1. A volcanic tremor signal that came next was what prompted the heightened alert level. 2. They've survived showers of rocks, choking ash and volcanic tremors . 3. There is a strong increase in the volcanic tremors , 4. Since then, low-level volcanic tremors have periodically sent ash and noxious gases into the air. 5. Eruptions and volcanic tremors were reported on 21 January and 22 January with activity subsiding on 23 January 2011. 6. Four small volcanic tremors were recorded after the eruption and the meteorological agency urged caution for those living near the mountain. 7. Gas emissions and volcanic tremors have been high for months and the volcano has expelled boulders and trickled lava in recent weeks. 8. Unlike earthquakes, which are characterized by the earth shifting along a fault, volcanic tremors are caused by the underground movement of magma. 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. 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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