Electric and Magnetic Field Changes Observed ... - GeoScienceWorld

Bulletin of the Seismological Society of America, Vol. 104, No. 3, pp. 1289–1298, June 2014, doi: 10.1785/0120130183

Electric and Magnetic Field Changes Observed during a Seismic Swarm in Pollino Area (Southern Italy) by Marianna Balasco, Vincenzo Lapenna, Gerardo Romano,* Agata Siniscalchi, Tony Alfredo Stabile, and Luciano Telesca

Abstract

Over the last few years, seismic activity in the Pollino area (a sector of the Calabro–Lucanian Apennines in southern Italy known as a seismic gap) has been very weak. However, in 2011 the seismicity gradually intensified, culminating in an earthquake of Mw 5.0 occurred on 25 October 2012. The depth of the 2011–2012 earthquake hypocenters ranges between 2 and 10 km; the seismicity results in two separate clusters and traces a north-northwest–south-southeast fracture more evident in the western sector. In this area, an MT station was installed on 26 September 2012 by the Institute of Methodologies for Environmental Analysis, National Research Council of Italy (IMAA-CNR), Italy, at about 50 km from another MT station operating since 2003 in the Agri Valley (Tramutola, southern Italy). Such a seismic swarm occurred in the Pollino area (more than 3600 events in last two years with local magnitude M L ≥ 0:1). It has provided a rare opportunity to study the earthquake-related temporal patterns of electromagnetic (EM) signals, and is potentially informative about ongoing seismogenic processes. In this study, we present several cases of EM field variations associated with the passage of seismic waves. The maximum amplitude of the electrical signals registered at the two MT sites and the earthquake magnitude are related by an attenuation factor that depends on the distance between the hypocenter and the MT station. Furthermore, at the two MT sites the maximum electrical anomalies seem to be more appreciable predominantly in different directions, indicating a certain directivity in the propagation of the electric field. A deep analysis of EM time series recorded during the mainshock M w 5.0 was performed. In particular, by applying time–frequency misfit criteria based on the continuous wavelet transform, we compared the electric field with seismic recordings, and we found a good waveform similarity between signals. Moreover, we also found an EM signal that significantly anticipates the theoretical first P-wave arrival at the Tramutola MT station.

Introduction The MT continuous monitoring was applied in active seismic areas (Eisel and Egbert, 2001; Johnston et al., 2006; Kappler et al., 2010) to detect anomalous electromagnetic (EM) behavior associated with earthquakes. Recent studies have reported significantly high amplitudes in EM signals during the passage of a seismic wave. Matsushima et al. (2002) observed electric and magnetic signals in association with the Izmit earthquake, on 17 August 1999 M w 7.4. Johnston et al. (2006) reported coseismic seismomagnetic effects in association with the 28 September 2004 M w 6.0 event. During the *Also at Institute of Methodologies for Environmental Analysis, National Research Council of Italy, C. da Santa Loja 5, 85050 Tito (PZ), Italy.

Bhuj earthquake that occurred on 26 January 2001 (M w 7.6), Abdul Azeez et al. (2009) observed significant changes in the amplitude and frequency of magnetotelluric time series. Zlotnicki et al. (2006) documented anomalous electric and magnetic signals not only after, but also before the seismic activity in the volcanic area. In order to check that MT variations are not due to the vibrations of MT sensors, Ujihara et al. (2004) proposed to set up two different arrays (electric cables and induction coils) in the same location, one buried in the ground (as usually is done in MT measurements) and the other in the air. When the seismic wave passed, both the arrays measured the same variation in the electric and magnetic fields.

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M. Balasco, V. Lapenna, G. Romano, A. Siniscalchi, T. A. Stabile, and L. Telesca

