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Anomalous Ultra Low Frequency signals observed by ground-based magnetometers that linked with seismic events are commonly accepted. These anomalous signals can be promising phenomena for constructing early Earthquake-warning systems. The present study analyzes high-resolution (1-s) geomagnetic data recorded at the Kototabang (KTB) station, Indonesia, and other remote reference stations to detect the occurrence of any anomalous ULF signals that may associated with a number of seismic events occurred in Sumatra, Indonesia. The amplitude of the ULF emissions in the Pc3 range (10-45 s) was examined during three big seismic events that occurred in Sumatra between 2004 and 2007. Results of data processing and analysis indicate the presence of anomalous enhancement in the Pc3 amplitude ratio (ZPc3/HPc3) at KTB station in association with the studied seismic events. On the other hand, there are no noticeable changes at other distant reference stations during the examined periods. In addition, there are no remarkable external geomagnetic disturbances during the studied seismic events as it reveals from the Disturbance storm time (Dst) index. So, we can conclude that the observed anomalous changes might be a possible signature related to the above-mentioned seismic events in Sumatra, Indonesia. 2. Ultra low frequency emissions and earthquake occurrence Geomagnetic pulsations, ULF waves, are natural magneto-hydrodynamic waves in the magnetosphere which can be classified as either

On 19 December 2009, the eastern part of Taiwan at approximate depth of 45 km was struck by a strong earthquake (Mw = 6.4). The epicenter was located about 20 km away from the Hualien (HLN) station in Taiwan. By analyzing data from the HLN station and Amami Oshima (AMA) in Japan as a remote reference, the geomagnetic east-west (D) and vertical (Z) components recorded at the HLN station are showing no correlation with those at the AMA station in December 2009. Anomalous variations of ∼10-15 nT started about one week before the occurrence of the earthquake and lasted for about two weeks. Also, an enhanced ULF signal in the range of Pc 3 (10-40 s) was observed a few days before the onset of the earthquake. Moreover, the polarization ratio (Z/H (north-south component)) of the Pc 3 amplitude at the HLN station decreased a few days preceding the earthquake. The mechanism behind these anomalous variations is not fully understood. However, we expect that crustal stress perturbations and underground conductivity changes associated with the earthquake played an important role for generating such observed geomagnetic variations.