The microzonation of vulnerable earthquake zone in Sorowako city using microtremor

Abstract

Micro-tremor analysis has been used on micro-zonation studies for Sorowako City area. This study is of importance since Sorowako City closes to the Matano Fault as an active seismic area. The city has been developed through mining and agriculture activities, despite prone to be vulnerable to earthquake hazards. This vulnerability increases due to heterogeneous local settlements and public facilities with non-engineering structures to support buildings design. This study aims to conduct micro-zonation over the vulnerable area from earthquake hazard covers an area of 121.3523° E – 121.3692° E and 2.5185° S – 2.5299° S. The acquisition data used portable seismograph instrument comprising of 14 observation points. The obtained signal data was then processed using gypsy software to determine the resonance factor, rock hardness level, and Kg Index. The results were formulated in three conclusions concerning vulnerability factors. The first factor is resonance phenomena. This vulnerable area caused resonances located along the Street of Merapi's settlement with a natural frequency of 0.68 Hz that resonates with seven large earthquakes of Matano Fault ranged in the interval of 0.65 – 0.82 Hz. The second factor is a cause of vulnerability which is considered to be the existence of soft rock composition with pra edominant frequency under 1 Hz that includes areas along Street of Merapi's Settlement, ship harbor, Magani Traditional Market, and bus station. The third factor is vulnerability of the areas viewed from the Kg index which gave value of greater than 30 Kg for areas of Kibait Church, Street of Merapi's Settlement, Al Ikliwan Great Mosque, and the adjacent area of Lake Matano. The latter is the result of combining frequency and amplitude of micro-tremor waves.

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