Remarkable tilt–strain anomalies preceding two seismic events in Friuli (NE Italy): their interpretation as precursors

ELSEVIER

Earth and Planetary Science Letters 170 (1999) 119–129 www.elsevier.com/locate/epsl

Remarkable tilt–strain anomalies preceding two seismic events in Friuli (NE Italy): their interpretation as precursors G. Dal Moro, M. Zadro * Dipartimento di Scienze della Terra, Universita` degli Studi di Trieste, Via Weiss, 1, 34127 Trieste, Italy Received 14 September 1998; accepted 19 April 1999

Abstract Crustal deformation data collected in the Friuli seismic area (NE Italy) through the employment of geodetic instrumentation are analysed to evaluate observed signals considered as preseismic. Once the disturbing action of atmospherical=hydrological factors on local crustal deformation data has been considered in previously published studies, two middle-term precursors (one tilt and one areal strain) are presented, as being the strongest and more evident signals recorded in the area after the 1976 destructive seismic events. An effort is also performed in their interpretation and modelling. Evaluations on the basis of micro-cracking of the source region and aseismic fault creep are done. Modelling based on trivial rheological models furnishes crustal viscosity values in agreement with the values obtained in the analysis of the silent earthquakes recorded in the same area before the 1976–77 destructive seismic events. We put forward the hypothesis that for both the presumably preseismic signals the viscosity plays an important role, although in one case slow dislocations could also have occurred. The two strong possible preseismic signals here considered support the hypothesis proposed by some authors that evident and reliable precursory signals can be detected only within a distance of 2–3 times the dimensions of the source area.  1999 Elsevier Science B.V. All rights reserved. Keywords: strainmeters; tiltmeters; earthquake prediction; precursors; rheology; viscoelasticity

1. Introduction The analysis of crustal-deformation data gathered according to several geodetic methodologies (tiltand strain-meters, GPS, VLBI and classical methods) have furnished some good results in studies regarding the seismotectonic phenomena and the development of seismic cycles. The analysis and modelling of pre-, co- and post-seismic deformational signals (e.g. [1–7]), besides the analysis of seismoŁ Corresponding

author. Tel.: C39 40 676-2257; Fax: C39 040 57-5519; E-mail: [email protected]

logical data, demonstrate the essential contribution of this kind of study for a comprehensive description of the seismogenetic processes. The complex relationships between stress and deformation can be usefully studied through the combined employment of seismological and deformational data able to describe both dynamic and static deformational phenomena acting inside the seismic zone. Several past results evidence, however, an essential problem regarding the dimensions of the seismically prone area where relevant precursor signals can be expected. In this connection a remarkable work is that by Takemoto [8,9]. From the analysis

0012-821X/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 0 9 5 - 3

120

G. Dal Moro, M. Zadro / Earth and Planetary Science Letters 170 (1999) 119–129

of deformational data gained in the Japanese region using geodetic instrumentation, Takemoto finds no evidence of precursory signals and through a magnitude–hypocentral distance relationship he hypothesizes that precursors can be revealed only within a distance of about two times the source dimension. With respect to the modelling of observed tectonic signals, a number of researchers dealt with the analysis of co- and preseismic deformational events. Elastic fault dislocation models are usually employed for the coseismic signals (e.g. [1,5,6,10]), while creep or preseismic events [2,3,11,12] are more commonly studied by rheological modelling. It has to be noted that different noise effects can influence the records so that possible seismotectonic signals are not easily recognised and an appropriate signal processing must be performed. Indeed, in addition to the well-known earth tide signals, several studies regarding crustal deformation in different geological settings show the role of hydrological, thermal and barometric factors in the recorded signal [13–17]. Hence, a propaedeutic noise analysis and suitable processing appear necessary in order to find possible seismotectonic signals obscured or altered by other effects. Regarding the deformational data we consider in the present paper, the noise caused by hydrological and barometric effects in our tilt=strain stations has already been analysed by Zadro et al. [18], Braitenberg and Zadro [19], Dal Moro [20] and Dal Moro and Zadro [21], so that the characteristics are well known for each station. The present paper regards middle (from two months to a few hours) preseismic signals recorded by the Friuli, northeast Italy network and partially reported in [20]. The NE Adria plate, where the Friuli seismic zone is located, represents one of the seismically most active areas in Italy and Slovenia [22,23]: there, due to the collision between the Adria plate and the European continent, two tectonic structural lineaments merge: the Alpine lineament, almost E–W-oriented with overthrust faulting, and the Dinaric lineament, almost NW–SE oriented, at present with strike-slip faults. The Friuli tilt=strain meter network was installed in 1977–79 with the aim of detecting the seismotectonic deformations able to describe the seismic cycle evolution in the long–middle and short term.

