G. Milana

Source mechanisms of explosions at Stromboli Volcano, Italy, determined from moment‐tensor inversions of very‐long‐period data

[1] Seismic data recorded in the 2–30 s band at Stromboli Volcano, Italy, are analyzed to quantify the source mechanisms of Strombolian explosions during September 1997. To determine the source-centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous elastic medium that takes topography into account. Two source centroids are identified, each representative of the distinct event types associated with explosive eruptions from two different vents. The observed waveforms are well reproduced by our inversion, and the two source centroids that best fit the data are offset 220 and 260 m beneath and � 160 m northwest of the active vents. The source mechanisms include both moment-tensor and single-force components. The principal axes of the moment tensor have amplitude ratios 1:1:2, which can be interpreted as representative of a crack, if one assumes the rock matrix at the source to have a Poisson ratio n = 1/3, a value appropriate for hot rock. Both imaged cracks dip � 60� to the northwest and strike northeast–southwest along a direction parallel to the elongation of the volcanic edifice and a prominent zone of structural weakness, as expressed by lineaments, dikes, and brittle structures. For our data set, the volume changes estimated from the moments are � 200 m 3 for the largest explosion from each vent. Together with the volumetric source is a dominantly vertical force with a magnitude of 10 8 N, consistent with the inferred movement of the magma column perched above the source centroid in response to the piston-like rise of a slug of gas in the conduit. INDEX TERMS: 7215 Seismology: Earthquake parameters; 7280 Seismology: Volcano seismology (8419); 8414 Volcanology: Eruption mechanisms; KEYWORDS: very-long-period seismicity, moment tensor inversions, eruption mehanics

Source and path effects in the wave fields of tremor and explosions at Stromboli volcano, Italy

The wave fields generated by Strombolian activity are investigated using data from small-aperture seismic arrays deployed on the north flank of Stromboli and data from seismic and pressure transducers set up near the summit crater. Measurements of slowness and azimuth as a function of time clearly indicate that the sources of tremor and explosions are located beneath the summit crater at depths shallower than 200 m with occasional bursts of energy originating from sources extending to a depth of 3 km. Slowness, azimuth, and particle motion measurements reveal a complex composition of body and surface waves associated with topography, structure, and source properties. Body waves originating at depths shallower than 200 m dominate the wave field at frequencies of 0.5–2.5 Hz, and surface waves generated by the surficial part of the source and by scattering sources distributed around the island dominate at frequencies above 2.5 Hz. The records of tremor and explosions are both dominated by SH motion. Far-field records from explosions start with radial motion, and near-field records from those events show dominantly horizontal motion and often start with a low-frequency (1–2 Hz) precursor characterized by elliptical particle motion, followed within a few seconds by a high-frequency radial phase (1–10 Hz) accompanying the eruption of pyroclastics. The dominant component of the near- and far-field particle motions from explosions, and the timing of air and body wave phases observed in the near field, are consistent with a gas-piston mechanism operating on a shallow (<200 m deep), vertical crack-like conduit. Models of a degassing fluid column suggest that noise emissions originating in the collective oscillations of bubbles ascending in the magma conduit may provide an adequate self-excitation mechanism for sustained tremor generation at Stromboli.

Broadband measurements of the sources of explosions at Stromboli Volcano, Italy

During September–October 1997, 21 three-component broadband seismometers were deployed on Stromboli Volcano at radial distances of 0.3–2.2 km from the active crater to investigate the source mechanisms of Strombolian explosions. In the 2–50 s band, the very-long period (VLP) signals associated with explosions are consistent with two stationary sources repeatedly activated in time. VLP particle motions are essentially linear and analyses of semblance and particle motions are consistent with a source centroid offset 300 m beneath and 300 m northwest of the active vents. Similar VLP waveforms are observed at all 21 stations, indicating that the seismograms are source-dominated. The VLP ground displacement response to each explosion may be qualitatively interpreted as: (1) pressurization of the conduit associated with the ascent of a slug of gas; (2) depressurization of the conduit in response to mass withdrawal during the eruption; and (3) repressurization of the conduit associated with the replenishment of the source with fluid.

