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focaL mechanisms of recent seismicity at campi fLegrei M. La Rocca1, D. Galluzzo2

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

Introduction. Campi Flegrei caldera is a complex volcanic field characterized by two nested calderas and tens of craters produced by the eruptive activity of the last 15 ka. Many structures are recognized from the ground morphology and many other have been inferred from a number of studies carried out during the last decades (Orsi et al., 2004). The most interesting phenomenon of the area is the bradyseism, an alternation of very slow ground subsidence and uplift. In the past the uplift phases have been accompanied by intense seismic activity, while subsidence phases are usually aseismic. At present a very slow uplift (two cm per year on average, http://www.ov.ingv.it/ov/en/campi-flegrei.html) is ongoing since 2000, after the subsidence of more than 1 meter that followed the bradyseismic strong uplift occurred in 1982-84. After that event, characterized by more than 16000 earthquakes of magnitude up to M4.2 (Aster et al., 1992), the seismicity rate was extremely low for about 15 years. Then the number of earthquakes increased again, roughly following the uplift trend, with several swarms of VT events during the last years. The magnitude or recent earthquakes is rarely greater than 2, but the shallow location (often less than 1 km) makes those events felt as ground shaking and audible rumble as well.

The source mechanisms of earthquakes is an important information to investigate the stress field in the area. Therefore the analysis of focal mechanisms of VT earthquakes is worth of attention from the seismologists. In this work we show the focal mechanisms of 36 VT earthquakes with 0.6 < Ml <2.5 occurred during the last decade.

Data analysis and results. Computation of focal mechanism and/or moment tensor is a routine analysis on earthquake data. For medium to large events it is computable automatically by evolute software. On the contrary, the estimate of focal mechanism becomes less reliable and more difficult as the magnitude decreases. This happens because small earthquakes are recorded only at short distances, thus an adequate coverage of the focal sphere is difficult or impossible to achieve. Moreover, small earthquakes produce mostly high frequency signals that are very difficult to fit by synthetic waveforms. In such cases the estimation of the moment tensor becomes challenging, while the computation of fault plane solution through the polarity of direct waves remains the unique effective method. In volcanic environment, usually characterized by high heterogeneous structures, the problem is further complicated by the amount of scattered signal that can mask the polarity of direct S wave.

The seismic monitoring network in the Campi Flegrei area has been greatly improved during the last 15 years, from 10 analog stations in 2000 (Castellano et al., 2002) to about 30 stations at present (La Rocca and Galluzzo, 2015). Current operative seismic stations are shown in Figure 1 by blue triangle (broad band) and circle (short period), while diamond represent stations no longer available but used in this work. With such a high number of stations, during the last years even earthquakes as small as M1 located in the central part of the caldera may have been recorded by more than 15 stations around the epicenter and in a range of distance up to 6-8 km. The most of VT events are located at depth between 1 km and 3 km bsl. The shallow location constitutes an advantage for the computation of focal mechanisms because the focal sphere is better covered by direct ray paths and even the lower hemisphere may be sampled by the farthest stations. We selected about 100 VT earthquakes with 0.1 < M < 2.0 recorded during the last 15 years and tried to estimate their focal mechanisms. We have used the software FOCMEC (Snoke, 1984, 1989) which allows the use of polarity of P, SV and SH waves. The first step of this analysis, after the source location, is the rotation of horizontal components in the radial and transverse directions. While determining the polarity of P waves is straightforward, the cases of SV and SH often is far from obvious. In many cases the observation of integrated signals (ground displacement)

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and filtered waveforms helped in the estimation of S wave polarity. However, many events remained undetermined due to the insufficient coverage of the focal sphere with the available observed phase polarity. We were able to compute the focal mechanism for 36 VT earthquakes recorded from 2005 to 2015 (Fig. 1). For at least 30 of them the solution is well constrained, with uncertainty of less than 10 degrees on the estimated strike, dip, and rake. Only for few events the solution is affected by uncertainty up to 30 degrees on one or more of the three angles. Of course this happens for the events characterized by poor data

quality, those of lower magnitude and/or those recorded by less than 10 stations. The increased number of stations after 2008 allowed for a better location of the source and a much better coverage of the focal sphere, thus permitting a reliable estimation of the focal mechanisms. Earthquakes occurred after 2010 in the central area of the caldera are located with gap smaller than 90 degrees, and the estimation of focal mechanisms has become feasible and reliable for any event of M>1, but in particularly favorable cases we were able to find a good solution even for events as small as M0.5.

