Electronic Theses and Dissertation

Penelitian ini mengenai sifat lempeng laut dari sinyal gempa dalam regional:kelompok kecepatan gelombang badan di pesisir Taiwan, bertujuan untuk melihat grup kecepatan dari getaran gempa dalam di daerah pesisir Taiwan sebagai salah satu sifat lempeng laut pada zona subduksi dengan menggunakan metode picking, dimana sinyal gempa dianalis dengan menggunakan perangkat lunak SAC (seismic analysis code) dan python 3. Hasil penelitian menunjukkan lokasi gempa sering terjadi di zona subduksi ryukyu dan diketahui beberapa stasiun mengalami dispersi. Stasiun yang mengalami dispersi akan memiliki keterlambatan waktu tiba 1 – 1.5 detik. Dispersi di sebabkan sinyal gempa yang terperangkap oleh kerak samudera. Hasil pengamatan menunjukkan frekuensi rendah tiba terlebih dahulu dibanding frekuensi tinggi. rata – rata nilai pengurangan kecepatan 8% pada gelombang P dan menandakan adanya fenomena guided wave. Guided wave hanya dapat diamati jika tereksitasi oleh sumber yang terletak di dekat atau di dalam saluran berkecepatan (Low velocity layer) di zona subduksi ryukyu. Sinyal yang merambat di sepanjang kerak samudera dengan kecepatan 7.6 Km/s akan terperangkap pada lapisan berkecepatan rendah dan mengalami pengurangan kecepatan sebesar 8%. Kecepatan sinyal yang telah mengalami pengurangan sebesar 8% akan dibagi dengan dominan frekuensi 1 – 3 Hz maka diketahui ketebalan kerak samudera 2.33 – 6.99 km.

Kata Kunci: Dispersi, Kecepatan, Sinyal, Guided wave, Frekuensi

The dispersed seismic signals are usually observed on the surface waves, however, at the subduction zone, the body wave signal has been found to be dispersed. The dispersed body-wave signal at the subduction zone only being observed caused by the deep regional earthquake signals. This observation is due to the presence of oceanic crust, which acts as the low-velocity layer, thus the dispersed body wave corresponds to the waveguide phenomenon. In this study, the body-wave dispersion was analyzed for its group velocity characteristics. A 0.5 Hz narrow-band passed technique and Hilbert transform were used to estimate the envelope of each band pass signal which was applied through Seismic Analysis Code (SAC) and python3 codes. The results indicate that the station that recorded the dispersed signals was located close to the trench and at 10 – 20 km depth from the plate interface. The delay of body-wave arrivals of P- and S-waves are between 0.5 – 1 second. The arrival of low-frequency signals then followed by high frequency is observed to be delayed at 8% velocity reduction, with a dominant frequency of 1 – 3 Hz. This indicates a guided wave phenomenon generated by the deep earthquakes which nucleated within the oceanic crust. Thus, if the velocity of 200 km depth is 8 km/s, it could be assumed that the thickness of the oceanic crust is about 2.45 – 7.36 km. Keywords: Dispersion, Velocity, Signal, Guided wave, Frequency