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La Rocca, Mario
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- PublicationOpen AccessCrustal Structure of the Seismogenic Volume of the 2010–2014 Pollino (Italy) Seismic Sequence From 3D P- and S-Wave Tomographic ImagesA tomographic analysis of Mt. Pollino area (Italy) has been performed using earthquakes recorded in the area during an intense seismic sequence that occurred between 2010 and 2014. 870 local earthquakes with magnitude ranging from 1.8 to 5.0 were selected considering the number of recording stations, the signal quality, and the hypocenter distribution. P- and S-wave arrival times were manually picked and used to compute 3D velocity models through tomographic seismic inversion. The resulting 3D distributions of VP and VS are characterized by high resolution in the central part of the investigated area and from surface to about 10 km below sea level. The aim of the work is to obtain high- quality tomographic images to correlate with the main lithological units that characterize the study area. The results will be important to enhance the seismic hazard assessment of this complex tectonic region. These images show the ductile Apennine platform (VP = 5.3 km/s) overlaying the brittle Apulian platform (VP=6.0 km/s) at depth of around 5 km. The central sector of the area shows a clear fold and thrust interface. Along this structure,most of the seismicity occurred, including the strongest event of the sequence (M W 5.0). High V P (>6.8 km/s) and high V P /VS (>1.9) patterns, intersecting the southern edge of this western seismogenic volume, have been interpreted as water saturated rocks, in agreement with similar geological context in the Apennines. These fluids could have played a role in nucleation and development of the seismic sequence. A recent study revealed the occurrence of clusters of earthquakes with similar waveforms along the same seismogenic volume. The hypocenters of these cluster events have been compared with the events re-located in this work. Jointly, they depict a 10 km × 4 km fault plane, NW-SE oriented, deepening towards SW with a dip angle of 40–45° . Instead, the volume of seismicity responsible for the M L 4.3 earthquake developed as a mainshock-aftershock sequence, occurring entirely within the average-to-low VP /VS Apennine platform. Our results agree with other independent geophysical analyses carried out in this area, and they could significantly improve the actual knowledge of the main lithologic units of this complex tectonic area.
140 46 - PublicationRestrictedRete Mobile e Laboratorio Analisi aVanzate (LAV) - Rendiconto 2011(2012-02)
; ; ; ; ; ; ; ;Cusano, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Galluzzo, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;La Rocca, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Petrosino, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Bianco, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Castellano, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Del Pezzo, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia; ; ; ; ; ; In the text179 39 - PublicationOpen AccessFocal mechanisms of recent seismicity at Campi Flegrei, ItalyThe present study analysed volcano tectonic earthquakes that occurred in the caldera of Campi Flegrei, Italy from2000 to 2019 to compute source focalmechanisms.Weuseddata froma largenumber of seismic stations to estimate reliable fault plane solutions for 71 events of a maximum duration magnitude (MD) ranging between 0.5 ≤ MD ≤ 2.5. We found that most solutions were of a normal type and were rake- angle negative focal mechanisms in 86% of the cases. Only a few earthquakes occurred on normal dip-slip faults; the majority occurred on varying degrees of oblique slip. Only one event had a pure reverse- mechanism, and it was located far from the caldera centre. The spatial distribution of the computed mechanisms exhibited a remarkable mix of fault orientations without any relationship to the location area or source depth. The predominance of normal kinematics on high-angle and nearly vertical faults and the very shallow source depth exhibited good agreement with the fact that seismicity at Campi Flegrei occurred during periods of ground uplift. In fact, ground uplift elongated the shallowest crust, thus reducing the normal stress on existing locked faults; this facilitated earthquakes, particularly those characterised by normal, dip-slip, and strike-slip mechanisms. In contrast, ground subsidence produced a horizontal shortening of the upper crust, which increased the normal stress on high-angle faults and reduced earthquake occurrence. Our results indicate that the driving force of the recent volcano-tectonic seismicity in the study region is the ongoing ground uplift.
