Seismologists
deploy networks (distributed arrangements of sensors) and arrays
(closely-spaced arrangements of sensors) in an attempt to extract
the Green’s functions for seismic energy propagating in a
volcano. But it is difficult to extract
the seismic source time function for the following reasons:
1)Unlike
tectonic earthquakes, which are often assumed to be decent approximations of double-couple
forces, the physical source motions of volcanic earthquakes are not
well understood (i.e., we can’t assume a certain type of source radiation
pattern or, in many cases, even assume much about the source
time duration).
2)Green’s
functions are complex due to:
a) complex velocity structure
in the volcano (i.e., waves move variably faster and slower
depending upon interior volcano rock properties, which are variable)
b) scattering (i.e., waves
reflect and radiate off of heterogeneities, which are plentiful
in a potentially mixed phase volcanic edifice).
c) attenuation (i.e.,
certain frequencies decrease in amplitude due to viscous forces)
d) multiple wave types (i.e.,
seismic waves in a solid medium comprise P-wave, S-wave
as well as various flavors of surface waves).