Annals 47, 1, 2004, 01/07def 29 ANNALS OF GEOPHYSICS, VOL. 47, N. 1, February 2004 Key words tectonomagnetism – seismomagnetic network – seismic activity – VLF – Central Italy 1. Introduction Magnetic and electromagnetic field natural signals are monitored in seismogenic areas in the search for possible correlations with seismic activity. This field of investigation covers phe- nomena that vary on a very wide range of fre- quencies from DC to VHF. This paper reports the results of the analysis of data coming from two subsets of electromagnetic investigations, the so-called tectonomagnetic field observa- tions and VLF natural electromagnetic field waves observation. As concerns tectonomagnetic field observa- tions, we refer to changes in the physical prop- erties of the crustal materials that could lead to local anomalous variations of the Earth’s mag- netic field. Magnetic changes due to stress field variations in the uppermost crustal layers have been predicted and observed historically and the term tectonomagnetism was introduced for this research field. Some indirect effects of the electrokinetic type caused by fluid flows have also been invoked as a possible cause for local magnetic anomalies. The term seismomagnet- ism is in particular referred to magnetic field changes associated with earthquake occurrence. Following the report that tectonic events, like earthquakes and volcanic eruptions, can produce observable variations in the absolute value of the geomagnetic field intensity, many groups have started such investigations. In the case of tectonomagnetic and seismomagnetic effects the scientific literature has many examples. Parrot and Johnston (1993), Hayakawa and Fujinawa (1994), Gasparini et al. (1997), Johnston (1997), Johnston and Parrot (1997), Mueller and John- ston (1998), Meloni et al. (2001) are dedicated to such topics. Seismomagnetic effects are de- tected with sensitive magnetometric instrumen- Tectonomagnetic and VLF electromagnetic signals in Central Italy Antonio Meloni (1), Domenico Di Mauro (1), Stefania Lepidi (2), Giuliana Mele (1) and Paolo Palangio (2) (1) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy (2) Osservatorio Geofisico di L’Aquila, Istituto Nazionale di Geofisica e Vulcanologia, L’Aquila, Italy Abstract Tectonomagnetic field observations from absolute magnetic field level measurements were undertaken in Cen- tral Italy in an area extending between latitude 41°N and 43°N and between longitude 13°E and 15°E. More- over, natural electromagnetic signals from a system of two VLF search coil wide-band antennas were collected at the geomagnetic observatory of L’Aquila (42º23′N, 13º19′E). The analysis of these data allowed the investi- gation of the electromagnetic properties of the study area at different time and spatial lengthscales. Tectono- magnetic field observations were obtained comparing data simultaneously recorded at three magnetometer sta- tions using L’Aquila Observatory as a reference for differentiation. We report on the time evolution of magnet- ic and electromagnetic indicators related to local and regional seismic activity. Mailing address: Dr. Antonio Meloni, Istituto Naziona- le di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy; e-mail: meloni@ingv.it 30 Antonio Meloni, Domenico Di Mauro, Stefania Lepidi, Giuliana Mele and Paolo Palangio tation in the near field of earthquakes only when magniude M is greater than 6 with an amplitude of about 1 nT or so (Johnston, 2002). According to many investigations, electro- magnetic phenomena in a wide range of fre- quencies ranging from ULF to VHF that take place in the atmosphere and ionosphere also show anomalous emissions in association with earthquakes. We will refer in this paper to measurements in the VLF band only. In this band transient signals and noise are generated by various natural and artificial phenomena of which radio atmospherics (or sferics for short and sometime statics) are one kind of typical impulsive electromagnetic signals that can propagate in the atmosphere for thousand kilo- meters. Generated by atmospheric lightning strokes, radio atmospherics travel in the Earth- ionosphere wave guide as a VLF wave with power concentrated mainly in the VLF range, from 0.1 to 10 kHz (see for a recent review Barr et al., 2000). Gokhberg et al. (1989) were among the first to report perturbations on VLF waves amplitude in association with earth- quakes. Other reports, by Hayakawa and Fuji- nawa (1994), Morgunov et al. (1994) and Par- rot (1995), followed. Anomalous VLF signals were also detected in the case of the 1995 Kobe earthquake (see Nagao et al., 2002). Electro- magnetic waves at ELF and VLF frequencies related to earthquakes were also detected from satellite instrumentation, see for example Molchanov et al. (1993) and Parrot (1994). 