A University of Washington researcher reports that volcanoes may scream before they erupt, and Costa Rica’s Arenal figured in the study.
The researcher is Alicia Hotovec-Ellis, a doctoral student in earth and space sciences. A new analysis of an eruption sequence at Alaska’s Redoubt Volcano in March 2009 shows that what is being called an harmonic tremor glided to substantially higher frequencies and then stopped abruptly just before six of the eruptions, five of them coming in succession.
It is not unusual for swarms of small earthquakes to precede a volcanic eruption, the university noted. They can reach a point of such rapid succession that they create a signal that is being called a harmonic tremor that resembles sound made by various types of musical instruments, though at frequencies much lower than humans can hear.
A French researcher, Philippe Lesage, reported in 2006 that he had noted frequencies being emitted by Arenal. He measured frequencies in the range of 0.9 to 2 hertz. He likened the activity to that of a clarinet with the sounds generated by volcano conduits that are filled with magma. He attributed the changes in frequency to variations in pressure in the conduits.
Ms. Hotovec-Ellis builds on the Arenal data. The significance is that monitors might be able to hear the low-frequency sounds to provide a warning for eruptions.
San Jose’s downtown is in sight of three active volcanoes.
“The frequency of this tremor is unusually high for a volcano, and it’s not easily explained by many of the accepted theories,” said Ms. Hotovec-Ellis of the Alaskan volcano, as quoted by the university in a summary.
She documented the rising tremor frequency, starting at about 1 hertz (or cycle per second) and gliding upward to about 30 hertz, the university said. In humans, the audible frequency range starts at about 20 hertz, but a person lying on the ground directly above the magma conduit might be able to hear the harmonic tremor when it reaches its highest point, it added.
Scientists at the U.S. Geological Survey’s Alaska Volcano Observatory have dubbed the highest-frequency harmonic tremor at Redoubt Volcano the screams because they reach such high pitch compared with a 1-to-5 hertz starting point.
Similar sounds have been reported at the volcano on the Caribbean island of Montserrat.