Volcanic tremors at Deception Island (South Shetland Islands, Antarctica)

Journal o f Volcanology and Geothermal Research, 53 (1992) 89-102

89

Elsevier Science Publishers B.V., Amsterdam

Volcanic tremors at Deception Island (South Shetland Islands, Antarctica ) Josep Vilaa, Joan Marti b, Ramon Ortiz c, Alicia Garcia c and Antoni M. Correiga aDGDGP-GEOFISICA, Facultat de Fisica, Universitat de Barcelona, and Laboratori d'Estudis Geofisics "Eduard Fontserk'" Institut d'Estudis Catalans, Marti i Franqu#s, 1, E-08028 Barcelona, Spain blnstitut de Ci~ncies de la Terra "'Jaume Almera", CSIC, Martl i Franqu~s, 3, E-08028 Barcelona, Spain CMuseo Nacional de Ciencias Naturales, CSIC, Josk Guti#rrez Abascal, 2, E-28006 Madrid, Spain (Received May 24, 1991; revised and accepted April 14, 1992)

ABSTRACT Vila, J., Marti, J., Ortiz, R., Garcia, A. and Correig, A.M., 1992. Volcanic tremors at Deception Island (South Shetland Islands, Antarctica). J. Volcanol. Geotherm. Res., 53: 89-102. Deception Island (62 ° 43' S, 60 ° 57' W) is an active volcano located at the spreading centre of the Bransfield Strait backarc on the southwestern side of the Scotia Sea region (Antarctica). Since 1986, the Spanish Antarctic National Program has supported geophysical and geological surveys addressed to the study of seismic and volcanic activities in the area. The present volcanotectonic activity is restricted to significant seismic activity and several fumarole fields. Numerous earthquakes, with magnitudes ranging from 0.1 to 2.2, have been recorded, most of them distributed along the main fractures of the island, which coincide with the regional structural trend of the Bransfield Strait back-arc. Several types of volcanic tremors have also been recorded, all of them with high spectral stability contents with few and well-defined spectral peaks. The location of the stations that record tremors, the correlation of seismic noise with the tremors and the geological characteristics of Deception Island, together suggest that the tremors are generally associated with geothermal noise originated in the uppermost ducts of the fumarolian system. If we associate the surficial ducts to harmonic oscillators, it is observed that the motion is basically unidirectional, as is expected in the degasification of an aquifer. Taking the main observed frequencies and applying the classical mechanical expressions of organ pipes, we obtain a duct length which agrees with the length of a water column necessary to maintain the equilibrium as indicated by the application of the Clausius-Clapeyron equation.

Introduction Volcanic activity generates a wide range of seismic signals that are classified, in a broad sense, as volcanic earthquakes and tremors. A volcanic earthquake is defined as a seismic event which occurs at or near the volcano and is associated with ruptures caused by volcanic activity, whereas a volcanic tremor is a continuous seismic disturbance with spectral conCorrespondence to: J. Vila, D G D G P - G E O F I S I C A , Facultat de Fisica, Universitat de Barcelona, and Laboratori d'Estudis Geofisics " E d u a r d FontserC', Institut d'Estudis Catalans, Marti i Franqu+s, 1, E-08028 Barcelona, Spain.

0377-0273/92/$05.00

tents characterized by few but well-defined spectral peaks. The origin of volcanic tremors may be attributed to different causes, ranging from oscillations of magmatic chambers (Kubotera, 1974) to resonance in emission ducts, geothermal noise produced in the uppermost ducts by gas phases (Schick et al., 1982a, b) or magma intrusions (Aki et al., 1977; Shaw, 1980). The analysis of volcanic tremors is a subject of great importance in the monitoring of volcanoes (Hurst, 1985). In general, the method of analyzing tremors is based on the study of the spectral content and the radiation diagram

© 1992 Elsevier Science Publishers B.V. All rights reserved.

90

J. VILA ET AL.

volcanic tremors recorded at Deception Island, and we discuss the origin of these seismic events on the basis of their spectral analyses and compare the results with the available geological information obtained during the same working period.

