Spatial mapping of the b value at Mount Etna, Italy, using earthquake data recorded from 1999 to 2005

[1] The spatial pattern of the b value of the frequency-magnitude relation has been analyzed using gridding techniques beneath Mount Etna, Italy. A regional data set of 2900 events with M-d (duration magnitude) >= 1.5 up to 15 km depth occurring between August 1999 and December 2005 has been used. Two regions with an abnormally high b value have been found, one centered beneath the southern part of the Valle del Bove, above the 6 km below sea level (bsl) deep basement, and the other beneath the summit region 2 km bsl east of the Central Craters. We can infer that these high b value anomalies are regions of increased crack density, and/or high pore pressure, related to the presence of nearby magma storage. This interpretation is supported by all the available geophysical evidence, such as tomographic studies and geodetic deformation measurements. The data set has also been subdivided into five periods, corresponding to different phases of volcanic activity: 2001 preeruption, 2001 eruptive, 2002-2003 preeruption, 2002-2003 eruptive, and 2002-2003 posteruption. The minimum magnitude of completeness, M-c, and the b value were computed for each period. A volume of anomalously high b values can be observed in each of these periods (except for the 2002-2003 preeruption interval). This approach has allowed the detection of the transient presence of magmatic intrusions during the various periods evaluated.