2. Area of investigation and geodynamic framework The study area extends between latitudes 41°N and 43°N and longitudes 12°E and 16°E (fig. 1). This area has been investigated in the last twelve years starting in July 1989 with the Fig. 1. Topography map of the study area showing L’Aquila Observatory and seismomagnetic stations used in this work; epicenters (white dots) of magnitude M ≥ 4.0 earthquakes occurred from 1989 to 2001 are also re- ported (see list in table I). 31 Tectonomagnetic and VLF electromagnetic signals in Central Italy intent of detecting electromagnetic and magnet- ic natural signals related to local seismic activi- ty. Such studies used magnetotelluric stations, magnetometer total field stations and VLF natu- ral wave detection systems (Mele et al., 1994; Meloni et al., 1996, 2001; Ernst et al., 1997). Central Italy has a long history of moderate- to-intense seismicity (Boschi et al., 1997; IN- GV, 2003) that makes this area suitable for studies of possible relations between electro- magnetic natural phenomena and seismic activ- ity. Even though rock magnetization in the Table I. Earthquakes with M ≥ 4.0 occurring in Central Italy in the study period. # Date Origin Time Lat. N Long. E Depth M d.m.y UT km 1 06.10.1989 17:38:27 42.17 15.59 5 4.1 2 22.12.1989 06:48:15 43.02 12.77 5 4.0 3 01.02.1990 06:24:15 42.13 15.58 5 4.1 4 16.07.1992 05:38:55 42.34 14.21 5 4.0 5 05.06.1993 19:16:18 43.13 12.67 5 4.3 6 02.06.1994 17:38:15 42.44 13.21 5 4.0 7 07.08.1994 06:31:12 41.95 13.37 5 4.0 8 30.09.1995 10:14:34 41.80 15.90 22 4.4 9 20.10.1996 19:06:56 42.56 13.16 5 4.0 10 19.03.1997 23:10:49 41.38 14.63 9 4.1 11 12.05.1997 13:50:16 42.83 12.54 5 4.6 12 26.09.1997 00:33:11 43.02 12.91 8 5.6 13 26.09.1997 09:40:24 43.02 12.93 5 5.8 14 03.10.1997 08:55:20 43.02 12.90 5 5.0 15 04.10.1997 15:07:19 42.92 12.98 4 4.1 16 04.10.1997 16:13:31 42.92 12.98 4 4.3 17 04.10.1997 18:47:46 42.92 12.97 5 4.3 18 12.10.1997 11:08:35 42.90 13.01 5 4.7 19 13.10.1997 11:01:44 42.88 13.05 3 4.0 20 13.10.1997 13:09:19 42.86 13.00 4 4.0 21 14.10.1997 15:23:09 42.91 12.97 5 5.5 22 15.10.1997 22:53:09 42.92 12.00 6 4.1 23 21.03.1998 16:45:09 42.98 12.90 5 4.1 24 15.08.1998 05:18:10 42.44 13.04 5 4.5 25 10.10.1999 15:35:52 42.69 13.13 5 4.0 26 11.03.2000 10:35:28 42.03 13.17 5 4.3 27 27.06.2000 07:32:32 41.00 13.04 5 4.0 28 16.12.2000 07:31:06 42.53 12.62 8 4.0 32 Antonio Meloni, Domenico Di Mauro, Stefania Lepidi, Giuliana Mele and Paolo Palangio study area is moderate to low (Molina et al., 1994; Chiappini et al., 2000), due to the pres- ence of carbonates and marls along with flysch deposits, the generally very low level of artifi- cial electromagnetic noise, makes it suitable for natural magnetic and electromagnetic field in- vestigations. Earthquake distribution in peninsular Italy mainly follows the NW-SE axis of the Apen- nine mountain belt. Focal mechanisms indicate that this chain is currently dominated by exten- sional deformation (e.g., Anderson and Jack- son, 1987; Amato et al., 1998) overimposed on older compressional features of Paleogene age. In the Central Apennines, where the study area is located, a M = 6.9 earthquake occurred in 1915 (e.g., Ward and Valensise, 1989) causing more than 32000 fatalities (Boschi et al., 1997). Moderate-magnitude events also occurred in re- cent years: the M = 5.4 Val Comino earthquake in 1984 (Console et al., 1989) and the 1997- 1998 Umbria-Marche seismic sequence, with the six largest earthquakes of magnitude rang- ing between 5.0 and 6.0 (e.g., Amato et al., 1998). In table I all M ≥ 4.0 earthquakes oc- curred from 1989 to 2001 are reported; epicen- ters are shown in fig. 1. Many studies have shown that in the phases of preparation of a seismic event and also in case of coseismic observations, patterns of time variations of electromagnetic or magnetic pa- rameters vary generally greatly in shape, time scales, phases and extent around the hypocen- tral area. Moreover, quantitatively defining an ‘anomaly’ is sometimes very difficult. For such reasons, besides the identification of magnetic and VLF wave signals and their relations to seismic activity, our investigation was also fi- nalized to improve the knowledge of the natural background electromagnetic phenomena in this region. 