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Experimental study of a VAN network in the French Alps
Tectonophysics, 224 (1993) 51-81 Elsevier Science Publishers B.V., Amsterdam
51
Experimental study of a VAN network in the French Alps Christophe Maron, Gilles Baubron, Louis Herbreteau
and Bernard Massinon
Laboratoire de D&e&on et de Giophysique, Commissariat ci l’&ergie Atomique, B.P. I.?, 91680 BruyZres-le-Cha^tei, France
(Received June 20,199l;
revised version accepted October 10, 1991)
ABSTRACT Maron, C., Baubron, G., Herbreteau, L. and Massinon, B., 1993. Experimental study of a VAN network in the French Alps. In: P. Varotsos and 0. Kulhanek (Editors), Measurement and Theoretical Models of the Earth’s Electric Field Variations Related to Earthquakes. Teclonophysics, 224: 51-81. In 1989, a telluric and magnetotelluric network was installed in the northern part of the French Alps by the Laboratoire de Detection et de Geophysique (LDG). The purpose of this experiment was to check the applicability of the Greek VAN method to earthquake prediction in a different geotectonic and seismic context from the Greek one. The sites were selected according to several criteria for geology and potential noise sources. A complete station consists of six telluric non-polarisable dipoles of Petiau type, 2 magnetometers of Mosnier type and an acquisition unit run by solar energy. The interesting signals are not correlated with the geomagnetic field variations. Anthropogenic noise, electrochemical variations and atmospheric storms are not listed. Synthetic graphs sum up the seismic and telluric activities during 3 months and 2 weeks. For the correlation between earthquakes and potential telluric precursors, we consider first the multiple epicentres, then the near-field ones and finally the most significant events. Some encouraging results are presented. This experiment must be continued over a longer period and the network completed with low-frequency radio-electric receiving stations.
1. Introduction
2. Experimental
In 1989, the Laboratoire de Detection et de Geophysique (LDG) began installing a network of three telluric monitoring stations and two magnetotelluric monitoring stations in the French Alps. The purpose of these stations is to validate the VAN method of predicting earthquakes in a seismotectonic and geological context which is different from that of Greece. The LDG undertook to achieve a two-fold objective: (1) to develop a reliable signal measurement, acquisition and transmission system; and (2) to demonstrate correlations between telluric signals that were not induced by variations in the Earth’s magnetic field and earthquakes in the Alps. This empirical work has been carried out using the properties of the seismic electric signals (SES) developed by Greek researchers, as well as their criteria for the selection of sites for installation of the stations.
The five monitoring sites are located in the Rhone-Alpes Region (Is&e, D&me, Ardeche, Savoie and Haute-Savoie), which provides a network that covers the northern Alps and the middle of the Rhbne Valley. In the northeast, the region is crossed by heavily industrialized, deep valleys (Is&e, Maurienne and Drac) which form the boundaries to crystalline massifs (Oisans, Beaufort and Belledonne). In the southwest, the Rh8ne Valley is surrounded by limest.one massifs. The area monitored, the French Alps and part of the Swiss and Italian Alps, is crossed by two major contact zones: the first, running northeast to southwest, extends from the upper Rhone Valley in Switzerland down to Cevennes; the second, concave toward the east, connects Mont Blanc to the Tende Pass in the region inland from Nice (Fig. 1). The seismicity of the area exhibits significant
0040-1951/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved
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EXPERIMENTAL
STUDY
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Fig. 1. Simplified geological map of the Western Alps (from C. Kerckhove, 1979). A = Peri-Alpine domains: Al = TertiaryQuaternary; A2 = Jura-Provence; A3 = Primary basement. B = Evternai Zone: Bf = Secondary-Tertiary cover; BZ = Paraautochthonous and Ultra-Helvetian; 53 = Permian and/or Carboniferous; 84 = Crystalline basement. C = Internal Zones: CI = Valaisanne zones and Niesen; C2 = Sub-Brianqonnaise and median plastic zones; C3 = Briansonnaise zone: Trias-Eocene cover; &id median; C4 = Brianpnnaise zone: Permo-Carboniferous sole; C5 = Internal crystalline massifs; C6 = Pre-Piedmontese zone and breccia nappe; C7 = Piedmontese zone s.1. (+Ligurian sequences) and ophiohtes; C8 = Exotic flyschs. Upper Pre-Alps; C9 = Austro-Alpine and South-Alpine. The two major tectonic events are indicated by a thick line.
spatial variations. However, several, more active zones are apparent on the seismicity instrument survey map of the last 28 yr (Fig. 21. From north to south are: Sion in Switzerland: the valley of Abondance, Vercors and Guillestre in France; Mount Viso and CunCo in Italy. France also has a number of historically documented hazardous seismic zones. In addition to those already mentioned, we may also note the northern region of the Mont Blanc massif, the Bauges and Belledonne massifs, the region around Nice, and the lower Durance Valley. Results obtained in Greece indicate that a station should preferably be sited on high-resistivity (crystalline) rocks along major faults. Since the magnitudes of earthquakes in the region are generally small, it is preferable to select sites near active zones. In addition, the sites must be far from large urban and industrial centres to enable detection of relatively weak SES. Practical criteria, such as the accessibility and proximity of a telephone line should also be considered. The purpose of the survey phase was to select sites that best meet a maximum of the above criteria, allowing for the constraints imposed by the region investigated. The various phases in selecting the five monitoring sites are described below. 3. Site seiection The site selection process included three phases: preliminary selection on maps, preliminary survey and instrument survey. In 1987 an initiation phase for magnetotelluric equipment and signal monitoring was carried out in the region around Nice. 3.1 1987: the initial phase From April to July, 1987, three monitoring stations were installed in the region inland from Nice, near seismic stations of the LDG and in the vicinity of the most active regions of the Alps, Mount Viso and Cun6o. The stations formed a miniature network intended for testing the equipment (three portable magnetotelluric stations) and for gaming initial experience with recording magnetotelluric signals. Numerous telluric signals
not correlated with the variations in the horizontal components of the local magnetic field were recorded. Eight of these signals, called detected telluric signals (DTS) exhibited the SES characteristics. At the same time. 78 seismic events. 14 of magnitude exceeding 3. were recorded and located within a IS&km radius of the network. This experiment was considered to be successful, with respect to the background noise quality and quantity of detected signals, over the short period considered. This conclusion encouraged the LDG to carry out a new experiment over a longer duration. 3.2 Preliminary selection on maps To simplify the preliminary survey, the regions for the installation of stations were pre-selected using geological and topological maps on the basis of: (1) Seismological criterion. Given the small magnitudes of the seismic events in the Alps, it was important to install the monitoring stations near active seismic zones. (2) Geological criterion. Since the quality and amplitude of the telluric signals increase with the resistivity of the subsurface rocks, it was expedient to site the stations on crystalline rocks. Furthermore, compact soil was sought out to provide stable foundations for the magnetometers. Following the advice of the VAN team at this time, the stations were sited along the two major contact zones which pass through the Alps. (3) Human criterion. The stations should be sited away from large industrial centres, which generate interference in the form of signals similar to the SES. These noise sources are mainly concentrated in the valleys. 3.3 Preliminary suruey (June 26-30,
1989)
During this week the regions selected on maps were surveyed to choose one or more candidate sites for instrument survey. Selection criteria included the size of the available land, accessibility and proximity of a telephone line: (1) Land surface: The telluric dipoles must be at least 100 m long in the N-S and E-W direc-
EXPERIMENTAL
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tions and be placed on the flattest possible surfaces. To avoid disturbing farmers as much as possible, the equipment was installed on the boundary of land plots. (2) Accessibility: The land must be accessible in all seasons to enable installation and quick repair of the station. (3) Proximity of a telephone line: Since it should be possible to interrogate the finally selected stations via telephone, passage close to a telephone line is preferable to permit rapid installation. 3.4 In~t~~ent
55
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.suruey
This survey was performed to assess the telluric background noise, which was not permitted to exceed 0.1 mV/lOO m. The background had to be free of anthropogenic noise; this noise is not correlated with the variations in the Earth’s magnetic field, is more intense during the day than at night and can be reproduced at fiied times from one day to another (e.g., electric trains). The instrument survey was conducted in three phases: (a) Isbre and D&me (July 17-August 3,1989); (b) Savoie and Haute-Savoie (September 7October 13, 1989); (c) Ardeche (November 14-28,1989, and April 2-11, 1990) f&-e (Site No. 1): The site selected was at Chichilianne (CHIC), at the foot of the limestone cliffs of the Vercors, 30 km from a zone that was active in 1962. The subsoil is a thick limestone alluvial cone. The background noise measured is very low (0.05 mV/lOO m). D&me (Site No. 2): The station is sited in the community of Le Pegue (PEGU) 25 km east of the RhBne Valley, which explains the presence of signals generated by the passage of trains. The noise level is nevertheless low (0.1 mV/lOO m). Satioie (Site No. 3,): The first site selected was IX Bersend (BERS) on a granite outcrop of one of the alpine tectonic faults. The background noise of the recorded signals was too high (10 mV/lOO m). An alternate terrain was found on a ridge at an altitude of 1800 m in contact with crystalline Iithologies (orthogneiss and granite) and sedimentary lithologies (carbon, gypsum,
dolomite and cellular dolomite, limestone and shale). This site (Sur F&e, SURF) has a background noise of between 0.5 and 1 mV/lOO m. caste-Sauoie (Site No. 4): Located near an active epicentre, the valley of Abondance, the site of Trtcout (TREC) lies on the northern boundary of the Chablais rock sheets 14 km south of Thonon-les-Bains. The background noise ranges from 0.15 to 0.30 mV/lOO m. Ardkhe (Site No. 5): Two phases were required for selection of a final site: (a) Berzeme (BERZ) Although background noise was not especially high (around 0.4 mV/lOO m), this site was not selected because of numerous interfering signals generated there by the passage of trains in the Rh8ne Valley 14 km to the east. (b) Sablieres (SABL) Located further to the west in the first Massif Central foothills, this land is situated on a shaley crest 45 km west of the Rhbne Valley, slightly north of the Cevennes fault. Due to the high resistivity of the soil, the background noise in a 24-h period is very high (about 10 mV/lOO m). The position of the stations is shown on the seismicity instrument survey map established during the last 28 years (Fig. 2). 4. The equipment The equipment used in the monitoring stations evolved throughout the phases described above. The magnetometers and electrodes are the same, but the signal acquisition equipment has been improved. 4.1 Magnetometers
Variations in the horizontal component of the local magnetic field are measured using two magnetometers: the D meter and the H meter, which record the variations in the E-W- and N-Sorientated components, respectively. One group of magnetometers will be installed at Chichilianne and another will be installed at Trecout. The magnetometers were made by the Applied Geophysics Laboratory of the French National Scientific Research Centre (Mosnier and Yvetot,
56
1977) at Orleans. Their operating principle is as follows: a magnet is hung by a wire between four vertical conducting plates placed at each of its end sections. The four plates are connected two by two diagonally to form two air capacitors. Their charges are equal if the magnet is parallel to the plates; they vary in phase opposition when the magnet pivots in response to the effect of a variation in the local magnetic field. Equilibrium is restored by a current generated through a
Helmholtz coil, which is mounted perpendicular to the centre line of the magnet. The sensitivity of the device is 0.02 nTeslas. 4.2 Electrodes The electrodes employed are non-polarizable Pb/PbCl, electrodes of the Petiau type, which are 63 cm long and 2.5 cm in diameter. They are made of lead bars coated with a mixture of plas-
Fig. 3. Plan of the VAN station of Chichilianne.
