Analisis Efek Tapak Lokal Berdasarkan Data Mikrotremor Menggunakan Metode Horizontal to Vertical Spectral Ratio (Studi Kasus: PT. Solusi Bangun Indonesia)

Metode Horizontal to Vertical Spectral Ratio (HVSR) dari rekaman mikrotremor sangat popular dalam konteks mikrozonasi seismik. Untuk tujuan mitigasi bencana, perlu dilakukan evaluasi geologi setempat guna mengetahui respon dinamika tanah yang dikenal efek tapak lokal. Penelitian dengan 34 titik data mikrotremor yang dilakukan di PT. Solusi Bangun Indonesia (Tuban Plant) ini bertujuan untuk menentukan persebaran nilai frekuensi natural (fo), faktor amplifikasi (Ao), indeks kerentanan seismik (Kg), kecepatan gelombang geser (Vs30), dan ketebalan sedimen (H). Hasil penelitian diperoleh nilai fo berkisar 0.64 – 61.53 Hz, Ao berada pada rentang 1.27 – 21.18, H berkisar 1.65 – 137.16 m, dan Kg antara 0.35 – 82.86. Keberadaan engineering bedrock berada pada kedalaman lebih dari 60 meter dengan nilai Vs sekitar 2000 – 2500 m/s berupa batugamping dolomitan. Daerah penelitian sampai kedalaman 30 meter mayoritas tergolong sebagai tanah tipe A berdasarkan klasifikasi Eurocode 8 dan tipe B berdasarkan klasifikasi SNI 1726-2012. Hal ini sesuai dengan daerah penelitian yang didominasi frekuensi natural tinggi, sedimen permukaan tipis, kecepatan gelombang geser tinggi, dan indeks kerentanan seismik rendah yang disebabkan oleh kondisi geologi setempat berupa batuan kompak, yakni batugamping Paciran, sehingga relatif stabil terhadap aktivitas seismik. Namun, ada pula daerah dengan respon frekuensi sangat rendah (0.63 dan 0.71 Hz), sedimen sangat tebal (117 dan 137 meter), serta indeks kerentanan seismik sangat tinggi (82.8 dan 23.5) yang disebabkan oleh geologi setempat berupa batulempung (clay) di sebelah ujung tenggara sehingga rentan mengalami goncangan seismik. Horizontal to Vertical Spectral Ratio (HVSR) method of microtremor recording is very popular in the context of seismic microzonation. For the purpose of disaster mitigation, it is necessary to conduct a local geological evaluation in oreder to determine the response of the soil dynamics known to the local site effects. This research with 34 microtremor data conducted at PT. Solusi Bangun Indonesia (Tuban Plant) aims to determine the natural frequency (fo), amplification factor (Ao), seismic vulnerability index (Kg), shear wave velocity (Vs30), and sediment thickness (H). The results obtained values to reach fo 0.64 – 61.53 Hz, Ao in the range 1.27 – 21.18, H ranges 1.65 – 137. 16 m, and Kg between 0.35 – 82.86. The existence of engineering bedrock at a depths of more than 60 meters with Vs value around 2000 – 2500 m/s containing dolomitic limestone. The majority of research areas to 30 meters depth are classified as type A based on Eurocode 8 and type B based on SNI 1726-2012. This is consistent with the study area which is dominated by high natural frequencies, thin surface sediments, high shear wave velocity, and low seismic vulnerability index caused by local geological conditions in the field of compact rocks, known to limestone from Paciran Formation, so that it is relatively stable against to seismic activity. However, there are areas with lowest frequencies (0.63 and 0.71 Hz), thickness sediments (117 and 137 meters), and highest seismic vulnerability indexes (82.8 and 23.5) due to local geology of clay at the southeastern tip so susceptible to seismic shocks