Even if the existence of anomalous changes in electric, magnetic, and electromagnetic fields correlated with seismic activity has been reported in several parts of the world, the physical mechanism is still unclear. Field and laboratory experiments confirm the existence of a coupling between seismic waves and the EM field. Several mechanisms have been proposed to explain these phenomena. Honkura et al. (2004) supposed that the passage of a seismic wave with velocity v through a conductive medium (Earth’s crust) can induce a movement of the ground-water ions contained in the rock matrix pores due to the presence of the Earth’s magnetic field B. They called this mechanism a seismo–dynamo effect. In contrast, the electrokinetic theory (Thompson and Gist, 1993; Pride, 1994; Garambois and Dietrich, 2001; Gao and Hu, 2010) maintains that the coupled seismic and EM wave propagation arises because of the electrochemical double layer that exists along the solid-grain and fluid-electrolyte boundaries of porous media. The electrokinetic theory involves the existence of three kinds of response field: 1. There may be a coseismic EM signal that is coupled with the seismic waves and that propagates with their velocity. The generation of this signal is known as seismoelectric conversion and implies both the existence of an electric field coupled to P waves and mainly of a magnetic field that correspondends to transversal waves (Garambois and Dietrich, 2001; Gao and Hu, 2010). 2. EM signals may be triggered directly by the seismic source and linked to the relative motion between the fluid and the solid phase (Hu and Gao, 2011) in the focal area. 3. EM signals generated when a seismic wave may traverse an interface with a contrast in electrical and/or mechanical properties. In this case, the EM signals propagate outside the support of the seismic waves with a higher speed (Pride and Haarsteen, 1996). A rigorous physical and mathematical formulation of the equations controlling such phenomena can be found in Pride (1994), in which Biot’s poro-elastodynamic equations are coupled to Maxwell’s equations of electrodynamics. This formulation provides a tool to model the seismoelectric coupling phenomena in fluid-saturated porous media. In volcanic environments, these anomalous MT signals have been interpreted as due to electrokinetic effects associated with the disturbance of the fluid-flow pattern in the volcanic system; however, we cannot exclude the possibility that the same mechanism is dominant also in proximity of faults where porous rocks saturated by an electrolytic fluid can be found.

Investigated Area The Pollino area is a sector of the Calabro–Lucanian Apennines that has long been considered a seismic gap, but it may contain seismogenic faults that have been relatively qui-

Figure 1.

Locations of the magnetotelluric station (CAMP MT and TRAM MT) of the seismic station (MCEL) and distribution of earthquake epicenters (ML ≥ 0:1) in the Pollino area (southern Italy, Calabro–Lucanian Apennines) from January 2012 to January 2013 within a radius of 50 km around the CAMP MT station. The star and square symbols in proximity to the CAMP MT station indicate the epicenter of the Mw 5.0 and the M w 4.3 earthquakes, respectively.

escent over historical time. This apparent lack of historical seismicity may be explained by the fact that the catalogs are shorter than the return periods of large events or the catalogs are incomplete due to the low density of population and the settlement pattern in this area (Cinti et al., 1997). Historical earthquakes in the Pollino area are reported in Data and Resources with a moderate magnitude M w 5.7 in 1693, M w 5.5 in 1708, and recently M w 5.6 in 1998, although the major faults in this area, such as the Castrovillari fault (Cinti et al., 1997), the Pollino fault (Michetti et al., 1997), and the Mercure fault (Papanikolaou and Roberts, 2007), represent a significant seismogenic potential (M w 6.5–7.0). In 2011, the seismicity in the area gradually intensified, and a seismic swarm took place. The depth of the 2011–2012 earthquakes ranges between 2 and 10 km approximately. Two separate clusters are clearly defined: the occidental and the oriental clusters. In particular, the former, which comprises most of the seismicity, delineates a north-northwest–south-southeast fracture. The major event occurred on 28 May 2012 (Mw 4.3) in the oriental cluster and on 25 October 2012 (M w 5.0) in the occidental one. The focal mechanisms evaluated for most of the seismic sequence (Totaro et al., 2013) show an extensional process with an antiApennine trend associated with a normal fault (northwest– southeast or north-northeast–south-southeast). As for other seismic swarms that occurred in Italy in the last decades (e.g., Lucente et al., 2010), in this case fluid presence may play a key role as a physical driving mechanism for occurrences of small events.

Acquisition Setup Since 2006, an MT station has been operating in continuous mode in Tramutola Agri Valley, southern Italy (latitude 40.2967° N, longitude 15.8056° E, elevation 890 m; Fig. 1), to study possible changes in the electric structure of the subsoil due to seismic events (Balasco et al., 2008). This area is characterized by several fault systems responsible for

Electric and Magnetic Field Changes Observed during a Seismic Swarm in Pollino Area (Southern Italy)

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Figure 2.

