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ANALISIS POLA PREKURSOR GEMPA BUMI KUAT SUMATRA PERIODE 2007-2012 BERDASARKAN EMISI ULF (Ultra- Low-Frequency) MENGGUNAKAN DATA GEOMAGNET

PhD Theses from JBPTITBPP / 2017-09-27 15:42:20
Oleh : SUAIDI AHADI (NIM: 32409006); Tim Pembimbing : Prof. Dr. A.Nanang T. Puspito; Dr. Gunawan Ibrahim; Prof. Dr. Sarmoko Saroso, S3 - Mathematics and Natural Sciences
Dibuat : 2014, dengan 7 file

Keyword : Sumatra, gempa bumi kuat, prekursor, waktu pendek, polarisasi ratio dan Lead time

Sebelas gempa bumi kuat (Mw > 5,0) di Sumatra pada periode 2007 – 2012 dipilih untuk studi analisis pola emisi Ultra Low Frequency (ULF) menggunakan data geomagnet yang berkaitan dengan prekursor gempa bumi. Tujuan dalam penelitian ini adalah mencari korelasi tren antara magnitudo gempa bumi dan jarak hiposenter terhadap lamanya waktu emisi anomali ULF (lead time). Pemilihan data gempa bumi yang di investigasi tersebut memiliki jarak hiposenter ≤ 550 km dari stasiun Kototabang (KTB). Di antara gempa bumi terpilih terdapat gempa bumi merusak, yaitu gempa bumi Padang 2009 (Mw=7,6) dan gempa bumi Mentawai 2010 (Mw= 7,8). Penelitian ini menggunakan stasiun referensi yang mewakili kondisi variasi medan geomagnet di ekuator, yaitu Davao (DAV) di Filipina dan Darwin (DAW) di Australia. Hal ini diperlukan untuk meyakinkan bahwa sumber emisi anomali ULF berasal dari aktivitas geomagnet global atau efek dari seismogenik. Indeks geomagnet Dst (Disturbance storm time) digunakan untuk mengetahui gangguan geomagnet di lintang rendah dan di ekuator dari aktivitas geomagnet global.


Data sampling magnetometer adalah 1 Hz atau 1 data/detik, sehingga frekuensi Nyquist ≤ 0,5 Hz. Analisis spektrum frekuensi pada saat hari gangguan (disturbance day), hari tenang (quiet day) maupun saat co-seismic dilakukan untuk mengetahui respons frekuensi yang dihasilkan pada ketiga fenomena tersebut. Analisis pola emisi ULF yang berkaitan dengan gempa bumi terpilih dilakukan pada setiap stasiun baik stasiun KTB maupun kedua stasiun referensi DAV dan DAW. Hal ini bertujuan untuk menentukan spektrum frekuensi yang digunakan untuk menentukan prekursor gempa bumi.


Metodologi yang digunakan untuk menganalisis pola emisi ULF adalah analisis pemrosesan spektral PSD (Power Spectrum Density) dengan memilih spektrum frekuensi yang diinginkan. Perhitungan yang digunakan adalah dengan metode Welch untuk Fs=1 Hz, lebar jendela =1024 dan mengunakan tipe jendela Hamming untuk komponen H dan Z. Hal ini dilakukan untuk analisis polarisasi power ratio SZ/SH. Selain itu digunakan metode diff (-0,5 0,05). Tanda prekursor gempa bumi ditunjukkan dengan adanya pola peningkatan emisi ULF secara sporadis sebelum terjadi gempa bumi.


Hasil penelitian diharapkan di kemudian hari dapat dibangun sistem peringatan dini gempa bumi di Sumatra berbasiskan pengamatan anomali emisi ULF geomagnet. Pada saat ini berkat kerjasama BMKG-ICSWSE (Kyushu University)-LAPAN telah terpasang tiga stasiun magnetometer di Sumatra, yaitu di Liwa (LWA), Gunungsitoli (GSI) dan Sicincin (SCN). Penelitian ini merekomendasikan agar menambahkan dua stasiun lagi yaitu di Meulaboh (MLB) Aceh dan Kepahiang (KPH) Bengkulu agar terbentuk cluster yang baik di Sumatra.

Deskripsi Alternatif :

Eleven strong earthquakes (Mw> 5.0) that hit Sumatra during 2007-2012 have been chosen in order to study the emission pattern of their Ultra Low Frequency (ULF) geomagnetic data that is related to earthquake precursors. The earthquakes were ≤ 550 km from the Kototabang station (KTB). Among the selected earthquakes, were devastating ones that happened in Padang in 2009 (Mw = 7.6) and in Mentawai in 2010 (Mw = 7.8). The locations of the reference stations in Davao (DAV), the Philippines, and Darwin (DAW), Australia, were selected because they both represent variations of the equatorial geomagnetic fields. They were also needed as reference stations to distinguish the source of the ULF emissions from global geomagnetic activities or seismogenic effects. The Dst geomagnetic indices (Disturbance storm time) were used to monitor the geomagnetic disturbances at low latitudes and at the equator. The purpose of this research is to find a trend correlation between the magnitude of the earthquake and hypocenter distance against the time duration of the ULF anomalous emissions (lead time).


Magnetometer data sampling was 1 Hz or 1 data/sec, so the Nyquist frequency was ≤ 0.5 Hz. The analysis of the frequency spectrum was conducted during global geomagnetic disturbances; on quiet days; and during the co-seismic activity to determine the frequency generated by the three different conditions. The analysis of the ULF emission patterns associated with earthquakes was done at the KTB station and the DAV and DAW reference stations. This research is also aimed to determine the earthquake precursors.


The methodology used to analyze the pattern of ULF emissions applied spectral processing analysis of power spectrum density (PSD) by selecting the desired frequency spectrum. The calculation used to analyze the SZ/SH polarization power ratio was the Welch method for Fs=1 Hz, with window width=1024, and the window type performed on the components H and Z was Hamming. In addition, this study used diff (-0.5

The results show a positive trend (+) with moderate corelation (moderate = 0.4 < R<0.599) and significant (F test = 0.01< 0.05) on the relationship between the earthquake energy against the duration of the ULF emissions (lead time); and has a negative trend (-) with low corelation (low = 0.2 0.05) on the hypocentre distance of the earthquake against the duration of the earthquake ULF emissions. This means that when there are two earthquakes that have the same magnitude but different focal distances, the emission of the earthquake that has the closest focal distance has a longer period compared to the earthquake with a farther focal distance.


Thus, the results of the study can be used as an option to develop an early warning system using earthquake precursors that are based on a network of geomagnetic stations. In terms of Sumatran earthquake precursor studies, geomagnetic network construction to study earthquake precursors was started in late 2012, between BMKG and ICSWSE (International Center Space Wheather and Education) University of Kyushu Japan. They built the first three stage magnetometers of Liwa (LWA, Lampung), Sicincin (SCN, West Sumatra), and Gunungsitoli (GSI, Nias). We recommend adding two stations in Meulaboh (MLB) Aceh and in Kepahiang (KPH) Bengkulu in order to form cluster in Sumatra.

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  • Tim Pembimbing : Prof. Dr. A.Nanang T. Puspito; Dr. Gunawan Ibrahim; Prof. Dr. Sarmoko Saroso, Editor: PKL-SMK

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