(Aki et al., 1977). However, most studies are confined to spectral analysis and its temporal variation, because the construction of the spatial distribution of the amplitude and phase of the seismic signal that defines the radiation diagram needs a large number of stations surrounding the source, which, in most cases, are few or non-existent. Since the 1986-1987 austral summer, a continuous survey is being carded out with the aim of monitoring seismic and volcanic activity at Deception Island by a Spanish working group with the financial support of the Spanish National Program of Antarctic Research. Several types of instrumentation were temporarily installed during these surveys, aimed especially at studying the seismic activity associated with volcanic activity at Deception. The data recorded during this survey have allowed the recognition of a wide spectrum of seismic signals caused by volcanic and tectonic phenomena, volcanic tremors being one of the most common features. This paper presents, for the first time, the

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The South Shetland Islands form an archipelago 550 km long, parallel to the northern extreme of the west coast of the Antarctic Peninsula (see Fig. 1). Geophysical data (e.g. Ashcroft, 1972; Barker and Griffiths, 1972; Barker, 1982; Parra et al., 1984) suggest that these islands lie on a continental plate restricted to the east by the Bransfield Strait backarc marginal basin, to the west by a well-defined trench zone, and to the north and south by transform faults. The South Shetland Trench (Fig. 2) represents the last surviving segment of a subduction zone that originally extended along the entire western margin of the Antarctic Peninsula, while the Bransfield Strait

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is taken to represent a contemporary episode of extension which has a back-arc geometry with respect to the possible subduction along the South Shetland Islands (see Hawkes, 1961; Barker, 1982; Parra et al., 1984; Pelayo and Wiens, 1989). Under the South Shetland Islands, the structure of the crust is of a continental type, with the first stages of its evolution similar to those from the western coast of the Antarctic Peninsula (Tarney et al., 1982). The thickness of the crust ranges from 30-32 km under the island-arc, to 40-45 km at the western coast of the peninsula. However, the crustal structure below the Bransfield Strait is typical of a rifting environment, with a thinned crust relative to the surrounding South Shetland Islands and the Antarctica Peninsula (Ashcroft, 1972; Guterch et al., 1987 ). Several models have been developed to explain the present-day tectonics and relative plate motions of the western side of the Scotia Sea region (see Fig. 2) (e.g. Fortsyth, 1975; Barker and Hill, 1980; Barker and Dalziel, 1983). Recently, Pelayo and Wiens (1989) have revised these models on the basis of the seismicity characteristic of that region. According to these authors, earthquakes are consistent with active subduction and back-arc spreading along the South Shetland Islands and Bransfield Strait. The results of their study

suggest continued but largely aseismic subduction along the South Shetland Trench, and that diffuse extension rather than typical organized mid-ocean spreading characterizes the Bransfield Strait where large normal faulting occurs. Thus, the tectonic model proposed by Pelayo and Wiens (1989) for western Scotia (see Fig. 2) includes active back-arc spreading in the Bransfield Strait, slow subduction along an active South Shetland Trench, and diffuse convergence between Antarctic and Scotia plates in the Drake passage. The edge of the Bransfield back-arc is defined by a spreading centre with which Deception, Penguin and Bridgeman islands and some submerged volcanic vents are associated. The geochemical character of volcanic rocks from these islands is transitional between calc-alkaline and MORB (Weaver et al., 1979). This fact appears to be consistent with a model of mantle diapirism and crustal fracturing during the first stages of back-arc spreading (Weaver et al., 1979). Deception Island is a young (<0.75 Ma, Smellie, 1988 ) horseshoe-shaped stratovolcano, 25 km in submerged basal diameter (Smellie, 1988) and about 15 km in diameter for the emerged zone, and has been very active during its entire evolution, although different

