Focal Mechanism for Determining Fault Planes Orientation in “GAMMA” Geothermal Field

Authors: Iwan Yandika Sihotang, Tommy Hendriansyah, Nanang Dwi Ardi
Keywords: microearthquake, fault planes, single event determination, double difference, focal mechanism
Conference: Indonesian Geothermal Association Conference Session:
Year: 2014 Language: English
Abstract: “Gamma” geothermal field is located in volcanic lines which associated by others geothermal field and controlled by geological processes as well as the activity of geothermal exploitation, so they are indicated by fault planes which became one of micro earthquake triggering factors. Fault planes identification with focal mechanism method towards micro earthquake data has been done by reviewed location and relocation of hypocenters, which used of Single Event Determination (SED) and Double Difference (DD). 211 data of 70.659 hypocenters was recorded by five micro earthquake recording stations in “Gamma” geothermal field during the period of March 2012 to October 2012. 211 hypocenters data is used for SED location determination, 60 data for DD relocation determination, and 4 data for focal mechanism solution. Hypocenters location and relocation of SED and DD methods spread off through northwest to southwest of “Gamma” geothermal field. Based on SED method result, distribution of hypocenter shows the cap rock elevation is located upon 500 meters and the geothermal reservoir is located about 500 meters to -1500 meters (beneath mean sea level point). Afterwards, the SED method result is relocated by DD method so that hypocenters distribution is centered on four fault planes, there are fault planes A, B, C and D which are located between production and injection wells. Determination of all fault planes obtained with focal mechanism solution which reviewed by initial movement of P waves arrival time, it indicate A, B, C fault planes are typed reverse lateral oblique and D fault plane is typed lateral strike slip.
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