T waves in Western Mediterranean Sea after the May 21, 2003 Algerian earthquake

• Main Authors: C. Eva, S. Solarino
• Format: Article
• Language: English
• Published: Istituto Nazionale di Geofisica e Vulcanologia (INGV) 2007-06-01
• Series: Annals of Geophysics
• Subjects: T waves; Ligurian Sea; SOFAR; frequency content
• Online Access: http://www.annalsofgeophysics.eu/index.php/annals/article/view/3058

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.