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In this paper, electromagnetic emissions recorded by a borehole TOA installment with three observing channels of CH1 (0.01 - 0.1 Hz), CH2 (0.1 - 1.0 Hz) and CH3 (1 - 9 kHz) before four large earthquakes with magnitudes more than 8.0 have been depicted. These abnormities present different fluctuating processes from one another. For the Wenchuan MS 8.0 earthquake on 12 May 2008, the nearest one among these four events and only 660 km from the TOA station, electromagnetic information appeared at least 5 months ago in two low frequency bands of CH1 and CH2 and it was subjected to an obvious fluctuating process with several developing stages: initial information, intensive anomaly and large amplitude signals. The typical pulse-like emissions in CH2 happened group by group with large various magnitudes, which can be of 10 mV in the climax period. While during this period, compatible wave-like information with little magnitudes also happened in CH1 channel and a few pulses in CH3. Anomalous emissions occurred about 4 months prior to the 25 April 2015 Nepal MS 8.1 event, 1560 km away from the TOA station. The abnormal information in CH2 also appeared group by group but with small various magnitudes, more than 2 mV during their climax. This process is also effective for the Sumatra MS 8.9 earthquake on 26 December 2004, 2500 km from the borehole TOA, only with a different duration of 2 months and less magnitudes of 0.1 mV in CH1 and 1 mV in CH2 in this case. However, there is no obvious fluctuation and only small constant amplitude signals being ~0.15 mV appeared during 2 weeks before the Japan MS 9.0 earthquake on 11 March 2011. It is the farthest one among these four events and beyond 4000 km from the observing station. So, we can make a conclusion that there is a near relationship between the properties of the abnormities associated with these four earthquakes, such as amplitudes, duration and signal types, and the distances from TOA station: on one hand, the amplitude and duration decreases as the distance increases; on the other hand, there is an evolution for emission properties from complex various magnitude signals to single equal magnitude ones as the distance changes to be far. However, one common feature of the anomalous information related to these four events is that almost electromagnetic emissions were collected in two low frequency bands of CH2 and CH1 instead of CH3 band, which means ULF band (0.01 - 1.0 Hz) is more sensitive than VLF band (1 - 9 kHz) at this TOA station.

open journal of earthquake research

Electromagnetic Emissions Recorded by a Borehole TOA Installment before Four Huge Destructive &amp;lt;i&amp;gt;M&amp;lt;/i&amp;gt;&amp;lt;sub&amp;gt;S&amp;lt;/sub&amp;gt; ≥ 8.0 Earthquakes in Asia