2. Stations While more complete documentation on the Friuli tilt=strain network can be found elsewhere (see e.g. [24–26]) some data regarding the stations are reported in the following. The five tilt=strain meter stations of the Friuli network were installed starting from 1977 just after the destructive 1976–77 earthquakes (e.g. [27]) with the aim to monitor the pre-, co- and post-seismic deformations. The network operating till 1995 (Fig. 1), consisted of four tiltmeter stations (Gemona, GE; Invillino, IN; Cesclans, CE; Barcis, BA) and one tilt=strain meter station (Villanova delle Grotte, VI). The GE, IN and CE stations are underground military fortifications built in 1930 in Trias–Giura calcarenites=calcilutites, Norian–Rhaetian dolomite and Pleistocenic conglomerate, respectively; VI and BA are caves: the first one in Eocene flysch and the second one in Paleocene limestone. At present, due to economic restrictions, only the VI station remains active, as well as the Trieste station with big horizontal Marussi pendulums, which is not considered in the present paper, being about 100 km from the epicentres. Two tiltmeters (EW and NS components) were operating in all the stations and the data were collected, as for the strain components, with hourly samplings. The tiltmeters are Marussi horizontal pendulums with Zo¨llner suspension, very stable due to their mechanical and geometrical dimensions (height 55 cm, length of the beam 45 cm, mass of the pendulum 700 g) as well as to their specially designed suspension inside a cast-iron conic housing of 45 kg. The amplification factor is computed from the free oscillation period automatically checked every month by recording for 10 min with 1 s sampling the sinusoidal oscillation following an electronic impulse. The free period is only slowly oscillating around the mean value during the year. The mean periods and the standard deviations evaluated over two years for the NS and EW components at CE and VI are: CENS D 80 š 5; CEEW D 79 š 2; VINS D 86:0 š 3; VIEW D 92 š 4. The resolution is about 0.04 arcsec (about 2 ð 10 7 rad) in the analogical records till 1990 and 0.001 arcsec (about 5 ð 10 9 rad) in the digital system after 1990. Three Cambridge invar wire strainmeters (ST2, ST3 and ST4) are operating at VI in the horizontal plane in

G. Dal Moro, M. Zadro / Earth and Planetary Science Letters 170 (1999) 119–129

121

Fig. 1. Stations for the crustal deformation monitoring in the Friuli (NE Italy) seismic area (1 D tilt stations, N D tilt–strain station).

the N128E, N27E and N68E directions; the resolution reaches about 1 ð 10 9 strain. Also for these instruments the recording was analog until 1990, and both analog and digital thereafter. VI is the most important station, being the oldest and the best equipped one. It is located 60 m under the topographical surface in a roughly ellipsoidal cave. From a geological and tectonic point of view, this area represents a very complex zone where the Alpine domain (with E–W-trending structures) merges into the Dinaric one (with NW–SE structures) determining a particular geological situation [23,28]. The analyses performed in the past on the rainfall and air pressure effects for the Friuli tilt=strain stations [20,21] show that, at least for the VI station, the hydrologically induced deformation is remark-

able, whereas the barometric one is almost negligible. In fact, while the highest deformation induced by air pressure variation can result in about 20–40 nstrain and 20–30 ms (about 1–1.5 ð 10 7 rad) for areal deformations and tilt components, respectively, the amplitude of the hydrological signals can reach about 500 nstrain for the areal deformation and 2000–2500 ms (about 1–1.2 ð 10 6 rad) for the tilt components. Daily rainfall data furnished by the Ufficio Idrografico e Mareografico of Venezia (Italy) are referred to Vedronza (1 km NW of VI station), while the watertable level is collected at Venzone (see Fig. 1) by the local administrative institution (Regione Autonoma Friuli Venezia–Giulia) with a sampling rate of 3 days.

