Microzonation of the liwa city on the great sumatera active fault and giant ranau volcanic complex in south sumatera, indonesia

Abstract

Liwa earthquake of February 16th, 1994 (6.5 Mb) was located in Sumatera fault zone shown the right-lateral strike-slip fault focal mechanism, where the west block of Liwa City region relatively moved to the north and east block to the South. During this earthquake, there were 201 people killed, 1,871 peoples injured and about 4,915 modern and traditional houses collapsed in Liwa City. In order to mitigate the potential geological hazard and risk in this area, Geological Agency of the Ministry of Energy and Mineral Resources of Indonesia, had conducted research on seismotectonic and seismic hazard microzonation in and around the Liwa City. The great Sumatera fault and giant Ranau volcanic complex in Liwa areas are part of active geotectonic features of oblique subduction zone activity in the western part of Sunda Arc. Geological and seismological impact of active geotectonic in this region is expressed by the destructive earthquakes epicenters, active fault and neotectonic evidences, active volcanoes and calderas as well as debris avalanche in and around volcanic complex (Ranau, Limau Kunci, Salingkuh, Gedung Surian and Ulumajus) along the Sumatera fault zone. Wide distribution of acidic tufaceous sand is a criterium of giant volcanic eruption in this region. Recent seismological phenomenon was expressed by three times historical destructive earthquakes, which have been occured in and around Liwa (1908, 1933 and 1994), with the maximum intensity IX-X MMI scale. The geological evidences following these destructive earthquakes are landslides, ground ruptures and phreatic eruptions in Suoh creater and Pasagi volcano. Through seismic hazard microzonation (microtremor) studies, Liwa City is divided into 5 (five) zonations of H/V predominant periode (<0.2 s, 0.2-0.4 s, 0.4-0.5 s, 0.5-0.8 s and >0.8 s). Mainly soil classification in the Liwa City is covered by shallow-deep stiff soil with Vs <350 m/s. Based on Indonesia soil clasification (SNI 1726:2012), Liwa City consist of SD (Intermediate Soil)-SE (Soft Soil).