Other applications (4) – Seismology of the atmosphere
Local infrasound and regional infrasound
both have their place. A recent study
by Johnson and Malone (in press) in EPSL is focused on using
regional airwave arrivals from the May 18 1980 Mount St. Helens
eruption (out to several hundred kilometers) to infer the
sequence of events occurring at the volcano.
This paper studies infrasound (and possible higher
frequency sonic) arrivals at seismic stations that behaved
as microphones. This paper utilizes an
inferred velocity structure of the atmosphere (from temperature
(T(z)) and wind (U(z)) profiles) to map acoustic raypaths
out to regional distances using a conserved ray parameter (p) where intrinsic
sound speed (c) is a function of temperature:
Some
important findings in this paper are the following:
1)Acoustic
rays turn back to earth from the stratosphere and thermosphere (150 km!)
2)There
were multiple large sources occurring in the first few minutes of the
eruption
3)At
least one source occurring several minutes after the eruption onset was
displaced about 10 km to the northwest.