92

periods of activity can be distinguished. It is presently the most active volcano of the South Shetland Islands-Antarctic Peninsula group, with eruptions known to have taken place in 1848, 1967, 1969 and 1970, and others that are maybe wrongly dated in 1912 and 1917. The composition of the Deception Island volcanic rocks is quite variable, ranging from basalt to rhyodacite. However, the basic and intermediate rocks are the most abundant volcanic products on this island (Weaver et al., 1979). Previous studies on the petrology and volcanology of Deception Island (e.g. Hawkes, 1961; Gonz~ilez-Femin and Katsui, 1970; Baker et al., 1975; Smellie, 1988, 1989) have suggested the existence of a caldera structure in the central part of the island which is thought to have been formed by the collapse of several pre-existing volcanic edifices (Hawkes, 1961 ) or by the collapse of a central stratovolcano (Gonz~ilez-Ferr~in and Katsui, 1970; Baker et al., 1975 ). Subsequent to this, volcanic activity would have continued, associated with the concentric faults around the edge of the caldera (Smellie, 1988). Recently, Marti and Baraldo (1990) have shown the first results of an integrated study of the geology and geophysics of Deception which has been developed by an international working group since 1986, and these authors have proposed a new stratigraphy for this volcanic island. These authors divide the island into two main groups of rocks, pre- and post-caldera, respectively, identifying two formations in the pre-caldera group: the Basaltic Shield Formation (BSF) and the Yellow Tuff Formation (YTF). The BSF mainly represents the unexposed basement of the island, which is formed of basaltic rocks. The YTF constitutes the main part of the island and comprises thick sequences of palagonitised pyroclastic flow deposits of andesitic composition and dry surge deposits with minor air-fall deposits and lava flows. The YTF can be divided into a lower member of massive pyroclastic flow deposits which form a nearly continuous outcrop that extends from

J. VILA ET AL.

Macaroni Point to the south of Entrance Point (see Fig. 3 ), and an upper member formed of pyroclastic surges and some associated air-fall and thin pyroclastic flow deposits, only appearing in small exposures. Phreatomagmatic episodes were common during the formation of the YTF, which seems to be derived, at least as for the lower member is concerned, from the explosive activity of a central vent which produced a radial distribution of its products. This central vent would have been destroyed during the formation of the caldera. Post-caldera volcanism, which occurs inside and outside the caldera rim, is mainly represented by several small vents which have acted independently, generating both strombolian and phreatomagmatic deposits. The present-day tectonic structure of Deception Island is defined by a nearly orthogonal normal fault system (Fig. 4). The set of faults oriented NE-SW can be interpreted as a

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Permanent Ice Post-Coldercl(ond Glacier) deposits Pr e-Cclidero deposits

Fig. 3. Simplified geological map of Deception Island, showing the distribution of the pre- and post-caldera volcanic deposits (after Marti and Baraldo, 1990).

VOLCANICTREMORSATDECEPTIONISLAND

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0 (63 03'24"5 , 60 55'24"W)

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Fig. 4. Simplified tectonic map of Deception Island, based on field geology, remote sensing and continuous seismic reflection studies (Marti et al., in prep.), showing the distribution of the main normal faults (ticks on downthrow side) and structural lineations (straight lines) as well as submerged volcanic vents (in black).

result of back-arc spreading in the Brans field Strait, whereas the N-S normal faults could be the consequence of the previous tectonic situation combined with compression still exerted by subduction at the South Shetland Trench (Marti et al., in prep. ). This fault system seems to have been a significant controlling factor on the distribution of volcanic activity during the entire evolution of the island. In addition, most of these faults are still active, as can be deduced from microseismicity analysis (Vila, 1992). The existence of the central depression, which has been interpreted as the result of an episode of caldera collapse (e.g. Hawkes, 196 l; Gonzfilez-Ferrfin and Katsui, 1970; Baker et al., 1975; Smellie, 1988, 1989), is clearly defined by morphological features, the bathymetry of Port Foster and continuous seismic reflection studies (Marti et al., in prep.). However, regional tectonics which are mainly determined by the Bransfield Strait back-arc spreading have exerted a strong influence on the evolution of Deception Island and may have masked the significance of local tectonics

related to the movement of magmas. In this sense, regional tectonics, which at Deception Island caused extension in all directions, apparently played a significant role in the formation of the central depression, or at least in modifying the original structure of the island. Another significant aspect in the geology of Deception Island is the existence of several shallow and confined water-saturated layers. These aquifers have been inferred from the presence of hydrothermally altered lithic clasts in the pyroclastic deposits (Marti and Baraldo, 1990) and by the study of the chemical composition of low-temperature fumaroles and thermal springs which occur at different points of the island (Martini and Giannini, 1988). The formation of these aquifers would be favoured by the existence of sea water in the central part of the island, and also by the high thermal gradient characteristic of Deception Island which favoured the melting of snow and percolation and accumulation of fresh water through the pyroclastic deposits. This may also explain the predominance of phreatomagmatic eruptions at Deception Island.

