| 1. | A volcanic tremor signal that came next was what prompted the heightened alert level.
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| 2. | They've survived showers of rocks, choking ash and volcanic tremors.
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| 3. | There is a strong increase in the volcanic tremors,
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| 4. | Since then, low-level volcanic tremors have periodically sent ash and noxious gases into the air.
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| 5. | Eruptions and volcanic tremors were reported on 21 January and 22 January with activity subsiding on 23 January 2011.
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| 6. | Four small volcanic tremors were recorded after the eruption and the meteorological agency urged caution for those living near the mountain.
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| 7. | Gas emissions and volcanic tremors have been high for months and the volcano has expelled boulders and trickled lava in recent weeks.
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| 8. | Unlike earthquakes, which are characterized by the earth shifting along a fault, volcanic tremors are caused by the underground movement of magma.
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| 9. | Gas emissions and volcanic tremors have been high for months and the volcano has spit red hot boulders and trickled lava in recent weeks.
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| 10. | Volcanic tremor normally indicates an imminent eruption, although it may also be caused by shallow intrusions of magma which do not reach the surface.
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