EIC Seismological Note No. 78 Jun. 6, 2000

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Preliminary Results of Teleseismic Body Wave Analysis

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Source Rupture Process of the Sumatera,

Indonesia, earthquake (Ms 7.9) of June 4, 2000

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< Outline >

  Late at night of June 4, a large earthquake (Ms 7.9) occurred
near the coast of southern Sumatera, Indonesia.  It caused more
than one hundred people dead and more than several hundreds injured. 
QED (quick epicenter determination) information due to USGS is:

Origin Time  16:28:25.6 (UT)
Epicenter    4.73 S, 102.05 E
Depth        5 km
Ms           7.9

< Data and Analysis >

   Vertical components of P-wave data at IRIS stations were
retrieved and converted into ground displacement.  The waveforms
suggest very complicated rupture process with mechanism change.
For the moment, however, we deterimined a fault mechanism for
the overall source process using point source approximation.
Next, with the fault mechanism fixed, we inverted the teleseismic
data into a series of subevents using Kikuchi and Kanamori (1991)'s
method. 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 = 4.56 S,  102.56 E
   (strike, dip, slip) = (300, 31, 90)
   Seismic moment  Mo = 7.8 x10**20 Nm (Mw = 7.9)
   Source duration T = 61 s
   Centroid depth H = 50 km
   Rupture area  S = 120 km x 60 km
   Averaged dislocation D = 1.5 m
   Averaged stress drop = 3.2 MPa

   The fault mechanism reveals a low-angle thrust with a strike of
NW-SE, suggesting that the present earthquake is an inter-plate
earthquake along a subduction zone (the Java Trench). However,
there are some discrepancies among the solutions derived by USGS
and Harvard Univ., and ours.  At this moment, we don't know the reason.
                                      (M. Kikuchi and Y. Yamanaka)