Stefano Gresta

Stress Directions and Shear-Wave Anisotropy: Observations from Local Earthquakes in Southeastern Sicily, Italy

The spatial distribution of 414 earthquakes (1.0 M L 4.6), recorded from 1994 to 2002 in southeastern Sicily (Italy), has been analyzed. The seismicity generally coincides with mapped Plio-Quaternary faults, including the north-north- west-south-southeast striking offshore fault system, which is the most important tectonic structure of the area. For the best located events, we computed 70 focal mechanisms by combining P-wave polarities with S-wave polarizations. A predom- inance of strike slip and normal faults was observed. Focal mechanisms were then inverted for stress tensor parameters by using the algorithm of Gephart and Forsyth. The results highlighted a region governed mainly by a north-northwest-south-south- east to northwest-southeast compressional stress regime. Moreover, anisotropy anal- ysis of shear waves showed a polarization of fast S waves coherently aligned with this stress direction. A finer-scale analysis of the stress tensor evidenced three regions characterized by slightly differing orientation of the greatest principal stress axis, r1. The eastern sector displays a nearly horizontal r1 trending northwest-southeast; the central sector is affected by a low-dip north-northwest-south-southeast r1; whereas, in the western sector, a north-northwest-south-southeast-oriented r1 with a higher dip angle, was detected. Finally, the comparison of the spatial distribution of seismicity occurring during 1994-2002, with locations of previous instrumental earthquakes and larger (M 5.0) historical events showed that the seismicity patterns are persistent.

Attenuation and source parameters of shallow microearthquakes at Mt. Etna volcano (Italy)

We estimated the attenuation laws of high-frequency seismic waves in the shallow crust (depth 5 km) and earthquake source parameters by using a se- lected data set of 320 shallow events (2.6 MD 4.2), recorded at Mt. Etna volcano during the last two flank eruptions occurring in 2001 and 2002-2003. The quality factor (Q) was estimated from spectra of P and S waves for 24 stations of the local permanent network by applying a spectral ratio technique. The results show varia- tions in both QP and QS as a function of frequency, according to the power law Q Q0f n , with n ranging between 0.3 and 1.3 for P waves and between 0.2 and 0.9 for S waves. As typical of volcanic environments, strong azimuthal variations of QP were also found, suggesting the presence of local strong lateral heterogeneities and/or of fluid-filled cracked volumes. After correction for attenuation, we estimated the source parameters (seismic moment, source radius, and stress drop) of a subset of 66 shallow events, under the assumption of a circular dislocation. The estimated seismic moments M0 range from 10 13 to 10 15 N m. The source radii (r) are confined between 100 and 1000 m and stress drop (Dr) ranges between 0.2 MPa and about 4 MPa. Combining the source parameters obtained in this study with those calculated by Pataneet al. (1997) for an old data set of smaller microearthquakes (10 9 M0 10 14 N m) recorded in the same area, we re-evaluated the scaling relationship between seismic moment (M 0 ) and corner frequency ( f c ) for the earthquakes with M 0 ranging between 10 13 and 10 15 N m. We confirm that microearthquakes at Mt. Etna seem not to obey a

Seismic and infrasonic evidences for an impulsive source of the shallow volcanic tremor at Mt. Etna, Italy

During a seismo-acoustic experiment we recorded volcanic tremor around the summit craters of Mt. Etna volcano. Tremor shows amplitude modulation, which disappear ≈ 900 m from the crater area. The infrasonic wavefield is coherent even at distances of ≈ 750 m. Time delay between infrasonic transient is stable around 1.3 s and is consistent with the position of the source in the Voragine crater. Amplitude modulation of tremor is well correlated (0.72) with infrasound amplitude with a time lag of 0.37 s. coherent with a shallow position of the source. Amplitude of volcanic tremor decays over increasing distances according to geometrical spreading of body waves. Tremor wavefield shows a linear polarization following the same time occourrence as the infrasonic pulses. Polarization azimuth indicates that wavefield rectilinearity is mostly due to P-waves. We infer that most of the volcanic tremor we recorded at Mt. Etna is generated by superimposition of small impulsive sources acting at 1–2 s rate caused by pressure instability during magma degassing.

Attenuation in Southeastern Sicily (Italy) by applying different coda methods

The attenuation in Southeastern Sicily has been investigated using 40aftershocks of the December 13 1990, earthquake. The quality factor ofcoda waves (Qc) was estimated in the frequency range 1.5–24 Hz,applying three different methods in time and frequency domains. On thewhole, a clear dependence of Qc on frequency was observed,according to the general law Q = Q0(f/f0)n . Thefrequency dependence relationships obtained from the analysis of codawaves at three lapse time windows (10, 20 and 30 seconds) show that, forall methods, Q0 (Qc at 1 Hz) significantly increases with lapsetime. In particular, Q0 is approximately 20 at short lapse time (10s) and increases to about 70 at longer lapse time (30 s). This is attributedto the fact that larger lapse times involve deeper parts of the crust andupper lithosphere which may be characterized by larger quality factors.Moreover, the value of the exponent n decreases with increasing codalengths from about 1.3 to 0.9, suggesting a decrease in heterogeneity ofthe medium with depth.Finally, Qc-values here found are of the same order as thosereported from other tectonic regions like the Anatolian Highlands orSouthern Spain, while significantly higher than in the neighboring volcanicarea of Mt. Etna.