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ter, NaCl and PbCl,. Each electrode is completely encapsulated in plastic; only the end section is active. During the inst~ment survey, the N-S and E-W orientated dipoles were 100 m long. During the final installation the electrodes were buried at a depth of 1 m and the dipoles had a variety of azimuth angles and lengths.
signal acquisition equipment have led to shorter data reception times (from 2 months to a few hours), collection of more significant data (more dipoles, more refined sampling) and greater autonomy (from 3.5 d to automatic control).
4.3 Signal ~cqu~it~~n
5.1 Storage
During the survey a portable, leak-tight box was used, with four signal acquisition channels (2 telluric signals with gains ranging from 20 to 1000, 2 magnetic signals with gain set at 10). The unit was powered by a 12-V battery. Sampling was performed at a rate of 1 sample every 12 s. Data were initially recorded on digital cassettes which could contain the data for 3.5 d. The cassettes were replaced by a local representative and sent to LDG, where they were read, demultiplexed and stored on tape. Since November 13th, 1989 the data storage medium employed has been a memory card with a capacity of 4 d, which was later extended to 8 d. After being replaced, the card was transferred directly to LDG. On March 9th, 1990, the Chichilianne station was equipped with longer telluric dipoles and three solar panels which charge three 12 V batteries, each used to supply the electronic equipment in parallel. These two phases were carried out at Le Pbgue on April 14th and March 6th, 1990. The boxes at these two stations were outfitted for daily telephone interrogation from the LDG on May 18th and June 12th, respectively. Sampling was carried out at a rate of 1 sample every 10 s and the signal acquisition time was several hours. For the present phase, a more complete and reliable electronic system was installed at Chichilianne on July 31st, 1990. A total of 12 channels can be recorded and transmitted automatically every night via telephone. We thus have two magnetometers and six telluric dipoles in the Is&e department: two N-S orientated (181 and 105 m), two E-W (130 and 70 m> and two with an azimuth angle of 345” (109 and 55 m>. A drawing of the present station at Chichilianne is given in Figure 3. It can be seen that improvements in the
The signals received via telephone on a microcomputer are processed in two phases. This processing is currently performed on Stm workstations, but it may be carried out on a microcomputer.
5. The data
5.1.1 Data uerij‘kation
The recordings are in the form of 5 min, multiplexed data blocks comprising 30 lines of data and 1 line of introduction. Each type of line starts with a specific identification code. Superfluous zeros are eliminated and the data are separated by commas. The introductory line includes, in addition to its identification code, the day number, the time of the first datum in the block and the voltage at the terminals of three batteries. The arrangement of the data may sometimes be upset at the local level. The most frequent abnormalities are: (1) one or several blocks partly or completely duplicated; (2) no block; (3) lines incomplete or too long. Alterations are made manually after a program lists the errors. 51.2 Demultiphing
and creation of new files
After alteration, if necessary, the received data are demultiplexed and stored in a file consisting of as many blocks as there are channels. A block starts with an introduction including the year, the day number, the hour, the minute and the second of the start of the file, the number of points, the station code and the channel. This is followed by the gain, the dipole length, the data proper and the end-of-block identification. These files, which may be of any length and start at any time, are
BW
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-----7
D.T.S./seisms
g--
._I
__.__
1st
1.
The distribution
in 1990,
Figs. 4-7.
distribution
I I
_..
from the 3rd quarter
D.T.S./seisms
,,g.._...
of DTS and earthquakes
quarter
“.
-.
of 1989 to the 2nd quarter
distribution
of 1YW.
in 1990,
I
2nd
gIJa+r
7
z 5 1
z g
EXPERIMENTAL
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1 mV NS CHIC
0
I
1
EW
L1 mV
NS
--I
--
LE P
0
EW
L1 mV
CHIC CHIC CHIC CHIC
D.T.S.: 27 July 09 27 July 13 29 July 15 30 July 08
1 2 3 4
Seisms: EW EW EW EW
00 15 00 30
-8.;::
1 24 July 00 18 2.7 Pinerolo 2 27 July 18 21 2.6 S.E. Cunec
0'350 _o.150 .
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f Ll mV D.T.S.: %:: CHIC CHIC CHIC CHIC CHIC
2 1 3 4 5 6 7
04 01 04 05 05 06 07
Augu Augu Augu Augu Augu Augu
Seisms
01 16 18 03 15 07 06
30 50 00 30 00 00 54
1 2 3 4 5 6
EW EW EW EW NS EW
06 07 08 14 14
:
Augu 04 38 3.5 05 03 2. z%%lnche Augu 04 25 3.1 E. Turin Augu 03 18 2.8 Izoard Augu 03 04 2.8 Serre-Poncon Augu 03 26 2.6 Serre-Poncon
r”=2
4
NS CHIC
0
I
EW
1 mv
Seisms:
D.T.S.: CHIC
1 27 Augu
08 45 EW
0.200
:;'I:
2 3 31 Augu
19 09 30 12 NS
8%: .
Figs. 8-30. Correlation
1 16 Augu 2 19 Augu 3 20 Augu
19 09 2.2 Ser~~;;~~;o" 00 20 2.9 N. 17 29 2.5 N.E. Cuneo
of DTS and seisms from the 2nd half of July 1989 to the 2nd half of June 1990. The DTS and the seisms are chronologically
and separately
numbered
from 1 on. Figs. 8-10.
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-
-
i
:
I
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.
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adjusted in relation to each other for each station to form 24 h files, starting at 090 UT. The signals are plotted, after adjustment for gain and dipole length, in groups of no more than
four channels, to a scale (in millivolts/100 m) that can be determined automatically or set by the user. The time scale and the plot length can be varied up to 1 cm/h for 24 h. It is also
L
SISMO
,
NS
0
CHIC EW
'
11 mV
1 mV NS
0
SURF EW
t 1 mV 1 mV NS
TREC
0
I
I 1
EW
t1 mV Seisms:
D.T.S.: CHIC SURF SURF TREC TREC
1 1 2 1 2
01 07 07 05 05
Otto Otto Otto Otto Otto
13 09 09 04 04
03 30 30 20 20
EW NS EW NS EW
1 2 3 4 5 6 7 8
-5%8 3:ooo 0.700 -0.250
01 02 04 05 05 05 06 10
Octc Octc Otto Otto Otto Otto Otto Otto
00 03 19 11 11 12 00 17
39 12 28 39 41 32 36 29
2.1 2.8 2.1 2.0 2.2 3.1 1.8 2.7
Chambeyron Chambeyr,Dn Dronero Mt Visa Mt Visa Les Gets Chambeyran Rumilly
1 mV J
NS CHIC
3
=
I
I
-
‘
18
EW
--+-
0
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Seisms:
D.T.S.: CHIC CHIC CHIC CHIC CHIC CHIC CHIC
1 2 3 4 5 6 7
19 19 19 19 22 22 25
Otto Otto Otto Otto Otto Otto Otto
13 13 20 20 09 09 03
00 00 00 00 30 30 00
1 2 3 4 5 6 7 8 9 10 11 12
NS EW NS EW NS EW NS
00 EW 9 27 25 Otto 03 09 20 gkI$E 8 CHIC 10 31 Otto 11 00 EW
-$=258 . Figs.13 and 14.
16 18 22 26 26 30 30 30 30 30 30 31
Otto Otto Otto Otto Otto Otto Otto Otto Otto Otto Otto Otto
04 06 07 02 04 05 06 08 11 13 13 03
34 46 17 40 44 32 06 06 24 09 44 19
2.7 2.6 2.5 2.2 2.8 2.2 3.0 2.7 3.7 2.5 3.0 2.8
1 mV
possible to plot the variations in voltage at the battery terminals in the 12-14 V range at a rate of 1 point every 5 min. 5.2 T@S
of the Earth’s magnetic field are extracted visually from the recordings. These signals are: (1) anthropogenic noise; (2) electrochemical variations around the electrodes; (3) atmospheric storms; (4) electronic offsets: at a certain period a small variation in battery voltage generated a
of s&aals
The telluric signals that cannot be correlated with the variations in the horizontal components
6
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mv
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Selama
D.T.S.:
:
Slon
1 mv 0 1 mV Seiame
Figs. 15 and 16.
:
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square-wave signal in the telluric signal acquisition channels. This phenomenon was particularly apparent in the morning and evening at the beginning and end of charging of the solar panels. (5) the detected telluric signals (DTS), which are of interest to us. The DTS are characterized by all of the channels in which they appear and,
for each one, by polarity, volts/100 m) and shape.
amplitude
(in milli-
5.3 Correlation graphs The sequence of DTS and earthquakes are displayed on graphs constructed in the following
8
-_-L-I M=.!
SISMO
'T
NS
/
CHIC
0
1
EW
-t
1 llv
mv
rl NS 0
BERZ EW
t 1 mV D.T.S.: CHIC CHIC
Seisms :
1 02 Dece 10 50 EW 2 06 Dece 09 06 NS
0.100 -0.400
1 2 3 4 5 6 7 8
01 02 02 03 04 12 13 15
Dece Dece Dece Dece Dece Dece Dece Dece
19 00 08 20 20 13 08 03
31 59 56 13 42 39 08 28
3.0 2.6 3.1 2.4 2.2 2.3 3.1 2.7
Serre-Poncon Neuchatel Mt Visa Sierre Cuneo Valdieri N.E. Guillestre Mte Nobio
r1 NS
!
EW
I
CHIC
mV
L1 mV 1 mV NS
BERZ EW 1 mV Seisms:
D.T.S.: CHIC CHIC CHIC CHIC
1 2 3 4
28 28 30 30
Dece Dece Dece Dece
13 13 13 13
48 48 42 42
NS EW NS EW
0.350 -ip; -0:oso
Figs.17and 18.
-1 mv NS -C
CHIC EW
-1 mv
-1 mV NS
L
LE P
EW
-0
-1 mV
NS
BERZ Et? I
L1
mV
Seisms:
D.T.S.:
4 14 Janu 03 34 2.
mV NS CHIC EW
mV mV NS
LE P EW mV mV NS
BERZ EW mV
D.T.S.: Cuneo
Bezre-Poncon onnsville . :y::our em00 en0 1. Figs.19and 20.
EXPERIMENTAL
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---d.-1 23,
65
5
NS
CHIC
EW
NS LEP
_ EW
1 nv NS
BERZ
0 EW
/ Ll mv D.T.S.:
Seisms: $01~;;
I,f,rche
St J. fiaurienne 1Okm S.W. Turin 1Okm S.W. Turin Abondance
NS 0
CHIC EW
NS LE
P EW
NS 0
BERZ EW
Seisms:
D.T.S.: ;z ;
NS 2 1 26 Febr 15 24 EW
LE P
3 27 Febr
17 10 NS
1 22 Febr
-0.250 0.500 0.500 Figs. 21 and 22.
