The Estimation of Seismic Site Amplification of Bukittinggi City, West Sumatera, Indonesia

Bukittinggi city area is exposed substantial risk of both, close to seismic source line and site amplification due to soft surface sediment / soil layer. The Great Sumateran Fault (GSF) which is crossing this area, notorious as a very active seismic source. Its responsible to some major earthquake in the vicinity of the fault line. Generally, Bukittinggi area is covered by volcanic product as pumiceous tuff. We applied combined array and single station microtremor measurement to characterize near surface sediment in this area. Based on analysis microtremor single station and array using spatial autocorrelation (SPAC) method, weathered surface layer thickness is in the range of 0-108 m with shear wave velocity in the range of 62-190 m/s, while the fresh pumiceous tuff is in the range of 375-629 m/s. The estimation of site amplification in this area which is indicated by mean amplification is in the range of 1-1.95. The high amplification zone is occupies the northernmost area and some area locally in the southern part of Bukittinggi city.

Microzonation of Cisarua District Using Horizontal Vertical Spectral Ratio

The Bandung region is part of the framework of the Indonesian tectonic system, namely the tectonic plate meeting zone, where the Indo Autralia plate is infiltrated under the Eurasian plate in a convergent manner. The subduction process produces an effect in the form of an active fault geological structure in the Bandung area. One of these active faults is the Lembang Fault, which has a length of ± 29 kilometers and a shear acceleration of 3 to 5.5 millimeters per year. The microtremor measurement method is a passive geophysical method that utilizes natural subsurface vibrations so that it can provide dominant frequency data and amplification factors for soil layers. Based on the results of seismic susceptibility research using microtremor measurements using the HVSR method in the Lembang Fault zone in Cisarua Sub-District, it can be seen that the distribution of the dominant frequency values tends to be influenced by lithology and topography. In the research area, it is known to have a dominant frequency value that varies due to the different types of lithological units. In general, the dominant frequency ranges from 1-3 Hz because it is dominated by tuff sand and tuff pumice, and areas composed of volcanic breccias have a dominant frequency value between 3-6 Hz. Meanwhile, the amplification factor value will be influenced by rock deformation and weathering. The area that has a very high amplification factor value is in the southeast of the study area with an A0 value greater than 5. This indicates that the area is composed of a layer of thick and not dense tuff sand

Mikrozonasi di Daerah Plampang, Nusa Tenggara Barat

ABSTRAK Provinsi Nusa Tenggara Barat (NTB) merupakan daerah dengan aktivitas kegempaan yang tinggi. Fenomena ini disebabkan oleh adanya aktivitas tektonik sebagai akibat pertemuan Lempeng Eurasia-Australia (zona subduksi) di bagian selatan dan Sesar Flores di bagian utara serta adanya keberadaan sesar-sesar lokal. Terkait dengan rencana pengembangan kawasan Samota di Pulau Sumbawa, NTB, perlu dilakukan suatu kajian kegempaan. Tujuan penelitian ini adalah memetakan indeks kerentanan seismik (Kg) melalui pengukuran mikrotremor dengan analisis menggunakan metode Horizontal to Vertical Spectral Ratio (HVSR). Hasil penelitian berupa peta kerentanan seismik daerah Plampang yang menunjukkan bahwa sisi utara lokasi penelitian memiliki indeks kerentanan seismik rendah yang ditandai dengan nilai amplifikasi kurang dari tiga jika dibandingkan daerah lainnya. Geologi sisi utara lokasi penelitian tersusun oleh batuan gunung api dengan karakteristik batuan keras, ketebalan sedimen sangat tipis, dan tersusun atas batuan Tersier atau lebih tua. Peta kerentanan seismik berguna sebagai acuan dalam mitigasi gempa bumi untuk mengurangi risiko yang ditimbulkan. ABSTRACT Nusa Tenggara Barat (NTB) Province is an area with intense seismic activity. This phenomenon is caused by tectonic activity as the result of the convergency of the Eurasia-Australia Plates (subduction zone) in the south and the Flores Fault in the north as well as the presence of local faults. Regarding the plan to develop the Samota area in Sumbawa Island, NTB, a study concerning earthquakes should be done. The purpose of this study is to map the seismic vulnerability index (Kg) through microtremor measurement by using the Horizontal to Vertical Spectral Ratio (HVSR) analysis method. The result of the study is a seismic vulnerability map of the Plampang area which its northern part has a low seismic vulnerability index indicated by the amplification factor value of less than three compared to other areas. The geology of the northern part of the Plampang area consists of volcanic rocks which has hard rock characteristic, very thin sediment thickness, and composed of Tertiary or older rocks. Seismic vulnerability maps can be useful as a reference for earthquake mitigation to reduce its risks.

The Soft Layer Thickness Estimation using Microtremor Measurement to Identify Landside Potential in Watukumpul, Central Java, Indonesia.

Watukumpul is an area that is prone to landslides, so determining the soft layer thickness is very important to identify the landslide potential. The soft layer thickness can be estimated using microtremor signal measurements which analyzed using the Horizontal to Vertical Spectral Ratio (HVSR). In this study,we measured microtremor signal of 33location around Watukumpul, Pemalang, Central Java area to determine soft layer thickness. Micretremor signal was analyzed based on theHVSR method using Geopsy software and follow the standard of the Sesame Europan Project. The results of the HVSR method are the HVSR curve that fulfills the reliable curve standard. HVSR curve shows that the dominant frequency of soft layer ranges from 1.36 – 7.62 Hz and the amplification values ranges from 9.00 – 41.45. The soft layer thickness value in the study area ranges from 17.58 - 103.60 meters. The high landslide potential area are located at W7, W8, W18, W30 and W32 where has thin soft layer and high soil slope.

Mitigasi Bencana Berdasarkan Nilai Shear Strain Tanah Akibat Gempa Bumi Pada Tempat Pembuangan Akhir (TPA) Ngipik, Gresik

Ngipik Landfill located in Gresik, East Java has a several active fault. Due to the condition, this area has high possibility of earthquake prone area. There is no bottom layer construction to prevent and control the leachate’s spread. This study aim to analyze the earthquake hazard map for the region based on the values of ground shear strain. The data was conducted by microtremor measurement and Plaxis modeling. The result of this study shows the shear strain value range between 3.5 x 10-3 to 12.5 x 10-3. The highest value is around the east and west-north area and indicated the high risk of damage under ground motion. The output of the research must consider as preliminary site mitigation of Ngipik Landfill.