GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L04305, doi:10.1029/2008GL036477, 2009
Low-frequency seismic signals recorded by OBS at Stromboli volcano (Southern Tyrrhenian Sea) Tiziana Sgroi,1 Caterina Montuori,1 Roberto Agrusta,2 and Paolo Favali1,2 Received 27 October 2008; revised 23 January 2009; accepted 27 January 2009; published 24 February 2009.
[ 1 ] Five three-component broadband ocean bottom seismometers (OBSs) were deployed on the seafloor around the Aeolian Islands (Southern Tyrrhenian Sea). By comparing OBSs digital seismograms, we found a lowfrequency seismicity recorded only at OBS05, the nearest seafloor station to Stromboli volcano. This seismicity appears in the form of a continuous seismic signal (tremor-like-signal) as well as a considerable number of shock-like events. We focused on recordings from OBS05 to verify their correlation with Stromboli volcanic activity. From the spectral analysis, we observed low-frequency events (LP events), superposed upon the continuous background noise (tremor). LP events and tremor, showing similar energy fluctuations and frequency content, appear to be produced by the same dynamic processes. We interpret this low-frequency seismicity as probably originating from a continuous uprising of gas bubbles from the deeper part of the Stromboli magmatic column. This could highlight the existence of a deeper source for low-frequency seismicity. Citation: Sgroi, T., C. Montuori, R. Agrusta, and P. Favali (2009), Low-frequency seismic signals recorded by OBS at Stromboli volcano (Southern Tyrrhenian Sea), Geophys. Res. Lett., 36, L04305, doi:10.1029/ 2008GL036477.
of underground magmatic activity. The seismic signals associated with the Strombolian explosions are accompanied by a forerunner signal that occurs several seconds before the eruption, during the time separating the formation of the gas pocket at different levels in the magmatic column and its reaching the surface. Stromboli produces different types of low-frequency seismic signals including explosionquakes (EQs), long period events (LP events) and tremors, with typical oscillation periods in the 0.2 – 2 s range [Chouet, 1996]. The processes generating these signals are supposed to be linked to the excitation of the volcanic conduit by a triggering mechanism. One plausible mechanism is the exhalation of gases from the fluid phase in a magmatic column. [4] In the last few years, Ocean Bottom Seismometers (OBSs) and seafloor observatories have frequently been used around the world to monitor the off-shore microseismicity and seismic signals also of volcanic origin [e.g., Butler et al., 2000; Caplan-Auerbach et al., 2001; Goslin et al., 2005; Sgroi et al., 2006]. Recently, OBSs, deployed on the East Pacific Rise, have recorded the micro-earthquake character of a mid-ocean ridge eruption, including precursory activity [Tolstoy et al., 2006].
2. Experiment and Seismic Data 1. Introduction [2] Stromboli is one of seven volcanic islands in the Aeolian Archipelago (Southern Tyrrhenian Sea, Italy). The island rises about 3,000 m from the seafloor and stands 924 m a.s.l. The volcano has been steadily erupting for over 2,000 years and perhaps even as long as 5,000 years [Allard et al., 1994]. The present eruptive behaviour is characterised by intermittent explosive activity during which wellcollimated gas jets, laden with molten lava fragments, burst into short eruptions lasting 5 – 15 s occurring at a typical rate of 3 – 10 events per hour. This persistent volcanic activity is sometimes interrupted by lava effusions or major explosions. Despite recent studies aimed at clarifying the volcano’s eruption dynamics, the spatial extent and geometrical characteristics of its plumbing system remain poorly understood. In fact, knowledge of the inner structure and the zone of magma storage is limited to the upper few hundred meters of the volcanic edifice [Chouet et al., 2003]. [3] Low-frequency seismic signals recorded on active volcanoes are usually interpreted as a direct consequence 1
Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy. Dipartimento di Scienze della Terra, Universita` degli Studi ‘‘La Sapienza,’’ Rome, Italy. 2
Copyright 2009 by the American Geophysical Union. 0094-8276/09/2008GL036477
[5] In the period December 2000 – May 2001 a network of Ocean Bottom Seismometers (OBS) and Ocean Bottom Hydrophones (OBHs) was deployed in the Southern Tyrrhenian Sea during the TYrrhenian Deep sea Experiment (TYDE) [Dahm et al., 2002]. The ocean bottom network consisted in 14 seafloor stations, deployed at depths ranging 1,500– 3,500 m b.s.l. (Figure 1, shows only the five OBS/ Hs used in this work). In particular, the OBS05 was deployed at a depth of 1,500 m, about 16 km SSE from the central axis of Stromboli Island. [6] During the TYDE experiment an intense microseismicity linked to both volcanic and tectonic processes was also recorded [Sgroi et al., 2006]. In this paper, we focused on the low-frequency data recorded by OBS05, representing the seismic manifestation of Stromboli volcanic activity.