122

G. Dal Moro, M. Zadro / Earth and Planetary Science Letters 170 (1999) 119–129

3. Observations From all the M ½ 3:3 earthquakes which occurred in the Friuli area in 1978–93 (57 events in the 46º–46.75ºN=12.25º–14ºE geographical window; see [29]), we have selected two strong middleterm signals in CE and VI, respectively. Additional signals of lower amplitude or possibly contaminated by atmospheric or hydrological factors were disregarded. Also signals marked by relevant data interruptions were not taken into account; this is the case, for instance, of the CENS record at the beginning of 1980 when the EW component shows no anomaly. The two ‘anomalous’ deformations are considered as precursory signals with respect to the pending seismic events occurring very near to CE and VI, respectively. 3.1. Ceslans 1988 To show how exceptional the occurrence of the identified preseismic signal (which summarises the effects of several subsequent seismic events, i.e. 30 M ½ 2:5 events in February 1988) is, we report in Fig. 2 the characteristic annual tilt vector trend for two years. The normal annual tilting mostly due to thermoelastic effects shows a stable and well known behaviour (see also Fig. 5a) which undergoes conspicuous deviations in the months preceding the February 1988 seismic events. If an anomalous behaviour seems present in August and September 1987, the tilting recorded in October appears to fit the normal seasonal trend. Successively, the November tilt deceleration proceeds a strong anomalous southward tilting (over 3 arcsec — about 1.5 ð 10 5 rad) observed in December. Neglecting much-articulated hypotheses about the total behaviour observed from August up to November, we will focus our attention on the December episode evidenced in Figs. 2 and 6. Concerning the EW component: a westward tilting is generally occurring during these months. Unfortunately, due to an electronic failure in the data acquisition, the EW component was not working during 14 days so that a large eastward tilt of about 10 arcsec (about 5 ð 10 5 rad) can be only supposed. For the above reason the NS component only will be considered in the following. Regarding the evaluation of the recorded anomalous signal, hy-

Fig. 2. CE station. (a) Normal annual tilting: behaviour of the tilt vector in 1990 and 1993 after filtering away diurnal and semi-diurnal tidal components (see also Fig. 5). (b) Behaviour of the tilt vector during January 15, 1987–July 1, 1988. Note the anomalous drifting after November–December 1987. The digits represent the beginning of each month and the circles the M ½ 3 earthquakes with epicentral distance 40 km from CE. Residual annual drift indicated by dashed arrows. Values expressed in arcsec.

drological factors already accounted for [20] cannot be responsible for this kind of behaviour in the CE tilt components due to lack of rainfall. Magnitude, location, epicentral distance and depth of the February 1988 M ½ 3:3 seismic events are reported in Table 1. The epicentral locations and fault plane solutions determined by Kravanja [30] show the typical alpine mechanism characterised by a NS compression with low-angle south-verging overthrust structures, E–W oriented, which in our case can be responsible for the observed preseismic southward tilting. Two coseismic tilt-steps have been observed for the M D 4:1 and M D 3:5 shocks (Fig. 5). They are of particular interest because they represent a par-

G. Dal Moro, M. Zadro / Earth and Planetary Science Letters 170 (1999) 119–129

123

Table 1 Principal events (M ½ 3:3) of the February 1988 and October 1991 seismic sequences; epicentral distances from CE and VI, respectively Date

Magnitude, M

N lat.–E long. (º)

Depth (km)

Epicentral distance (km)

CE ’88

Feb. 1 ’88 Feb. 1 ’88 Feb. 4 ’88

4.1 3.5 3.8

46.35–13.08 46.36–13.10 46.35–13.10

5.1 9.6 7

1.8 3.3 3.3

VI ’91

Oct. 5 ’91 Oct. 5 ’91

3.9 3.4

46.24–13.31 46.26–13.31

19.5 10.9

2.9 2.3

tial sudden recovery of the slow preseismic tilting, whereas it is difficult to recognise possible postseismic long-term recovering effects. 3.2. Villanova 1991 The August 13–November 30, 1991 VI data are presented in Fig. 3: the strainmeter signals were considered precursors of the seismic events occurring very near to the station (see Table 1). They show a wide extension (200, 450 and 900 nstrain for ST2 ST3 and ST4, respectively) which starts about nine days before the October 5 M D 3:9 earthquake and lasts over one week. In the following we have taken into consideration the above signal although we believe the observed extension may have been altered (and probably reduced) by the effects of the September 27 rainfall: indeed, as discussed in Dal Moro and Zadro [21], rainfall-induced deformation is always characterised by a contraction (Fig. 3, cases b, c, d and f). The not easily recognisable behaviour of the tilt components (the influence of hydrological factors is not so clear as in the extensometric case — see [21]) make it impossible to evaluate the clinometric signal: we can only deduce that possible tilt signals are not greater than 70 ms (0.35 µrad) for both components. Seasonal and yearly tilt components are generally much lower than in the CE case and, from a comparison with the ‘normal trend’, a long-term (months) drift possible precursor of the 1991 event is not evident. Magnitudes, locations, epicentral distances, and depths of the October 1991 M ½ 3:3 seismic events are reported in Table 1. In this case we can also observe that the preseismic deformation is summing up effects of two M ½ 3:3 near shocks (seventeen M ½ 2:5 in October 1991).

