#TITLE_ALTERNATIVE#

• Main Author: AHADI (NIM: 32409006); Tim Pembimbing : Prof. Dr. A.Nanang T. Puspito; Dr. Gunawan Ibrahim; , SUAIDI
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/19499
• Institution: Institut Teknologi Bandung

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 &#8804; 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). <br /> <br /> <br /> Magnetometer data sampling was 1 Hz or 1 data/sec, so the Nyquist frequency was &#8804; 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. <br /> <br /> <br /> 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 <diff <0.5), which was the spectrum ratio of the same component used at the monitoring stations of KTB and at both reference stations of DAV and DAW, which was diff Hktb/Href and Zktb/ Zref. The onset time of earthquakes is determined by the standard deviation (pz + &#61540;, pz-&#61540; ) of the ratio SZ / SH. The onset time of seismogenic activities was first determined when the activities were registered below the standard deviation pattern. The onset time of the earthquake until the moment the event occurred was used to analyze Single Station Transfer Function, which is the inversion method of the X, Y and Z components that aim to determine the direction of anomalous conductivity of the earthquake source. The results of this study used the frequency spectrum of seismogenic activities, i.e. f= 0.02 -0.06 Hz. The onset time of anomalous ULF emissions was found during a short period (<1 month) before the time of the earthquake. The typical pattern of anomalous ULF emissions was found in the form of increased ULF emissions a few days before the earthquake. This relies on the earthquake’s energy sources (magnitude, seismic index, volume of rupture, rupture radius, and the preparation radius), and the hypocenter distance to the KTB geomagnetic station. <br /> <br /> <br /> 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 <R < 0.399) and no significant (F test = 0.133 > 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. <br /> <br /> <br /> 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.