3. Data Analyses [7] Temporal and spectral analyses have been performed in order to characterise the low-frequency seismicity. The comparison among the vertical components of continuous seismic signals by all OBSs allowed us to point out the different features from the pattern of seismic signals on the OBS05. Figure 2a shows an hour-long data sample of the seismicity simultaneously observed at five OBSs. Three small amplitude events, having low-frequency character-
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Figure 1. Southern Tyrrhenian multibeam map [Marani et al., 2004]. The insert shows the location of the area in the Tyrrhenian Sea. The triangles indicate the five OBS/Hs of the seafloor array used in this work (OBS05, the nearest to Stromboli volcano, is in red). istics, were only displayed on OBS05. This seismicity consisted in several hundreds of shock-like events (LP events) superposed upon a sustained background noise. Records have been band-pass filtered between 0.5– 4 Hz
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with a 4-pole zero-phase-shift Butterworth filter in order to exclude oceanic microseismic noise, typical of the 0.1– 0.5 Hz band [Webb, 1998; Arciniega-Ceballos et al., 2000], while the spectral peaks in the 0.5– 4 Hz band are commonly associated with a volcanic origin of the seismic signals [Aki and Koyanagi, 1981; Chouet, 1996]. [8] The LP events are generally characterised by emergent onsets and harmonic signatures. Most of the LP events display quasi-monochromatic signatures and different wave-trains are always well-visible (Figure 2b). Typically, the durations of these signals range from 30 to over 100 s and the features are quite similar to those seen in LP events at other active volcanoes [Chouet, 1996; ArciniegaCeballos et al., 2000]. For the single LP event, the comparison among 3-component recordings showed that the amplitude was always greater for the horizontal components than for the vertical one, often by a factor of two or three (Figure 2b). Although there is variability in the waveforms of the events, the signal onsets and their behaviour in time are also similar from event to event, again consistent with the repetitive action of a non-destructive source. [9] Spectral analysis was performed by computing the Power Spectral Density (PSD) on acceleration seismograms for each tapered window to investigate the short-time evolution of the signals. Figure 2c shows the comparison of the spectra for the vertical component of each OBS. All the stations analysed are characterised by spectral levels
Figure 2. (a) Examples of vertical component signals recorded by the five OBSs. In the dark grey box a local earthquake is shown, recorded at all OBSs. On the OBS05 seismogram 3 low-frequency events (light grey boxes) are visible, events not recorded by the other OBSs. (b) The 3-components of the velocity seismogram of a LP event (0.5 – 4 Hz band-pass filtered) recorded by OBS05. Different quasi-monochromatic wave-trains are well visible. (c) Spectra calculated on the five OBSs, computed on the first 2000 seconds of the recording, to exclude from our analysis the tectonic event occurred at 2480 s. A more energised spectral band between 1 and 3 Hz is visible only on OBS05; PSD values for this OBS are higher than values for the other ones. (d) Comparison between spectra performed on a LP event (red trace) and tremor signature (green trace) using the seismic signals recorded at OBS05. Spectral content is similar for both seismic signals, but spectral amplitude of LP event is about 10 dB greater than tremor signal in the range 1 –2 Hz. 2 of 5