3. Data analysis 3.1. Observations by proton procession magnetometer network Proton Procession Magnetometers (PPMs) and optical pumping magnetometers are gen- erally used for reliable tectonomagnetic stud- ies (Parsons and Wiatr, 1982; Mueller and Johnston, 1998). The installation of a seismo- magnetic network in the area of study started in middle 1989. Data are gathered by means of PPMs installed in a network with station sepa- ration of a few tens of kilometers. Magneto- meters simultaneously measure the total mag- netic field intensity with a sampling rate that has been variable in the last ten years (4-10-15 min) and is now set to 15 min. The present net- work consists of three stations shown in fig. 1 with their acronyms. Measured values are av- eraged and differentiated with respect to L’Aquila Geomagnetic Observatory to filter out the contributions from external sources and from regional inhomogeneities of the Earth’s magnetic field. After processing, the total magnetic field data are plotted and search- ed for possible local field changes in order to relate them to earthquake occurences in a de- fined area around the instruments magnetic network. 3.2. Observations by VLF antennas in L’Aquila Time variability of natural signals in VLF band is monitored in the L’Aquila Observatory by means of two search coil wide band anten- nas operating in the range 15-40 kHz. The sys- tem has 4 band pass filters centered at 15, 20, 30 and 40 kHz respectively. Radiometer sensi- tivity is 200 fT at 40 Hz and data are sampled at 1 Hz and stored with a 16 bit resolution, af- ter averaging every 60 s. The main purpose of this monitoring procedure is to investigate the time behaviour of recorded natural radio sig- nals in the VLF band to study its time evolution and stability. The time plots, here produced on- ly for 20 and 40 kHz, were also related to seis- mic activity as recorded in the defined area around L’Aquila Observatory. Some authors have pointed out that VLF waves monitoring could be more advantageous if an artificial source is used (see Molchanov and Hayakawa, 1998; Saraev et al., 1999). However in this area the overall low artificial electromagnetic noise in this band has facilitated the VLF natural waves data acquisition. 33 Tectonomagnetic and VLF electromagnetic signals in Central Italy 3.3. Results Figure 2 reports the daily mean values for the Earth’s magnetic field intensity from AQU and the seismomagnetic stations in their time evolution. A steady increasing secular variation of about 25 nT/yr is observed at all stations; synchronous spikes are due to solar activity, some interruptions are caused at the seismo- magnetic stations because of instrumental fail- ure (this figure is an update of Meloni et al., 2001). Figure 3 shows five-day means of simple differences for the total geomagnetic field be- tween the AQU reference station and RIT, MDM, and CVT stations. Vertical lines in the plot indicate the occurrence of the most intense earthquakes (Md ≥ 4.0); see also table I for the complete list. The sequence of September-Oc- tober 1997, where 4 earthquakes with Md 5.0 to 6.0 took place in the Umbria-Marche region, on Fig. 2. Time variation of total magnetic field F (daily means) at L’Aquila Observatory (AQU) and the seismo- magnetic stations (RTI, MDM and CVT; see also fig. 1 for map location). 34 Antonio Meloni, Domenico Di Mauro, Stefania Lepidi, Giuliana Mele and Paolo Palangio F ig . 3. F iv e da ys a ve ra ge s of d if fe re nc es b et w ee n L ’A qu il a G eo m ag ne ti c O bs er va to ry ( A Q U ) an d th e se is m om ag ne ti c st at io ns C V T ,M D M a nd R T I fr om 1 98 9 to 2 00 1. V er ti ca l ba rs i nd ic at e ea rt hq ua ke s w it h M ≥ 4. 