STUDI MIKROSEISMIK UNTUK MENDETEKSI PERUBAHAN GARIS PANTAI DENGAN INDIKATOR INDEKS KERENTANAN SEISMIK, PEAK GROUND ACCELERATION DAN GROUND SHEAR STRAIN DI PROVINSI BENGKULU

DRS. MUHAMMAD FARID, MS, Prof. Dr. H. Kirbani Sri Brotopuspito

2014 | Disertasi | S3 Ilmu Fisika

Pantai Provinsi Bengkulu secara geografisterletak didekat zona subduksi antara lempeng tektonik India-Australia dan lempeng tektonik Eurasia, sehingga sering dilanda gempabumi. Banyak penelitian telah dilakukan untuk menghubungkan antara kekuatan gempabumi dengan dampak kerusakan yang ditimbulkannya. Penelitian ini bertujuan untuk: (1) mengetahui karakteristik indeks kerentanan seismik berdasarkan data mikroseismik, peak ground accelerationdan ground shear strainpada setiap tipe pantai di Provinsi Bengkulu; (2) mengetahui korelasinya dengan perubahan garis pantai. Untuk keperluan penelitian ini telah dilakukan pengambilan data mikroseismik sebanyak 127 titik di sepanjang Pantai Bengkulu dengan menggunakan seismometer periode pendek 3 komponen. Lama pengukuran untuk tiap-tiap titik adalah 30 menit. Pengambilan data tidak dilakukan dengan cara gridding, akan tetapi dengan menetapkan tipe pantaikemudian diambil berbagai titik pengukuran. Jumlah data yang diambil padasetiap tipe pantai berbeda-beda bergantung luasannya. Untuk tipe pantai datar berpasir halus diambil sebanyak 21 titik, pantai datar berbatu 12 titik, pantaidatar berlumpur 12 titik, pantai datar berpasir kasar 20 titik, pantai curam berbatuan lava 15 titik, pantai curam berbatuan lempung 20 titik, pantai curam berbatu lempung tufaan 16 titik, dan Pantai curam bertufa pasiran batu 15 titik. Data lain sebagai pendukung adalah data historis gempabumi dari tahun 1900 sampai dengan tahun 2009. Data diolah menggunakan Geopsy untuk mendapatkan spektrumnya. Validasi spektrum mikroseismik mengacu pada aturan SESAME 2004, dan diperoleh 122 spektrum yang validdan 5 spektrum tidak valid. Validasi dilakukan untuk memperoleh puncak yang jelas, diperoleh 97 spektrum yang memenuhi kriteria jelas dan 30 spektrum tidak memenuhi kriteria jelas. Analisis data mikroseismik dilakukan secara kuantitatif melalui proses Horizontal to Vertical Spectral Ratio (HVSR), sedangkan data historik gempabumi diproses dengan menggunakan persamaan atenuasi. Ada 2 persamaan atenuasi yatu (1) persamaan Atenuasi Kanai dan persamaan atenuasi Fukushima-Tanaka. Hasil penelitian menunjukkan bahwa nilai rata-rata indeks kerentanan seismik berdasarkan mikroseismik berubah mengikuti tipe pantai. Nilai rata-rata indeks kerentanan seismik untuk (1) tipe pantaidatar berpasir halus adalah 2,1; (2) pantai datar berlumpur adalah 7,6; (3) pantai datar berbatu adalah 0,5; (4) pantai datar berpasir kasar adalah2,9; (5) pantai curam berbatuan lava adalah 1,3; (6) pantai curam berbatuan lempung adalah 16,1; (7) pantai curam bertufa pasiran batu adalah 7,9; dan (8) pantai curam berbatu pasir tufaan adalah 8,8. Nilai PGA hasil olahan data menggunakan persamaan Atenuasi Kanai (merupakan niali PGA di lapisan sedimen) antara 432 – 2672 gal.Nilai PGA hasil olahan data dengan persamaan Atenuasi Fukushima-Tanaka (merupakan nilai PGA di basement) antara 92,1 – 194 gal. Nilai ground shear strainuntuk (1) tipe pantai datar berpasir halus adalah 4×10 -4 ; (2) pantai datar berlumpur adalah 1,1×10 -3 pantai datar berbatu adalah 1,0×10 -4 ; (4) ) pantai datar berpasir kasar adalah 6,0×10 -4 ; (5) pantai curam berbatuan lava adalah 3,0×10 -4 ; (6) pantai curam berbatuan lempung adalah 3,3×10 -3 ; (7) pantai curam bertufa pasiran batu adalah 1,1×10 -3 ; (8) pantai curam berbatu pasir tufaan adalah 1,4×10 -3 . Hasil penelitian memberikan kesimpulan: (1) Karakteristik indeks kerentanan seismik berdasarkan mikroseismik, peak ground accelerationdan ground shear strainberubah mengikuti tipe pantai; (2) Ada kecenderungan hubungan antara indeks kerentanan seismik dengan laju perubahan garis pantai; (3) Tidak ada hubungan/korelasi antara peak ground accelerationdengan laju perubahan garis pantai; (4) Ada kecenderungan hubungan antara ground shear straindengan laju perubahan garis pantai. ; (3)