Rupture mechanism and source parameters of Umbria-Marche mainshocks from strong motion data

A long sequence of earthquakes causing few casualties and considerable damage in a wide zone struck Central Italy starting on September 26, 1997. Theearthquakes are characterized by normal faulting mechanism, with a NE-SW(anti-Apenninic direction) tension axis. In this paper we analyze the accelerometric recordings collected by the accelerograph stations belonging to the National Accelerograph Network. About 10 stations were triggered by the mainshocks of the sequence. In particular, a small size foreshock and the two mainshocks that occurred on September,26 (00:33(GMT) MW = 5.7 and 09:40 MW = 6.0) have been recorded by two digital 3-C accelerometers located at near source distances (within 30 km from the faults). These records are relevant to investigate the detail of therupture kinematics, due to the close epicentral distance and azimuthallocation relative to the fault orientation and geometry. Using a trial and error approach we modeled the source mechanism through the fit of the arrival times, the apparent source time duration, the main polarization features and the entire waveforms of the recorded signals, in order to get some insight on the rupture evolution, the location of the fracture origin point and the fault geometry. Based on this fault kinematic model, inferences on fault slip distribution are obtained by modeling the S acceleration waveform, comparing the ray theory synthetics with 1–5 Hz band filtered ground velocity records.The final model shows that the seismic ruptures occurred along two adjacent,sub-parallel, low angle dipping normal faults. Ruptures bothnucleated from the fault bottom and propagated up-dip, showing differentrupture velocity and length. The presence of a transfer zone (barrier)can be suggested by the mainshocks rupture evolution. This transfer zonehas probably controlled the amplitude increase of local stressreleased by the first rupture at its NW edge which triggered about 9 hourslater the second rupture. The inferred model was used to compute the predictedground acceleration in the near source range, using a hybridstatistical-deterministic approach.A similar trial and error method has been also applied to the October 14, 199715:23 earthquake (MW = 5.6). The inferred kinematic model indicates a rupture nucleating from the faultbottom and propagating up-dip, toward the SE direction. Thus the three mainshocks ruptured distinct fault segments, adjacent and slightly offsetfrom one to another.

The 1997 Umbria‐Marche (central Italy) Earthquake Sequence: Insights on the mainshock ruptures from near source strong motion records

A small size foreshock and the two mainshocks of the Umbria Marche earthquake sequence which occurred on September 26, 1997 have been recorded by two digital 3C accelerometers located at near source distances. The close epicentral distance and azimuthal location relative to the fault orientation and geometry make these records relevant to look at the detail of the rupture kinematics. S-wave polarizations, apparent source time duration and waveforms from strong motion records are used to constrain the location of the fracture origin point, the fault geometry, the final slip distribution, size and mechanism of the events. The final model shows that the seismic ruptures occurred along two adjacent, sub-parallel, low angle dipping normal faults. The relative timing, location and geometry of the mainshock faults suggest the presence of a transfer zone (barrier) which has probably controlled the amplitude increase of local stress released by the first rupture at its NW edge which triggered about 9 hours later the second rupture.

A detailed analysis of two seismic sequences in Abruzzo, Central Apennines, Italy

A three-component digital seismic network has been installed along central Apennines since the end of 1991. Two seismic sequences having main shocks of magnitudes 3.9 and 3.7 were recorded in August 1992 and June 1994, respectively. A detailed analysis of these sequences, including multiplet relocation, fault-plane solutions and source parameter estimation, is performed in the present paper. A correlation analysis allowed us to recognize a number of correlated events in the two sequences which were used for relative locations using a master event technique. This analysis allowed to obtain a better alignment of epicentral data along two almost orthogonal directions, following an Apenninic and an anti-Apenninic trend. For the two sequences, fault-plane solutions were evaluated by using a first arrival technique, resulting in mechanisms with predominant normal faulting for the 1992 and 1994 swarms. S-wave polarization analysis allowed to check the stability of the previous solutions and to reduce their range of uncertainty. The same technique was also applied to derive the composite fault-plane solutions from the aftershocks, resulting in solutions which are in good agreement with those derived from the main shocks of both sequences. Source parameters were then derived from the three-component records of 28 well-recorded events with seismic moment in the range 8.5 × 1010–1.0 × 1014 Nm. Stress drops ranged in the interval 0.3–52.3 bar and source radii were of the order of 100 m. Their scaling relations are in good agreement with other results derived from the analysis of other Italian earthquakes that occurred in regions of predominantly normal faulting tectonics (Apennines and Calabrian arc).

