Publication Date:
2020-12-15
Description:
Seismogram envelopes recorded at Campi Flegrei caldera show diffusive characteristics as
well as steep amplitude increases in the intermediate and late coda, which can be related to
the presence of a non-uniformly scattering medium. In this paper, we first show the results of
a simulation with a statistical model considering anisotropic scattering interactions, in order
to match coda-envelope duration and shape.We consider as realistic parameters for a volcanic
caldera the presence of large square root velocity fluctuations (10 per cent) and two typical
correlation lengths for such an heterogeneous crust, a = 0.1 and 1 km. Then, we propose the
inclusion of a diffusive boundary condition in the stochastic description of multiple scattering,
in order to model intermediate and late coda intensities, and particularly the sharp intensity
peaks at some stations in the caldera. Finally, we show that a reliable 2-D synthetic model
of the envelopes produced by earthquakes vertically sampling a small region can be obtained
including a single drastic change of the scattering properties of the volcano, that is, a caldera
rim of radius 3 km, and sections varying between 2 and 3 km. These boundary conditions are
diffusive, which signifies that the rim must have more scattering potential than the rest of the
medium, with its diffusivity 2–3 orders of magnitude lower than the one of the background
medium, so that the secondary sources on its interface(s) could enhance coda intensities. We
achieve a good first-order model of high-frequency (18 Hz) envelope broadening adding to the
Monte Carlo solution for the incident flux the secondary source effects produced by a closed
annular boundary, designed on the caldera rim signature at 1.5 km depth. At lower frequencies
(3 Hz) the annular boundary controls the intermediate and late coda envelope behaviour, in a
way similar to an extended diffusive source. In our interpretation, the anomalous intensities
observed at several stations and predicted by the final Monte Carlo solutions are mainly due
to the diffusive transmission reflection from a scattering object of increased scattering power,
and are controlled by its varying thickness.
Description:
This work was carried out under the HPC-Europa2 project (project
number: 228398) with the support of the European Commission
Capacities Area-Research Infrastructures Initiative. We thank the
whole staff at EPCC (Edinburgh Parallel Computing Centre) in
Edinburgh and particularly Dr. Adam Carter for their help in both
developing and parallelizing the code. The challenging comments
and suggestions of the editor and two anonymous reviewers helped
both in focusing the aim and in overcoming the strong limits of a
previous version of the paper.
Description:
Published
Description:
1102–1119
Description:
3.1. Fisica dei terremoti
Description:
JCR Journal
Description:
restricted
Keywords:
Numerical solutions;
;
Seismic anisotropy;
;
Seismic attenuation
;
Seismic tomography
;
Wave scattering and diffraction
;
Calderas
;
04. Solid Earth::04.06. Seismology::04.06.09. Waves and wave analysis
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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