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Sub-Segmentasi Sesar Pada Segmen Kumering Antara Danau Ranau Hingga Lembah Suoh, Lampung Barat

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Sub-Segmentasi Sesar Pada Segmen Kumering Antara Danau Ranau Hingga Lembah Suoh, Lampung Barat JURNAL LINGKUNGAN DAN BENCANA GEOLOGI Journal of Environment and Geological Hazards JLBG ISSN: 2086-7794, e-ISSN: 2502-8804 Akreditasi LIPI No. 692/AU/P2MI-LIPI/07/2015 e-mail: [email protected] - http://jlbg.geologi.esdm.go.id/index.php/jlbg Sub-Segmentasi Sesar Pada Segmen Kumering Antara Danau Ranau Hingga Lembah Suoh, Lampung Barat Subdivision Of Segmentation In Kumering Segment Between Ranau Lake To Suoh Valley, West Lampung Sonny Aribowo1,2, Dicky Muslim1, Winantris1, Danny H. Natawidjaja3, Mudrik R. Daryono3 1Universitas Padjadjaran, Bandung 2Loka Uji Teknik Penambangan dan Mitigasi Bencana, Liwa, LIPI 3Pusat Penelitian Geoteknologi LIPI, Bandung Naskah diterima 08 November 2017, selesai direvisi 2017, dan disetujui 2017 e-mail: [email protected] ABSTRAK Sesar Sumatra merupakan sesar mendatar yang tersusun atas beberapa segmen. Evolusi sesar memainkan peranan penting terhadap retakan permukaan akibat gempabumi dan juga pencabangan sesar. Segmen Kumering merupakan sesar yang menjadi sumber gempabumi yang bersifat merusak pada tahun 1933 dan 1994. Kami melakukan delineasi pada Segmen Kumering berdasarkan interpretasi pada citra SRTM dengan resolusi 30 m untuk memberikan pandangan yang paling mutakhir mengenai sesar yang menjadi sumber gempa tersebut. Kami juga melakukan tinjauan untuk dimensi dari sub-segmen dalam hubungannya dengan offset sesar maksimum. Dari hasil interpretasi citra teridentifikasi 12 sub-segmen yang dibatasi oleh adanya step dan/atau tekukan serta hilangnya jejak morfologi pada kenampakan citra. Kompleksitas sesar mendatar tergambarkan dalam hubungan antara jumlah step dan atau tekukan dengan panjang maksimum offset sesar. Tren linear menunjukkan bahwa jarak offset maksimum yang semakin panjang pada segmen sesar yang lebih panjang dan memiliki jumlah step dan/atau tekukan yang semakin sedikit. Hasil penelitian memberikan gambaran yang lebih baik secara resolusi. Hasil ini juga membuktikan adanya pencabangan sesar yang berasosiasi dengan sesar utama pada lingkungan vulkanik. Kata kunci: sesar mendatar, segmen sesar, offset sesar, SRTM30, Kumering, Sesar Sumatra ABSTRACT Sumatran fault is well known as highly segmented strike-slip fault. The evolution of fault segmentation plays crucial role to the dimension of earthquake ruptures as well as fault splays. Kumering Segment of the Sumatran Fault Zone allegedly as the source of Liwa’s 1933 and 1994 earthquakes. To update the prediction of geometrical attribute of strike-slip system in the Sumatran Fault Zone,we delineate sub-segmentation in the Kumering Segment based on SRTM30 imagery. We studied the dimension of each subsegments and correlate them to the maximum fault offset. From imageries interpretation, we identify twelve subdivision of Kumering Segment bounded by step and the subdued of geomorphic trace. We show strike-slip complexity by relationship between number of steps with maximum fault offset length. Linear trend shows that our data fit with previous study, which concluded that faults