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voLcanic tremor and Low frequency earthquakes at mt. vesuvius M. La Rocca1, D. Galluzzo2

1 Università della Calabria, Cosenza, Italy2 Istituto Nazionale di Geofisica e Vulcanologia – Osservatorio Vesuviano, Napoli, Italy

Introduction. Seismic activity in volcanic environment is characterized by peculiar events not observed in tectonic regions. Beside volcano-tectonic (VT) earthquakes, low frequency (LF) earthquakes, long period events (LP) and volcanic tremor are observed at hundreds of active volcanoes worldwide (Chouet, 2003; McNutt et al., 2005). LF events are characterized by corner

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frequency between 1 Hz and 5 Hz, emergent onset, shallow location, and not well defined P-S direct phases. On the other hand, typical features of volcanic tremor are the emergent onset and long duration (from minutes to months) compared with normal earthquakes, while the spectrum is usually rich of low frequency, with the most of energy in the 1 – 5 Hz range (Konstantinou and Schlindwein, 2002), the same of LF events. In the most of documented cases, both LF events and volcanic tremor are obviously related with eruptive activity, occurring before and during eruptions. Therefore volcanic tremor and LF events are the most important precursor phenomena in case of volcanic unrest. The features of seismic signals are well matched by source models based on the interaction of fluids (both magma and gas) with the feeding system (Chouet, 1996, 2003). The resonance of a fluid filled cavity excited by some triggering perturbation is often adopted as source model of LF events. The interaction of magmatic gas with the hydrothermal system is also believed to play an important role in the generation of volcanic tremor and LF earthquakes.

At Mt Vesuvius for many decades after the last eruption of 1944, only VT earthquakes of low magnitude have been recorded (Del Pezzo et al., 2004; D’Auria et al., 2013). LF earthquakes are very rare (Cusano et al., 2013), while LP signals related with the volcanic activity have never been observed. Short bursts of volcanic tremor were first recognized in the seismic wavefield in 2012, after the installation of a seismic array named VAS (La Rocca et al., 2014, 2015, 2016). In this work we compare volcanic tremor and LF events with VT earthquakes to gain some insight about their source features.

Data analysis and results. Seismic activity at Mt Vesuvius is monitored by a network of more than 20 stations and one array of 10 short period stations (La Rocca and Galluzzo, 2014, 2015). Array data are analyzed in several narrowband frequencies in the range 1 Hz – 5 Hz with the aim of identifying coherent phases of low amplitude in the background signal. This analysis permitted the discovery of many short bursts of volcanic tremor and some small LF

Fig. 1 – Results of array analysis with the method Semblance, showing the occurrence of a tremor burst. BackazimuthResults of array analysis with the method Semblance, showing the occurrence of a tremor burst. Backazimuth and slowness of windows characterized by semblance greater than 0.6 are plotted by bold dark symbol to emphasize the most important phases.

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events. An example of array analysis showing the occurrence of a tremor episode is plotted in Fig. 1. Results of array analysis show coherent phases characterized by small incidence angle (slowness in the range 0.1 s/km – 0.4 s/km), indicating a source located at some depth below the crater. The analysis of more than 4 years of array data and the careful inspection of signals previously recorded by the local monitoring network permitted the discovery of at least two dozens of tremor episodes occurred during the last decade. The array offers a further advantage consisting in the signal stacking, which improves the signal to noise ratio of coherent signals, thus permitting a better identification of seismic phases in low amplitude and emergent events like volcanic tremor and LF earthquakes.

After the unexpected discovery of volcanic tremor, we committed particular attention and performed detailed analysis to any LF signals detected during the last decade. From such study we realized that pure LF events attributable to sources involving the interaction of fluids with rock are very rare at Vesuvius. In fact the most of LF signals recorded during the last decade revealed to be VT earthquakes with anomalously low frequency (La Rocca and Galluzzo, 2016). An example of such event is shown in Fig. 2a as recorded by three seismic stations installed upon the crater, likely around the epicenter. The predominant role of P and S direct waves in the seismograms is evident at all stations as observed usually for VT earthquakes, while the low frequency content is evident in Fig. 2b. More than 20 earthquakes with these features, hereafter referred to as LFVT, have been identified and studied. All of them are characterized by similar low frequency spectra, but the most important feature is the Ts-Tp > 1.2 s, as described below.