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Figure 3. Waveforms and spectrograms of vertical component of OBS05 performed on 200-s window for (a) LP event and (b) tremor. (c) Waveform and spectrogram of vertical component of land station performed on 60-s window for explosion-quake. Warm colours (red, yellow and green) define the dominant spectral amplitudes; cooler colours (light to dark blue) define lower amplitudes and background. within the noise reference curves [Peterson, 1993], except for OBS05 that is 20 dB above the higher spectral boundary in the frequency range 1– 3 Hz. Hence, the spectral analysis was focused on the signals recorded only at OBS05. [10] Figure 2d shows the comparison of spectra computed on a LP event and tremor, calculated on the same 100-s duration. Both seismic signals display dominant spectral peaks around 1 and 3 Hz, although at about 1 Hz the LP events show a PSD value 10 dB higher than the tremor signal. Between 3 – 4 Hz both signals display a peak, having the same spectral amplitude. These spectra suggest that tremor and LP events can be distinguished by the content of radiated energy within the same frequency bands. The first one ranges between 1.0 and 1.3 Hz and the second spans 3.1– 3.7 Hz. This bimodal nature of tremor and LP event spectra is further demonstrated in spectrograms (Figure 3). Figures 3a and 3b show an example of OBS05 vertical component spectrograms for a LP event and a record of the same length for tremor. The predominant frequencies of LP events and tremor show a strictly similarities within a frequency band of 0.8– 1.5 Hz. The frequency band around 3.2 Hz is also well-evident for both recordings. The similar spectral content in both signals provides additional evidence supporting the argument of a common source for LP events and tremor signals.
a short-period seismometer, was installed about 100 m from the Stromboli crater at an altitude of about 800 m and was run by the University of Udine (courtesy of Roberto Carniel). Seismic recordings from this land station are related to explosion-quakes (Figure 3c) and volcanic tremor. No LP events are recorded. [13] We compared the occurrence of LP events recorded by OBS05 with that related to EQs recorded from the land station (http://www.swisseduc.ch/stromboli/volcano/ seismik/index-en.html). The number of LP events observed at OBS05 and the temporal pattern of EQs recorded by the land station are similar. This suggests a common origin of the two types of signals. In order to emphasise the significant spectral peaks, 16 one-minute long tremor windows were stacked for both land station and OBS05 seismograms. The comparison between the two stacked spectra is shown in Figure 4b. Both spectra exhibit a similar pattern. Small frequency content differences observed at the two stations may be ascribed to site and propagation effects. Nevertheless, common spectral peaks are also visible, which may be due to source effects, even if the spectral amplitude of the peaks is different. This can demonstrate the common origin of the signals, also evidenced from the common bimodal nature of the spectra.