Records of magnetic field (H x − Hy ) and electric (Ex − Ey ) at the TRAM MT station from 01:06:00 to 1:09:00 on 28 May 2012. The origin time of the event M w 4.3 is at 01:06:27 (UTC). The dashed lines indicate the origin time of the earthquake.

the occurrence of historical seismic events, such as the 1857 Mw 7.0 earthquake (Mallet, 1862). Therefore, the area has a high seismic risk also posed by the presence of many towns as well as two artificial dams and oil wells. However, the seismicity of the area has been quite rare so far. During the seismic swarm that occurred in the Pollino region, anomalous magnetic and electric signals were recorded by the MT station installed in Tramutola (hereafter TRAM), located at about 50 km from the seismic swarm. In September 2012, the appearance of such anomalies led to the installation of another MT station in Campotenese (hereafter CAMP; latitude 39.8941° N, longitude 16.0844° E, elevation 1487 m) in which the seismic swarm was very dense. Both stations are equipped with an MT24LF receiver (EMI Schlumberger), two orthogonal induction coils (BF4, EMI Schlumberger) that measure the time-varying magnetic field (Hx;NS and Hy;EW ), and two 50 m electrical dipoles to measure the electric field in the surface plane (Ex;NS and Ey;EW ). The TRAM station is also equipped with a vertical induction coil (Hz ) and a second dipole Ey;EW . It is worth noting that x, y, and z directions correspond to northward, eastward, downward directions in the geomagnetic coordinate system, respectively. The declination angle is about 3° at our observation sites. The horizontal coils are buried in the trenches 0.5 m deep, whereas the vertical coil and the PbPbCl2 electrodes are placed in drilled holes 0.5 m in depth. The frequency of electric and magnetic data recording is set to 6.25 Hz with a 24-bit A/D system. Actually, the system oversamples at 500 Hz, and then it returns the data at 6.25 Hz to prevent the aliasing effect.

Data Analysis Through a visual inspection, the EM data measured at both stations during earthquake occurrences show a very similar variation to that of the seismograms.

Figure 2 shows, as an example, the EM signal recorded at the TRAM station on 28 May 2012 at 01:06:27 related with the earthquake (M w 4.3) occurred in the Pollino area (latitude 39.859°, longitude 16.118°) at 55.5 km from the TRAM station. The recording lasts three minutes: the amplitude of the x component of the magnetic field and that of the y component of the electric field increases significantly a few seconds after the origin time of the earthquake (Fig. 2). Similar examples can also be found in the CAMP station for other subsequent earthquakes (Fig. 3). Table 1 reports the peak amplitude values of the electric and magnetic field observed for the September–November 2012 events with M L ≥ 2:0. It is worth noting that the table is not complete for 2 ≤ M L ≤ 3 due to the high number of earthquakes (261 events). Some common features can be evidenced: 1. The minimum magnitude threshold above which anomalous changes in the electric field at the TRAM station are detectable is ML 3:0, whereas it is M L 2:0 at the CAMP station. The magnetic CAMP records do not always show earthquake-related anomalies in the amplitude; however, when they appear, they are more clearly visible in H x . 2. The anomalous signals are always visible a few seconds after the earthquake occurrence. 3. The anomalous signals observed in the CAMP station precede those observed at the TRAM station. 4. The maximum amplitude of the anomalies is reached a few seconds after its beginning. 5. For all the electric CAMP records, the Ey amplitude is at least twice as great as that of Ex . For the electric TRAM records, the Ex component shows a larger amplitude with respect to Ey . The different behavior of the two electric components may indicate the existence of anisotropy in the subsoil electrical structure of the two sites. 6. The initial part of the electric anomalies is not always coupled with an anomalous magnetic signal.

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Figure 3. Examples of electric (Ex − Ey ) and magnetic signals (Hx − Hy ) recorded between October and November 2012 at the CAMP MT station related to events with different magnitudes. The dashed lines indicate the origin time of the earthquakes. Because the amplitude of seismic waves decreases with the increase in the distance from the hypocentral area, a similar feature is expected for the EM anomalies associated with seismic events. Indeed, heuristically a relationship can be stated between the amplitude of the anomalous signals, the magnitude of the earthquake, and the distance of the hypocenter from the station. Even if the exact relationship between seismic waves and EM anomalies is unknown, we have considered the case in which the attenuation factor is the reciprocal and the square reciprocal of the distance between the earthquake focus and the station location. Figure 4 shows the relationship between the magnitude of the events in Table 1 versus the product of the maximum peak amplitude of the Ey (CAMP station) and Ex (TRAM station) signals with the attenuation factor, as defined previously. We selected the CAMP station Ey component and the TRAM station Ex component because, as reported in Table 1, they show the maximum amplitudes. A good linear correlation can be found in semilog scales for the anomalies recorded