Fig. 3. Deformational data recorded in the August 13–November 30, 1991 period by the instruments of VI. From above: NS and EW tilt components, ST2, ST3 and ST4 strain components, well level at Venzone and daily precipitation at Vedronza (about 1 km NW of VI). The reported M D 3:9 earthquake occurred on October 5, 1991. Grey-filled areas are typical rainfall-induced deformations, also indicated by the letters b, c, d, e and f; a shows the beginning of the preseismic anomalous wide extension discussed in the text. Due to the different sampling intervals (daily for rainfall and three days for watertable), watertable variations may appear to precede rainfall for 1–3 days (see November events).

124

G. Dal Moro, M. Zadro / Earth and Planetary Science Letters 170 (1999) 119–129

No significant coseismic steps are evident in the data with regard to this event.

4. Data interpretation The phenomena that can be considered in the attempt at an interpretation of the deformational precursory events discussed in the present study are the following: crustal deformations induced by microcracking processes occurring in the source region; fault creep events; and the rheological response of crustal masses to stress variations. We will put particular emphasis on the rheological modelling since the exponential trend of the two observed signals is typical of rheological models generally used for modelling geological phenomena. 4.1. Micro-cracking in the source region Considering the M ½ 3 events which occurred in the Friuli area during 1978–93, we have calculated the preseismic surface deformations theoretically induced by a solid soft inclusion caused, as in the dilatancy theory, by a micro-cracking process in the source region. According to the model of Dobro-

volsky et al. [4], this deformation " is a function of magnitude and epicentral distance. In Fig. 4 the calculated " values are reported versus epicentral distance: the highest theoretical preseismic deformations (evaluated for mean crustal properties — see [4]) are just associated to the two events selected in the present study. The theoretical precursory deformation is represented in these figures for VI and CE, while for BA, GE, and IN the representation is omitted since the values are very low due to the high station–epicentre distances. Considering the amplitude of the observed signals (about 3 s D 1.5 ð 10 5 rad for the CE 1988 tilt event and 900 nstrain of areal dilatation for the VI 1991 signal), for both events the theoretical estimation according to Dobrovolsky et al. is larger for about one order of magnitude. This could at least partially be explained by taking into account the superficial positioning of the soft inclusion considered by Dobrovolsky et al. in order to obtain an ‘upper estimate’ for the induced surface deformation. In any case, it can be of particular interest to point out that the two events showing the highest theoretical deformation (for CE and VI, respectively) were in fact preceded by anomalous precursory deformational signals considered in the present study.

Fig. 4. Preseismic theoretical deformations " ½ 1 ð 10 8 for the M ½ 3 Friuli seismic events (dots) in the 1978–93 period versus the epicentral distance from CE (a) and VI (b) stations according to Dobrovolsky et al. [4].

G. Dal Moro, M. Zadro / Earth and Planetary Science Letters 170 (1999) 119–129

4.2. Evaluation according to the dislocation model As is demonstrated by the dislocation theory in the interpretation of some creep-related observations [2,3,11], another possible explanation of slow deformation signals is represented by aseismic dislocation events occurring along a fault surface. For meaningful results this kind of modelling should be supported by an adequate number of independent signals observed in other nearby stations. Since we do not have such evidence, at least not above the noise level, once realistic assumptions are made, only some rough evaluations are carried out. Considering a dislocation model like, for example, the Okada one [31,32], a continuous slow slip across the fault area can be considered in order to obtain a corresponding temporal evolution in the surface deformation, being the deformational components linear functions of the slip value. It follows that, once the geometry of the fault and the final slip value are known, the temporal evolution is determined by the specific law considered for the slip increase. According to this, the amount of the final total slip and the fault geometry are the only parameters we can consider. Different and more complex models describing an increase in the fault dimensions could be also evaluated; nevertheless, in our case, the limited constraints furnished by the available observations make a comparison of different and articulated modellings clearly meaningless. Regarding the possible characteristics of the fault and the relationships between the different parameters and the magnitude, some information has been inferred from empirical studies (e.g. [33]). Indeed, many authors have treated the problem, which presents several difficulties as being strongly related to local mechanical and tectonic features as well as to the magnitude definition itself. If, according to Okada’s [31,32] notations, we adopt a length 2L being twice the width W [34,35] we obtain a fault area A that can be related to the magnitude M as well as to the seismic moment Mo (e.g. [33,35]). Hence, for a given fault — i.e. magnitude — fixing typical values for the rigidity ¼ of the upper crustal materials (¼ ³ 20 GPa), it is then possible to calculate the slip amount U from Mo D ¼AU. Some dislocation models were tested in order to identify the faults possibly responsible of the ob-