5. Discussion and Conclusions 4. Comparison Between OBS05 and Land Station Tremor Recordings [11] From OBS05 recordings, we collected a dataset of low-frequency seismic signals made up of several thousands of LP events, while continuous background-noise data were also available throughout the experimental period. The frequency of LP events typically occurred at a rate of about 2 – 5 events per hour (occasionally up to 10 –12 events per hour). A total of 5578 LP events, recorded by OBS05 from December 22, 2000 to March 31, 2001, were collected (Figure 4a). [12] The low-frequency seismic signals from OBS05 were compared with those recorded at the only one land station placed on top of Stromboli volcano during the TYDE experiment. The land seismic station, equipped with
[14] For the first time ever at Stromboli, low-frequency seismic signals associated to the volcanic activity were recorded by an OBS deployed at 1500 km b.s.l., about 16 km SSE from the central axis of Stromboli volcano. These signals were recognised as LP events and volcanic tremor. From 22 December 2000 to 31 March 2001, 5578 LP events were recorded by OBS05. Their occurrence and their waveforms are typical of volcanic environment. The consistent features of the emergent onset of the first arrivals, but also the absence of the S-wave arrivals, further suggest that LP events were caused by magmatic activity. Moreover, the duration of LP events is significantly longer than those of tectonic earthquakes and their amplitude is substantial lower. It is evident from waveform and spectral features that LP events are characterised by scattering of
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Figure 4. (a) Histogram of the daily occurrence rate of the 5578 LP events recorded by OBS05 (December 22, 2000 – March 31, 2001). (b) Comparison between stacked spectra computed by the use of 16 1-minute-long windows of tremor signal recorded by the land station and OBS05. Common spectral peaks, having a significance of more than 20%, are reported on both spectra. seismic waves, linked to the path source-station. This scattering is evident from the dispersion of high frequencies on OBS05 spectra and from the long duration of the tail of LP signal. Although there is variability in the waveforms of the events, the signal onsets and their behaviour in time are also similar from event to event, consistent with the repetitive action of a non-destructive source. [15] The comparison with the low-frequency seismicity recorded at the only land station available at the time of the TYDE experiment is highly significant. The stacked spectra computed on 1 minute-long tremor windows on land station and OBS05 were compared. As a result, tremor signals recorded at OBS05 and land stations can be associated to the same source dynamics, having similar frequency content. Moreover, both EQs and LP events show the same spectral content but differ substantially in the waveform. While the seismic energy of EQ is concentrated at the onset of the signal, seismic energy of the LP event is smeared over the entire signal duration. Similar observations have been made for Mt. Vesuvius by Wegler [2003], who explained the signal types by a different degree of scattering due to propagation path. Nevertheless, we think that EQs
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and LP events do not share the same source. As shown in Figure 2b, the amplitude of LP events recorded on the vertical component of OBS05 is in the order of 10 5 m/s. These values recorded at a distance of 16 km from the centre of Stromboli, are the same as found for EQs recorded on land stations placed near the summit of Stromboli [Neuberg et al., 1994]. Starting from these results, it is more reliable to focus on a deep seismic source in the generation of LP events. A deeper component of the source dynamics during eruptions is not unexpected if we consider the associated mass removal process. As gases escape from the top of the conduit, liquid magma within the conduits moves into the void left by the escaping gases. Although this process mostly disrupts the column of liquid perched above the location where the gas slug is released, it is also expected to induce weaker fluid motions in the deeper parts of the conduit below the volume of the slug release. Given the location of OBS05, it is therefore natural to presuppose that the receiver on the seafloor may be more sensitive to detecting these deeper components in the eruption process. Different studies suggest the existence of a deeper source for LP events at Stromboli. Francalanci et al. [2005] defined the existence of a magma reservoir at about 3 km in depth below Stromboli, where crystallisation, recycling, magma mixing and degassing processes occur. From spectroscopic measurements performed during both quiescent degassing and explosions on Stromboli volcano, Burton et al. [2007] demonstrated that gas slugs originated from as deep as the volcano-crust interface (3 km), where both structural discontinuities and differential bubble-rise speed can promote slug coalescence. The observed decoupling between deep slug genesis and shallow (250 m) explosion-quakes may be a common feature of Strombolian activity, determined by the geometry of plumbing system. [16] The use of a permanent array of OBS placed around Stromboli may turn out to be a precious tool in monitoring volcano activity and focusing on its dynamics. A denser spatial coverage of seafloor broadband receivers all around the volcano will be required to further quantify the source mechanism of the explosions, map the geometry of the magma/gas transport system beneath the volcano and to monitor potential eruptions at Stromboli. [17] Acknowledgments. The authors wish to thank Roberto Carniel who kindly provided the seismic data referring to explosion-quakes and volcanic tremor recorded by land station. Thanks are due also to Thomas Braun, Giuliano Milana and Stephen Monna for their helpful suggestions and to Bernard Chouet for his enlightening discussions on conduit structures and dynamics of Stromboli. James Famiglietti and the anonymous reviewer are acknowledged for their constructive comments on the present work.
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[email protected])
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