both at the CAMP (dashed black line) and at the TRAM stations (black line). In Figure 4a (in which the attenuation is given by the reciprocal of the distance between the hypocenter and the station location), the regression lines corresponding to CAMP and TRAM data have an approximately similar y intercept. In Figure 4b (in which the attenuation is given by the square reciprocal of the distance), the two regression lines have the same slope, but their y intercepts differ. Such offset can be explained considering the distance between the TRAM station and the hypocenters is always about one order magnitude larger than that between the CAMP station and the hypocenters. Such features of the EM signals suggest that they can be explained in the framework of the electrokinetic theory, mainly as coseismic EM phenomena that are coupled to seismic waves. In order to investigate this phenomenon, the Mainshock of Mw 5.0 section will be focused on the analysis of the EM anomalies related to the major event that occurred on 25 October 2012 with M w 5.0.

23:05:24 01:06:27 08:28:39 20:28:28 02:51:57 21:27:51 00:08:57 23:16:01 12:06:32 01:59:52 10:13:14 17:07:36 17:48:02 03:03:44 03:37:46 17:50:44 12:37:31 05:56:46 09:32:33 10:40:24 11:11:57 20:41:11 02:43:46 02:25:09 02:40:08 09:19:52 13:52:18 00:31:53 23:20:35 23:12:19 16:08:58 10:47:32 04:12:42 23:35:08 23:24:54 23:56:34 08:40:14 05:45:32 23:07:17 02:56:52 01:20:54

6.3 3 7.5 8.1 7.8 7.9 7.4 8.3 8.7 9 8 7.8 9.8 5.1 8.8 7.2 5.8 3 8.5 9.2 6.3 8.9 8.2 6.6 7 4.2 8.9 6.4 7.6 10.1 8.3 7.7 3.6 10 6.1 8 8.2 9.3 8 8.6 6

Depth (km)

5 4.3 3.7 3.6 3.5 3.3 3.3 3.3 3.3 3.3 3.2 3.2 3.2 3.2 3.1 3.1 3.1 3 3 3 3 3 3 2.9 2.9 2.9 2.9 2.8 2.8 2.7 2.7 2.6 2.6 2.5 2.4 2.3 2.3 2.3 2.3 2.2 2

Mw

10.4 – 10.8 11.6 10.4 11.6 10.8 11.8 13.1 12.0 10.8 11.8 13.4 9.1 12.9 10.2 10.0 5.1 11.6 12.2 10.3 10.9 12.9 9.9 10.6 8.9 13.7 10.8 9.6 13.5 11.1 10.5 6.3 12.4 8.1 10.6 11.3 11.1 11.6 12.0 8.0

d (km)

148.67 – 8.67 11.07 5.73 3.20 3.20 3.87 1.47 0.40 4.93 1.07 2.40 26.27 – 3.47 1.60 4.40 2.27 1.87 6.27 1.33 2.67 1.87 0.93 2.13 1.47 0.93 4.53 0.93 1.73 0.80 1.73 0.80 1.20 0.53 0.40 1.73 0.40 0.27 0.63

410.13 – 16.40 22.00 17.73 5.87 6.27 6.67 3.20 7.60 8.80 3.73 5.20 – – 9.60 4.53 12.00 5.07 4.00 16.80 3.07 5.33 5.47 2.53 3.07 2.80 2.00 11.73 1.33 3.87 1.47 5.73 1.60 2.53 1.60 0.80 3.07 1.07 0.93 1.6

Ey (μV=m)

CAMP MT Station Ex (μV=m)

2.04 – 0.104 0.084 0.08 0.024 0.02 0.036 0.032 0.048 0.068 0.048 – 0.144 – 0.052 0.028 0.08 0.024 – – – – 0.032 – – – – 0.036 – – – – – – – – 0.02 – – –

H x (nT)

0.74 – – – 0.012 – – 0.004 – – 0.024 – – – – – – – – – – – – 0.008 – – – – 0.008 – – – – – – – – – – – –