Correction to “Source mechanisms of explosions at Stromboli Volcano, Italy, determined from moment-tensor inversions of very-long-period data”

[1] Seismic data recorded in the 2–30 s band at Stromboli Volcano, Italy, are analyzed to quantify the source mechanisms of Strombolian explosions during September 1997. To determine the source-centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous elastic medium that takes topography into account. Two source centroids are identified, each representative of the distinct event types associated with explosive eruptions from two different vents. The observed waveforms are well reproduced by our inversion, and the two source centroids that best fit the data are offset 220 and 260 m beneath and 160 m northwest of the active vents. The source mechanisms include both moment-tensor and single-force components. The principal axes of the moment tensor have amplitude ratios 1:1:2, which can be interpreted as representative of a crack, if one assumes the rock matrix at the source to have a Poisson ratio n = 1/3, a value appropriate for hot rock. Both imaged cracks dip 60 to the northwest and strike northeast–southwest along a direction parallel to the elongation of the volcanic edifice and a prominent zone of structural weakness, as expressed by lineaments, dikes, and brittle structures. For our data set, the volume changes estimated from the moments are 200 m for the largest explosion from each vent. Together with the volumetric source is a dominantly vertical force with a magnitude of 10 N, consistent with the inferred movement of the magma column perched above the source centroid in response to the piston-like rise of a slug of gas in the conduit.

Velocity and spectral characteristics of the volcanic tremor at Etna deduced by a small seismometer array

Abstract A small L-shaped short-period seismic array was set up at Etna Volcano to measure the wave velocity and the incoming azimuth of the volcanic tremor generated by the volcanic activity at Etna in non eruptive-conditions. Cross-correlation techniques were used to measure the phase-shifts between the array stations. An approximately N-S direction of the propagation direction was estimated, coincident with the azimuth of the crater area of Etna. An apparent velocity of less than 500 m/s was estimated, suggesting a surface propagation of the tremor waves. Spectral analysis, confirming previous results, shows velocity spectra peaked at approximately 2 Hz. A localized source, coincident with the crater area, agrees with our data and could generate the volcanic tremor at Etna in non-eruptive stages.

Waveguide effects in very high rate GPS record of the 6 April 2009, Mw 6.1 L'Aquila, central Italy earthquake

This work has partially benefited from the activities performed in the NERA project (Network of European Research infrastructures for earthquake risk Assessment and mitigation, 262330), funded by the European Commission FP7 program, and in the FIRB-Abruzzo project, funded by the Italian Ministery of Education, University and Research.

Strong Motion Records of the 2002 Molise, Italy, Earthquake Sequence and Stochastic Simulation of the Main Shock

The far field radiation inferred from accelerometric data recorded by the Italian Strong Motion Network (RAN) during the October 31, 2002, Molise, Italy, earthquake indicates a strong anisotropy in PGA distribution that presents its maximum values in the eastern part of the epicentral area. This study explores an interpretation of source directivity using a stochastic approach to produce synthetic seismograms accounting for source dimensions and rupture evolution. An E-W strike-slip fault, with a dip close to 90° and a fault plane size of (8×6) km2, reproduces satisfactorily both peak accelerations and spectral shapes of the recorded data when the rupture enucleates from the western edge at a depth of about 20 km and propagates eastward. This approach allows us to put constraints on seismic source characteristics and can be used to simulate ground motion for the most damaged areas where strong motion data are not available.