As per recent reports, it has been found that small earthquakes proceed towards volcanic eruption by making a harmonic tremor. The sound is not audible to humans, but it is said that it is a mix of a number of musical instruments.

In 2009, an analysis was carried out by Alaska's Redoubt Volcano. During its evaluation, it was found that the harmonic tremor reach at quiet a high frequency.

Alicia Hotovec-Ellis from University of Washington was of the view that it is not common for a volcano of such kind to witness such tremors. Ellis continued by affirming that for now, it has not been known yet as what causes earthquakes to have harmonic tremor.

However, Ellis thinks that she knows what causes harmonic tremors in volcanoes. Ellis was of the view that the harmonic tremors might be taking place as magma is under pressure to pass through a small place to reach at the centre of the mountain.

"Because there's less time between each earthquake, there's not enough time to build up enough pressure for a bigger one", said Hotovec-Ellis.

Ellis affirmed that one of their main aims is to know that why harmonic tremors stops before volcanic explosion.