"Comparison of Focal Mechanisms and Source Parameters of Volcano-tectonic Earthquakes between Active and Normal Periods at Lokon Volcano, North Sulawesi, Indonesia"

Lokon volcano complex as a part of Sangihe arc system is located in Tomohon City - Minahasa Regency - North Sulawesi, Indonesia. Increase in seismic activity began in June 2007. The number of volcanic earthquake increased rapidly and volcanic tremor was recorded continuously on 7 December 2007. After the activity, seismicity decreased gradually until the end of May 2008. We compared characteristic of VT earthquake during active period in December 2007 and normal period in May 2008. VT earthquakes were distinguished into two types, the deep type (VTA) and shallow type (VTB). Hypocenter in December 2007 composed of 3 cluster at depths 300 – 600 m, 600 – 1100 m and 1100 – 1400 m beneath the crater. Deep one corresponds to VTA type, middle and shallow are hypocenters of VTB type. Seismicity in the middle cluster (deep VTB) was high in December 2007, however it declined significantly in normal period in May 2008. This cluster may correspond temporal shallow magma storage. Focal mechanism of VT earthquakes was estimated assuming double couple. Mechanism of VTA is normal fault with tension axis nearly N-S direction, both in active and normal periods. It is interpreted to be related to inflation of magma storage at depth 4 – 6 km due to magma migration from deeper part. Focal mechanisms of VTB were reverse fault in active period whereas normal fault in normal period. In the active period, reverse fault type was caused by contraction stress due to depression of shallow magma storage by emission of gas.

Sketch of magma system of Lokon Volcano in December 2007, as active period