Kontribusi Kecepatan Gelombang -S (Vs) untuk Karakterisasi Zona Zona Kritis Geologi (Critical Zone)

Handoyo Handoyo

doi:10.23960/jgrs.ft.unila.673

Handoyo Handoyo Teknik Geofisika, Institut Teknologi Sumatera

DOI: https://doi.org/10.23960/jgrs.ft.unila.673

Keywords: critical zone, kurva dispersi, MASW, zona sesar

Abstract

Zona Kritis (Critical Zone/CZ) merupakan lapisan dekat permukaan bumi tempat berlangsungnya interaksi antara batuan, tanah, air, atmosfer, dan biota yang berperan penting terhadap evolusi bentang alam, sistem hidrologi, dan bencana geologi. Penelitian ini bertujuan untuk mengkarakterisasi struktur bawah permukaan dangkal serta mengevaluasi pengaruh sistem sesar aktif terhadap ketebalan Zona Kritis menggunakan model kecepatan gelombang geser (Vs) hasil metode Multichannel Analysis of Surface Waves (MASW). Penelitian dilakukan pada Sistem Sesar Carrascoy dan Sesar Alhama de Murcia (AMF), Semenanjung Iberia bagian tenggara, Spanyol. Akuisisi data seismik dilakukan menggunakan 240 kanal perekaman dengan geofon vertikal 10 Hz dan sumber seismik accelerated weight drop 200 kg. Kurva dispersi yang dihasilkan kemudian diekstraksi dan diinversi menggunakan algoritma inversi Occam untuk menghasilkan model Vs 1D yang kemudian diinterpolasi menjadi penampang pseudo-2D Vs. Model Vs yang dihasilkan mampu memberikan pencitraan bawah permukaan yang andal hingga kedalaman sekitar 150 m dengan rentang nilai Vs antara 200–2000 m/s. Zona berkecepatan rendah (<600 m/s) yang diinterpretasikan sebagai Zona Kritis menunjukkan variasi ketebalan sekitar 15–30 m pada bagian selatan profil dan meningkat hingga 45–50 m di sekitar zona sesar aktif. Beberapa sesar buta yang sebelumnya belum terpetakan, yaitu F3–F8, berhasil diidentifikasi berdasarkan anomali Vs rendah pada kisaran 600–900 m/s yang mengindikasikan zona batuan retak dan terlapukkan intensif. Hasil penelitian juga menunjukkan bahwa ketebalan CZ meningkat secara signifikan di sekitar zona kerusakan sesar dan pada area dengan kemiringan topografi yang lebih curam. Selain itu, model Vs mampu mengidentifikasi sub-lapisan internal CZ, yaitu lapisan dangkal dengan ketebalan sekitar 10 m dan Vs ~400 m/s serta lapisan di bawahnya dengan ketebalan sekitar 30 m dan Vs antara 400–600 m/s. Hasil penelitian ini menunjukkan bahwa pencitraan Vs resolusi tinggi menggunakan metode MASW efektif untuk memetakan arsitektur Zona Kritis, mengidentifikasi struktur sesar tersembunyi, serta mengevaluasi pengaruh tektonik dan topografi terhadap perkembangan CZ pada lingkungan geologi aktif.

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