05 26 2.4 W. Cuneo
NS CHIC
EW mV mV
i NS
3
I ,
LE P EW ,
5 mV mV
i
NS
BERZ
-0
EW mV D.T.S.: LE P
SISMO
--i_e l‘
Saisms:
1 01 Marc 0
II
I,
I,
>o mV
NS
CHIC
EW mV mV NS
-i-
LE P EW
mV mV NS
0
BERZ EW
mV D.T.S.: LE P
1 19 Marc
19 42 NS
Seisms: 0.300
Figs. 23 and 24.
67
EXPERIMENTALSTUDYOFAVAMNEIWORK
~1 mV NS -0
CHIC EW
1 mV
I
NS
LE P
0
EW
1 mV
1
mv
NS
BERZ
C
~ EW
1 mV D.T.S.:
Seisms : 1 2 3 4 2
02 05 05 05 $3
Apri Apri Apri ix:
7 15 Apri
05 15 18 23 $2 1
21 49 18 03 39 08
2.0 2.4 2.1 2.4 2.3 1.9
SOS el Mt eiso Mt Viso St Sauveur/Tine Mt St Thabor Laurent du V
07 50 4.2 5Okm E. Nice
1
Ll mV
3 1 mV
NS LE P EW
1 mV -1 mV NS BERZ
-0
EW
-1 mV D.T.S.:
Seisms:
Figs. 25 and 26,
1 2 3 4 5 7 6
19 19 20 21 21 26 21
05 50 2.2 Verdaches Apri 07 36 2.4 Cuneo Apri 15 18 2.0 N. Cuneo Apri 05 43 2.0 N.W. Cuneo Apri 21 11 2.7 25km Sospel Apri 22 01 09 56 2.2 2.1 33km Sian Sospel
8 9 10 11 12 13 14
27 27 27 28 28 29 29
Apri 01 49 2.4 N. Mt Rose 12 06 2.6 S.W. Berne Apri 23 22 2.6 Mf Vanoise Apri 12 23 2.4 N. San Remo Apri 22 24 2.9 Sierre Apri 12 47 2.2 Sierre Apri 16 50 2.5 S. Mt Visa
hX
manner: First, a horizontal time axis is drawn. Then one vertical bar for each signal is added. This bar is proportiona to the intensity or magnitude of the signal and includes a representation of the polarity (bold for a positive signal or an earthquake, thin for a negative signal). The earthquakes are indicated over the time axis. The DTS are indicated above the time axis if they are measured on the N-S channel or below for the E-W channel. A station malfunction is indicated by an interruption of the time axis. A line section parallel to this axis indicates that the data of the telIuric signal acquisition channel cannot be used. These graphs are constructed for periods of 3 months (37.5 mm for 15 d), which gives an overview of the seismic and telluric activities (Figs. 4 -71, and for periods of 15 d (10 mm for 24 h), which enables correlation of DTS and earthquakes (Figs. 8-30).
All the DTS and earthquakes which occur in the Alps between the Rh&ne Valley and the longitude 8”E, the Mediterranean coast and latitude 47”N (an area of about 60,000 km’) arc recorded. In addition, major earthquakes at a greater distance are indicated without their aftershocks; the earthquakes in the region of Nice are an example of these. The earthquakes are located on the basis of info~ation provided by the geophysicist who prepares the weekly report of earthquakes to the LDG. 6. Results 6.1 Comprehsiue
anafysis
From July l&h, 1989, to June 3Oth, 1990, there were 188 earthquakes originating from 74 different epicentres: 47 epicentres had only one period
mV
mV rl
NS
mV
.-0
LE P
EW
NS
SABL
-_-
r
-1
mV
rl
m-J
-0
EW -1
Fig. 27.
mV
EXPERIMENTAL
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,r
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1,
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NETWORK
19
69
m
’
>I
mV
NS CHIC EW
NS
LE P EW 1
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mV
mV NS
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1
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1
D.T.S.: LEP LE P SABL
116May 2 21 May 1 22 May
00 36EW 16 48 EW 05 30 NS
mv
Seisms:
w. seyne
I8.$$s
Co1 de Larche Puget-Theniers Argentera
161000
hi P ~~:era Authon Barre Ecrins Authon Authon S. Mt Visa Authon Fig. 28.
of activity and 27 had at least two, including a total of 142 earthquakes, including CunCo (22 events) and Mount Viso (18 events). This gives a seismicity that exhibits significant spatial variations, as shown in Figure 31. It can be seen that the earthquakes are located especially to the south and southeast of the LDG network. The magnitude distribution is given in Table 1. Only 17
events exceeded a magnitude of 3, which is considered by the VAN group to be a minimum for detection. These events are listed in ‘Table 2 and are shown in Figure 32. The three earthquakes at a distance of less than 100 km have small magnitudes (3.1). Five of these epicentres were involved only once (Les Gets, Thonon, Guillestre, Bussoleno and Abondance). During the same period,
70
113 DTS were recorded at all stations. Very few were present at several stations simultaneously. 6.2 Correlation of DTS and earthquakes The graphs prepared every quarter do not show any general correlation, except in the second half of June, when a significant increase in the number of DTS at Le Pegue was followed by a rise in seismic activity at the end of June. The same phenomenon was observed at Chichilianne in September 1989. The tests performed to establish a correlation between DTS and earthquakes are based on the principle proposed by the VAN researchers (Varotsos et al., 1984). Earthquakes with the same epicentre exceeding a given magnitude limit are preceded by DTS, which always appear on the
same group of channels with the same polarities. for each earthquake. However, investigations will also be made into the earthquakes which occurred near some of our stations, as well as for the most violent earthquakes. 6.3 Multiple epicentres There are 27 multiple epicentres, which are located and listed in decreasing order for the number of events and in increasing order for the distance from Chichilianne in Appendix I. Several of these epicentres are worthy of note: (1) Co1 de Vars. Six events took place within 3 d and were preceded by six positive DTS on the N-S channel from September 6th to 9th. The delay time was from 16 to 17 d (Figs. 33-36). (2) Chambeyron. From September 27th to Ck-
ij
CHIC
i
i
NSS
mV
EW
1 T_f
:
1 mV mV
NS LEP
EW mV
mV NS SABL
-
-
.-0
EW mV
D.T.S.:
Fig. 29.
EXPERIMENTAL
STUDY
OF A VAN
71
NEWORK
1990, occurred while the stations were inoperative. The one that occurred on June 7th, 1990, was not preceded by any DTS with the same characteristics. (4) Mount Viso. A total of eighteen events occurred on Mount Viso: eight before the Le Pegue station was installed and ten when it was operating together with Chichilianne. Of these ten events, four occurred when the stations were inoperative (those of January 25th, April 5th and April 29th). Of the other six, four were preceded at Le Pegue on the E-W channel by negative DTS (earthquakes of May 31st, June 4th, June
tober 6th, four events were preceded by four negative DTS, 5-7 d before, on the E-W channel of Chichilianne (Figs. 37-40). The fifth earthquake occurred when none of the stations was operational. (3) Digne. A crisis north of Digne began on June 28th and six events took place before June 30th. They were preceded by an increase in the number of DTS at Le Pegue, 95 km to the west-southwest; there were simultaneous negative DTS on the N-S and E-W channels on June 19th, 23rd and 24th, that is, 7-9 d before the earthquakes (Fig. 41). The event of May 6th,
IJ
i,
Llr”=* II
'-0
-1 mV NS
I
CHIC
-0
EW
11 ’
1
mv
1 mv
7
3
NS
I
LE P EW
I ‘
I
0
10
1 mv
1 mV NS SABL
--
--
--
f
0
EW 1 mv
Seisms:
D.T.S.: CHIC 1 LE P 1 LE P 2 LE P 3 LE LE P P 4 5 LE P 6 LE P 7 LE P 8 LE P 9 LE P 11 10 SABL
20 17 18 19 19 19 20 23 23 24 28 24
June June June June June June June June June June June
15 15 09 01 09 09 11 07 07 20 06 20
30 54 30 36 30 30 48 06 06 30 06 30
EW EW NS NS NS EW EW NS EW NS EW
1 22 June 04 33 EW
1 2 3 4 5 6 7 8 9 10 11 12 13 14
:;*gg; 0:250 -0.750 I!.;Z~ 18_:$;: 4:ooo Fig. 30.
17 23 24 25 27 27 28 29 29 29 29 30 30 30
June June June June June June June June June June June June June June
17 10 19 03 21 21 06 00 01 08 18 03 0 18
46 32 34 20 25 28 03 52 19 55 08 $8
2.2 2 3 2'0 217 2.3 2.7 2.3 1.9 3.1 2.8 2.4 3.8
Sisteron Bor o Mt $iso Suse Valberg
Valberg N. Digne S. Mt Visa N. Di ne N. Di t ne N. Di ne g.EDi ne uneo 37 211 N: bigne
TABLE 2 Events of
M > 3
Date Maptitude:
46 46
45
August 6 August 7 September 24 September 29 October 5
4
5
6
7
24th and June 29th). The DTS occurred on May 16th, May 21st, June 17th and June 28th, that is, 15-19 d before (Figs. 42-44). The two other earthquakes, including the one of magnitude 3.3, were not preceded by DTS with the same characteristics. It should be noted, however, that the operation of the station in the D&me was inconsistent during this period. Other correlations are feasible, but only for two earthquakes. This is because there were only
m
N
FkrceEtage
< 2.1 2.1-2.5 2.6-3.0 3.1-3.5 3.6-4.0 4.1-4.5 > 4.5
21 82 68 11 2 3 1
11.2 43.6 36.2 5.9 1.1 1.6 0.5
3.2
3.2 3.1 3.1 3.3 3.3 3.1
215 NE 16OE 95 ESE 145 ESE 170 NNE 20 SE (T&out) 185 NNE 120 ESE IOOE 130 ENE 120 ESE 90 SE
3.7 4.0
120 ESE I85 NNE
3.6 - 4.0
Mount Viso Abondance
4. I - 4.5
September 30
Sion
4.3
December 26 April 15
Nice East Nice
4.5 4.2
205 NNE 6 SE (Trkout) 200 SE 220 SE
Turin SW
4.8
160E
Magnitude
The magnitude distribution
Azimuth angle ’
8
Fig. 31. Seismic@ in southeast France. 16/07/89-30/X/90.
TABLE 1
M
3.1 3.1 3.1 3.1
Thonon Mount Viso Guillestre Bussoleno Mount Viso N. Digne
October 30 February 14 Magnitude
43
3.s Zermatt East Turin Vars Dronero Les Gets
November 26 December 2 December 13 January 20 June 4 June 29 Magnitude
44
Location 3.1-
>
February 11
4.6
’ Relative to Chichilianne station.
and to the nearest operational
two events at the same location or because, for the others, the VAN stations were not operational. The two earthquakes at Argentera on May 22nd, 1990, which were preceded at Le P&ue by two negative DTS on the E-W channel, belong to the first category (Fig. 42). The earthquakes of Mercantour and Sospel belong to the second category. Regarding the Mercantour earthquakes, the station in the DrGme was not installed on December 29th, 1989, but it was on January 18th and March 29th. Two positive DTS were recorded on the N-S channel, on January 12th and March 19th, that is, 6 and 10 d, respectively, before the earthquake (Fig. 45). As for -1, the stations of Chichifianne and Le P&gue were inoperative on October 18th, 1989, and on April 21st, 1990. However, the two earthquakes of February 5th and April 2nd were preceded at Le P&ue by two
EXPERIMENTAL
STUDY OF A VAN NETWORK
73
e Fig. 32. From top to bottom
and from left to right: location
of the seisms of magnitude
(
over 3.0, 3.5,4.0
and 4.5 (scale: 1 : 4 500000).