Shores of Bengkulu Province are geographically located in the subduction zone between the Indian-Australian plate and the Eurasian plate, so it is often hit by earthquakes. Many researchs have been done to correlate the power of the earthquake with the impact of damages. This research aims to: (1) determine the characteristics of the seismic vulnerability index based on microseismic data, peak ground acceleration and ground shear strain on any types of beach in Bengkulu province and correction with the rate of change shorelines. For the purposes of this research, it has been carried out taking 127 points of microseismic data along the shore of Bengkulu by using short-period seismometers with 3 components. The measurements for each point was 30 minutes. Data collection was not done by gridding, but by specifying the type of beach. Data collection points aretaken for each type of shore. The amount of data taken at each type of beach based onthe areaof type shore. For the type of flat fine sandy beach, it was taken 21 points, a flat rocky beach was taken 12 points, 12 points of beach muddy flat, flat sandy beaches rugged was taken 20 points, steep rocky lava beach was taken 15 points, steep rocky coast clay was taken 20 points, steep rocky coast tuffaceous clay was 16 points, and tufa steep sandy beach stone was taken 15 points. Historical data of earthquakes in 1900 to 2009 are other supporting data. The data were processed using Geopsy to obtain the spectrum. The validation of microseismic spectrum to obtain a reliable spectrum refers to the rules of SESAME 2004, obtained 122 reliable spectrums and unreliable spectrums. The validation is done for obtaining clear peak, obtained 97 spectra that show clear peaks and 30 did not show the clear spectral peaks. The microseismic data were analyzed quantitatively through Horizontal to Vertical Spectral Ratio (HVSR), while the earthquake history of the data were processed by using the attenuation equation. There are two attenuation equations, (1) Kanai and (2) Fukushima-Tanaka equation. The results showed that the average value of the seismic vulnerability index based on microseismic changed with the type of shore . The average value for the seismic vulnerability index (1) the type of flat fine sandy beach is 2.1 , (2) a muddy flat beach is 7.6 , (3) a flat rocky beach is 0.5 , (4 ) coarse sandy flat beach is 2.9 , (5) a steep rocky lava beach is 1.3 , (6) the steep rocky shore clays is 16.1 ; (7) the steep shore of tuffacous sandy stone is 7.9 , and (8) the steep shore of stones sandy tuff is 8.8 . PGA value of the processed data using equations Attenuation Kanai (PGA value in the sediment) is between 432-2672 gal . PGA value of data processed by the attenuation equations Fukushima - Tanaka (a PGA values in sedimentary layers) is between 92.1 to 194 gal . Ground value of shear strain for ( 1 ) the type of flat fine sandy beach is 4.0×10 -4 , (2) flat muddy beach is 1.1×10 -3 , (3) a flat rocky beach is 1.0×10 -4 ; (4) coarse sandy flat beach is 6.0×10 -4 , (5) a steep beach rocky lava is 3.0×10 -4 , (6) the steep rocky shore clays is 3.3×10 -3 ; (7) the steep shore of tuffacous sandy stone is 1.1×10 -3 ; (8) the steep shore of stones sandy tuff is 1.1×10 -3 . The results of the study provides conclusions: (1) Characteristics of the seismic vulnerability index based onmicroseismic, peak ground acceleration and ground shear-strain changes according to the type of beach; (2) There is a relationship tendency between seismic vulnerability index to the rate of shoreline change, (3) There is no correlation between peak ground acceleration with shoreline change; (4) There is a relationship tendency between ground shear strain with shoreline change.

Kata Kunci : Indeks Kerentanan Seismik, Peak Ground Acceleration, Ground Shear Strain, Laju Perubahan Garispantai, Provinsi Bengkulu.