Time reverse location of seismic long-period events recorded on Mt Etna

We
present the first application of a time reverse location method in a
volcanic setting, for a family of long-period events recorded on Mt
Etna. Results are compared with locations determined using a full
moment tensor grid search inversion and cross-correlation method.
From June 18th to July 3rd, 2008, 50 broadband
seismic stations were deployed on Mt Etna, Italy, in close proximity
to the summit. Two families of long-period events were detected with
dominant spectral peaks around 0.9 Hz. The large number of stations
close to the summit allowed us to locate all events in both families
using a time reversal location method. The method involves taking the
seismic signal, reversing it in time, and using it as a seismic
source in a numerical seismic wave simulator where the reversed
signals propagate through the numerical model, interfere
constructively and destructively, and focus on the original source
location. The source location is the computational cell with the
largest displacement magnitude at the time of maximum energy current
density inside the grid. Before we located the two long-period
families we first applied the method to two synthetic datasets and
found a good fit between the time reverse location and true synthetic
location for a known velocity model. The time reverse location
results of the two families show a shallow seismic region close to
the summit in agreement with the locations using a moment tensor full
waveform inversion method and a cross-correlation location method.