Figs. 33-36.
The DTS preceding
the seisms of September
1oQl c:2w +yp--
09lQ9/89
chid9 252
1989 at the Co1 de Vars.
-
0.25 BV/llOl
I
c:zoo
chid9 264 21109l89
1Oh
21/09/B9
chic*9 261
-
t I
I
Figs. 37-40. The DTS preceding the seisms of September-October
lo.25 nV/l~Om
I
I
i I
/
I
i
1989 at Chambeyron.
I
/
l
i
/
/
.,.,-L-J
Figs. 41-44.
- .- _._.,
”
The DTS preceding
the seisms of June-July 1990 at Digne (41). The DTS preceding the seims of May-June Argentera (42). The DTS preceding the seisms of May-June 1990 at the Mount
1990 at the Mount Viso (43 and 44).
Viso and the aeisms of Ma) 72. !99f? at
EXPERIMENTAL
STUDY
OF A VAN
NETWORK
Figs. 49-51.
1‘9 s:lW
_-
The DTS preceding the scism of October
26/89/0 l#E:lO
cUdI
lob
w9l89
mm
I
fl
-.--.
September
the seism of September 30, 1989 at Sion (5 1).
10. 1989 at Rumilly (49). The DTS preceding
-
30. 1989 at Sion (50)
The DTS prcccdinp
the WIWI
EXPERIMENTAL
STUDY
OF A VAN
79
NETWORK
positive DTS on the N-S channel on January 20th, 1990 (choice of two signals), and on March 19 (Figs. 45 and 46). Finally, the two earthquakes at Turin on February 11th were preceded by one negative DTS on each of the N-S and E-W channels of Le Pegue on January 27th, as was the earthquake of March 11th (DTS with same characteristics were recorded on March 1st) (Fig. 47). The other epicentres have not yielded results of this type. Several explanations may be given: (1) magnitudes often small relative to the epicentre distances; (2) stations not sensitive at the epicentres; (3) non-continuous signal acquisition.
TABLE 3 Array of the multiple epicentres (July 1989-June 1990) Epicentre, distance and azimuth from Chichilianne Cuneo 160”ESE
6.4 Near-field epicentres Results obtained in Greece show that, for a given magnitude and epicentre-dipole pair, the amplitude of the SES varies inversely with the distance, r, between the epicentre and the station. This means that it may be possible to detect significant telluric signals before earthquakes near a station in service. This was the case for the earthquake at Les Gets on October 5th CM = 3.11, 20 km southeast of the T&out station, which was in service at the time. It was preceded by a positive DTS on the N-S channel and a negative DTS on the E-W channel 8 h before the earthquake (Fig. 48). No similar signals were recorded at the Sur F&e station 50 km to the south at the same time. An earthquake at Rumilly on October lOth, 1989 (A4 = 2.7), 55 km northwest of Sur F&e, was preceded at this station by a positive DTS on the E-W and N-S channels on October 7th (Fig. 49). However, no signals were recorded at Trecout (60 km to the northeast) at the same time. These last results should be treated with caution because the epicentres mentioned were active only once. In addition, other epicentres near operating stations did not give the same results (Romans and Bourg d’oisans) and are not in conflict with the experiments conducted in Greece. 6.5 Significant events Significant events in the region are those with a magnitude of more than 3.5. The Sion earth-
Mount Viso 120”E
Sion-Sierre 205”NNE
ML
Date
2.6 2.5 2.8 2.4 2.8 2.2 2.2 1.7 1.9 2.4 3.0 2.8 2.4 2.7 2.6 2.4 2.0 2.0 2.4 2.5 2.4 2.2 2.5 2.0 2.2 3.7 3.0 2.4 2.2 3.1 2.4 2.4 2.1 2.5 2.9 2.6 3.3 2.5 2.0 1.9 4.3 2.7 2.9 2.7 2.9 2.4 3.0 3.0 2.9 2.2 3.0 2.8
July August September September October December December December December January January January February March March April April April May May May June September October October October November November November December January April April April May June June June June June September September November November November December March April April April June June
27 20 2 15 26 4 21 29 29 10 10 17 22 21 22 19 20 21 6 8 10 30 6 5 5 30 3 26 30 2 25 5 5 29 31 4 4 6 24 29 30 30 15 17 28 3 2 26 28 29 3 3
80 TABLE 3 (continued)
TABLE 3 (continued)
Epicentre, distance and azimuth from Chichilianne
ML
Digne WSE
1.9 2.7 2.3 3.1 2.8 2.4 1.8 2.1 2.1 2.0 2.1 2.5 2.3 2.2 2.2 2.8 2.4 2.3 2.4 2.6 2.5 2.4 2.8 2.6 2.2 3.0 2.3 2.5 2.5 2.8 3.1 2.2 2.3 2.9 2.2 3.0 2.7 2.5 3.0 2.8 2.6 2.1 2.8 1.8 2.2 2.6 2.0 2.0 2.7 2.1 2.6 2.5 2.7 3.0
Authon 8O”SSE
San Rem0 205”SE
Serre-Porqon WSE
Co1 de Vars 95”ESE
Montreux 205”NE
Chambeyron 105”ESE
Sospel 1WSE
Brianp 85”E
Date
May June June June June June June June May 26 May 26 May May May June June March April May May May May May August August August December January June September September September September September September October October October October October October September October October October June October February April April April November December January March
Epicentre, distance and azimuth from Chichilianne 6 7 28 29 29 29 30 30
Turin 16O”E
Parpaillon WESE Allos
105”SE 28 29 31 1 7 20 28 8 8 10 13 14 14 14 16 1 17 2 23 24 24 24 25 25 30 30 30 30 30 31 27 1 2 6 6 18 5 2 21 21 5 30 14 4
Valberg 130”SE Mercantour 14U’ESE Mount Pelat 1lOOSE Col de Larche 1lU’ESE Suse 124WSE Dronero 145”ESE Argentera 1WSE Mount Ceppo 195”SE Nice 2O@%E 2OO’SE Zermatt 215”NE Neuchatel WNNE
Ml
Date
3.1
August February February March March March June December March June December June June December January March
4.8 2.1 3.0 2.1 2.7 2.0 2.1 2.2 2.1 2.9 2.3 2.7 2.3 2.6 2.2 2.9 2.1 2.1 2.2 2.4 2.1 3.1 2.1 2.3 1.8 2.5 2.6 4.5 4.2 2.1 2.5 3.2 2.9 2.5 2.6
May May February May March June September October May May June June December April November November August August November December
I
II II II
24 26 8 18 6 12 16 27 27 29 18 29 7 7 3 20 31 25 29 4 22 22 10 10 26 15 6 6 6 19 30 2
The multiple epkcntres are listed in decreasing order of the number of events and in increasing order of the dktance from Chichilianne.
quake on September 3Oth, 1989 was preceded at T&out by a negative DTS on the N-S channel on September 24th and a positive DTS on the two channels at Sur F&e at the same time (Figs. 50 and 51). Other earthquakes occurred at Sion and Sierre, while none of the stations in Savoie were in service. The Turin earthquake of February llth, 1990, followed by an aftershock, was preceded at Le Pbgue by a negative DTS on both channels. An earthquake at the same location on March llth,
EXPERIMENTAL
STUDY
OF A VAN
81
NETWORK
1990, was preceded by a DTS with the same characteristics. No similar signals were recorded for the other epicentres. Nice is at least 200 km from the LDG stations and the Mount Viso earthquake of November 3rd, 1989, occurred while the Le Pegue station was not in service.
magnitude 04,) of about 5. This has encouraged us to complete the VAN network in the Alps with several low-frequency radioelectric receiving stations to advance further the research being conducted on the prediction of earthquakes.
7. Conclusion
Kerckhove, C., 1979. Notice explicative de la feuille Gap a 1:250.000. Bureau de Recherches Geologiques et Mini&es. Kinoshita, M., Uyeshima, M. and Uyeda, S., 1989. Earthquake prediction research by means of telluric potential monitoring. Progress Rep. 1: installation of monitoring network. Bull. Earthquake Res. Inst. Univ. Tokyo, 64: 255-311. Meyer, K., Varotsos, P., Alexopoulos, K. and Nomicos, K., 1985. Efficiency test of earthquake prediction around Thessaloniki from electrotelluric precursors. Tectonophysics, 120: 153-161. Mosnier, J. and Yvetot, P., 1977. Nouveau type de variometres horizontaux a asservissement de champ et capteur capacitif. Ann. Geophys., 33 (3): 391-396. Petiau, G. and Dupes, A., 1980. Noise, temperature coefficiency and long time stability of electrodes for telluric observation. Geophys. Prospect., 28: 792. Uyeshima, M., Kinoshita, M., Iino, H. and Uyeda, S., in press. Earthquake prediction research by means of telluric potential monitoring. Progress Rep. 2: preliminary study on Teshikaga channel 2 signals and the sismicity in the off Kushiro region. Bull. Earthquake Res. Inst. Univ. Tokyo. Varotsos, P., Alexopoulos, K. and Nomicos, K., 1984. Physical properties of the variations of the electric field of the earth preceding earthquakes. I. Tectonophysics, 110: 7398. Varotsos, P., Alexopoulos, K. and Nom&s, K ,1984. Physical properties of the variations of the electric field of the earth preceding earthquakes, II. Determination of epicenter and magnitude. Tectonophysics, 110: 93-125. Varotsos, P., Alexopoulos, K. and Nomicos, K., 1987. Physical properties of the variations of the electric field of the earth preceding earthquakes. III. Tectonophysics, 136: 335-339.
The first year of experiments conducted using the VAN method made it possible to design a reliable signal recording and acquisition system. In addition, the selection of installation sites using stringent criteria enabled measurement of signals with characteristics which seem similar to SES for four epicentres which were active on several occasions (Co1 de Vars, Chambeyron, Digne and Mount Visa). However, each earthquake was not always preceded by one telluric signal. The data were interpreted placing emphasis on making the transition to routine phases of file verification, data storage and signal plotting. In addition, a program for the automatic processing of telluric data (and the extraction of the component induced by magnetic field variations) is under preparation. Efforts should be continued for a longer period (2 yr) before any first conclusion. The station in Savoie should be installed and all stations should be provided with the new electronic units. Radioelectric interference has now been detected in California, several days before the Loma Prieta earthquake (October l&h, 1989, M, = 6.6). Similar results were obtained in Japan about 100 km southwest of Tokyo for earthquakes with a
References
51
Experimental study of a VAN network in the French Alps Christophe Maron, Gilles Baubron, Louis Herbreteau
and Bernard Massinon
Laboratoire de D&e&on et de Giophysique, Commissariat ci l’&ergie Atomique, B.P. I.?, 91680 BruyZres-le-Cha^tei, France
(Received June 20,199l;
revised version accepted October 10, 1991)
ABSTRACT Maron, C., Baubron, G., Herbreteau, L. and Massinon, B., 1993. Experimental study of a VAN network in the French Alps. In: P. Varotsos and 0. Kulhanek (Editors), Measurement and Theoretical Models of the Earth’s Electric Field Variations Related to Earthquakes. Teclonophysics, 224: 51-81. In 1989, a telluric and magnetotelluric network was installed in the northern part of the French Alps by the Laboratoire de Detection et de Geophysique (LDG). The purpose of this experiment was to check the applicability of the Greek VAN method to earthquake prediction in a different geotectonic and seismic context from the Greek one. The sites were selected according to several criteria for geology and potential noise sources. A complete station consists of six telluric non-polarisable dipoles of Petiau type, 2 magnetometers of Mosnier type and an acquisition unit run by solar energy. The interesting signals are not correlated with the geomagnetic field variations. Anthropogenic noise, electrochemical variations and atmospheric storms are not listed. Synthetic graphs sum up the seismic and telluric activities during 3 months and 2 weeks. For the correlation between earthquakes and potential telluric precursors, we consider first the multiple epicentres, then the near-field ones and finally the most significant events. Some encouraging results are presented. This experiment must be continued over a longer period and the network completed with low-frequency radio-electric receiving stations.