0. A t hi ck v er ti ca l li ne r ep or ts t he s ei sm ic s eq ue nc e of S ep te m be r- O ct ob er 1 99 7 th at t oo k pl ac e in U m br ia -M ar ch e re gi on s N W o f L ’A qu il a. 35 Tectonomagnetic and VLF electromagnetic signals in Central Italy the NW sector of the monitored area, is report- ed as a gray band. In January-February 1990 and only at that time, an anomalous variation was clearly detected at MDM station. Figure 4 reports the time variability of natural electromagnetic signals, measured at L’Aquila Observatory in the VLF band by means of two search coil wide band antennas, for 20 and 40 kHz. This variability is shown for the year 2001 by means of 1 min data from March to Septem- ber with an interruption caused by technical fail- ure for twenty days in July and August. 4. Discussion and conclusions The plots of magnetic field differences be- tween seismomagnetic stations and L’Aquila Observatory show a scatter around the mean difference values; the scatter amplitude increas- es with the distance from L’Aquila Observatory showing the limits in the removal of external magnetic field variation gradients. Although a moderate seismicity occurred in the study peri- od and sometimes anomalies have been detect- ed from this network, there is no clear evidence of a one-to-one relation between magnetic a- nomalies and seismic events (see also Meloni et al., 1998). Some variations of moderate ampli- tude occur in correspondence with the Umbria- Marche in 1997 while no other significant ef- fects appear in the following years here report- ed up to 2001. The weak evidence of magnetic field changes during the observational period indicates that magnetic long term crustal origin effects do not frequently occur in this area. Fur- thermore, the weak to moderate energy seismic activity that occurred in this observation period Fig. 4. VLF 20 kHz and 40 kHz one minute data for part of the year 2001. 36 Antonio Meloni, Domenico Di Mauro, Stefania Lepidi, Giuliana Mele and Paolo Palangio (1989-2001) is probably one of the causes that did not allow us to find an evident correlation between local magnetic field variations and earthquake occurrence. VLF wave characteristics recorded at any station are mainly influenced by ionospheric pa- rameters. Among these, the ionospheric height and consequently the profile of electron density in the lower D layer are connected primarily to solar radiation. Of course many other possible external sources of ionization can also con- tribute (cosmic rays, whistler induced particle precipitation and other). An anomalous en- hancement of VLF waves amplitude in L’Aquila was reported in Meloni et al. (2001) for Sep- tember and October 1997. Since at that time no other ionospheric effects were detected at the ionospheric station of Rome (only about 100 km away) an external cause for the anomaly that should operate at a larger scale was reasonably excluded. The anomaly consisting in a signifi- cant increase in signal amplitude and daily range of the order of a factor three above the preceding quiet level, was instead observed in correspondence with the Umbria-Marche se- quence ca. 75 km from the L’Aquila Observato- ry lasting for about two months. For this reason, Meloni et al. (2001) considered an influence from the Umbria-Marche earthquakes possible. A qualitative explanation of such effect may be related to rock creeping that takes place in the earthquake focal area and the intensive strain rate in the surrounding crust, that can give a me- chanical background for the generation of elec- tromagnetic anomalous fields. As concerns VLF signals, since more evidence is necessary before final conclusions can be reached, a continuous monitoring was set up to have a better assess- ment in case other possible earthquake electro- magnetic related phenomena in this area were recorded. Figure 4 shows the time plots for 20 and 40 kHz by their 1 min values, here shown only for part of 2001 as an example. No other sinificant variations comparable to those record- ed in the 1997 events (see Meloni et al., 2001) were recorded and at the same time no other rel- evant earthquakes in the area were occurred. In conclusion, seismomagnetic and VLF da- ta gathered in recent years were used to obtain a better knowledge of the natural tectonomag- netic and electromagnetic phenomena observed in this region. Data were also analyzed to eval- uate if results could be used to search for possi- ble correlations to seismic activity. No evidence of time variations of crustal magnetic and earthquake occurrence was clearly found. 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