94

Seismic instrumentation During the Antarctic Summer Expeditions 1986-1987 and 1987-1988, a network of five analog drum recorder stations were installed in order to evaluate the seismic activity at Deception. In the 1988-1989 survey a digital seismic network was set up, consisting of 6 FM telemetry stations set at the VHF 170 MHz band. The electronic instruments and the software used were especially developed to operate successfully under Antarctic conditions (Ortiz and Vila, 1990 ). Figure 5 shows the location of the seismic digital network. The analog network (1987 and 1988 surveys) has the same stations except GNS. The absence of other radio emissions in the zone allowed low-power transmitters, from 10 to 20 mW, to cover distances of over 10 km. Three-element directional antennas were used for the transmitters and omnidirectional antennas for the receivers. The central unit included the set of receivers, a demodulator and the digital recording system. The digital unit used a 14-bit digitalanalogical converter controlled by a dedicated CMOS microprocessor, which operated at 64 or 128 samples per second. Data were digitally

Fig. 5. Location of the seismic digital network installed in 1988-1989 and subsequent surveys. The analogical network ( 1987 and 1988 surveys) have the same stations except GNS.

J. VILAET AL.

recorded on a portable PC, on RAM-DISK, and periodically transferred to a 3.5" floppy disk. The detection algorithm employed was the STA/LTA (Short Term and Long Term Average, Lee and Stewart, 1981 ) to which were added digital high-pass and low-pass filters. All the geophones were of the vertical component, 1 Hz, Mark Productions L4C type.

Seismicity at Deception Island The successive surveys (Vila, 1992; Vila et al., 1992) showed that the seismic activity around Deception Island remained stationary, at approximately 1,000 seismic events per month. In general, these appear in clusters, i.e. a series of events occurred in a small zone, separated by short intervals of time. The distribution of the elapsed time events (Vila, 1992) shows that the interval of time between earthquakes was less than 10 min. The large proportion of intervals of several minutes between events and the departure from a Poisson distribution suggest a clustering of seismic events in the time sequence (see e.g. Del Pezzo et al., 1984). However, the distribution of located events with RMS less than 0.35 s, shown in Figure 6, lies around the system of fractures which crosses the island (see Fig. 4), emphasizing the fact that most of the faults are still active. Particularly significant is the lineation of a large number of epicentres parallel to the main NE-SE lineated fault system which corresponds to the regional tectonic trend defined by the Bransfield Strait Rift (see e.g. Pelayo and Wiens, 1989). Because of this system of fractures, it is impossible, from the point of view of seismology, to separate Deception Island from the rest of the Brans field Rift. The predominance of pyroclastic deposits and the horseshoe morphology of the island provides a high level of background noise, so it was not possible to operate the seismic instrumentation with a high gain. Thus, many events were only registered by one or two stations and could

95

VOLCANIC TREMORS AT DECEPTION ISLAND

TABLE 1 (9

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Fig. 6. Distribution of located events with RMS less than 0.35 s corresponding to the 1988-1989 survey. The events are distributed throughout the island, grouped by zones and generally aligned in accordance with the directions of the main fractures (see text and Fig. 4).

not be located, although they are still useful for statistical analysis. The magnitude M of the events was calculated from the duration of the event by means of the expression (Lee and Stewart, 1981 ): M = 21ogT+ 0.00356- 0.87 where T is the duration of the event in seconds and 6 is the epicentral distance in kilometres. Measured magnitudes range between 0.1 and 2.2, the lower boundary depending on the meteorological conditions that determined the noise level. Exceptionally, in 1991 a 3.2 magnitude local event occurred at the NE sector of the island. The corresponding seismic energy E was calculated by the expression (Richter, 1958): logE= 9.9 + 1 . 9 M - 0.024M 2 where M is the local magnitude as computed previously. It is shown that during the periods under study, the released energy remains constant, with an average value of 3.0E 13 erg/day (Vila, 1992). The number of events N occurring for a given magnitude M has been used for the determi-

