Electronic Theses and Dissertation

Penelitian ini dilakukan di Kecamatan Bahorok, Kabupaten Langkat, Provinsi Sumatera Utara, lokasi penelitian memiliki luas wilayah 25 km. Menurut Cameron, dkk (1982), pada lokasi penelitian terdapat empat formasi batuan yaitu, formasi Baong (Tmb), Formasi Belumai (Tmpb) dan Tuff toba (Qvt) dan Endapan Aluvium (Qh). Adapun yang alasan dilakukan penelitian di daerah tersebut, karena Kecamatan Bahorok, yang menjadi lokasi penelitian berada di jalur sesar Segmen Semangko dan punggung Bukit Barisan sehingga rawan terdampak gempa bumi dari daerah sekitar. Penelitian ini bertujuan untuk mengetahui Indeks Kerentanan Seismik di wilayah Kecamatan Bahorok berdasarkan frekuensi dominan dan nilai amplifikasi di lokasi penelitian. Pemetaan geologi dilakukan dengan menggunakan metode pengamatan langsung di lapangan dan analisis petrografi untuk mengetahui kondisi litologi dan bentang alam di lokasi penelitian sedangkan untuk studi kasus, dilakukan pengukuran Mikrotremor untuk menentukan Indeks Kerentanan Seismik di Kecamatan Bahorok. Pengukuran Mikrotremor dilakukan sebanyak delapan titik yang tersebar di kawasan kecamatan Bahorok, data pengukuran ini kemudian dianalisis menggunakan metode Horizontal to Vertical Spectra Ratio (HVSR), dari metode ini diperoleh dua parameter penting yaitu frekuensi dominan dan amplifikasi. Hasil penelitian menunjukkan bahwa wilayah Kecamatan Bahorok memiliki empat satuan geomorfologi, meliputi perbukitan dan lereng denudasional, dataran, badan sungai, dan dataran banjir. Kondisi geologi di lokasi penelitian terdiri dari lima satuan batuan, yaitu satuan lempung karbonatan (Tmb), satuan batu gamping terumbu (Tmpb), satuan batu pasir glaukonit (Tmpb), satuan tufa riodasit (Qvt), dan satuan konglomerat (Qh). Analisis mikrotremor menghasilkan tiga zonasi kerentanan seismik di Kecamatan Bahorok: zona dengan kerentanan rendah mencakup 76% wilayah, zona dengan kerentanan sedang mencakup 12%, dan zona dengan kerentanan tinggi mencakup 12% wilayah.

Kata Kunci: Pemetaan Geologi, Mikrotremor, HVSR, Frekuensi Dominan, Amplifikasi dan Indeks Kerentanan Seismik

This research was conducted in Bahorok District, Langkat Regency, North Sumatra Province, covering an area of 25 km². According to Cameron et al. (1982), the study area contains four rock formations: the Baong Formation (Tmb), Belumai Formation (Tmpb), Toba Tuff (Qvt), and Alluvial Deposits (Qh). The reason for conducting this study in this area is that Bahorok District is located along the Semangko Segment Fault and the Barisan Mountain ridge, making it susceptible to earthquakes from surrounding regions. The objective of this research is to determine the Seismic Vulnerability Index in Bahorok District based on dominant frequency and amplification values at the study sites. Geological mapping was carried out using direct field observations and petrographic analysis to understand the lithology and landscape conditions of the area. For the case study, microtremor measurements were conducted to determine the Seismic Vulnerability Index in Bahorok District. Microtremor measurements were taken at eight points scattered throughout Bahorok District. The data were analyzed using the Horizontal to Vertical Spectral Ratio (HVSR) method, from which two important parameters were obtained: dominant frequency and amplification. The results show that Bahorok District consists of four geomorphological units, including hills and denudational slopes, plains, river channels, and floodplains. The geological conditions comprise five rock units: carbonate clay (Tmb), reef limestone (Tmpb), glauconitic sandstone (Tmpb), rhyodacite tuff (Qvt), and conglomerate (Qh). Microtremor analysis produced three seismic vulnerability zones in Bahorok District: a low vulnerability zone covering 76% of the area, a moderate vulnerability zone covering 12%, and a high vulnerability zone covering 12% of the area. Keywords: Geological Mapping, Microtremor, HVSR, Dominant Frequency, Amplification, and Seismic Vulnerability Index.