125

served preseismic deformations. The depths d of the faults were chosen on the basis of the mean hypocentral locations of the incoming seismic events [29] and considering the local seismotectonic character, while considering the alpine overthrust structures (low angle inverse fault) that dominate the local seismotectonics (e.g. [22,23,27,30]), the dip Ž D 30º and an inverse fault mechanism were fixed for both the episodes. For the M D 4:1 main episode recorded in the CE 1988 seismic sequence, a fault at d D 5 km (d D depth of the lower edge of the fault — see [31,32]) with 2L D 6 km, W D 3 km and total slip U2 D 30 cm (U2 D slip perpendicular to the fault strike) represents a reliable source able (in a little area centred on the epicentre) to produce a tilt of the same order of magnitude as observed. Regarding the M D 3:4 preseismic deformation of VI 1991, the fault was posed at a depth d D 10 km corresponding to that of the highest local seismic activity [36] and considering that aseismic slip involves a large structure connected with the hypocentral volume of the incoming main shock. Fault parameters able to produce the observed signals are: length 2L D 4:4 km, width W D 2:2 km and slip U2 D 20 cm. The above source parameters, if referred to single seismic events, correspond to magnitudes M ³ 5 and M ³ 4:6 for the CE and VI cases, respectively, thus a little greater than those observed in the main shocks (especially for VI). 4.3. Rheological modelling As described by several authors (e.g. [37], Chap. II), some useful mechanical analogies are commonly considered in order to represent the behaviour of materials with respect to stress variations. Suitable combinations of springs and dashpots can be employed to describe the mechanical response of linear elastic, viscoelastic or viscous materials. For their particular geological meaning, the modelling of precursory signals considered here will be in the following performed with respect to three rheological models: the Kelvin–Voigt (KV) viscoelastic model, the purely viscous Newton (V) model and the KV C V model represented by the two previous elements combined in series. Constant stress (in our case a stress increase) and no initial strain are the fundamental assumptions.

126

G. Dal Moro, M. Zadro / Earth and Planetary Science Letters 170 (1999) 119–129

The Kelvin–Voigt (KV) viscoelastic model is represented by one spring and one dashpot combined in parallel and is described by: ²    ½¦ ¼ A 1 exp ".t/ D t (1) ¼ KV where ".t/ is the deformation at time t, A is the active stress and ¼ and KV are rigidity and viscosity of the medium, respectively. A simple dashpot is able to represent (in the mechanical analogy) a perfectly viscous material (V model). The deformation is given by: ".t/ D

At V

(2)

being V the viscosity of the considered element. Combining in series the two previous elements we obtain a model we will call KV C V. Also employed for some modelling related to geophysical processes (e.g. [38], pp. 268–281), the total deformation is represented by the sum of Eqs. 1 and 2. In the following, the rheological modelling of the two identified precursory signals is presented. Starting from about two months before the February 1, 1988 M D 4:1 event, the anomalous tilt signal recorded at CE has been modelled both in the Kelvin–Voigt (KV) and the purely viscous (V) rheological regime. In the present case, the KV C V model has been disregarded as being clearly inconsistent with the observed data. The NS observed and modelled tilts are represented in Fig. 5b. As already noted, the EW component has not been considered due to a lack in the data. Rigidity was fixed to a typical crustal value (10 GPa) so that viscosity and active stress values were consequently varied to obtain an acceptable fit with the observed data. Viscosity is clearly related to the temporal evolution while active stress determines the amplitude of the signal. Several viscosity and active stress values have been tested (see Fig. 5b) and a good agreement with the observed data has been obtained by adopting the values reported in Table 2. The viscosity values agree with those (1 ð 1016 –1 ð 1017 P) found by Bonafede et al. [39] for deep viscoelastic fault gouges in interpreting the silent earthquakes recorded by the Grotta Gigante horizontal pendulums [40] before the 1976–

Fig. 5. (a) Daily data filtered for diurnal and semi-diurnal earth tides. Normal annual behaviour and the anomalous one in the months around the CE 1988 events are evident. Missing data are interpolated by means of a polynomial (symbols indicate interruptions lasting more then 10 days). (b) Modelling of the precursory signal revealed by the NS tilt component of CE before the M D 4:1 event of February 1, 1988. Kelvin–Voigt model (KV): viscosity KV D 2 ð 1017 P with active stress A D 0.20 (A1) and 0.25 MPa (A2); viscosity KV D 2 ð 1017 P with active stress A D 0.20 (B1) and 0.25 MPa (B2). Purely viscous model (V) for V D 3 ð 1017 P (A D 0.20 MPa). For all the models the rigidity modulus ¼ is 10 GPa.

