Sains Malaysiana 38(1): 69-76(2009)

Analyzing seismogram of the Earthquake C051099C, PNGon UGM,

PMG and CHTO Observation Stations to Understand the S and P Wave Velocity Structure

(Analisis Seismogram Gempa Bumi C051099C, PNG di Stesen Cerapan UGM, PMG

dan CH70 untuk Memahami Struktur Gelombang Halaju S dan P)

Bagus Jaya Santosa

Prodi Geofisika, FMIPA

Institut Teknologi Sepuluh Nopember

Jl. Arif Rahman Hakim 1, Surabaya 60111

Received  :  18 February 2008 / Accepted :  18 March 2008

ABSTRACT

This research, compares the observed seismogram of the PNG earthquake, C051099C with its synthetics in UGM, Wanagama, Indonesia, PMG, Port Moresby, PNG and CHTO, Chiang May, Thailand. The synthetic seismogram is calculated using preliminary earth model, which are the IASPEI91 and the anisotropic version of PREM. The seismogram comparison is conducted after imposing a low pass filter whose corner frequency is fixed at 20 mHz. We have found a real discrepancy on the travel time and waveform of some wave phases, namely P, S and Rayleigh and Love surface waves, by seismogram comparison in time domain. To correct the discrepancies, we need to adjust the earth structures, which include the depth of the Moho reflector, the velocity gradient of b_h, and the propagation velocity of the P and S waves. The correction has been conducted in the earth layering system from the upper mantle down to the CMB, so that the excellent seismogram fitting was obtained for nearly all phases of the targeted waves. The used wave length for analysing is about 150 km. It turns out that the waveform of the body and surface waves is sensitive to the change of velocity structure. The analysis of repetitive ScS depth phases at closer distance stations gives better opportunity to investigate the S velocity structure near the CMB, something not used by other seismologists.

Keywords:  Crust thickness; fitting of P and S; seismogram analysis; ScS and surface waves

ABSTRAK

Penyelidikan ini dibandingkan seismogram yang dicerap semasa gempa bumi PNG, C051099C dengan sintetik di UGM, Wanegame, Indonesia, PMG Port Moresby, PNG dan CHT0, Chiang May, Thailand. Seismogram sintetik dikira menggunakan model bumi awal iaitu  IASPEI91 dan versi anisotropi PREM. Perbandingan seismogram dijalankan selepas mengenakan turas lepasan rendah dengan frekuensi ditetapkan pada 20mHz.  Kami mendapati ketaksamaan dalam masa jalanan dan bentuk gelombang sebahagian fasa iaitu gelombang P, S dan Rayleigh dan Love melalui perbandingan dalam domain masa. Untuk membetulkan ketaksamaan ini, struktur bumi telah diubah termasuklah kedalaman pembalik Mohor, cerun halaju b_h dan halaju rambatan gelombang P dan S.  Pembetulan telah dibuat dalam sistem lapisan bumi dari mantel atas sehingga ke CMB, supaya pemadanan seismogram yang baik di perolehi bagi semua fasa gelombang yang dituju.  Panjang gelombang yang digunakan ialah 150 km. Didapati bentuk gelombang bagi gelombang jasad dan permukaan adalah sensitif terhadap perubahan dalam struktur halaju. Analisis berulang fasa kedalaman ScS pada stesen yang lebih hampir memberikan peluang yang lebih baik untuk mengkaji struktur halaju S berhampiran CMB, sesuatu yang belum pernah digunakan sebelum ini.

Kata kunci: Analisis seismogram; ketebalan kerak; pemadanan P dan S; ScS dan gelombang permukaan

REFERENCES/RUJUKAN

Bagus J.S., 1999. Moeglichkeiten und Grenzen der Modellierung vollstaendiger langperiodischer Seismogramme, Doktorarbeit, Berichte Nr. 12, Inst. fuer Geophysik, Uni. Stuttgart

Bagus J.S., 2001, Mempelajari Model Bumi Berlapis dengan Seismogram, Majalah IPTEK, 13, No 3: 25-33.

Bulland, R. and Chapman, C., 1983, Travel time Calculation, BSSA, 3, 1271-1302.

Dziewonski, A.M. and Anderson, D.L., 1981. Preliminary Reference Earth model, Phys. of the Earth and Plan. Int. 25: 297-356.

Dalkolmo, J., 1993. Synthetische Seismogramme fuer eine sphaerisch symmetrische, nichtrotierende Erde durch direkte Berechnung der Greenschen Funktion, Diplomarbeit, Inst. fuer Geophys., Uni. Stuttgart

Friederich, W. and Dalkolmo, J., 1995. Complete synthetic seismograms for a spherically symmetric earth by a numerical computation of the green's function in the frequency domain, Geophys. J. Int. 122: 537-550.

Gubbins, D., 1990, Seismology and Plate Tectonics, Cambridge University Press, Cambridge.

Kennett, B.L.N., 1991. IASPEI 1991, Seismological Tables, Research School of Earths Sciences, Australian National University.

Keyser, M., Ritter, J.R.R, Jordan , M., 2002. 3-D shear wave velocity structure ofthe Eifel plume, Germany , Earth and Plan. Sci. Letters, 203: 59-82.

Souriau, A. & Poupinet, G., 1991. A study of the outermost liquid core using differential travel times of the SKS, SKKS and S3KS phases, Phys. of the Earth and Plan. Int., 68.  1-2: 183-199.

Wittlinger, G., Vergne, J., Tapponnier, P., Farra, V., Poupinet, G., Jiang, M., Su, H., Herquel, G. and Paul, A., 2004. Seismic imaging of subducting lithisphere and Moho offsets beneath western Tibet , Earth and Plan. Sci. Letters 221: 117-130.

Wysession, M.E., Valenzuela, R.W., Zhu, A. and Bartkö L., 1995. Investigating the base of the mantle using differential travel times, Phys. of the Earth and Plan. Int., 92. 1-2: 67-84.

Zhao, D, 2004. Global tomographic imaging of mantle plumes and subducting slabs: Insight into deep Earth dynamics, Phys. of the Earth and Plan. Int. 146: 3-34.