No preseismic evidence from hydrogeochemical parameters on the occasion of the April 29, 1991, Georgian earthquake, Caucasus

Tectonophysics, Elsevier

Science

353

213 (1992) 353-358 Publishers

B.V., Amsterdam

No preseismic evidence from hydrogeochemical parameters on the occasion of the April 29,1991, Georgian earthquake, Caucasus G. Areshidze ‘, F. Bella ‘, P.F. Biagi ‘, M. Caputo b, G. Della Monica b, A. Ermini P. Manjgaladze a, G. Melikadze d, V. Sgrigna b, D. Zilpimiani a ” Institute of Geophysics of the Academy of Sciences of Georgia, Z.Ruckhadze, 1, 380093 Tbilisr, Georgra

‘,

’ Dipartimento di Fisica, Unicersit6 La Sapienza, P. le A. Moro, 2, I-00185 Rome, Italy ’ Dipartimento di Scienze della Terra, UniraersitciLa Sapienza, P. le A. Moro, 2, l-00185 Rome, Italy ’ Gruzgeologia, Moseshrili, 24, 380079 Tbilisi, Georgia (Received

January

15. 1992; revised version

accepted

April 22, 1992)

ABSTRACT Areshidze, G., Bella, F., Biagi, P.F., Caputo, M., Della Monica, G., Ermini, A., Manjgaladze, P., Melikadze, and Zilpimiani, D., 1992. No preseismic evidence in some hydrogeochemical parameters on the occasion 1991, Georgian earthquake, Caucasus. Tectonophysics, 213: 353-358.

G., Sgrigna. V. of the April 29,

The behaviour of helium content in thermal waters and of water level in deep wells was investigated on the occasion of the strong (M = 6.9) Georgian earthquake which occurred in the Caucasus on April 29, 1991. Data were collected from a measurement network located in Georgia. Just one significant preseismic variation appeared in the water level which seemed to be connected with the Georgian earthquake. In other wells, only some co-seismic and post-seismic effects are visible. No statistically viable variation was noted in the helium content. In contrast, clear anomalies in these parameters were observed on the occasion of the 1988 Spitak earthquake with the same magnitude which occurred in the Caucasus. We prepose that the location of the epicentre with respect to the measurement network was the reason for this disagreement. We therefore suggest the introduction of a coefficient c into relationships that give the size of the premonitory area as a function of the magnitude of the forthcoming earthquake. This coefficient, ranging from 0 to 1. can take into account the extent to which the focal zone belongs to different tectonic structures and the extent to which the measurement network coincides with these structures.

Introduction

In a previous paper (Areshidze et al., 1992) we presented some results of the recent strong Spitak (December 7, 1988; M = 6.9) earthquake, which occurred in the Caucasus (Fig. 1). We considered data collected in Georgia, concerning the water level in deep wells, the helium content in thermal waters, electromagnetic signals (EME) revealed by surface and underground antennae and a F(ar) function based on travel times of P

and S waves (Slavina and Tagizade, 1989). Fairly clear anomalies in the above-mentioned parameters were interpreted as being phenomena related to this earthquake. More recently, the Georgian earthquake (April 29, 1991) occurred in the Caucasus with the same magnitude (M = 6.9) as the Spitak earthquake (Fig. 1). In this paper we present the behaviour of the water level in wells and of the helium content in thermal waters on the occasion of the Georgian earthquake. Equipment

Correspondence di Scienze

to: P.F. Biagi, Universita

della Terra,

Piazzale

‘La Sapienza’.

Aldo Moro 2, I-00185

0 1992 - Elsevier

Science

sites

Dip. Rome,

Italy.

0040-1951/92/$05.00

and measurement

Publishers

Figure 1 shows the location of the measurement sites of geophysical and hydrogeochemical B.V. All rights

reserved

G. ARESHIDZE

354

ET AI.

TURKEY

Fig. 1. Map showing symbols

indicate

the location

measurement

of measurement

sites of some geophysical

sites used for this study. The epicentres

main fault systems in the Caucasus

parameters the data of which have been at our disposal since 1985. Data are not always complete; either because some measurement sites were put into operation at different periods or due to some form of disturbance of the equipment. The hatched symbols indicate the measurement sites used for this study. Instruments and measurement techniques have been described in Areshidze et al. (1992). Some characteristics of the boreholes and of the wells are listed in Table 1.

Results

TABLE

parameters

in Georgia.

(a) and Spitak (b) earthquakes,

are also shown. 1

Characteristics

of boreholes

Site

h

T

Flow rate

(m)

PC)

(I/s)

and wells

Helium boreholes VAR

1360

56

1

LIS

2255

62

30

BOR

1320

44

25

OKT

1194

30

1

BAK

3510

24

24

SAB

2867

66

4

TSA

3855

80

100

SBR

2555

33

14

Water level wells MAR

The Georgian earthquake of April 29, 1991 with a magnitude M = 6.9 occurred at 09 12 45.5 UT; its epicentral coordinates are C$= 42.42”N and A = 43.67”E (Seismological Notes, 1991). The hypocentre was estimated to lie at a depth of

and geochemical

of the Georgian

3500

80

LIS

330

18

BOR

1339

14

AKA

1400

14

KOB

2000

22

GOR

1500

20

AXA

3505

28

Hatched with some

PRESEISMIC

EVIDENCE

FROM

HYDROGEOCHEMICAL

355

PARAMETERS

helium content from July lst, 1989 to February 28, 1991. It must be noted that in July 1989 the aftershock activity of the Spitak earthquake was practically complete. Figure 2 shows some statistically significant variations in the helium content, mainly in 1990. These variations are not more than 30 days long and their scattering over the 3~ value is not more than 20%. They seem not to be connected with the Georgian earthquake.

about 10 km. This event was the strongest to have occurred in the Caucasus since the Spitak earthquake. Figure 2 shows the variations in the helium content in thermal waters of the BAK, BOR, OKT, LIS and VAR boreholes during the period January lst, 1990 to June 30, 1991. A vertical line indicates the time of the Georgian earthquake. The dotted lines indicate the &3a value at each site, calculated from the average value of the HELIUM

CONTENT

i+isli

0.8 BAK

VAR 0.6

Fig. 2. Variation

in the helium

1991. Dotted

content

lines indicate

in the BAK, BOR,

OKT, LIS and VAR boreholes

the + 3a levels. The vertical

during

the period

line shows the time of the Georgian

January

earthquake.