Among the 36 computed mechanisms two thirds (24) are classifiable as normal faults (red in Fig. 1), while the reverse solutions are only 2 (magenta in Fig. 1). Solutions with nearly vertical fault (dip slip and strike slip mechanisms) counts 10 earthquakes (orange in Fig. 1). Fig. 2 shows the values of the three angles found for all focal mechanisms shown in Fig. 1. Their distribution is quite homogeneous with regard to strike, while solutions with dip smaller

Fig. 1 – Map of Campi Flegrei caldera showing the results of focalMap of Campi Flegrei caldera showing the results of focal mechanisms analysis. Blue symbols represent the seismic stations used in this analysis and/or currently installed. The size of beach ball is proportional to magnitude (0.6 <– M <– 2.5).

Fig. 2 – Dip (red) and rake (blue) angles versus strike of the focal mechanisms shown in Fig. 1.Dip (red) and rake (blue) angles versus strike of the focal mechanisms shown in Fig. 1.

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than 30 degrees are only two. Rake angle is negative for 32 out of 36 solutions, indicating a predominance of normal mechanism (88% of the total amount). The analyzed earthquakes are all very small (M<–2.5) and their location is spread in a wide area of the caldera. Therefore it is difficult to associate their sources with the main geological structures recognized in the area. It seems more likely that the studied seismicity is associated mostly with many secondary, small faults, rather than with few larger faults.

Discussion and conclusions. Focal mechanisms at Campi Flegrei were computed for the strongest earthquakes of 1982-84 bradyseismic episode (Gaudiosi and Iannaccone, 1984; De Natale et al., 1985), and revised recently through a joint seismological - deformative approach (D’Auria et al., 2015). In any cases the results show the predominance of normal mechanisms, the same indication given by our analysis on low magnitude recent earthquakes.

The predominance of normal mechanisms suggest near horizontal orientation of the tensile stress field likely responsible of the observed seismicity. This feature, the spread epicenter distribution and the shallow location (less than 3 km) seem compatible with a small horizontal elongation of the shallow crust produced by the ongoing uplift phase. On the other hand, the horizontal elongation reduces the normal stress acting on dip and nearly vertical faults, thus favoring the occurrence of dip slip and strike slip earthquakes, of which type we have observed at least 10 out of 36. Our simple interpretation fit very well the observation that VT earthquakes occur only during ground uplift and not during subsidence periods. In fact during subsidence the shallowest part of the crust undergoes a shortening, thus the normal stress on existing dip and nearly vertical faults increases and this condition reduces the occurrence of earthquakes. Another implication of our simple model is that the role of regional stress field is negligible in the generation of Campi Flegrei VT earthquakes.References Aster, R.C., Meyer, R.P., De Natale, G., Zollo, A., Martini, M., Del Pezzo, E., Scarpa, R., Iannaccone, G. (1992).

Seismic investigation of the Campi Flegrei Caldera, in “Volcanic Seismology”, Proc. Volcanol. Ser. III (Springler Verlag, New �ork).

Castellano M., Buonocunto C., Capello M., La Rocca M. (2002), Seismic surveillance of active volcanoes: the Osservatorio Vesuviano Seismic Network (OVSN – Southern Italy). Seism. Res. Lett., Vol. 73, 2, pp. 168-175.

D’Auria L., B. Massa, E. Cristiano, C. Del Gaudio, F. Giudicepietro, G. Ricciardi, C. Ricco (2015). Retrieving the Stress Field Within the Campi Flegrei Caldera (Southern Italy) Through an Integrated Geodetical and Seismological Approach. Pure Appl. Geophys. 172, 3247–3263.

Del Gaudio C., I. Aquino, G. P. Ricciardi, C. Ricco, R. Scandone (2010). Unrest episodes at Campi Flegrei: A reconstruction of vertical ground movements during 1905-2009. J. Vol. Geoth. Res., 195, 48-56. doi:10.1016/j.jvolgeores.2010.05.014.

De Natale G., A. Zollo, A. Ferraro, J. Virieux (1985). Accurate fault mechanism determinations for a 1984 swarm at Campi Flegrei caldera (Italy) during an unrest episode: Implications for volcanological research. J. Geophys. Res., 100 B12, 167-185.

Gaudiosi G., G. Iannaccone (1984). A preliminary study of stress pattern at Phlegraean Fields as inferred from focal mechanisms. Bull. Volcanol. 47-2.

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.

Orsi G., M. A. Di Vito, R. Isaia (2004). Volcanic hazard assessment at the restless Campi Flegrei caldera. Bull. Volcanol., 66, 514-530, doi: 10.1007/s00445-003-0336-4.

Snoke J.A., J.W. Munsey, A.C. Teague, G.A. Bollinger (1984). A program for focal mechanism determination by combined use of polarity and SV -P amplitude ratio data. Earthquake Notes, 55, #3, 15.

Snoke J.A. (1989). Earthquake Mechanisms, Encyclopedia of Geophysics (D. E. James Ed.). Van Nostrand Reinhold Company, New �ork, 239–245.