1. Introduction
2. Experimental
In 1989, the Laboratoire de Detection et de Geophysique (LDG) began installing a network of three telluric monitoring stations and two magnetotelluric monitoring stations in the French Alps. The purpose of these stations is to validate the VAN method of predicting earthquakes in a seismotectonic and geological context which is different from that of Greece. The LDG undertook to achieve a two-fold objective: (1) to develop a reliable signal measurement, acquisition and transmission system; and (2) to demonstrate correlations between telluric signals that were not induced by variations in the Earth’s magnetic field and earthquakes in the Alps. This empirical work has been carried out using the properties of the seismic electric signals (SES) developed by Greek researchers, as well as their criteria for the selection of sites for installation of the stations.
The five monitoring sites are located in the Rhone-Alpes Region (Is&e, D&me, Ardeche, Savoie and Haute-Savoie), which provides a network that covers the northern Alps and the middle of the Rhbne Valley. In the northeast, the region is crossed by heavily industrialized, deep valleys (Is&e, Maurienne and Drac) which form the boundaries to crystalline massifs (Oisans, Beaufort and Belledonne). In the southwest, the Rh8ne Valley is surrounded by limest.one massifs. The area monitored, the French Alps and part of the Swiss and Italian Alps, is crossed by two major contact zones: the first, running northeast to southwest, extends from the upper Rhone Valley in Switzerland down to Cevennes; the second, concave toward the east, connects Mont Blanc to the Tende Pass in the region inland from Nice (Fig. 1). The seismicity of the area exhibits significant
0040-1951/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved
background
++ +++ +++ *++ *:+*
DAUPHINt
1
77JA3
jTJ-JJc9
_-.._---
CK19
EXPERIMENTAL
STUDY
OF A VAN
NETWORK
4
5
6
7
8
46
0 0 8 . I
ml ml ml ml ml
> < < < <
5.0 5.0 4.5 4.0 3.5 4
5
6
7
8
Fig. 1. Simplified geological map of the Western Alps (from C. Kerckhove, 1979). A = Peri-Alpine domains: Al = TertiaryQuaternary; A2 = Jura-Provence; A3 = Primary basement. B = Evternai Zone: Bf = Secondary-Tertiary cover; BZ = Paraautochthonous and Ultra-Helvetian; 53 = Permian and/or Carboniferous; 84 = Crystalline basement. C = Internal Zones: CI = Valaisanne zones and Niesen; C2 = Sub-Brianqonnaise and median plastic zones; C3 = Briansonnaise zone: Trias-Eocene cover; &id median; C4 = Brianpnnaise zone: Permo-Carboniferous sole; C5 = Internal crystalline massifs; C6 = Pre-Piedmontese zone and breccia nappe; C7 = Piedmontese zone s.1. (+Ligurian sequences) and ophiohtes; C8 = Exotic flyschs. Upper Pre-Alps; C9 = Austro-Alpine and South-Alpine. The two major tectonic events are indicated by a thick line.
spatial variations. However, several, more active zones are apparent on the seismicity instrument survey map of the last 28 yr (Fig. 21. From north to south are: Sion in Switzerland: the valley of Abondance, Vercors and Guillestre in France; Mount Viso and CunCo in Italy. France also has a number of historically documented hazardous seismic zones. In addition to those already mentioned, we may also note the northern region of the Mont Blanc massif, the Bauges and Belledonne massifs, the region around Nice, and the lower Durance Valley. Results obtained in Greece indicate that a station should preferably be sited on high-resistivity (crystalline) rocks along major faults. Since the magnitudes of earthquakes in the region are generally small, it is preferable to select sites near active zones. In addition, the sites must be far from large urban and industrial centres to enable detection of relatively weak SES. Practical criteria, such as the accessibility and proximity of a telephone line should also be considered. The purpose of the survey phase was to select sites that best meet a maximum of the above criteria, allowing for the constraints imposed by the region investigated. The various phases in selecting the five monitoring sites are described below. 3. Site seiection The site selection process included three phases: preliminary selection on maps, preliminary survey and instrument survey. In 1987 an initiation phase for magnetotelluric equipment and signal monitoring was carried out in the region around Nice. 3.1 1987: the initial phase From April to July, 1987, three monitoring stations were installed in the region inland from Nice, near seismic stations of the LDG and in the vicinity of the most active regions of the Alps, Mount Viso and Cun6o. The stations formed a miniature network intended for testing the equipment (three portable magnetotelluric stations) and for gaming initial experience with recording magnetotelluric signals. Numerous telluric signals
not correlated with the variations in the horizontal components of the local magnetic field were recorded. Eight of these signals, called detected telluric signals (DTS) exhibited the SES characteristics. At the same time. 78 seismic events. 14 of magnitude exceeding 3. were recorded and located within a IS&km radius of the network. This experiment was considered to be successful, with respect to the background noise quality and quantity of detected signals, over the short period considered. This conclusion encouraged the LDG to carry out a new experiment over a longer duration. 3.2 Preliminary selection on maps To simplify the preliminary survey, the regions for the installation of stations were pre-selected using geological and topological maps on the basis of: (1) Seismological criterion. Given the small magnitudes of the seismic events in the Alps, it was important to install the monitoring stations near active seismic zones. (2) Geological criterion. Since the quality and amplitude of the telluric signals increase with the resistivity of the subsurface rocks, it was expedient to site the stations on crystalline rocks. Furthermore, compact soil was sought out to provide stable foundations for the magnetometers. Following the advice of the VAN team at this time, the stations were sited along the two major contact zones which pass through the Alps. (3) Human criterion. The stations should be sited away from large industrial centres, which generate interference in the form of signals similar to the SES. These noise sources are mainly concentrated in the valleys. 3.3 Preliminary suruey (June 26-30,
1989)
During this week the regions selected on maps were surveyed to choose one or more candidate sites for instrument survey. Selection criteria included the size of the available land, accessibility and proximity of a telephone line: (1) Land surface: The telluric dipoles must be at least 100 m long in the N-S and E-W direc-
EXPERIMENTAL
STUDY
OF A VAN
tions and be placed on the flattest possible surfaces. To avoid disturbing farmers as much as possible, the equipment was installed on the boundary of land plots. (2) Accessibility: The land must be accessible in all seasons to enable installation and quick repair of the station. (3) Proximity of a telephone line: Since it should be possible to interrogate the finally selected stations via telephone, passage close to a telephone line is preferable to permit rapid installation. 3.4 In~t~~ent
55
NETWORK
.suruey
This survey was performed to assess the telluric background noise, which was not permitted to exceed 0.1 mV/lOO m. The background had to be free of anthropogenic noise; this noise is not correlated with the variations in the Earth’s magnetic field, is more intense during the day than at night and can be reproduced at fiied times from one day to another (e.g., electric trains). The instrument survey was conducted in three phases: (a) Isbre and D&me (July 17-August 3,1989); (b) Savoie and Haute-Savoie (September 7October 13, 1989); (c) Ardeche (November 14-28,1989, and April 2-11, 1990) f&-e (Site No. 1): The site selected was at Chichilianne (CHIC), at the foot of the limestone cliffs of the Vercors, 30 km from a zone that was active in 1962. The subsoil is a thick limestone alluvial cone. The background noise measured is very low (0.05 mV/lOO m). D&me (Site No. 2): The station is sited in the community of Le Pegue (PEGU) 25 km east of the RhBne Valley, which explains the presence of signals generated by the passage of trains. The noise level is nevertheless low (0.1 mV/lOO m). Satioie (Site No. 3,): The first site selected was IX Bersend (BERS) on a granite outcrop of one of the alpine tectonic faults. The background noise of the recorded signals was too high (10 mV/lOO m). An alternate terrain was found on a ridge at an altitude of 1800 m in contact with crystalline Iithologies (orthogneiss and granite) and sedimentary lithologies (carbon, gypsum,
dolomite and cellular dolomite, limestone and shale). This site (Sur F&e, SURF) has a background noise of between 0.5 and 1 mV/lOO m. caste-Sauoie (Site No. 4): Located near an active epicentre, the valley of Abondance, the site of Trtcout (TREC) lies on the northern boundary of the Chablais rock sheets 14 km south of Thonon-les-Bains. The background noise ranges from 0.15 to 0.30 mV/lOO m. Ardkhe (Site No. 5): Two phases were required for selection of a final site: (a) Berzeme (BERZ) Although background noise was not especially high (around 0.4 mV/lOO m), this site was not selected because of numerous interfering signals generated there by the passage of trains in the Rh8ne Valley 14 km to the east. (b) Sablieres (SABL) Located further to the west in the first Massif Central foothills, this land is situated on a shaley crest 45 km west of the Rhbne Valley, slightly north of the Cevennes fault. Due to the high resistivity of the soil, the background noise in a 24-h period is very high (about 10 mV/lOO m). The position of the stations is shown on the seismicity instrument survey map established during the last 28 years (Fig. 2). 4. The equipment The equipment used in the monitoring stations evolved throughout the phases described above. The magnetometers and electrodes are the same, but the signal acquisition equipment has been improved. 4.1 Magnetometers
Variations in the horizontal component of the local magnetic field are measured using two magnetometers: the D meter and the H meter, which record the variations in the E-W- and N-Sorientated components, respectively. One group of magnetometers will be installed at Chichilianne and another will be installed at Trecout. The magnetometers were made by the Applied Geophysics Laboratory of the French National Scientific Research Centre (Mosnier and Yvetot,
56
1977) at Orleans. Their operating principle is as follows: a magnet is hung by a wire between four vertical conducting plates placed at each of its end sections. The four plates are connected two by two diagonally to form two air capacitors. Their charges are equal if the magnet is parallel to the plates; they vary in phase opposition when the magnet pivots in response to the effect of a variation in the local magnetic field. Equilibrium is restored by a current generated through a
Helmholtz coil, which is mounted perpendicular to the centre line of the magnet. The sensitivity of the device is 0.02 nTeslas. 4.2 Electrodes The electrodes employed are non-polarizable Pb/PbCl, electrodes of the Petiau type, which are 63 cm long and 2.5 cm in diameter. They are made of lead bars coated with a mixture of plas-
Fig. 3. Plan of the VAN station of Chichilianne.