Fig. 2 – Seismograms of a LFVT earthquake at three summit stations (a) and their spectra (b). P and S direct wavesSeismograms of a LFVT earthquake at three summit stations (a) and their spectra (b). P and S direct waves typical of VT events are highlighted by background colored boxes. Stacked array signals of a tremor with many P-S wave pairs marked by color bar and label (c).

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The source location of volcanic tremor is affected by uncertainty much larger than high frequency VT earthquakes due to the lack of impulsive phases recognizable at the local network stations. A careful inspection of the signal emergent onset and signal envelope at the many stations available at Vesuvius permits to locate the tremor epicenter in the crater area, but determining the source depth with a sufficient precision is much more difficult. We pursued such aim through a comparison of volcanic tremor with LFVT events and regular VT earthquakes.

A careful visual inspection of the three component seismograms of the stations characterized by the highest signal to noise ratio, and particularly those of the array stacked signals, shows the presence in the tremor signals of many P-S wave pairs with constant Ts-Tp. This feature has been observed for many of the analyzed tremors in the array stacked signals, and sometimes it is seen also at the summit stations when the signal to noise ratio is high enough. As an example, Fig. 2c shows the beginning of the tremor 20120329 at VAS array. The array stacked signals contain many pulses on the vertical component (P waves) followed after about 1.35 s by a corresponding pulse on at least one of the horizontal components (S waves). The constant Ts-Tp suggest that such P-S wave pairs are small VT earthquakes of low frequency, or in other words they are LFVTs as those described above. The estimated Ts-Tp time of such LFVT events in the volcanic tremor is in the range 1.25 s <= Ts-Tp <= 1.6 s at the summit stations. Since it is not possible to recognize the same phase at many stations due to the chaotic nature of the wavefield and the low signal to noise ratio, we can not locate the source of individual P-S wave pairs through the inversion of time picking at the network stations. Nonetheless the estimated Ts-Tp time is the most important information to constrain the depth of tremor source. The Ts-Tp of volcanic tremor estimated in the range 1.25 s <= Ts-Tp <= 1.6 s at the summit stations corresponds to a source depth between 5 km and 6.5 km below sea level (La Rocca and Galluzzo, 2016).

LFVTs have two very important features in common with volcanic tremor: the same Ts-Tp time and the same frequency contents. A first straightforward conclusion inferred from this consideration is that the tremor is simply a sequence of LFVTs. We carried out a detailed analysis of these features and compared these events with regular high frequency VT earthquakes with the aim of gaining some insight about their source properties. We did this comparative analysis by taking into account regular VT located as near as possible to the same depth of tremor and LFVTs. We estimated the corner frequency of displacement spectra for all events characterized by Ts-Tp > 1.0 s at the summit stations, those near the epicenter. The results, shown in Fig. 3, depict two different groups of sources that do not merge to each other. VT earthquakes with Ts-Tp < 1.2s have corner frequency between 15 Hz and 22 Hz, while volcanic tremor and LFVTs all have Ts-Tp > 1.2 s and corner frequency smaller than 6 Hz. We searched the data recorded during the last 25 years looking for VT earthquakes with Ts-Tp > 1.2s and corner frequency greater than 6 Hz, but could not find any. We also searched for LFVTs and tremor characterized by Ts-Tp < 1.2 s, but could not find any. This result suggest the existence of a transition zone between shallow seismicity characterized by the typical brittle failure that produces high frequency VT earthquakes, and a deeper volume where shear failure radiates energy at much lower frequency. Such transition zone is located at depth corresponding to Ts-Tp = 1.2 s at the summit stations, which is estimated to be about 5 km bsl.

Discussion and conclusions. Vesuvius is one of the few cases where bursts of volcanic tremor are observed at a closed conduit, quiescent volcano, without any apparent relationships with other volcanic phenomena. The presence of P-S wave pairs suggests LF shear failure as source model, rather than the interaction of fluids with surrounding rock. On the other hand, LFVTs and tremor form a group well separated from high frequency VTs with regard to corner frequency and source depth (Fig. 3), thus we conclude that their striking difference must be related with the rock properties at depth where they are located. Our results indicate a significant change of the medium mechanical properties with depth, reasonably in terms of stiffness, temperature, and perhaps the presence of nearly melt material. A significant change of these

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properties at depth below 5 km bsl (roughly corresponding to Ts-Tp = 1.2 s) would explain the lower frequency contents of tremor and LFVTs in terms of lower stress drop compared with the VT located at shallower depth. Moreover the lack of any sources located deeper than about 6.5 km bsl suggests a further increase of temperature below that depth, in good agreement with the presence of melt material inferred to lay below 8 km bsl.

