Djedi S. Widarto

Geoelectric potential difference monitoring in southern Sumatra, Indonesia — Co-seismic change—

Five geoelectric potential difference (electric field, here after) monitoring stations have been in operation since September 1997 in anarea near Liwa town, southern Sumatra, Indonesia, to examine the relationship between electric field changes and earthquakes. Short-term electric field variations were found to correspond mainly to geomagnetic activity, while long-term variation was mostly gradual shift and was clearly correlated neither precipitation nor ground water level variations. Co-seismic electric field changes ranging between 1 and 8 mV were observed for five mb > 5 earthquakes at multiple stations during September ∼ December 1997. The epicenters of the earthquakes were in the Indian Ocean within about 170 km from the monitoring sites.

Numerical modeling of a hydrothermal system around Waita volcano, Kyushu, Japan, based on resistivity and self-potential survey results

A self-potential survey has been conducted around Waita volcano, Kyushu, Japan. A large negative anomaly, generally interpreted as a surface recharge zone, has been observed at medium altitude. However, combined resistivity and self-potential modeling suggests that this anomaly is not necessarily related to surface recharge but to a high permeability column within the body of the volcano. This column encourages lateral flow, which results in a mixture of meteoric water and hot fluid flowing laterally towards the Takenoyu geothermal system. Such a combined modeling analysis has proved to be a useful technique for imaging subsurface fluid circulations.