have longer segments and fewer steps when their offsets increase. This article also intended to obtain better understanding in characterizing source of earthquake triggered by right lateral Sumatran Fault. Our results provide better resolution for fault segmentation. The results may also reveal the orientation of secondary fault formed by splaying associated with first order fault in the volcanic environment. Keywords: strike-slip fault, fault segment, fault offset, SRTM30, Kumering, Sumatran Fault. 31 Jurnal Lingkungan dan Bencana Geologi, Vol. 8 No. 1, April 2017: 31 - 45 PENDAHULUAN Sumatra merupakan salah satu sumber dari beberapa Sesar mendatar memiliki arsitektur yang kompleks gempabumi yang merusak di Pulau Sumatra dengan sejumlah segmen yang memiliki panjang (Supartoyo dan Surono, 2008). Identifikasi dari bervariasi yang terpisahkan oleh adanya sesar tekukan yang terjadi pada sistem sesar mendatar yang paralel (steps) (de Joussineau dan Aydin, juga sangat penting dalam memperkirakan 2009). Segmentasi sesar dan evolusi segmen sesar kejadian gempabumi pada masa mendatang. memiliki hubungan yang relevan dengan dinamika Segmen sesar memiliki peranan secara langsung dan ukuran dari besaran offset akibat pergeseran terhadap dinamika dan ukuran dari retakan yang pada jalur sesar (de Joussineau dan Aydin, 2009). terjadi pada saat gempabumi (Barka dan Kadinsky- Dalam sebuah sistem sesar mendatar yang menjadi Cade, 1988; Shaw dan Dieterich, 2007; Wesnousky, sumber gempabumi, sesar bukanlah merupakan 2006). Gempabumi yang bersifat merusak telah sebuah struktur planar yang sederhana, tetapi terjadi di daerah Liwa, Kabupaten Lampung terdapat juga tekukan (bends), daerah yang Barat, Provinsi Lampung pada tahun 1933 dan tertekan (jogs), percabangan (branches) dan step 1994 (Widiwijayanti drr., 1996; Soehaimi drr., (Shaw, 2006). Daerah tekukan pada sistem sesar 2002) Gempabumi yang terjadi bersumber dari mendatar terdapat elemen seperti tekukan terlepas pergerakan Segmen Kumering yang merupakan (releasing bends) dan tekukan tertahan (restraining salah satu segmen dari 19 segmen Sesar Sumatra bends), tekukan sesar terlepas dan tekukan sesar (Gambar 2) yang memiliki karakteristik sesar tertahan adalah lokasi dimana terjadi deformasi mendatar (strike-slip) menganan atau dekstral transtensional dan transpresional (Gambar 1) (Sieh dan Natawidjaja, 2000). Sesar Sumatra, (Cunningham dan Mann, 2007). Umumnya fitur- dalam hal ini merupakan sesar mendatar yang fitur struktur yang terbentuk dalam konfigurasi terjadi karena pengaruh subduksi miring (oblique) sesar mendatar pada skala regional dapat terekam dimana tegangan antar lempeng dipartisi ke dalam pada ekspresi geomorfik pada skala yang lebih sistem strike-slip yang paralel di dalam zona kecil (Burbank dan Anderson, 2012). depan busur maupun busur belakang (Sieh dan Penelitian mengenai struktur geologi di daerah Natawidjaja, 2000). tektonik aktif penting dilakukan karena sesar Nomenklatur Segmen Kumering merujuk Gambar 1. Konfigurasi sesar mendatar dan struktur yang terbentuk di dalamnya (dimodifikasi dari Christie-Blick dan Biddle, 1986; Ye drr, 2015) 32 Sub-Segmentasi Sesar Pada Segmen Kumering Antara Danau Ranau Hingga Lembah Suoh, Lampung Barat kepada Sieh dan Natawidjaja (2000). Pemilihan Tatanan Geologi