H y (nT)

49.6 50.3 50.8 – – – – – – – – – – – – – – – – – – – – –

45.8 47.4 46.3 46.3 47.6

49.8 56.1 47.1 48.9 50.6 48.6 48.2 49.2 45.1 47.5

D (km)

146.67 – 4.13 7.87 4.80 5.20 4.40 3.07 3.47 1.73 – 1.47 1.87 – – 1.07 – 1.20 1.07 1.87 – – – – – – – – – – – – – – – – – – – – –

152.0 3.6 3.07 3.60 3.20 2.80 2.40 1.73 2.40 1.73 – 1.47 1.20 – – 1.07 – 0.93 1.33 1.20 – – – – – – – – – – – – – – – – – – – – –

Ey (μV=m)

TRAM MT Station Ex (μV=m)

– – – – – – – – – b.d. – – – – – – – – – – – – – – – – – – – – – – – – – –

0.32 0.028 – –

H x (nT)

0.26 – – – – – – – – – – – – – b.d. – – – – – – – – – – – – – – – – – – – – – – – – – –

H y (nT)

0.22 – – – – – – – – – – – – – b.d. – – – – – – – – – – – – – – – – – – – – – – – – – –

H z (nT)

This table reports the peak-to-peak semiamplitude values of electric and magnetic fields observed for all events occurring in the Pollino area and recorded by MT stations with Mw ≥ 3:0 and some with Mw ≥ 2:0 in the investigated period. d and D are the distances from the hypocenter of the earthquake to the CAMP and TRAM stations, respectively, corrected for the elevation; b.d. stands for bad data; and the dash means the changes in electric and magnetic fields are inappreciable.

2012/10/25 2012/05/28 2012/11/25 2012/10/01 2012/10/18 2012/10/01 2012/10/02 2012/10/25 2012/11/05 2012/11/22 2012/10/28 2012/11/03 2012/11/25 2012/11/30 2012/10/28 2012/11/02 2012/11/28 2012/09/28 2012/10/04 2012/10/23 2012/11/08 2012/11/25 2012/11/28 2012/10/26 2012/10/26 2012/10/28 2012/10/28 2012/10/26 2012/10/26 2012/10/25 2012/10/26 2012/10/29 2012/10/30 2012/10/25 2012/10/29 2012/10/25 2012/10/26 2012/09/28 2012/10/28 2012/10/26 2012/10/26

Time (UTC) (yyyy/mm/dd hh:mm:ss)

Table 1 Amplitude Values of Electric and Magnetic Fields Observed for Some Earthquakes

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Figure 4.

Peak amplitude values of the electric fields signal (Ey component for the CAMP station and Ex component for the TRAM station) multiplied by (a) the hypocentral distance and (b) square of the hypocentral distance of the Pollino earthquakes versus the magnitude of events. Open and solid dots represent the values recorded at the CAMP and the TRAM MT stations, respectively. Regression lines (dashed for CAMP station) and coefficients of determination are also displayed.

Mainshock of M w 5.0 During the investigated period, the largest earthquake in the Pollino area occurred on 25 October 2012 at 23:05:24 UTC. Both the MT stations (CAMP and TRAM, at 9 and 49 km hypocentral distance, respectively) were operating and recorded EM anomalies (Fig. 5). Contrary to what is observed for the smallest earthquakes (Table 1 and Fig. 3), even the magnetic components showed clear anomalous fluctuations. At the beginning of the earthquake-related signals recorded at the TRAM station (Fig. 5b), great variations can be observed in the Hx and Hy components (highlighted by the dotted line in Fig. 5). Afterward, the H y component decreases rapidly, whereas the Hx increases again in correspondence to the shear waves. Therefore, more attention should be paid to magnetic field changes although their measurements are generally left

