Estimasi Respon Tanah Sedang di Beberapa Lokasi di Kota Medan Akibat Skenario Terburuk Gempa Sumatera
DOI:
https://doi.org/10.5614/jts.2007.14.1.2Keywords:
Spektrum respon percepatan, Respon tanah, Skenario gempa Sumatera.Abstract
Abstrak. Kota Medan terletak ~80 km dari patahan strike-slip Sumatera dan ~300 km dari pertemuan lempeng Indo-Australian dan Eurasian. Kota ini pernah bergetar hebat tahun 1936 akibat gempa Kutacane (segmen patahan Tripa) MS=7,2 yang menyebabkan sejumlah bangunan rumah rusak. Karena tidak sepenuhnya aman, studi ini melakukan sejumlah kajian skenario gempa terhadap Kota Medan berdasarkan mekanisme dan magnitude gempa, jarak terdekat dan kondisi geoteknik. Skenario gempa di kawasan Nias berkekuatan Mw=9,3 dan gempa Mw=7,8 di segmen Tripa/Renun digunakan sebagai skenario terburuk, bersama dengan gempa Aceh 26/12/2004, Nias 28/03/2005 dan Renun 01/04/1921. Kemudian tiga lokasi tanah di Medan dianalisa dengan memakai riwayat waktu percepatan gempa yang berasal dari 21 rekaman nyata gempa-gempa subduksi dan patahan aktif di belahan dunia lain. Percepatan batuan dasar diperoleh sebesar 0,1002 g dan 0,1420 g, masing-masing akibat gempa subduksi Nias dan segmen patahan Tripa/Renun. Sebanyak 153 respon spektrum percepatan tanah dihasilkan, terdiri dari 81 spektrum akibat gempa subduksi dan 72 spektrum akibat gempa patahan. Respon tanah ini menunjukkan gejala yang harus diwaspadai karena melampaui spektrum rencana tanah sedang dan mencapai nilai atas spektrum rencana tanah lunak SNI-1726-2003 Wilayah 3, pada hampir seluruh perioda waktu.Abstract. Medan is located ~80 km away from Sumatran strike-slip fault and ~300 km away from boundary of Indo-Australian and Eurasian plates. Medan has been experienced tremor causing damage of some buildings in 1936 due to Ms=7,2 earthquake sourced from Kutacane (Tripa fault segment). Since the city was not fully safe, this study conducts an investigation to the effect of worst scenario earthquakes for Medan based on source mechanism, magnitude, closest source-to-site distance, and geotechnical condition. The subduction earthquakes of Mw=9,3 in Nias region and Mw=7,8 in Tripa/Renun fault segments are employed as the worst scenario earthquakes for Medan. The earthquakes of Aceh 26/12/2004, Nias 28/03/2005, and Renun 01/04/1921 are took into account as well. Three locations in Medan are then assessed using 21 synthetic time histories accelerations sourced from ground motion record of subduction and strike-slip earthquakes from other countries. The bedrock peak ground accelerations (PGA) found for Nias subduction region and Tripa/Renun fault segment reach 0,1002 g and 0,1420 g, respectively. This study results 153 soil surface spectrum response accelerations that consist of 81 spectrums and 72 spectrums affected by subduction and transform faults earthquakes. The soil responses due to worst scenario earthquakes show apprehensive trend, which beyond the design spectrum of medium soil in Zone 3 of SNI-1726-2003. It almost reaches soft soil design spectrum at all over periods as well.
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