183 31 - PublicationOpen AccessRete Mobile e Laboratorio Analisi aVanzate (LAV) - Rendiconto I semestre 2011(2011-07)
; ; ; ; ; ; ; ;Cusano, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Galluzzo, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;La Rocca, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Petrosino, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Bianco, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Castellano, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Del Pezzo, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia; ; ; ; ; ; In the text144 1473 - PublicationRestrictedA Seismic Array in the Town of Pozzuoli in Campi Flegrei (Italy)Many active volcanic areas in the world are densely inhabited. Large towns are built at the foot of volcanoes, or inside volcanic fields, resulting in a very high volcanic hazard due to the combination of high volcanic risk and a large population. Vesuvius, Campi Flegrei, and Etna (Italy); Marapi (Indonesia); Teide (Tenerife, Canary Islands); Guagua Pichincha (Ecuador); Popocatepetl (Mexico); Aso, Sakurajima, and Unzen (Japan); and Mayon and Pinatubo (Philippines) are only a few of the active volcanoes sited in densely populated areas. Monitoring these volcanoes with modern technology is critical to mitigating volcanic hazard through the detection of any possible precursor phenomena. Volcanic eruptions are usually preceded by a wide variety of observable phenomena, such as an increase of seismic activity, ground deformation, and variations of the gas emissions. Therefore a continuous monitoring of seismic activity to detect the most common signals possibly related to imminent volcanic activity, such as volcano-tectonic earthquakes, low-frequency (or long-period) events, and volcanic tremor (McNutt 2005; Chouet 2003), is an important task. The use of borehole instruments in highly urbanized environments may increase the signal to noise ratio (SNR), improving the capability to detect very small signals. The Hi-net in Japan (Okada et al. 2004) and the seismic network of the Auckland volcanic field in New Zealand (Ashenden et al. 2011) are two notable examples. Volcanic tremor is observed at many volcanoes before and during eruptions. The analysis of signals characterized by emergent onset, lack of phases recognizable as direct P and S waves, and long duration (referred to as “tremor-like signals” throughout this paper) requires continuous data recorded by dense arrays. In this paper we describe a short-period seismic array, named ARF, installed since August 2010 under the town of Pozzuoli, in the center of the Campi Flegrei caldera (Italy). The aim of the ARF array is the detection of volcanic tremor, whenever should it show up. One year of continuous data have been analyzed with array techniques tuned to detect coherent low-frequency signals, possibly related to volcanic activity. We describe the main features of seismic noise, its relationship with human activity, and compare the signals recorded at 16 m depth with those recorded at surface. Detailed analysis of some local events, including volcano-tectonic earthquakes, artificial explosions, and artificial tremor-like signals, are used to check the array performances. These events provide important insight for establishing the amplitude threshold and frequency range of detectable tremor-like signals. The results of array analysis suggest that the ARF array is suitable to detect coherent signals with root mean square (rms) > 1 micron/s in the 1–5 Hz frequency band.
108 5 - PublicationRestrictedTesting Small-Aperture Array Analysis on Well-Located Earthquakes,(2008-04)
; ; ; ; ; ; ;La Rocca, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Galluzzo, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Malone, S.; Department of Earth and Space Sciences ;McCausland, W.; Department of Earth and Space Sciences ;Saccorotti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Del Pezzo, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia; ; ; ; ; We have here analyzed local and regional earthquakes using array techniques with the double aim of quantifying the errors associated with the estimation of propagation parameters of seismic signals and testing the suitability of a probabilistic location method for the analysis of nonimpulsive signals.We have applied the zero-lag cross-correlation method to earthquakes recorded by three dense arrays in Puget Sound and Vancouver Island to estimate the slowness and back azimuth of direct P waves and S waves. The results are compared with the slowness and back azimuth computed from the source location obtained by the analysis of data recorded by the Pacific Northwest seismic network (PNSN). This comparison has allowed a quantification of the errors associated with the estimation of slowness and back azimuth obtained through the analysis of array data. The statistical analysis gives σBP 10° and σBS 8° as standard deviations for the back azimuth and σSP 0:021 sec= km and σSS 0:033 sec =km for the slowness results of the P and S phases, respectively. These values are consistent with the theoretical relationship between slowness and back azimuth and their uncertainties. We have tested a probabilistic source location method on the local earthquakes based on the use of the slowness estimated for two or three arrays without taking into account travel-time information. Then we applied the probabilistic method to the deep, nonvolcanic tremor recorded by the arrays during July 2004. The results of the tremor location using the probabilistic method are in good agreement with those obtained by other techniques. The wide depth range, of between 10 and 70 km, and the source migration with time are evident in our results. The method is useful for locating the source of signals characterized by the absence of pickable seismic phases.145 27 - PublicationRestrictedObservations of high-frequency scattered waves using dense arrays at Teide Volcano(1997-12)