The corner frequency of seismic waves radiated by a shear failure depends by several parameters. Low frequency may be caused by very low rupture velocity or by very low stress drop. Weak fault subject to low normal stress in a rock volume with temperature not much lower than the brittle failure limit seems the best candidate for the

source of tremor and LFVTs at Vesuvius. Unfortunately a reliable estimation of such physical parameters is not possible at the moment. Attempts to estimate the rupture velocity and stress drop from the analysis of available signals are in progress, but preliminary results are not encouraging. In fact our LFVTs and tremor are characterized by low signal to noise ratio, which makes difficult some analysis and the interpretation of results. On the other hand, considering the current seismicity rate we have to wait many years to collect a sufficient number of good quality events for a better estimation of the source parameters.References Chouet, B.A., 1996. Long-period volcano seismicity: its source and use in eruption forecasting. Nature, vol. 380, pp.Nature, vol. 380, pp.

309-316, 1996. Chouet, B.A., 2003. Volcano seismology. Pure Appl. Geophys., 160, 739-788, 2003.Geophys., 160, 739-788, 2003. Cusano P., Petrosino, S., Bianco, F., Del Pezzo, E., 2013. The first Long Period earthquake detected in the background

seismicity at Mt. Vesuvius. Annals of Geophysics, 56, 4, 2013, S0440; doi:10.4401/ag-6447. D’Auria, L., Esposito, A.M., Lo Bascio, D., Ricciolino, P., Giudicepietro, F., Martini, M., Caputo, T., De Cesare, W.,

Orazi, M., Peluso, R., Scarpato, G., Buonocunto, C., Capello, M., Caputo, A., 2013. The recent seismicity of Mt. Vesuvius: inference on seismogenic processes. Annals of Geophysics, 56, 4, S0442; doi: 10.4401/ag-6448.uvius: inference on seismogenic processes. Annals of Geophysics, 56, 4, S0442; doi: 10.4401/ag-6448.

Del Pezzo, E., Bianco, F., Saccorotti, G., 2004. Seismic source dynamics at Vesuvius volcano, Italy. J. Volc. Geoth.Seismic source dynamics at Vesuvius volcano, Italy. J. Volc. Geoth.J. Volc. Geoth. Res., 133, 23-29.

Konstantinou, K. I., Schlindwein, V., 2002. Nature, wavefield properties and source mechanism of volcanic tremor: a review. J. Volc. Geoth. Res. 119, 161-187, 2002.

La Rocca M., D. Galluzzo (2016). Volcanic tremor at Mt Vesuvius associated with low frequency shear failures. Earth Planet. Sci. Lett., 442, 32-38, doi: 10.1016/j.epsl.2016.02.048.

La Rocca, M., Galluzzo, D., 2015. Seismic monitoring of Campi Flegrei and Mt. Vesuvius by stand alone instruments.Seismic monitoring of Campi Flegrei and Mt. Vesuvius by stand alone instruments. Annals of Geophysics, 58, 5, S0544; doi: 10.4401/ag-6748.

La Rocca, M., Galluzzo, D., 2014. Seismic monitoring of Mt. Vesuvius by array methods. Seism. Res. Lett., vol 85,Seismic monitoring of Mt. Vesuvius by array methods. Seism. Res. Lett., vol 85,vol 85, n 4, 809-816, doi: 10.1785/0220130216.

McNutt, S.R., 2005. Volcanic seismology. Annu. Rev. Earth Planet. Sci., 32, 461-491.

Fig. 3 – Corner frequency versus Ts-Tp of tremor, LFVTs,Corner frequency versus Ts-Tp of tremor, LFVTs, and VT earthquakes. The existence of two groups of results without a smooth transition from one to the other is the most important result of our analysis.