77 destructive Friuli seismic sequence. The recorded coseismic tilt-steps result in opposite signal signs with respect to the slow precursory tilting, thus giving a partial recovery of the accumulated preseismic deformation. Similarly to the CE case, the modelling results for the VI extensometric precursory signal (areal deformation ∆ D "1 C"2 , with "1 and "2 the principal

G. Dal Moro, M. Zadro / Earth and Planetary Science Letters 170 (1999) 119–129 Table 2 Characteristic rheological values for the modelling of the two preseismic events

V model KV model KV C V model

Active stress (kPa)

Viscosity (P)

Event

200 21.5 200–250 21.5 19

3 ð 1017 12 4.5 ð 1016 2 ð 1017 2.5 ð 1016 KV D 2.5 ð 1016 V D 5 ð 1017

CE ’88 VI ’91 CE ’88 VI ’91 VI ’91

Rigidity ¼ D 10 and 20 GPa for the CE 1988 and the VI 1991 events, respectively. Viscosity values in poises (P).

axes of the deformation) are reported in Fig. 6. Three different rheological models are tested: Kelvin–Voigt (KV), purely viscous (V) and KV C V. Also in that case several viscosity and active stresses have been tested although only the more realistic ones are shown in Fig. 6 and reported in Table 2. Viscosity values appear of the same order of magnitude as those deduced for the CE 1988 episode.

Fig. 6. Modelling of the precursory signal revealed by the areal deformation at VI before the October 5, 1991, M D 3:9 event. Reported are the observed areal deformation (solid line) and the modelled ones. Shaded area represents a typical rainfall-induced deformation. Kelvin–Voigt (KV) model for viscosity KV D 2.5 ð 1016 P and active stress A D 21.5 kPa. Viscous (V) model for V D 4.5 ð 1016 P and A D 21.5 kPa and KV C V model for KV D 2.5 ð 1016 P, V D 5 ð 1017 P and active stress A D 19 kPa. For all the models the rigidity modulus ¼ is 30 GPa.

127

5. Discussion and conclusions Deformational data observed in the Friuli (NE Italy) seismic area at five tilt–strainmeter stations were considered to evaluate possible preseismic signals after the effects of the noise induced by hydrological and barometric factors were previously evaluated [20,21]. A couple of different, significant signals lasting weeks to months were analysed as precursory to two very near earthquakes of magnitudes 4.1 and 3.9 (epicentral distances 1.8 and 2.9 km from CE and VI, respectively). Other possible signals have been disregarded as being masked by noise of hydrological origin and, moreover, as being very low due to larger epicentral distances, in agreement with the analyses conducted by Takemoto [8,9]. Considering deformational data of the Japanese seismic area, Takemoto concludes that significant and reliable deformational precursory changes can be expected only within a distance of about two times the radius of the source dimension: the empirical M–r (magnitude– distance) relationship proposed by Takemoto [8] on the base of levelling and tilt=strain data as ‘criterion of earthquake prediction’ is M D 1:96 log r C 3:84, with r being twice the source dimension. In our case, the two events (M D 4:1 for CE and M D 3:9 for VI) are characterised by an epicentral distance comparable with the dimensions of the corresponding seismic sources. It is clear that the above empirical relationship strongly depends on the local seismotectonics and could appear as rather difficult to apply in different geological contexts. Although some hydrological effect could have partly influenced the VI signal, an evaluation of both observed signals was performed with respect to different hypotheses. Theoretical preseismic deformations calculated for all the M ½ 3 events in the 1978–93 period on the base of the model of Dobrovolsky et al. [4] as the result of a solid soft inclusion determined by a micro-cracking process (for mean crustal mechanical properties) show the highest values just for the two episodes selected in the present study. The dimensions of the slow dislocations theoretically able to produce the observed signals were also modelled considering fault creep events. Two dislocation models were inferred on the base of the hypocentral location of the incoming events: for the

128

G. Dal Moro, M. Zadro / Earth and Planetary Science Letters 170 (1999) 119–129

CE 1988 signal a dislocation at depth d D 5 km with a length of 6 km, width 3 km and total slip of 30 cm (M ³ 5); for the VI 1991 event a fault (posed at depth d D 10 km) with a length of 4.4 km, width 2.2 km and slip 20 cm (M ³ 4:6). In the CE 1988 case, a valuable hypothesis for the interpretation of the observed signal could be a slow step-wise dislocation caused by micro-cracking processes lasting for several tens of days: the theoretically accumulated elastic energy corresponds to the energy released by an earthquake of magnitude M ³ 5, therefore of the same order of magnitude as the total energy released by the shocks that occurred in the area during the February 1988 seismic crisis. The partial recovery of the precursory tilting realised in the coseismic tilt-steps is also remarkable, tilt-steps being in good agreement with the synthetic deformation fields associated to dislocation sources modelled on the base of the available seismological data [29,30]. For the VI case, however, the disagreement between the theoretical elastic energy (corresponding to M ³ 4:6) appears large when compared with the observational data, also taking into account the short duration of the recorded deformational episode, which was a few days. Some common rheological models were finally considered in order to infer the viscoelastic properties of the involved crustal materials. Once typical values were fixed for the rigidity of the upper crust, the viscosity values obtained by fitting the observed data were found ranging from 1 ð 1016 to 1 ð 1017 P depending on the event and model considered (see Table 2), thus resulting in good agreement with the values proposed by Bonafede et al. [39] in the modelling of the silent earthquakes recorded by the horizontal pendulums of the Grotta Gigante [40] before the destructive 1976–77 Friuli seismic events. With respect to a possible post-seismic recovery, there is no clear evidence for both CE-VI cases. A diluted post-seismic deformation perhaps occurred, as seems to be the CE case, but it faded into the annual tilting. As a conclusion, we believe that in both examined preseismic signals the viscosity plays an important role, as evidenced by the characteristic exponential trends. Moreover we believe that a sequence of small step-wise dislocations also occurred,

