T waves in Western Mediterranean Sea after the May 21, 2003 Algerian earthquake
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Abstract
Aim of this paper is to discuss on the T phases generated after the mb 6.5 earthquake that shook Algeria on May 21,
2003. The seismograms, recorded by a cluster of seismic stations located on the coast facing the Ligurian Sea, Northern
Italy, some 800 km N-NE from the source, represent a good database able to shed some light on the recognition,
propagation and characteristics of these quite uncommon phases. The occurrence and the recording of T phases are
in fact due to particular conditions, and require both particular characteristics of the bathymetric slope and the existence
of a clear path between the instruments and the earthquakes source: these constraints are exactly realized in
the north-western part of the Mediterranean Sea, whose coasts have been affected several times in the past by similar
events. The preliminary investigations on the complex recorded seismogram show two different behaviours for
stations close the coast and inland. In both cases, two distinct T phases (namely T1 and T2) are observed. In one case
they have apparent velocities close to an average SOFAR channel, and are thus the recording of direct T phases. In
particular, T1 is probably a precursor due to some scattering, while T2 is the direct T wave. Conversely, the recordings
of the stations inland show apparent velocities that suggest back conversion of the original T to P and S waves
and a crustal path. The frequency content of the T phases, as derived from the spectral analysis, reveals marked amplitude
peaks also in the range 1-3 Hz, conversely to what was proposed by other authors for similar occurrences in
other parts of the world. Since the geometry and shape of the SOFAR channel vary, it is highly likely that the spectrum
is biased by the water conditions and the frequency content might change in different seas. Finally, the attenuation
of the T phase does not depend on the actual distance of the receiver from the source but rather from the backconversion
point: the amplitude varies thus with the in-land path and decreases proportionally to x?1.
2003. The seismograms, recorded by a cluster of seismic stations located on the coast facing the Ligurian Sea, Northern
Italy, some 800 km N-NE from the source, represent a good database able to shed some light on the recognition,
propagation and characteristics of these quite uncommon phases. The occurrence and the recording of T phases are
in fact due to particular conditions, and require both particular characteristics of the bathymetric slope and the existence
of a clear path between the instruments and the earthquakes source: these constraints are exactly realized in
the north-western part of the Mediterranean Sea, whose coasts have been affected several times in the past by similar
events. The preliminary investigations on the complex recorded seismogram show two different behaviours for
stations close the coast and inland. In both cases, two distinct T phases (namely T1 and T2) are observed. In one case
they have apparent velocities close to an average SOFAR channel, and are thus the recording of direct T phases. In
particular, T1 is probably a precursor due to some scattering, while T2 is the direct T wave. Conversely, the recordings
of the stations inland show apparent velocities that suggest back conversion of the original T to P and S waves
and a crustal path. The frequency content of the T phases, as derived from the spectral analysis, reveals marked amplitude
peaks also in the range 1-3 Hz, conversely to what was proposed by other authors for similar occurrences in
other parts of the world. Since the geometry and shape of the SOFAR channel vary, it is highly likely that the spectrum
is biased by the water conditions and the frequency content might change in different seas. Finally, the attenuation
of the T phase does not depend on the actual distance of the receiver from the source but rather from the backconversion
point: the amplitude varies thus with the in-land path and decreases proportionally to x?1.
Article Details
How to Cite
Solarino, S. and Eva, C. (2007) “T waves in Western Mediterranean Sea after the May 21, 2003 Algerian earthquake”, Annals of Geophysics, 50(5). doi: 10.4401/ag-3058.
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