EIC Seismological Note No. 66 Sep. 21, 99
Source Rupture Process of the central Taiwan earthquake of September 21, 1999
OutlineA 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 AnalysisVertical 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.
ResultsThe 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).