Roberto Scarpa

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.

Source parameters of the 1908 Messina Straits, Italy, earthquake from geodetic and seismic data

[1]xa0We carry out a nonlinear joint inversion of P wave first-motion polarities and coseismic surface displacement data of the 1908 Messina earthquake. We model the earthquake using a single planar fault: Slip is at first assumed to be uniform across the whole fault, then independent in a small set of coplanar subfaults, and finally smoothly variable across the fault. The first two steps are accomplished using a global minimization technique. The main features of the retrieved model are very robust and independent of the seismic velocity profile, of the presence of questionable bench marks, and of the assumed residual distribution. The along-strike component of slip is about half the along-dip component, in agreement with the direction of the extensional stress axis retrieved from geological observations. Surface vertical displacement is consistent with tsunami data and with the morphology of the Messina Straits.

Inversion of source parameters from near‐ and far‐field observations: An application to the 1915 Fucino earthquake, central Apennines, Italy

We use a nonlinear inversion approach for determining source parameters of the Fucino Ms=6.9 earthquake that occurred along central Apennines, Italy, in 1915, taking into account both near-field surface deformations and far-field first-motion polarities. This event was one of the most destructive in Italy this century, causing over 30,000 casualties. Altitude and leveling measurements were performed before (1862) and after (1917) the event, along a circular route consisting of 18 benchmarks. Polarities of first arrivals do not constrain a unique fault-plane solution, while the one obtained by inverting geodetic data, with relevant constraints provided by neotectonic data, disagrees with P wave polarities. Our joint inversion, based on a robust fitting technique, has resulted in only one well-defined solution, satisfying geophysical data as well as surface-faulting distribution and paleoseismological information.

Preliminary Results from a Field Experiment on Volcanic Events at Kilauea Using an Array of Digital Seismographs

We carried out a field experiment at Kilauea Volcano for two months in 1988, during which the volcano was active and generated gas-piston events, long-period events and tremors. Six three-component digital instruments recorded 200 h of useful signal and 12 GEOS 6-channel recorders collected more than 200 events with a total amount of information of about 10 gigabits. Preliminary analyses of the data revealed that seismic spectra of all these volcanic events show many narrow spectral peaks, and we were able to establish that these peaks are not due to propagation path or recording site effect but due to source effect. We also found that, the movement of gas bubbles plays an important role in the excitation of gas-piston events, and possibly also long-period events because of the similarity in spectral feature between them. We propose preliminary source models for gas-piston events, long-period events and volcanic tremor based on fluid-gas filled crack models, and discuss outstanding problems with these models.