1990-June

Fig. 3. Variation

in the water

level in the KOB, AXA, The vertical

GOR,

Figure 3 shows the water level trends in the KOB, AXA, GOR, BOR and LIS wells during the period January Ist, 1990 to August 3&t, 1991. A vertical line indicates the time of the Georgian earthquake. A clear trough-shaped disturbance appears at the LIS site, with a duration of about 2 months. No correlation was found with meteorological data collected from a nearby weather station; so, this disturbance appears to be a preseismic

effect.

At

the

KOB,

AXA,

GOR

BOR wells only some co- to post-seismic are visible.

and

effects

and LIS wells, during

the period

January

lWO-August

1901.

earthquake.

at each borehole used for the Georgian quake have practically the same standard

earthdevia-

tion as the ones considered for the Spitak event. Notwithstanding the fact that the Spitak and the Georgian earthquakes had the same magnitude and equidistant

their epicentres were approximately from the network we used, the results

obtained now are in complete Areshidze et al. (1992).

contrast

to those in

As regards water level, in the study concerning the Spitak earthquake, it was only possible to use data from a single well (MAR, Fig. 1); notwithstanding this, we pointed out significant preseismic and co- to post-seismic variations (Areshidze

Discussion In the

BOR

line shows the time of the Georgian

previous

section

was

noted

that

the

Georgian earthquake seems not to have produced any statistically noticeable variation in the helium content data at our disposal. In contrast, on the occasion of the Spitak earthquake very evident and significant variations (a lo-1.50% scattering over the 3a value and a duration from 1 to 2 months) in the helium content were observed (Areshidze et al., 1992). The boreholes in the two studies are almost the same and the helium data

et al., 1992). In the case of the Georgian earthquake, we had data from more wells our disposal than for the previous study. In spite of this, as shown in the previous section, only the LIS well responded clearly to the preparation phase of the earthquake, while only co-seismic and post-seismic effects appeared in the other wells. The absence of such effects at the LIS site suggests that changes in the water level at this site are governed by significantly high inertial processes (such as water diffusion). In

PRbSElSMlC

EVIDENCE

FROM

WYDROGEOCHE~~ICAL

357

PARAMETEUS

this way, the trough-shaped disturbance at the LIS site can be attributed not to local but to some remote stress rcadjustmcnt processes that may occur in the focal zone of the forthcoming earthquake. In any case, the results we obtained here concerning the water level are disappointing with respect to those obtained previousty (Areshidze et ai., 1992k In the studies concerning earthquake precursors, it is usually supposed that the dimensions of the zone for monitoring preseismic effects depend on the magnitude of the forthcoming event, according to empirical relations proposed by Dobrovolsky et al. f1979f, Odekov (1986) and Rikitake (1987). The radius of this zone for the Georgian earthquake may be estimated in this way as ranging from about 250 to 1000 km. The results obtained from the network shown in Figure 1 ~measurem~~t sites identified with hatched symbols) evidently do not confirm these estimations. For the Spitak event, the size of the earthquake preparation zone does not contradict the abovementioned empiricaf relationships (Areshidae et al., 1992). The epicentre of the Spitak earthquake is Jocated in the Lesser Caucasus while the epicentre of the Georgian earthquake is in the southern slope of the Greater Caucasus. These two tectonic units are seismog~n~tically different from each other and are separated by the Transcaucasian Massif (Adamia et al., 1991). The measurements network is located in the latter area. In the light of the above, the results obtained here and in Areshidze et al. (1992f indicate that the empirical relations to estimate the size of the “premonitory area” of an earthquake do not take into account the location of the monitoring network with respect to the seismogenetic structure which the forthcoming earthquake belongs to. In empirical relationships that give r (radius of the premonitory area) as a function of M (magnitude of the earthquake), a multiplicative coefficient, c, should be inserted, ranging from 0 to unity. This coefficient should characterize the stress/strain transmission through different types of tectonic discontinuities between the focal zone and the measurement site (or network). In this way, the extreme values of the coefficient should

correspond to perfect and loose contacts at the d~scoutinuities (Bella et al., 1990; Biagi et al., 1990). It must be noted that the. coefficient c can depend also on the type of the geochemical and geophysical parameter under study, due to their possible different coupling with the stress variation (Rikitake, 1987). Conclusions The results obtained in this paper have shown that strong earthquakes sometimes do not produce precursory phenomena in some parameters, even for measurement sites located within the “premonitory area”. We have justified such a lack of preseismic signals by supposing that the readjustment of the stress that produces carthquake precursors is affected significant& by the pattern and properties of discontinuities between the focal zone and the measurement site (or network). In view of this. the knowledge of a proper coefficient in the relationship that defines the size of the ~rernonjto~ area could reduce the uncertainty concerning the Iocation of the epicentre of the forthcoming earthquake. In addition, it could help to define the characteristics of the boundary zone rigidity in some discontinuitics.

This work was supported by the Ministro della Universitri e della Ricerca Scientifica e Tecnologica, The Academy of Sciences of Georgia and the CNR of Italy. References A&n&, S.,

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