Figure 5. Records of electric and magnetic field at (a) the CAMP and (b) the TRAM sites. The dotted line indicates the beginning of the signal changes recorded at the TRAM station. The origin of the x axis corresponds to origin time of the event M w 5.0 at 23:05:24 (UTC). out in the literature because of the possible shaking effect of the sensors. Almost coincident with the inception of the anomaly recorded at the TRAM station, a sudden increase of amplitudes in the Ex and Hy channels at the CAMP station can be observed. After an initial coseismic oscillation (typical of all the anomalies considered in this paper), the Ey component shows a sudden decay of the signal amplitude, which starts in correspondence to the incoming of the magnetic signal recorded in the TRAM station. To understand the origin of such anomalies, we compared them with the seismic traces. Figure 6 shows the comparison between the east–west component of the electric field (Ey ) measured at the TRAM station and the east–west component (VelEW) of the seismic signal recorded at the Monticello (MCEL, latitude 40.3249° N; longitude 15.8019° E; elevation 960 m) station, which is the seismic station of the Istituto Nazionale di Geofisica e Vulcanologia (INGV) seismic network nearest to the TRAM site. The traces recorded at the MCEL seismic station were deconvolved by the instrument response and were resampled with the same sampling rate

Electric and Magnetic Field Changes Observed during a Seismic Swarm in Pollino Area (Southern Italy)

Figure 6. Homologous components along the east–west direction of (top) the ground-motion velocity (mm=s) recorded at Monticello seismic station and (bottom) the electric field (mV=m) recorded at the TRAM station. Earthquake origin time (origin), theoretical P- and S-wave arrival times (Pth and Sth) at both stations, and the first motion (F pick) of the electric field are also reported. The dashed line indicates the Sth–Pth differential time at a station, whereas dotted line represents the Sth–F differential time at the TRAM station. The color version of this figure is available only in the electronic edition. (6.25 Hz) of the EM signal. A similar comparison could not be performed for the CAMP station because of the absence of a close seismic station. According to the seismic velocity model proposed by Maggi et al. (2009) for the Lucanian Apennines and Bradano foredeep (that includes both the Pollino and the Val d’Agri areas), theoretical travel times were computed for P and S waves at both stations using the NLLoc package (Lomax et al., 2000), and the corresponding arrival times (Pth and Sth picks, respectively) are reported in Figure 6. The theoretical Pwave arrival time is consistent with the observed first motion at the MCEL station, while there is an evident disagreement between the theoretical P-wave arrival time and the observed first motion (F pick in Fig. 6) at the TRAM station. This lag at the TRAM station suggests that the initial part of the EM anomaly is not associated with the P-wave arrival, but it can be an EM signal. An additional proof of such hypothesis arises from the computation of the travel-time differences between Pth, Sth, and F picks. Indeed, because the MCEL station is at a distance of 3.05 km from the hypocenter with respect to the TRAM station, the Sth–Pth differences (dashed lines in Fig. 6) are 8.3 s and 7.8 s at the MCEL and TRAM stations, respectively, whereas the Sth–F difference (dotted line in Fig. 6) is 8.9 s. The latter is not physically possible if the F pick is associated to a P wave. Two possible physical models can be considered to explain the existence of such an EM signal that precedes the Pwave arrival. The first one, postulated by Pride’s equation (Pride and Haartsen, 1996), involves the generation of an EM signals, the presence of which was successfully detected in small-scale field experiments (see e.g., Garambois and Dietrich, 2001), that propagate outside the support of the

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seismic wave with a much higher velocity. The necessary condition to the generation of these signals is the existence of an interface between two media with different electrical or mechanical properties. According to Valoroso et al. (2011), a zone of high V P =V S values, associated with fluid-filled cracks in the upper portion of the Apulia carbonate platform hosting hydrocarbon reservoirs, is located at 3 km deep in the Agri Valley. Hence, such an anomalous zone may be responsible for the generation of the EM signals detected at the TRAM station as a first arrival and at the CAMP station as changes in the signal features after the first arrival (dotted line in Fig. 5). The second possible mechanism of generation of these signals is linked to the change in the EM response due to a rupture along a fault and a fluid-pressure re-equilibration following the earthquake. According to this theory, an EM anomalous signal is generated directly in the focal area during the earthquake occurrence. The related electric anomalies may not be reduced to zero immediately, but they may decay slowly and last for hundreds of seconds or even longer (Pride et al., 2004; Hu and Gao, 2011). The magnetic anomalies affecting the initial part of TRAM signals could be hence explained as the expression of a signal emitted in the focal area in the later part of the rupture process. In both the hypotheses, the anomalous signal should be observed earlier and with a larger amplitude at the station closer to the source, which is the TRAM station if the source is assumed to be in the Agri Valley (first hypothesis) and the CAMP station if the source is assumed to be in the Pollino area (second hypothesis). It is hard, however, to uniquely identify the EM signal starting time and amplitude at the CAMP station because of too low a sampling rate (0.16 s) and a superposition to the seismic-wave-related anomaly within the EM signal. Finally, we quantitatively compared the seismic and EM recordings using the globally normalized time–frequency misfit criteria developed by Kristekova et al. (2006). This method is based on the continuous wavelet transform (the analyzing wavelet is the Morlet wavelet) and has been further extended (Kristekova et al., 2009) in order to perform a quantitative comparison of time signals through the additional evaluation of the goodness of fit (with discrete values from 0 to 10). This criterion has been largely used by several authors to validate synthetic seismograms (e.g., Stabile et al., 2009), to test the rupture models of complex seismic sources (e.g., Benjemaa et al., 2007), and to identify acoustic waves caused by the explosions (Kristekova et al., 2008). Recently, the continuous wavelet transform has also been used to compare the electric field with the ground-motion velocity in the time–frequency domain (Matsushima et al., 2013). Before applying the time–frequency misfit and goodness-of-fit criteria, we aligned the seismic and EM recordings with respect to the theoretical S-wave arrival time at the stations (Sth picks in Fig. 6), removed the mean from all traces, and then normalized. The time–frequency envelope misfit (TFEM) and the time–frequency envelope goodness of fit (TFEG) between the electric field variation recorded at the TRAM station and