; ; ; ;Del Pezzo, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;La Rocca, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Ibanez, J.; University of Granada, Spain; ; A study of the seismic coda of seven small earthquakes recorded on the Teide Volcano-Canary Islands (Spain) was carried out using a temporary, small- aperture, 12-station, seismic array. The purpose was to measure backazimuth, ap- parent velocity and the type of waves that compose the coda in the frequency range 4 to 6 Hz. We used the zero-lag cross-correlation (ZLC) method to obtain the com- ponents of the wave vector and three-component analysis techniques based on the covariance matrix of the signal in the time domain to investigate the polarization properties of the signals. The results show that a great part of the coda signals in the analyzed frequency range are almost uncorrelated, while a low number of isolated correlated arrivals show an apparent slowness between 2 × 10-3 and 2.5 × 10-4 sec/m and an almost random distribution of backazimuths. The correlated arrivals have been interpreted as generated by strong scatterers, probably related to the presence, in the study area, of surface topography irregularities (the volcanic cone and the caldera rim). The wave type varies waves with some Rayleigh components.148 31 - PublicationOpen AccessScattering and absorption imaging of a highly fractured fluid-filled seismogenetic volume in a region of slow deformation(2020)
; ; ; ; ; ; ; ; ; ; ; Regions of slow strain often produce swarm-like sequences, characterized by the lack of a clear mainshock-aftershock pattern. The comprehension of their underlying physical mechanisms is challenging and stilldebated. We used seismic recordings from the last Pollino swarm (2010–2014) and nearby to separate and mapseismic scattering (from P peak-delays) and absorption (from late-time coda-wave attenuation) at different fre-quencies in the Pollino range and surroundings. High-scattering and high-absorption anomalies are markers of afluid-filled fracture volume extending from SE to NW (1.5–6 Hz) across the range. With increasing frequency,these anomalies approximately cover the area where the strongest earthquakes occurred from the sixteenthcentury until 1998. In our interpretation, the NW fracture propagation ends where carbonates of the LucanianApennines begin, as marked by a high-scattering and low-absorption area. At the highest frequency (12 Hz) theanomalies widen southward in the middle of the range, consistently marking the faults active during the recentPollino swarm. Our results suggest that fracture healing has closed small-scale fractures across the SE faults thatwere active in the past centuries, and that the propagation offluids may have played a crucial role in triggeringthe 2010–2014 Pollino swarm. Assuming that thefluid propagation ended at the carbonates barrier in the NWdirection, fractures opened new paths to the South, favoring the nucleation of the last Pollino swarm. Indeed, therecently active faults in the middle of the seismogenic volume are marked by a high-scattering and high-absorption footprints. Our work provides evidence that attenuation parameters may track shape and dynamicsoffluid-filled fracture networks in fault areas.385 48 - PublicationOpen AccessObservations of volcanic earthquakes and tremor at Deception Island - Antarctica(1999-06)
; ; ; ; ; ; ; ; ;Alguacil, G.; Instituto Andaluz de Geofisica, Universidad de Granada, Spain ;Almendros, J. C.; Instituto Andaluz de Geofisica, Universidad de Granada, Spain ;Del Pezzo, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Garcia, A.; Museo Nacional de Ciencias Naturales, CSIC, Madrid, Spain ;Ibañez, J. M.; Instituto Andaluz de Geofisica, Universidad de Granada, Spain ;La Rocca, M.; Dipartimento di Fisica, Università di Salerno, Baronissi (SA), Italy ;Morales, J.; Instituto Andaluz de Geofisica, Universidad de Granada, Spain ;Ortiz, R.; Museo Nacional de Ciencias Naturales, CSIC, Madrid, Spain; ; ; ; ; ; ; Deception Island - South Shetlands, Antarctica is site of active volcanism. Since 1988 field surveys have been carried out with the aim of seismic monitoring, and in 1994 a seismic array was set up near the site of the Spanish summer base in order to better constrain the source location and spectral properties of the seismic events related to the volcanic activity. The array was maintained during the Antarctic summer of 1995 and the last field survey was carried out in 1996. Data show the existence of three different groups (or families) of seismic events: 1) long period events, with a quasi-monochromatic spectral content (1-3 Hz peak frequency) and a duration of more than 50 s, often occurring in small swarms lasting from several minutes to some day; 