EXPERIMENTAL
STUDY
OF A VAN
57
NETWORK
ter, NaCl and PbCl,. Each electrode is completely encapsulated in plastic; only the end section is active. During the inst~ment survey, the N-S and E-W orientated dipoles were 100 m long. During the final installation the electrodes were buried at a depth of 1 m and the dipoles had a variety of azimuth angles and lengths.
signal acquisition equipment have led to shorter data reception times (from 2 months to a few hours), collection of more significant data (more dipoles, more refined sampling) and greater autonomy (from 3.5 d to automatic control).
4.3 Signal ~cqu~it~~n
5.1 Storage
During the survey a portable, leak-tight box was used, with four signal acquisition channels (2 telluric signals with gains ranging from 20 to 1000, 2 magnetic signals with gain set at 10). The unit was powered by a 12-V battery. Sampling was performed at a rate of 1 sample every 12 s. Data were initially recorded on digital cassettes which could contain the data for 3.5 d. The cassettes were replaced by a local representative and sent to LDG, where they were read, demultiplexed and stored on tape. Since November 13th, 1989 the data storage medium employed has been a memory card with a capacity of 4 d, which was later extended to 8 d. After being replaced, the card was transferred directly to LDG. On March 9th, 1990, the Chichilianne station was equipped with longer telluric dipoles and three solar panels which charge three 12 V batteries, each used to supply the electronic equipment in parallel. These two phases were carried out at Le Pbgue on April 14th and March 6th, 1990. The boxes at these two stations were outfitted for daily telephone interrogation from the LDG on May 18th and June 12th, respectively. Sampling was carried out at a rate of 1 sample every 10 s and the signal acquisition time was several hours. For the present phase, a more complete and reliable electronic system was installed at Chichilianne on July 31st, 1990. A total of 12 channels can be recorded and transmitted automatically every night via telephone. We thus have two magnetometers and six telluric dipoles in the Is&e department: two N-S orientated (181 and 105 m), two E-W (130 and 70 m> and two with an azimuth angle of 345” (109 and 55 m>. A drawing of the present station at Chichilianne is given in Figure 3. It can be seen that improvements in the
The signals received via telephone on a microcomputer are processed in two phases. This processing is currently performed on Stm workstations, but it may be carried out on a microcomputer.
5. The data
5.1.1 Data uerij‘kation
The recordings are in the form of 5 min, multiplexed data blocks comprising 30 lines of data and 1 line of introduction. Each type of line starts with a specific identification code. Superfluous zeros are eliminated and the data are separated by commas. The introductory line includes, in addition to its identification code, the day number, the time of the first datum in the block and the voltage at the terminals of three batteries. The arrangement of the data may sometimes be upset at the local level. The most frequent abnormalities are: (1) one or several blocks partly or completely duplicated; (2) no block; (3) lines incomplete or too long. Alterations are made manually after a program lists the errors. 51.2 Demultiphing
and creation of new files
After alteration, if necessary, the received data are demultiplexed and stored in a file consisting of as many blocks as there are channels. A block starts with an introduction including the year, the day number, the hour, the minute and the second of the start of the file, the number of points, the station code and the channel. This is followed by the gain, the dipole length, the data proper and the end-of-block identification. These files, which may be of any length and start at any time, are
BW
-,-.-
-----7
D.T.S./seisms
g--
._I
__.__
1st
1.
The distribution
in 1990,
Figs. 4-7.
distribution
I I
_..
from the 3rd quarter
D.T.S./seisms
,,g.._...
of DTS and earthquakes
quarter
“.
-.
of 1989 to the 2nd quarter
distribution
of 1YW.
in 1990,
I
2nd
gIJa+r
7
z 5 1
z g
EXPERIMENTAL
STUDY
OF A VAN
59
NETWORK
1 mV NS CHIC
0
I
1
EW
L1 mV
NS
--I
--
LE P
0
EW
L1 mV
CHIC CHIC CHIC CHIC
D.T.S.: 27 July 09 27 July 13 29 July 15 30 July 08
1 2 3 4
Seisms: EW EW EW EW
00 15 00 30
-8.;::
1 24 July 00 18 2.7 Pinerolo 2 27 July 18 21 2.6 S.E. Cunec
0'350 _o.150 .
1
J
I
,
t
N=2
*
L,1
SISMO
1
,
5
6
7
I
I
10
I,
It
I,
1,
I,
0
r
1 mV
I
NS CHIC
-
I
I '
EW
>
t
0
I
,
1 mV NS LEP
C
EW
f Ll mV D.T.S.: %:: CHIC CHIC CHIC CHIC CHIC
2 1 3 4 5 6 7
04 01 04 05 05 06 07
Augu Augu Augu Augu Augu Augu
Seisms
01 16 18 03 15 07 06
30 50 00 30 00 00 54
1 2 3 4 5 6
EW EW EW EW NS EW
06 07 08 14 14
:
Augu 04 38 3.5 05 03 2. z%%lnche Augu 04 25 3.1 E. Turin Augu 03 18 2.8 Izoard Augu 03 04 2.8 Serre-Poncon Augu 03 26 2.6 Serre-Poncon
r”=2
4
NS CHIC
0
I
EW
1 mv
Seisms:
D.T.S.: CHIC
1 27 Augu
08 45 EW
0.200
:;'I:
2 3 31 Augu
19 09 30 12 NS
8%: .
Figs. 8-30. Correlation
1 16 Augu 2 19 Augu 3 20 Augu
19 09 2.2 Ser~~;;~~;o" 00 20 2.9 N. 17 29 2.5 N.E. Cuneo
of DTS and seisms from the 2nd half of July 1989 to the 2nd half of June 1990. The DTS and the seisms are chronologically
and separately
numbered
from 1 on. Figs. 8-10.
_^ -.
-
-
i
:
I
’ :
.
EXPERIMENTAL
STUDY
OF A VAN
61
NETWORK
adjusted in relation to each other for each station to form 24 h files, starting at 090 UT. The signals are plotted, after adjustment for gain and dipole length, in groups of no more than
four channels, to a scale (in millivolts/100 m) that can be determined automatically or set by the user. The time scale and the plot length can be varied up to 1 cm/h for 24 h. It is also
L
SISMO
,
NS
0
CHIC EW
'
11 mV
1 mV NS
0
SURF EW
t 1 mV 1 mV NS
TREC
0
I
I 1
EW
t1 mV Seisms:
D.T.S.: CHIC SURF SURF TREC TREC
1 1 2 1 2
01 07 07 05 05
Otto Otto Otto Otto Otto
13 09 09 04 04
03 30 30 20 20
EW NS EW NS EW
1 2 3 4 5 6 7 8
-5%8 3:ooo 0.700 -0.250
01 02 04 05 05 05 06 10
Octc Octc Otto Otto Otto Otto Otto Otto
00 03 19 11 11 12 00 17
39 12 28 39 41 32 36 29
2.1 2.8 2.1 2.0 2.2 3.1 1.8 2.7
Chambeyron Chambeyr,Dn Dronero Mt Visa Mt Visa Les Gets Chambeyran Rumilly
1 mV J
NS CHIC
3
=
I
I
-
‘
18
EW
--+-
0
-_IO -i
Seisms:
D.T.S.: CHIC CHIC CHIC CHIC CHIC CHIC CHIC
1 2 3 4 5 6 7
19 19 19 19 22 22 25
Otto Otto Otto Otto Otto Otto Otto
13 13 20 20 09 09 03
00 00 00 00 30 30 00
1 2 3 4 5 6 7 8 9 10 11 12
NS EW NS EW NS EW NS
00 EW 9 27 25 Otto 03 09 20 gkI$E 8 CHIC 10 31 Otto 11 00 EW
-$=258 . Figs.13 and 14.
16 18 22 26 26 30 30 30 30 30 30 31
Otto Otto Otto Otto Otto Otto Otto Otto Otto Otto Otto Otto
04 06 07 02 04 05 06 08 11 13 13 03
34 46 17 40 44 32 06 06 24 09 44 19
2.7 2.6 2.5 2.2 2.8 2.2 3.0 2.7 3.7 2.5 3.0 2.8
1 mV
possible to plot the variations in voltage at the battery terminals in the 12-14 V range at a rate of 1 point every 5 min. 5.2 T@S
of the Earth’s magnetic field are extracted visually from the recordings. These signals are: (1) anthropogenic noise; (2) electrochemical variations around the electrodes; (3) atmospheric storms; (4) electronic offsets: at a certain period a small variation in battery voltage generated a
of s&aals
The telluric signals that cannot be correlated with the variations in the horizontal components
6
L_I_... I
’ f
SUM0
NS CHIC
’
1
’ ~
*
I
M-2
f
,_L_
1
1 -0
’
EW
mv
-1 mV
NS BERZ EW
Selama
D.T.S.:
:
Slon
1 mv 0 1 mV Seiame
Figs. 15 and 16.
:
EXPERIMENTAL
STUDY
OF A VAN
63
NETWORK
square-wave signal in the telluric signal acquisition channels. This phenomenon was particularly apparent in the morning and evening at the beginning and end of charging of the solar panels. (5) the detected telluric signals (DTS), which are of interest to us. The DTS are characterized by all of the channels in which they appear and,
for each one, by polarity, volts/100 m) and shape.
amplitude
(in milli-
5.3 Correlation graphs The sequence of DTS and earthquakes are displayed on graphs constructed in the following
8
-_-L-I M=.!
SISMO
'T
NS
/
CHIC
0
1
EW
-t
1 llv
mv
rl NS 0
BERZ EW
t 1 mV D.T.S.: CHIC CHIC
Seisms :
1 02 Dece 10 50 EW 2 06 Dece 09 06 NS
0.100 -0.400
1 2 3 4 5 6 7 8
01 02 02 03 04 12 13 15
Dece Dece Dece Dece Dece Dece Dece Dece
19 00 08 20 20 13 08 03
31 59 56 13 42 39 08 28
3.0 2.6 3.1 2.4 2.2 2.3 3.1 2.7
Serre-Poncon Neuchatel Mt Visa Sierre Cuneo Valdieri N.E. Guillestre Mte Nobio
r1 NS
!
EW
I
CHIC
mV
L1 mV 1 mV NS
BERZ EW 1 mV Seisms:
D.T.S.: CHIC CHIC CHIC CHIC
1 2 3 4
28 28 30 30
Dece Dece Dece Dece
13 13 13 13
48 48 42 42
NS EW NS EW
0.350 -ip; -0:oso
Figs.17and 18.
-1 mv NS -C
CHIC EW
-1 mv
-1 mV NS
L
LE P
EW
-0
-1 mV
NS
BERZ Et? I
L1
mV
Seisms:
D.T.S.:
4 14 Janu 03 34 2.
mV NS CHIC EW
mV mV NS
LE P EW mV mV NS
BERZ EW mV
D.T.S.: Cuneo
Bezre-Poncon onnsville . :y::our em00 en0 1. Figs.19and 20.