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Figure 7. Time–frequency envelope misfit (TFEM, top panels) and time–frequency envelope goodness of fit (TFEG, bottom panels) between the electric field (Ex , Ey ) and the ground-motion velocity (VelNS, VelEW). Electromagnetic (EM) and seismic traces are aligned with respect to the theoretical S-wave arrival times (Sth) at stations (vertical dashed lines in the figure). the ground-motion velocity recorded at the MCEL seismic station are reported in Figure 7. In particular, the comparison of Ex (electric field measured along the north–south direction) against VelNS (ground-motion velocity measured along the north–south direction) is displayed on the left panels of Figure 7 and the comparison of the correspondent components along the east–west direction (Ey and VelEW, respectively) on the right panels. Along both north–south and east– west directions, the discrete goodness-of-fit values (for details, see Kristekova et al., 2009) are generally excellent or good and never poor (discrete goodness-of-fit value lower than 4). Looking again at Figure 7, the misfits of amplitudes are lower than 60%, and the largest differences are evident particularly from 32 to 52 s and in the 0.3–0.5 Hz frequency range, in which the electric field amplitudes are lower (light gray in TFEM panels) than the ground-motion velocity amplitudes. On the other hand, the amplitudes of the electric fields are generally higher (dark gray in TFEM panels) than

the ground-motion velocity amplitudes for frequencies above 1 Hz. This suggests that the EM signal has generally a higher frequency content toward the seismic signal that is consistent with an electrical field proportional to the acceleration of the seismic wave, as it is expected during coseismic effects (Mahardika et al., 2012). Moreover, we cannot exclude that some differences could be due to local site effects, because the MT TRAM site is at a hypocentral distance of 3.05 km from the MCEL site.

Conclusions and Discussions The presence of the two MT stations in the proximity of the Pollino area, during the seismic swarm, has provided a rare opportunity to study the earthquake-related temporal patterns of the MT response also with very low-magnitude events. In association with the occurred seismic swarm (the investigated period was October–November 2012), the elec-