2) volcanic tremor, with a spectral shape similar to the long period events but with a duration of several minutes (2-10); 3) hybrid events, with a waveform characterised by the presence of a high frequency initial phase, followed by a low frequency phase with characteristics similar to those of the long period events. The high frequency phase of the hybrid events was analysed using polarisation techniques, showing the presence of P waves. This phase is presumably located at short epicentral distances and shallow source depth. All the analysed seismic events show back-azimuths between 120 and 330 degrees from north (corresponding to zones of volcanic activity) showing no seismic activity in the middle of the caldera. Particle motion, Fourier spectral and spectrogram analysis show that the low frequency part of the three groups of the seismic signals have similar patterns. Moreover careful observations show that the high frequency phase which characterises the hybrid events is present in the long period and in the tremor events, even with lower signal to noise ratios. This evidence suggests that long period events are events in which the high frequency part is simply difficult to observe, due to a very shallow source and/or hypocentral distance higher than that of hybrids, while the tremor is composed of rapidly occurring hybrid events. We propose a possible interpretation for the three groups of seismic events. These may be generated by multiple pressure-steps due to the rapid phase change from liquid to vapour in a shallow aquifer which comes in contact with hot materials. The pressure change can put a crack in resonance or excite the generation of multiple surface waves modes in the shallow layered structure.188 343 - PublicationOpen AccessA seismic survey at Colima volcano (Mexico)(2008-04-13)
; ; ; ; ; ; ;Breton, M. ;Orozco-Rojas, J. ;Ibanez, J. ;Veneruso, M. ;Del Pezzo, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;La Rocca, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Galluzzo, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Petrosino, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Cusano, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Bianco, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Breton, M.; Observatorio Vulcanologico de Colima – Università di Colima (Mexico) ;Orozco-Rojas, J.; Observatorio Vulcanologico de Colima – Università di Colima (Mexico) ;Ibanez, J.; Instituto Andaluz de Geofisica - Universidad de Granada (Spain) ;Veneruso, M.; Centro Regionale di Competenza “Analisi e Monitoraggio del Rischio Ambientale” (AMRA); ; ; ; ; ; ;Observatorio Vulcanologico de Colima – Università di Colima (Mexico) ;Observatorio Vulcanologico de Colima – Università di Colima (Mexico) ;Instituto Andaluz de Geofisica - Universidad de Granada (Spain)Centro Regionale di Competenza “Analisi e Monitoraggio del Rischio Ambientale” (AMRA)In the period 2-6 April 2007 a seismic survey was carried out at Solfatara Volcano, (Campi Flegrei, Southern Italy) with the aim of inferring the shallow structure and evaluating local site effects. Five circular seismic arrays equipped with 1-Hz 3-component Mark LE3Dlite sensors, were installed in the Solfatara crater. Each array consisted of 4 sensors, 3 of them evenly spaced (120°) around the circumference and the fourth placed at its center. The arrays were designed with radii of 5, 10, 25, 50 and 100 m. Some stations of the arrays shaped two orthogonal profiles of about 250-m-length, roughly oriented in the N-S and E-W directions. A further seismic station was installed outside the crater, on the Eastern rim. The particular geometry of the station deployment was adopted to apply the spatial autocorrelation technique (Aki, 1957) and its modifications (MSPAC, Bettig et al., 2001, CCA, Cho et al., 2004), to the data recorded by each array and to infer shear-wave velocity models for different areas of the crater. Further information about the shallow structure will come from the application of Nakamura’s technique (1989) to microtremor recorded at each sampled site. The two orthogonal profiles oriented N-S and E-W will be useful to map possible variations of the resonance frequencies and amplification values along the N-S and E-W directions. The results of the preliminary spectral analysis of some samples of seismic noise recorded during the 2-6 April 2007 survey at Solfatara Volcano, are already indicative of differences among the spectral content of the microtremor recorded in different areas of the crater. In particular the most evident differences are observed between the recordings of the stations located in the central part of the crater and those deployed in the Northern and Eastern areas. Moreover, the seismic noise recorded outside the crater has spectral characteristics that are very different from those observed for the array stations. The observed differences in the spectral content of the seismic noise could be due both to the presence of horizontal velocity contrasts and to variations of the thickness of the shallower layers. The high density of the deployment and the large number of the sampled sites will allow to obtain a detailed shallow velocity structure, to investigate about the presence (or not) of lateral heterogeneities and to map resonance frequencies and amplifications values in different areas of the crater.154 243