EXPERIMENTAL
SISMO
STUDY
OF A VAN
NETWORK
---d.-1 23,
65
5
NS
CHIC
EW
NS LEP
_ EW
1 nv NS
BERZ
0 EW
/ Ll mv D.T.S.:
Seisms: $01~;;
I,f,rche
St J. fiaurienne 1Okm S.W. Turin 1Okm S.W. Turin Abondance
NS 0
CHIC EW
NS LE
P EW
NS 0
BERZ EW
Seisms:
D.T.S.: ;z ;
NS 2 1 26 Febr 15 24 EW
LE P
3 27 Febr
17 10 NS
1 22 Febr
-0.250 0.500 0.500 Figs. 21 and 22.
05 26 2.4 W. Cuneo
NS CHIC
EW mV mV
i NS
3
I ,
LE P EW ,
5 mV mV
i
NS
BERZ
-0
EW mV D.T.S.: LE P
SISMO
--i_e l‘
Saisms:
1 01 Marc 0
II
I,
I,
>o mV
NS
CHIC
EW mV mV NS
-i-
LE P EW
mV mV NS
0
BERZ EW
mV D.T.S.: LE P
1 19 Marc
19 42 NS
Seisms: 0.300
Figs. 23 and 24.
67
EXPERIMENTALSTUDYOFAVAMNEIWORK
~1 mV NS -0
CHIC EW
1 mV
I
NS
LE P
0
EW
1 mV
1
mv
NS
BERZ
C
~ EW
1 mV D.T.S.:
Seisms : 1 2 3 4 2
02 05 05 05 $3
Apri Apri Apri ix:
7 15 Apri
05 15 18 23 $2 1
21 49 18 03 39 08
2.0 2.4 2.1 2.4 2.3 1.9
SOS el Mt eiso Mt Viso St Sauveur/Tine Mt St Thabor Laurent du V
07 50 4.2 5Okm E. Nice
1
Ll mV
3 1 mV
NS LE P EW
1 mV -1 mV NS BERZ
-0
EW
-1 mV D.T.S.:
Seisms:
Figs. 25 and 26,
1 2 3 4 5 7 6
19 19 20 21 21 26 21
05 50 2.2 Verdaches Apri 07 36 2.4 Cuneo Apri 15 18 2.0 N. Cuneo Apri 05 43 2.0 N.W. Cuneo Apri 21 11 2.7 25km Sospel Apri 22 01 09 56 2.2 2.1 33km Sian Sospel
8 9 10 11 12 13 14
27 27 27 28 28 29 29
Apri 01 49 2.4 N. Mt Rose 12 06 2.6 S.W. Berne Apri 23 22 2.6 Mf Vanoise Apri 12 23 2.4 N. San Remo Apri 22 24 2.9 Sierre Apri 12 47 2.2 Sierre Apri 16 50 2.5 S. Mt Visa
hX
manner: First, a horizontal time axis is drawn. Then one vertical bar for each signal is added. This bar is proportiona to the intensity or magnitude of the signal and includes a representation of the polarity (bold for a positive signal or an earthquake, thin for a negative signal). The earthquakes are indicated over the time axis. The DTS are indicated above the time axis if they are measured on the N-S channel or below for the E-W channel. A station malfunction is indicated by an interruption of the time axis. A line section parallel to this axis indicates that the data of the telIuric signal acquisition channel cannot be used. These graphs are constructed for periods of 3 months (37.5 mm for 15 d), which gives an overview of the seismic and telluric activities (Figs. 4 -71, and for periods of 15 d (10 mm for 24 h), which enables correlation of DTS and earthquakes (Figs. 8-30).
All the DTS and earthquakes which occur in the Alps between the Rh&ne Valley and the longitude 8”E, the Mediterranean coast and latitude 47”N (an area of about 60,000 km’) arc recorded. In addition, major earthquakes at a greater distance are indicated without their aftershocks; the earthquakes in the region of Nice are an example of these. The earthquakes are located on the basis of info~ation provided by the geophysicist who prepares the weekly report of earthquakes to the LDG. 6. Results 6.1 Comprehsiue
anafysis
From July l&h, 1989, to June 3Oth, 1990, there were 188 earthquakes originating from 74 different epicentres: 47 epicentres had only one period
mV
mV rl
NS
mV
.-0
LE P
EW
NS
SABL
-_-
r
-1
mV
rl
m-J
-0
EW -1
Fig. 27.
mV
EXPERIMENTAL
STUDY
SISMO ’
,r
OF A VAN
1,
,e
NETWORK
19
69
m
’
>I
mV
NS CHIC EW
NS
LE P EW 1
I
mV
mV NS
SABL
0
-
1
EW
1
D.T.S.: LEP LE P SABL
116May 2 21 May 1 22 May
00 36EW 16 48 EW 05 30 NS
mv
Seisms:
w. seyne
I8.$$s
Co1 de Larche Puget-Theniers Argentera
161000
hi P ~~:era Authon Barre Ecrins Authon Authon S. Mt Visa Authon Fig. 28.
of activity and 27 had at least two, including a total of 142 earthquakes, including CunCo (22 events) and Mount Viso (18 events). This gives a seismicity that exhibits significant spatial variations, as shown in Figure 31. It can be seen that the earthquakes are located especially to the south and southeast of the LDG network. The magnitude distribution is given in Table 1. Only 17
events exceeded a magnitude of 3, which is considered by the VAN group to be a minimum for detection. These events are listed in ‘Table 2 and are shown in Figure 32. The three earthquakes at a distance of less than 100 km have small magnitudes (3.1). Five of these epicentres were involved only once (Les Gets, Thonon, Guillestre, Bussoleno and Abondance). During the same period,
70
113 DTS were recorded at all stations. Very few were present at several stations simultaneously. 6.2 Correlation of DTS and earthquakes The graphs prepared every quarter do not show any general correlation, except in the second half of June, when a significant increase in the number of DTS at Le Pegue was followed by a rise in seismic activity at the end of June. The same phenomenon was observed at Chichilianne in September 1989. The tests performed to establish a correlation between DTS and earthquakes are based on the principle proposed by the VAN researchers (Varotsos et al., 1984). Earthquakes with the same epicentre exceeding a given magnitude limit are preceded by DTS, which always appear on the
same group of channels with the same polarities. for each earthquake. However, investigations will also be made into the earthquakes which occurred near some of our stations, as well as for the most violent earthquakes. 6.3 Multiple epicentres There are 27 multiple epicentres, which are located and listed in decreasing order for the number of events and in increasing order for the distance from Chichilianne in Appendix I. Several of these epicentres are worthy of note: (1) Co1 de Vars. Six events took place within 3 d and were preceded by six positive DTS on the N-S channel from September 6th to 9th. The delay time was from 16 to 17 d (Figs. 33-36). (2) Chambeyron. From September 27th to Ck-
ij
CHIC
i
i
NSS
mV
EW
1 T_f
:
1 mV mV
NS LEP
EW mV
mV NS SABL
-
-
.-0
EW mV
D.T.S.:
Fig. 29.
EXPERIMENTAL
STUDY
OF A VAN
71
NEWORK
1990, occurred while the stations were inoperative. The one that occurred on June 7th, 1990, was not preceded by any DTS with the same characteristics. (4) Mount Viso. A total of eighteen events occurred on Mount Viso: eight before the Le Pegue station was installed and ten when it was operating together with Chichilianne. Of these ten events, four occurred when the stations were inoperative (those of January 25th, April 5th and April 29th). Of the other six, four were preceded at Le Pegue on the E-W channel by negative DTS (earthquakes of May 31st, June 4th, June
tober 6th, four events were preceded by four negative DTS, 5-7 d before, on the E-W channel of Chichilianne (Figs. 37-40). The fifth earthquake occurred when none of the stations was operational. (3) Digne. A crisis north of Digne began on June 28th and six events took place before June 30th. They were preceded by an increase in the number of DTS at Le Pegue, 95 km to the west-southwest; there were simultaneous negative DTS on the N-S and E-W channels on June 19th, 23rd and 24th, that is, 7-9 d before the earthquakes (Fig. 41). The event of May 6th,
IJ
i,
Llr”=* II
'-0
-1 mV NS
I
CHIC
-0
EW
11 ’
1
mv
1 mv
7
3
NS
I
LE P EW
I ‘
I
0
10
1 mv
1 mV NS SABL
--
--
--
f
0
EW 1 mv
Seisms:
D.T.S.: CHIC 1 LE P 1 LE P 2 LE P 3 LE LE P P 4 5 LE P 6 LE P 7 LE P 8 LE P 9 LE P 11 10 SABL
20 17 18 19 19 19 20 23 23 24 28 24
June June June June June June June June June June June
15 15 09 01 09 09 11 07 07 20 06 20
30 54 30 36 30 30 48 06 06 30 06 30
EW EW NS NS NS EW EW NS EW NS EW
1 22 June 04 33 EW
1 2 3 4 5 6 7 8 9 10 11 12 13 14
:;*gg; 0:250 -0.750 I!.;Z~ 18_:$;: 4:ooo Fig. 30.
17 23 24 25 27 27 28 29 29 29 29 30 30 30
June June June June June June June June June June June June June June
17 10 19 03 21 21 06 00 01 08 18 03 0 18
46 32 34 20 25 28 03 52 19 55 08 $8
2.2 2 3 2'0 217 2.3 2.7 2.3 1.9 3.1 2.8 2.4 3.8
Sisteron Bor o Mt $iso Suse Valberg
Valberg N. Digne S. Mt Visa N. Di ne N. Di t ne N. Di ne g.EDi ne uneo 37 211 N: bigne
TABLE 2 Events of
M > 3
Date Maptitude:
46 46
45
August 6 August 7 September 24 September 29 October 5
4
5
6
7
24th and June 29th). The DTS occurred on May 16th, May 21st, June 17th and June 28th, that is, 15-19 d before (Figs. 42-44). The two other earthquakes, including the one of magnitude 3.3, were not preceded by DTS with the same characteristics. It should be noted, however, that the operation of the station in the D&me was inconsistent during this period. Other correlations are feasible, but only for two earthquakes. This is because there were only
m
N
FkrceEtage
< 2.1 2.1-2.5 2.6-3.0 3.1-3.5 3.6-4.0 4.1-4.5 > 4.5
21 82 68 11 2 3 1
11.2 43.6 36.2 5.9 1.1 1.6 0.5
3.2
3.2 3.1 3.1 3.3 3.3 3.1
215 NE 16OE 95 ESE 145 ESE 170 NNE 20 SE (T&out) 185 NNE 120 ESE IOOE 130 ENE 120 ESE 90 SE
3.7 4.0
120 ESE I85 NNE
3.6 - 4.0
Mount Viso Abondance
4. I - 4.5
September 30
Sion
4.3
December 26 April 15
Nice East Nice
4.5 4.2
205 NNE 6 SE (Trkout) 200 SE 220 SE
Turin SW
4.8
160E
Magnitude
The magnitude distribution
Azimuth angle ’
8
Fig. 31. Seismic@ in southeast France. 16/07/89-30/X/90.