Electric and Magnetic Field Changes Observed during a Seismic Swarm in Pollino Area (Southern Italy) tric and magnetic field variations, recorded in two MT stations, were clearly observed a few seconds after the earthquake occurrence. We found that, except for those related to the mainshock (Mw 5.0), all the recorded anomalies are linked only to the passage of seismic waves near the MT monitoring stations. The maximum amplitude of the electrical signals recorded at the MT stations and the magnitude of earthquakes are related by an attenuation factor, which depends on the distance between the hypocenter and the MT station. Additional signal amplitudes of the order of 1μV=m in the electrical channels and of 0.01 nT in the magnetic channels are distinguishable from the background thanks to the high sensitivity of the acquisition system (24 bit A/D converter) and to the corner frequency of the analyzed earthquakes (3 ≤ M w ≤ 5) that falls into a narrow frequency range (0.5–5 Hz), known as the dead band, in which the power spectrum of the natural EM field has a minimum which produces low-amplitude MT signals. Moreover, the induction coil response in this frequency band has a good amplification factor (300 mV=nT); when an unusual stronger natural EM field band occurs in this frequency, the seismoelectric effects are undetectable (b.d. in Table 1). We also observed that in the CAMP station the y component of the electric field always shows the larger anomalies associated with the earthquake occurrence, whereas at the TRAM station the anomalous variations are more visible in the x component of the electric field. We suppose these differences in the two electrical components depend on the presence of possible electrical resistivity anisotropy in the subsoil of the two sites (see Mahardika et al., 2012). The M w 5.0 mainshock is characterized by features that suggest the coexistence of two phenomena. Besides the seismoelectric conversions, evidence of a possible EM independent signal can be found in the channels of the two MT monitoring stations at time T 23:05:33:12 UTC. In the TRAM records, an anomalous oscillation of the magnetic field was observed just before (less than 1 s) the arrival of the seismic-wave-related anomaly; almost at the same time in the CAMP records, a sudden decay of Ey and an increasing amplitude of the other channels were detected. According to the physical electrokinetic theory, an EMindependent signal can be generated either when the seismic waves approach the mechanical and electrical interfaces located beneath the Agri Valley (Pride’s model) or directly in the focal area during the earthquake occurrence and the subsequent fluid diffusing process (Hu and Gao, 2011). Because the anomalous signal amplitudes recorded at the CAMP station seem to be larger by one order of magnitude than those measured at TRAM, the second mechanism is more plausible. This hypothesis is also supported by the observed discharge curve in the CAMP Ey component. The ∼9 s lag between the beginning of the earthquake and the anomaly occurrence may be explained as the time necessary for the fluids to flow into the void spaces generated by the propagation of the rupture. As the saturated condition is reached, the EM signal is generated. Furthermore, this is consistent with the

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swarm-type distribution of the analyzed earthquakes occurring in the Pollino area since 2011; in fact, one of the most likely underlying physical driving mechanisms for swarms of small events is fluid flow (Shearer, 2012, and references therein). A wavelet-based quantitative comparison between EM and seismic recordings in correspondence with the M w 5.0 event demonstrated a good waveform similarity of the signals within a large range of frequencies. After the S-wave arrival, the maximum amplitude of electric field signals appears shifted to higher frequency with respect to the seismic traces (Fig. 7). At much higher frequencies, a similar phenomenon was also observed by Honkura et al. (2009); however, the mechanism discussed by Honkura et al. (2009), in which the maximum amplitude of the signals was observed at frequencies compatible with the cyclotron resonant ones of the relevant ions present in the groundwater (seismodynamo effect), is different from that discussed in our paper, which is related to electroseismic phenomena as described by Hu and Gao (2011). The EM observations herein shown indicate almost stable and repeatable coseismic effects known in the literature as seismoelectric conversion, even in correspondence of moderate and small seismic events (Mw < 4). In order to demonstrate the occurrence of EM independent waves and quantitatively verify their similarity with the ones predicted by theoretical models (i.e., Hu and Gao, 2011), a network of stations equipped with full MT components and seismic sensors should be realized. In this way, it is also possible to avoid problems related to different arrival times of seismic waves and local site effects on recordings.

Data and Resources Seismic data used in the present study are available to the public through the websites of the databases http://iside.rm .ingv.it, http://cnt.rm.ingv.it/tdmt.html, and CPTI11, la versione 2011 del Catalogo Parametrico dei Terremoti Italiani, Milano, Bologna by A. Rovida, R. Camassi, and P. Gasperini (http://emidius.mi.ingv.it/CPTI, last accessed July 2013).

Acknowledgments This study was supported by the Department of Civil Protection and National Institute of Geophysics and Volcanology in the framework of the “Short Term Earthquake Forecasting” S3-Project. The authors would like to thank anonymous reviewers for their comments, which have significantly enhanced the clarity and quality of this paper.

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Institute of Methodologies for Environmental Analysis National Research Council of Italy C. da Santa Loja 5 85050 Tito (PZ), Italy [email protected] [email protected] [email protected] [email protected] (M.B., V.L., T.A.S., L.T.)

Department of Earth and Geo-Environmental Science University of Bari Via Orabona 4 70125 Bari, Italy [email protected] [email protected] (G.R., A.S.) Manuscript received 10 July 2013; Published Online 27 May 2014