Geodynamic behaviour of eastern and western sides of Mount Etna

Abstract The differing behaviour of the eastern and western flanks of Mount Etna has been investigated by analyzing morphological, structural, volcanic and seismic data. Morphological and structural analyses have pointed out a higher “degree” of tectonization on the eastern side. From the volcanological point of view, the analysis of some parameters of adventive eruptions during the last three centuries did not evidence great differences between the two sides of the volcano. Only higher lava outputs on the eastern flank have been remarked. Historic and recent seismic activity has been analyzed for the number and magnitude of events, strain energy release and frequency-magnitude law. The shallowest and strongest earthquakes occur on the eastern side of the volcano, strictly linked to a regional structural system trending NNW-SSE. The western side is mostly characterized by deeper earthquakes that seem to control the volcanic activity. Lower seismic energy releases and higher values of the b coefficient in the Gutenberg-Richters (1956) law can be observed on the western side. The differences observed between the two sides of the volcano have been interpreted in terms of both regional and local tectonics, with the result that a higher extensional stress field is attributed to the eastern side of the volcano.

High-Precision Relative Locations of Two Microearthquake Clusters in Southeastern Sicily, Italy

In November 1999 and January 2000, two microearthquake swarms occurred in southeastern Sicily (Italy). They were analytically located in the depth range 17-25 km, some kilometers northward from the buried front of a regional foredeep, below the active thrust zone of the Sicily mountain chain. Their hypocentral distribution showed two distinct clusters, and comparison of the waveforms revealed clearly that the two swarms formed two distinct families of multiplet events. This led us to (1) carry out a precise relocation relative to two chosen master events of the families and (2) better define the geometrical structure of the two clusters. The cross-spectral method was applied to obtain precise readings of the wave onsets. SH wave onsets were used instead of P waves, as they showed clearer onsets and a good signal-to-noise ratio. Residuals of the relative locations showed small values, no more than several meters on average. The vertical extent of the two relocated clusters was 500 and 250 m, respectively, while the horizontal extent was 250 m. Hypocenters of the first cluster clearly delineate a north-northwest-trending plane with almost vertical dip, matching one nodal plane of the focal mechanism obtained as a composite solution of all events of the cluster. Given the considerable gap angles, because of unfavorable network geometry with respect to the events, the stability of our results was tested carrying out a Monte Carlo experiment. Varying the onset times randomly in the range of ±5 msec, a dispersion of the locations less than 10 m in longitude and less than 50 m both in latitude and depth was found. Similar results were obtained when comparing relocations carried out with different master events. Thus, the overall geometrical characteristics of the clusters were not affected seriously by random errors. Considering the geostructural framework of the region, together with the location and time evolution of the two clusters, fluids of plutonic origin are suggested as the trigger mechanism. Manuscript received 26 June 2002.

Inferences on the main volcano-tectonic structures at Mt. Etna (Sicily) from a probabilistic seismological approach

We analysed earthquakes at Mt. Etna for the period 1983–1991 using a method that weights uncertainties in hypocentral location. Three-dimensional distributions of ‘hypocentral probability’ and ‘energy density’ were studied, and two first-order volcano-tectonic structures identified. The first, on the northern and western sides, is roughly NE–SW oriented, and strongly marks the northernmost limit of earthquake occurrences in the volcano region; the second, NNW–SSE trending, affects the south-eastern flank of the volcano, and is evidence for an almost aseismic uprise of magma along it. Both structures fit well with the geodynamic framework of eastern Sicily. On the contrary, there is no evidence for a main magma chamber, as postulated in the literature.

Volcanic tremor at Mount Etna (January 1984–March 1985): its relationship to the eruptive activity and modelling of the summit feeding system

Spectral analyses of volcanic tremor at Etna during January 1984–March 1985, have been performed and the relationship between tremor energy and observed volcanic phenomena have been examined. The highest energy levels have been observed during the paroxysmal phases of eruptions, whereas a gradual decrease was linked to the lowering of eruptive activity. Amplitude variations with time of some spectral frequency peaks (0.95, 1.20, 1.45, 1.65, 1.80 and 2.40 Hz) have been compared with volcanic activity at the summit craters, and on the basis of these results a new schematic diagram for the feeding system of the summit vents is proposed.

Feeding Mechanism of Eruptive Activity at Mt. Etna Based on Seismological and Petrological Data

Seismic and petrological data are analyzed with reference to the eruptive phenomena at Mt. Etna from 1974–1984. Seismological data show that most earthquakes occur within the upper 16 km of the crust and very few below 30 km. Focal solutions for events at depth between 7 and 16 km show evidence of both normal and thrust faults, due to an unstable stress field, whereas at shallower levels only normal mechanisms are observed. In most cases, enhanced seismic activity preceded the beginning of adventive eruptions, but neither upward migration nor clear concentration of epicenters near the eruptive area (except for the shallowest events; h < 1 km) was observed.

Review of seismological studies at Mount Etna

This paper reports the present state of seismological research at Mt. Etna.A schematic classification of the earthquakes that occur on the volcano is proposed, based on both seismogram and spectrum features.We have made both focal solutions and estimates of earthquake source parameters (stress drop values between 2 and 20 bars and small source dimensions).The crust of Etna thus appears as an extremely heterogeneous medium that does not permit great stress accumulation. The coexistence of an extensional regime with an older and deeper compressive one seems confirmed at depths greater than about 7 km.Eruptive and seismic phenomena occur mainly along the principal structural trends of the volcano, but often the directions of the eruptive fractures and the earthquake concentration during the same eruption do not coincide.Tectonics seem to play an important role in controlling seismo-volcanic behaviour.