TABLE 1
M
3.1 3.1 3.1 3.1
Thonon Mount Viso Guillestre Bussoleno Mount Viso N. Digne
October 30 February 14 Magnitude
43
3.s Zermatt East Turin Vars Dronero Les Gets
November 26 December 2 December 13 January 20 June 4 June 29 Magnitude
44
Location 3.1-
>
February 11
4.6
’ Relative to Chichilianne station.
and to the nearest operational
two events at the same location or because, for the others, the VAN stations were not operational. The two earthquakes at Argentera on May 22nd, 1990, which were preceded at Le P&ue by two negative DTS on the E-W channel, belong to the first category (Fig. 42). The earthquakes of Mercantour and Sospel belong to the second category. Regarding the Mercantour earthquakes, the station in the DrGme was not installed on December 29th, 1989, but it was on January 18th and March 29th. Two positive DTS were recorded on the N-S channel, on January 12th and March 19th, that is, 6 and 10 d, respectively, before the earthquake (Fig. 45). As for -1, the stations of Chichifianne and Le P&gue were inoperative on October 18th, 1989, and on April 21st, 1990. However, the two earthquakes of February 5th and April 2nd were preceded at Le P&ue by two
EXPERIMENTAL
STUDY OF A VAN NETWORK
73
e Fig. 32. From top to bottom
and from left to right: location
of the seisms of magnitude
(
over 3.0, 3.5,4.0
and 4.5 (scale: 1 : 4 500000).
Figs. 33-36.
The DTS preceding
the seisms of September
1oQl c:2w +yp--
09lQ9/89
chid9 252
1989 at the Co1 de Vars.
-
0.25 BV/llOl
I
c:zoo
chid9 264 21109l89
1Oh
21/09/B9
chic*9 261
-
t I
I
Figs. 37-40. The DTS preceding the seisms of September-October
lo.25 nV/l~Om
I
I
i I
/
I
i
1989 at Chambeyron.
I
/
l
i
/
/
.,.,-L-J
Figs. 41-44.
- .- _._.,
”
The DTS preceding
the seisms of June-July 1990 at Digne (41). The DTS preceding the seims of May-June Argentera (42). The DTS preceding the seisms of May-June 1990 at the Mount
1990 at the Mount Viso (43 and 44).
Viso and the aeisms of Ma) 72. !99f? at
EXPERIMENTAL
STUDY
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NETWORK
Figs. 49-51.
1‘9 s:lW
_-
The DTS preceding the scism of October
26/89/0 l#E:lO
cUdI
lob
w9l89
mm
I
fl
-.--.
September
the seism of September 30, 1989 at Sion (5 1).
10. 1989 at Rumilly (49). The DTS preceding
-
30. 1989 at Sion (50)
The DTS prcccdinp
the WIWI
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79
NETWORK
positive DTS on the N-S channel on January 20th, 1990 (choice of two signals), and on March 19 (Figs. 45 and 46). Finally, the two earthquakes at Turin on February 11th were preceded by one negative DTS on each of the N-S and E-W channels of Le Pegue on January 27th, as was the earthquake of March 11th (DTS with same characteristics were recorded on March 1st) (Fig. 47). The other epicentres have not yielded results of this type. Several explanations may be given: (1) magnitudes often small relative to the epicentre distances; (2) stations not sensitive at the epicentres; (3) non-continuous signal acquisition.
TABLE 3 Array of the multiple epicentres (July 1989-June 1990) Epicentre, distance and azimuth from Chichilianne Cuneo 160”ESE
6.4 Near-field epicentres Results obtained in Greece show that, for a given magnitude and epicentre-dipole pair, the amplitude of the SES varies inversely with the distance, r, between the epicentre and the station. This means that it may be possible to detect significant telluric signals before earthquakes near a station in service. This was the case for the earthquake at Les Gets on October 5th CM = 3.11, 20 km southeast of the T&out station, which was in service at the time. It was preceded by a positive DTS on the N-S channel and a negative DTS on the E-W channel 8 h before the earthquake (Fig. 48). No similar signals were recorded at the Sur F&e station 50 km to the south at the same time. An earthquake at Rumilly on October lOth, 1989 (A4 = 2.7), 55 km northwest of Sur F&e, was preceded at this station by a positive DTS on the E-W and N-S channels on October 7th (Fig. 49). However, no signals were recorded at Trecout (60 km to the northeast) at the same time. These last results should be treated with caution because the epicentres mentioned were active only once. In addition, other epicentres near operating stations did not give the same results (Romans and Bourg d’oisans) and are not in conflict with the experiments conducted in Greece. 6.5 Significant events Significant events in the region are those with a magnitude of more than 3.5. The Sion earth-
Mount Viso 120”E
Sion-Sierre 205”NNE
ML
Date
2.6 2.5 2.8 2.4 2.8 2.2 2.2 1.7 1.9 2.4 3.0 2.8 2.4 2.7 2.6 2.4 2.0 2.0 2.4 2.5 2.4 2.2 2.5 2.0 2.2 3.7 3.0 2.4 2.2 3.1 2.4 2.4 2.1 2.5 2.9 2.6 3.3 2.5 2.0 1.9 4.3 2.7 2.9 2.7 2.9 2.4 3.0 3.0 2.9 2.2 3.0 2.8
July August September September October December December December December January January January February March March April April April May May May June September October October October November November November December January April April April May June June June June June September September November November November December March April April April June June
27 20 2 15 26 4 21 29 29 10 10 17 22 21 22 19 20 21 6 8 10 30 6 5 5 30 3 26 30 2 25 5 5 29 31 4 4 6 24 29 30 30 15 17 28 3 2 26 28 29 3 3
80 TABLE 3 (continued)
TABLE 3 (continued)
Epicentre, distance and azimuth from Chichilianne
ML
Digne WSE
1.9 2.7 2.3 3.1 2.8 2.4 1.8 2.1 2.1 2.0 2.1 2.5 2.3 2.2 2.2 2.8 2.4 2.3 2.4 2.6 2.5 2.4 2.8 2.6 2.2 3.0 2.3 2.5 2.5 2.8 3.1 2.2 2.3 2.9 2.2 3.0 2.7 2.5 3.0 2.8 2.6 2.1 2.8 1.8 2.2 2.6 2.0 2.0 2.7 2.1 2.6 2.5 2.7 3.0
Authon 8O”SSE
San Rem0 205”SE
Serre-Porqon WSE
Co1 de Vars 95”ESE
Montreux 205”NE
Chambeyron 105”ESE
Sospel 1WSE
Brianp 85”E
Date
May June June June June June June June May 26 May 26 May May May June June March April May May May May May August August August December January June September September September September September September October October October October October October September October October October June October February April April April November December January March
Epicentre, distance and azimuth from Chichilianne 6 7 28 29 29 29 30 30
Turin 16O”E
Parpaillon WESE Allos
105”SE 28 29 31 1 7 20 28 8 8 10 13 14 14 14 16 1 17 2 23 24 24 24 25 25 30 30 30 30 30 31 27 1 2 6 6 18 5 2 21 21 5 30 14 4
Valberg 130”SE Mercantour 14U’ESE Mount Pelat 1lOOSE Col de Larche 1lU’ESE Suse 124WSE Dronero 145”ESE Argentera 1WSE Mount Ceppo 195”SE Nice 2O@%E 2OO’SE Zermatt 215”NE Neuchatel WNNE
Ml
Date
3.1
August February February March March March June December March June December June June December January March
4.8 2.1 3.0 2.1 2.7 2.0 2.1 2.2 2.1 2.9 2.3 2.7 2.3 2.6 2.2 2.9 2.1 2.1 2.2 2.4 2.1 3.1 2.1 2.3 1.8 2.5 2.6 4.5 4.2 2.1 2.5 3.2 2.9 2.5 2.6
May May February May March June September October May May June June December April November November August August November December
I
II II II
24 26 8 18 6 12 16 27 27 29 18 29 7 7 3 20 31 25 29 4 22 22 10 10 26 15 6 6 6 19 30 2
The multiple epkcntres are listed in decreasing order of the number of events and in increasing order of the dktance from Chichilianne.
quake on September 3Oth, 1989 was preceded at T&out by a negative DTS on the N-S channel on September 24th and a positive DTS on the two channels at Sur F&e at the same time (Figs. 50 and 51). Other earthquakes occurred at Sion and Sierre, while none of the stations in Savoie were in service. The Turin earthquake of February llth, 1990, followed by an aftershock, was preceded at Le Pbgue by a negative DTS on both channels. An earthquake at the same location on March llth,
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1990, was preceded by a DTS with the same characteristics. No similar signals were recorded for the other epicentres. Nice is at least 200 km from the LDG stations and the Mount Viso earthquake of November 3rd, 1989, occurred while the Le Pegue station was not in service.
magnitude 04,) of about 5. This has encouraged us to complete the VAN network in the Alps with several low-frequency radioelectric receiving stations to advance further the research being conducted on the prediction of earthquakes.
7. Conclusion
Kerckhove, C., 1979. Notice explicative de la feuille Gap a 1:250.000. Bureau de Recherches Geologiques et Mini&es. Kinoshita, M., Uyeshima, M. and Uyeda, S., 1989. Earthquake prediction research by means of telluric potential monitoring. Progress Rep. 1: installation of monitoring network. Bull. Earthquake Res. Inst. Univ. Tokyo, 64: 255-311. Meyer, K., Varotsos, P., Alexopoulos, K. and Nomicos, K., 1985. Efficiency test of earthquake prediction around Thessaloniki from electrotelluric precursors. Tectonophysics, 120: 153-161. Mosnier, J. and Yvetot, P., 1977. Nouveau type de variometres horizontaux a asservissement de champ et capteur capacitif. Ann. Geophys., 33 (3): 391-396. Petiau, G. and Dupes, A., 1980. Noise, temperature coefficiency and long time stability of electrodes for telluric observation. Geophys. Prospect., 28: 792. Uyeshima, M., Kinoshita, M., Iino, H. and Uyeda, S., in press. Earthquake prediction research by means of telluric potential monitoring. Progress Rep. 2: preliminary study on Teshikaga channel 2 signals and the sismicity in the off Kushiro region. Bull. Earthquake Res. Inst. Univ. Tokyo. Varotsos, P., Alexopoulos, K. and Nomicos, K., 1984. Physical properties of the variations of the electric field of the earth preceding earthquakes. I. Tectonophysics, 110: 7398. Varotsos, P., Alexopoulos, K. and Nom&s, K ,1984. Physical properties of the variations of the electric field of the earth preceding earthquakes, II. Determination of epicenter and magnitude. Tectonophysics, 110: 93-125. Varotsos, P., Alexopoulos, K. and Nomicos, K., 1987. Physical properties of the variations of the electric field of the earth preceding earthquakes. III. Tectonophysics, 136: 335-339.
The first year of experiments conducted using the VAN method made it possible to design a reliable signal recording and acquisition system. In addition, the selection of installation sites using stringent criteria enabled measurement of signals with characteristics which seem similar to SES for four epicentres which were active on several occasions (Co1 de Vars, Chambeyron, Digne and Mount Visa). However, each earthquake was not always preceded by one telluric signal. The data were interpreted placing emphasis on making the transition to routine phases of file verification, data storage and signal plotting. In addition, a program for the automatic processing of telluric data (and the extraction of the component induced by magnetic field variations) is under preparation. Efforts should be continued for a longer period (2 yr) before any first conclusion. The station in Savoie should be installed and all stations should be provided with the new electronic units. Radioelectric interference has now been detected in California, several days before the Loma Prieta earthquake (October l&h, 1989, M, = 6.6). Similar results were obtained in Japan about 100 km southwest of Tokyo for earthquakes with a
References