1 PROCEEDINGS, 7 th ITB International Geothermal Workshop 2018 Institut Teknologi Bandung, Bandung, Indonesia, March 21–22, 2018 IDENTIFICATION OF PERMEABILITY STRUCTURES USING MOMENT TENSOR AND FOCAL MECHANISM ANALYSIS OF MEQ DATA AT WAYANG WINDU GEOTHERMAL FIELD Nur Inna Alfianinda 1 , M. Syamsu Rosid 1 and Wahyuddin Diningrat 2 1 Geothermal Magister Program, Universitas Indonesia, Depok 16424, Indonesia 2 Star Energy Geothermal (Wayang Windu) Ltd., Wisma Barito Pacific, Slipi, Jakarta 11410 e-mail: syamsu.rosid@ui.ac.id, ni.alfianinda@outlook.com ABSTRACT Rock permeability is an important parameter in improving drilling success ratio and monitoring of geothermal reservoir. Its existence is controlled by fracture due to stress. Identification and analysis of fracture characteristics can be used to optimize the productivity. MEQ (microearthquake) is a method that can be used to determine the existing of permeable zones controlled by fractures or faults. MEQ data not only map the permeable zone distribution based on its hypocenter, but also characterize the fracture zones based on analysis of focal mechanism and moment tensor. Moment tensor inversion has done using MEQ data by utilizing three components of local waveform. The result of this study indicate that the distribution of fractures that control permeability has dominant strike orientation of NW-SE and NE-SW direction. The result of focal mechanism shows the dominance of fracture type of strike-slip, oblique, and normal. At the North field at elevation of about -1 km a.s.l. upwards is dominated by implosive components of moment tensor correlated with volume decreases and closing tensile cracks. It can be assumed to have correlation with the production activity that is typical of highly exploited. While at the North field at elevation of about -1 km a.s.l. downwards shows explosive components. These indicate the presence of active hydrothermal steam and can also be assumed to be associated with deep fluid activity against hot rock. Then, in a shallow elevation at the South field shows that events in that zone are aseismic. Meanwhile, in a deeper elevation has explosive-dominated components indicating volume increases and opening tensile cracks. Those are giving an information that permeability at the South zone is quite large. In addition, the shear failure components of moment tensor resulted in deep elevation indicate a tectonic stress that triggers an active fault. There is shear component as well of normal fracture in very shallow elevation that is assumed to be related with ground/rock movement on the surface. Keywords: Geothermal, microearthquake, moment tensor, focal mechanism, permeability. INTRODUCTION Background Wayang Windu geothermal field (WWGF) is a volcanic-hosted geothermal field that has been actively produced. The geothermal field is located in a volcanic zone of the extinct strato-volcanoes and has a reservoir characteristic that is the transition of vapor-dominated system at the North and liquid-dominated system at the South (Bogie et al., 2008). This field has been in operation since 2000 with total capacity of unit-1 and unit-2 reaching 227 MW. Unit-1 and unit-2 generators are supported by 28 production wells and 5 injection wells (Masri et al., 2015). Rock permeability is an important parameter in increasing drilling success ratio and monitoring of geothermal reservoir. Its existence is controlled by fracture due to stress. One method for determining the presence of permeable zone controlled by fractures is MEQ (microearthquake). Identification and analysis of fracture characteristics can be used to optimize productivity. Microearthquake data not only map the permeable zone distribution based on the hypocenter distribution, but also characterize the fracture zone based on the analysis of focal mechanism and moment tensor. Analysis of focal mechanism and moment tensor can be done by moment tensor inversion method that utilizes three components of local waveform. The analysis of focal mechanism and moment tensor of this study aims to identify microseismic sources, including fracture types and parameters that can be used to detect sub-surface geological structures that control permeability. In addition, analysis of focal mechanisms and moment tensor are used to identify the factors that trigger microearthquake and factors that affect the permeability distribution based on the moment tensor component results. From all the above descriptions, this study was conducted with a view to identify the source of