particularly in the CE case as a consequence of micro-cracking=dilatancy processes.

Acknowledgements This study has been supported partly by EC research grant No. IC15 CT96-0205 and partly by contracts MURST ex 40% and ex 60%, contractor M. Zadro. The authors like to express their gratitude to Prof. C. Ebblin for his valuable suggestions and several helpful discussions. The authors are also grateful to Prof. Hans-Joachim Ku¨mpel, Prof. S. Takemoto and Dr. Malte Westerhaus for their remarkable suggestions, which have highly improved the present paper. [AC]

References [1] S. McHugh, M.J.S. Johnston, An analysis of coseismic tilt changes from an array in central California, J. Geophys. Res. 82 (1977) 5692–5698. [2] S. McHugh, M.J.S. Johnston, Dislocation modelling of creep-related tilt changes, Bull. Seismol. Soc. Am. 68 (1978) 155–168. [3] S. McHugh, M.J.S. Johnston, C.E. Mortensen, The implications of creep-related tilt observations on San Andreas fault in Central California, in: A. Vogel (Ed.), Terrestrial and Space Techniques in Earthquakes Prediction Research, Vieweg, Braunschweig, 1979, pp. 161–179. [4] I.P. Dobrovolsky, S.I. Zubkov, V.I. Miachkin, Estimation of the size of earthquake preparation zones, Pure Appl. Geophys. 117 (1979) 1025–1044. [5] A. McGarr, I.S. Sacks, A.T. Linde, S.M. Spottiswoode, R.W.E. Green, Coseismic and other short-term strain changes recorded with Sacks–Evertson strainmeters in a deep mine, South Africa, Geophys. J.R. Astron. Soc. 70 (1982) 717–740. [6] F.K. Wyatt, Measurements of coseismic deformation in Southern California: 1972–1982, J. Geophys. Res. 93 (1988) 7923–7942. [7] M.T. Gladwin, R.L. Gwyther, J.W. Higbie, R.G. Hart, A medium term precursor to the Loma Prieta earthquake?, Geophys. Res. Lett. 18 (1991) 1377–1380. [8] S. Takemoto, Some problems on detection of earthquakes precursors by means of continuous monitoring of crustal strains and tilts, J. Geophys. Res. 96 (1991) 10377–10390. [9] S. Takemoto, Recent results obtained from continuous monitoring of crustal deformation, J. Phys. Earth 43 (1995) 407–420. [10] M.J.S. Johnston, A.T. Linde, M.T. Gladwin, R.D. Borcherdt, Fault failure with moderate earthquakes, Tectonophysics 144 (1987) 189–206.

