| Summary: | The flow of conducting ocean water generates weak magnetic signals relative to the main field. These signals lead to magnetic anomaly observed as variations in the vertical component b! z of time- varying secondary field and the horizontal component b! H during past tsunamis across the Pacific and Indian oceans. In this study, the maximum amplitude of b! z was predicted using theoretical approach and the prediction was compared with magnetogram provided by INTERMAGNET and/or BCMT for tsunamigenic events and cases with no tsunami threats in Indonesian regions. The focus of this study is thus to examine whether theory used to estimate tsunami-magnetic signals is consistent in terms of accuracy with real-time field records from the two world wide magnetic institutions. For three events where tsunami occurred, including the 2004 Aceh event, it was found that frozen-flux theory provides a useful tool for best estimates of b! z with its application to b! H signals is here limited with caution. While b! z is a measure of tsunami wave height offshore, b! H is likely to be a good indicator for tsunami propagation direction. For four recent earthquakes of no tsunami potential after shocks the b! z signal provided no anomaly with convincing signs of tsunami absence were, in the same period of time, given by almost zero declination. The results for all cases considered in the present study confirm that detection of magnetic anomaly owing to tsunami passage prior to tsunami arrivals at coastal zones is possible. This is of primary importance for tsunami early warning in the country.
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