nomenklatur tersebut merupakan nomenklatur yang Daerah penelitian merupakan bagian dari Lajur paling update dan berdasarkan pada hasil analisis Barisan yang terletak sejajar dengan Pulau Sumatra yang cukup valid. Nomenklatur ini menggantikan (Gafoer drr., 1994). Daerah penelitian terletak di beberapa nomenklatur seperti segmen Semangko bagian selatan Lajur Barisan dengan Danau Ranau (Katili dan Hehuwat, 1967), Sesar Sukabumi berperan sebagai daerah depresi yang terbentuk (Koswara dan Santoso, 1995; Suwijanto drr., karena mekanisme transtensional Sesar Sumatra. 1996) dan segmen Ranau – Suoh (Bellier dan Daerah penelitian termasuk ke dalam Peta Geologi Sébrier, 1994; Pramumijoyo drr., 1994, Soehaimi Lembar Kotaagung (Amin drr., 1994) dan Peta drr., 2013). Segmen Ranau – Suoh terdiri dari 7 Geologi Lembar Baturaja (Gafoer drr., 1994) sub-segmen (Soehaimi drr., 2013; Soehaimi drr., (Gambar 2). 2014). Daerah penelitian tersusun oleh 3 kelompok batuan Maksud dari penelitian ini adalah mendapatkan yaitu Batuan Vulkanik Kuarter, Batuan Piroklastik pola struktur geologi detail pada daerah yang aktif Kuarter – Tersier dan Batuan Vulkanik Tersier secara tektonik. Sedangkan tujuan dari penelitian (Gafoer drr., 1994; Amin drr., 1994, Natawidjaja ini adalah mengetahui sub-segmentasi sesar dan Kesumadharma, 1993; Pramumijoyo drr., aktif pada Segmen Kumering. Selain itu juga 1994; Suwijanto drr., 1996). Secara rinci masing- menggambarkan kompleksitas sesar mendatar masing kelompok batuan ini dibagi lagi ke dalam yang dilihat dari hubungan antara jumlah step beberapa satuan batuan (Koswara dan Santoso, dan atau tekukan dengan panjang maksimum 1995) antara lain : offset sesar. Sub-segmentasi (subdivision of Batuan gunungapi Kuarter (Qhv dan Qv) yang segmentation) terbagi berdasarkan kepada seksi tersusun atas batuan gunungapi Seminung yang sesar (fault section). Seksi sesar (fault section) berupa lava andesit basaltis dan breksi lahar dengan adalah bagian dari segmentasi yang terbagi sisipan tuf pasiran; batuan gunungapi Kukusan dikarenakan perubahan arah strike dan/atau berupa lava andesit, batuan gunungapi Pesagi perubahan kinematik gerak sesar (Daryono, 2016). Gambar 2. A. Tatanan tektonik Pulau Sumatra dan segmentasi Sesar Sumatra. B Segmentasi Sesar Sumatra di bagian selatan. Segmentasi Sesar Sumatra berdasarkan Sieh dan Natawidjaja (2000). Lingkaran merah pada gambar sebelah kiri dan garis merah pada gambar sebelah kanan menunjukkan lokasi dari Segmen Kumering. 33 Jurnal Lingkungan dan Bencana Geologi, Vol. 8 No. 1, April 2017: 31 - 45 berupa lava andesit dan breksi lahar dan batuan berumur Kuarter dan Tersier di daerah penelitian gunungapi Sekincau berupa breksi lahar. Batuan tersebut terpotong oleh Sesar Sumatra. Sesar gunungapi ini berumur Plistosen – Holosen. Sumatra merupakan sesar transform dipengaruhi oleh subduksi miring (oblique) dimana tegangan Batuan piroklastik yang tersusun atas Tuf Ranau antar lempeng dipartisi ke dalam sistem strike-slip (QTr) (van Bemmelen, 1949; Marks, 1956; Bellier yang paralel di dalam zona depan busur maupun drr., 1999; Gasparon, 2005) atau Tuf Liwa berumur busur belakang (Sieh dan Natawidjaja, 2000) dan Plio-Plistosen. Tuf Ranau yang diambil dari daerah berhubungan dengan pemekaran
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