G. Dal Moro, M. Zadro / Earth and Planetary Science Letters 170 (1999) 119–129 [11] M. Dragoni, M. Bonafede, E. Boschi, On the interpretation of slow ground deformation precursory to the 1976 Friuli earthquake, Pure Appl. Geophys. 122, 781–792, 1984=85. [12] F. Bella, P.F. Biagi, M. Caputo, G. Della Monica, A. Ermini, P.V. Manjgaladze, V. Sgrigna, D.O. Zilpimiani, Possible creep-related tilt precursors obtained in the Central Apennines (Italy) and in the Southern Caucasus (Georgia), Pure Appl. Geophys. 144 (1995) 277–300. [13] R.J. Edge, T.F. Baker, G. Jeffries, Borehole tilt measurements: aperiodic crustal tilt in an aseismic area, Tectonophysics 71 (1981) 97–109. [14] J. Peters, C. Beaumont, Borehole tilt measurements from Charlevoix, Que´bec, J. Geophys. Res. 90 (1985) 12791– 12806. [15] H.J. Ku¨mpel, J.A. Peters, D.R. Bower, Nontidal tilt and water table variations in a seismically active region in Quebec, Canada, Tectonophysics 152 (1988) 253–265. [16] T. Yamauchi, Anomalous strain response to rainfall in relation to earthquake occurrence in the Tokai area, Japan, J. Phys. Earth 35 (1987) 19–36. [17] M. Westerhaus, Tidal tilt modification along an active fault, in: H. Wilhelm, W. Zu¨rn, H.G. Wenzel (Eds.), Tidal Phenomena, Lecture Notes in Earth Sciences 66, Springer, Heidelberg, 1997, pp. 311–339. [18] M. Zadro, E. Plenizio, C. Ebblin, C. Chiaruttini, Influence of groundwater table level variations and of rainfall on tilting in the Friuli area, N-E Italy, Acta Geophys. Pol. 35 (1988) 323–338. [19] C. Braitenberg, M. Zadro, Strains, seismicity and rain, Proceeding of the XXIV General Assembly of the European Seismological Commission, 1994, September 19–24, Athens, 1995, pp. 1216–1224. [20] G. Dal Moro, Crustal Deformations in the Friuli Seismic Area: Observations and Models (in Italian), Ph.D. Unpublished Thesis, University of Trieste, 1997, 190 pp. [21] G. Dal Moro, M. Zadro, Subsurface deformations induced by rainfall and atmospheric pressure: tilt=strain measurements in the NE-Italy seismic area, Earth Planet. Sci. Lett. 164 (1998) 193–203. [22] A. Amato, P.F. Barnaba, I. Finetti, G. Groppi, B. Martinis, A. Muzzin, Geodynamic outline and seismicity of Friuli Venezia–Giulia region, Boll. Geofis. Teor. Appl. 72 (1976) 217–255. [23] G.B. Carulli, R. Nicolich, A. Rebez, D. Slejko, Seismotectonics of the Northwest external Dinarides, Tectonophysics 179 (1990) 11–25. [24] M. Zadro, Use of tiltmeters for the detection of forerunning events in seismic areas, Boll. Geod. Sci. Aff. 38 (1978)

129

597–618. [25] M. Zadro, Tilt and strain variations in Friuli, NE-Italy, after the 1976 earthquake, in: M. Dragoni, E. Boschi (Eds.), Proc. Int. School of Solid Earth Geophysics — Earthquake Prediction, Ettore Majorana Centre for scientific Culture — Istituto Nazionale di Geofisica, Roma, 1992, pp. 295–313. [26] G. Rossi, M. Zadro, Long-term crustal deformations in NE Italy revealed by tilt–strain gauges, Phys. Earth Planet. Inter. 97 (1997) 55–70. [27] J. Cipar, Teleseismic observations of the 1976 Friuli, Italy earthquake sequence, Bull. Seismol. Soc. Am. 70 (1980) 963–983. [28] C. Ebblin, Orientation of stresses and strains in the Piedmont area of the Eastern Friuli, NE Italy, Boll. Geofis. Teor. Appl. 19 (1976) 559–579. [29] OGS, Earthquakes catalogue for the Eastern Alps region, OGS, Trieste, computer file, 1995. [30] S. Kravanja, Stress Field in NE Italy from Digital Waveforms (in Italian), Unpublished degree thesis, University of Trieste, 1994. [31] Y. Okada, Surface deformation due to shear and tensile faults in a half-space, Bull. Seismol. Soc. Am. 75 (1985) 1135–1154. [32] Y. Okada, Internal deformation due to shear and tensile faults in a half-space, Bull. Seismol. Soc. Am. 82 (1992) 1018–1040. [33] D.L. Wells, K.J. Coppersmith, New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, Bull. Seismol. Soc. Am. 84 (1994) 974–1002. [34] K. Abe, Reliable estimation of the seismic moment of large earthquakes, J. Phys. Earth 23 (1975) 381–390. [35] R.J. Geller, Scaling relations for earthquake source parameters and magnitudes, Bull. Seismol. Soc. Am. 66 (1976) 1501–1523. [36] G. Bressan, R. De Franco, F. Gentile, Seismotectonic study of the Friuli (Italy) area based on tomographic inversion and geophysical data, Tectonophysics 207 (1992) 383–400. [37] J.C. Jaeger, Elasticity, Fracture and Flow, Methuen, London, 1962, 208 pp. [38] J.G. Ramsay, Folding and Fracturing of Rocks, McGrawHill, New York, 1967, 568 pp. [39] M. Bonafede, E. Boschi, M. Dragoni, A physical model of the long-period precursor to the 1976 Friuli earthquake, Boll. Geofis. Teor. Appl. 24 (1982) 93–108. [40] C. Chiaruttini, M. Zadro, Horizontal pendulum observations at Trieste, Udine Meeting on the Friuli earthquakes, Dec. 1976, Boll. Geofis. Teor. Appl. 19 (1976) 441–455.