Preliminary Result of Microearthquake Monitoring at Namora-I-Langit and Silangkitang, Sarulla, North Sumatra, Indonesia

Abstract

Namora-I-Langit (NIL) and Silangkitang (SIL) are two exploited geothermal fields within Sibualbuali WKP. SIL started commercial generation of 110 MW on Mar 18, 2017. Currently, 4 producers and 9 injectors support SIL full power generation. The NIL geothermal field has been operated 220 MW since May 2018 with the support of 11 production and 10 injection wells. NIL is a volcanic hydrothermal system that associates with distributed permeability while SIL is fault-controlled permeability system that relates with extensional regime along main Great Sumatra Fault (GSF). Since early 2020, the monitoring of microearthquake (MEQ) has been running in NIL and SIL. The MEQ network consists of 18 permanently installed seismometers which covers a total area of 224 km2. The instruments record 3-4 events in average per day within 300-days monitoring. Four MEQ clusters are consistently detected, two clusters in NIL and other two in SIL. One cluster which is located South of NIL seems to correlate with non-benign water downflow where the clay cap is thin or absence. The intensity of MEQ occurrence and downflow process were dictated by production activity. In SIL, changing of injection activity coincides with the variation of MEQ event rate in East cluster. Those clusters are interpreted due to geothermal exploitation activities in this area. Further MEQ analysis needs to be carried out to understand the interplay between the geothermal conceptual model, geological structure, and injection/production activity.