fig. 1.  Main source parameters derived
are as follows:

   Epicenter = 23.82N, 120.89E
   (strike, dip, slip) = (26, 27, 82)  (HRV CMT)
   Seismic moment  Mo = 2.4 x10**20 Nm (Mw = 7.5)
   Source duration T = 28 s
   Centroid depth H = 11 km
   Rupture area  S = 80 km x 40 km
   Averaged dislocation D = 2.2m
   Averaged stress drop = 3.3 MPa


   The mechanism reveals a reverse fault with a strike of NNE-SSW.
We took a shallow dipping plane as a fault plane where the eastern
side rode over the western side.   The results indicates that a small
introductory rupture with a duration of 10 s was followed by a few
bigger subevents.  The rupture front velocity is about 2.5 km/s.
The rupture eventually swept over the length of 50 km to NE and 30 km
to SW and the width of about 40 km.
   The total moment release of this earthquake source is more than
10 times as large as that of the Hyogo-ken Nanbu, Japan  earthquake
of Jan. 17, 1995.
                                        (M. Kikuchi and Y. Yamanaka)

Refer also to the result by Yagi & Kikuchi (1999).