In applying this expression only events occurring inside a radius of less than 20 km have been used. As can be seen in Table 1, results of the linear fit applied to the 1986-1987 and 1987-1988 surveys show great stability for parameters a and b, as well as a large correlation coefficient r. Volcanic tremors From the point of view of the seismograms, the distinction between earthquakes and tremors has basically been dependent on duration, although there is no definition for its specific time duration. Recently, however, Seidl et al. (1990) have reported seismic tremors, with the above-mentioned spectral characteristics, of very short duration, and developed a method for its spectral analysis based on MESA (Maximum Entropy Spectral Analysis). Thus, time duration can no longer be used to identify volcanic tremors. As stated by Schick and Mugiono ( 1991, p. 2 ), "volcanic tremor is only a collective name for numerous kinds of seismic signals generated by volcanic activity", the only common characteristics being their spectral contents. In the context of the present study, the recorded events have been classified in relation to the morphological characteristics of the signal, that is, a small variation of the envelope in terms of time (du-

96

J. VILAETAL.

ration of the signal) and large stability of its spectral contents. As we have indicated above, volcanic tremors are frequently recorded at Deception Island, especially by the stations located at those zones with a higher thermal gradient and fumarolic emissions. These stations are BAS and FUM, located at Fumarole Bay, and TEL and RBT which are close to the last eruption centres ( 1967, 1969 and 1970). The tremors recorded at each zone have different characteristics. While the tremors recorded at BAS and F U M tend to be of long duration (more than 20 min) and are characterized by low-frequency contents, tremors recorded at TEL and RBT are of a shorter duration (from tens of seconds to a few minutes) and show higherfrequency contents, and very often are triggered by small earthquakes. Low-frequency tremors with a duration of less than one minute are usually recorded in both zones. One of the most interesting features of the tremors recorded at Deception Island is that the increase of the amplitude at the beginning of the tremor and its decrease at the end occurs in a few seconds, independently of the duration of the tremor, whereas in other volcanic areas this interval may be as long as 450 s (Schick et al., 1982a, b ). Figure 7 shows a small portion of a long-duration event of about 40

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TIME (s) Fig. 7. Examples of tremors. (A) Small portion of longduration tremor of about 40 min recorded at BAS station on January 29, 1988. (B) Sample of noise recorded at the same station before the occurrence of the tremor. Both registers were recorded with an instrumental gain of 66 dB. It is easily seen that the amplitude of the noise is lower than the tremor (signal/noise ratio > 2).

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min (A) along with a sample of background noise previous to its occurrence (B). In general, the detection of a tremor is easy when there is a good signal-to-noise ratio (as can be seen in Fig. 7 ) and by considering the fast increase of the amplitude of the recordings when the event starts. However, in many cases the variation of the amplitude of the tremor is more difficult to observe because the severe meteorological conditions provide high-amplitude noise, with low-frequency components provided by the surrounding sea and high frequencies by the wind. Although it is not helpful for the identification of the tremors, when a presumed tremor has occurred (usually identified by a characteristic frequency of the tremor superimposed on the frequency contents of the background noise), a comparison made in the frequency domain between the presumed tremor and the preceding noise gives us the tremor as a residual spectrum, defined as the signal minus the background noise. In order to model the long-duration tremors, a study of their occurrence was made taking into account several factors, such as the local seismicity of the whole island, meteorological conditions or studies of seismic series located near the stations where the tremors were re-

VOLCANIC TREMORS AT DECEPTION

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Fig. 9. Spectral analysis. (A) F F T spectral analysis of the long-duration tremor with signal/noise ratio > 2 (see Fig. 7 ). The corresponding residual of a tremor (solid line) is obtained by subtracting from the averaged spectra of samples of tremors (long-dashed line) the averaged spectra of samples of noise (short-dashed line). (B) F F T spectral analysis of the long-duration tremor with signal/ noise ratio ~ 1 (see Fig. 8) by applying the same method as in (A).

dently and their spectra averaged. To eliminate the noise contents of the tremor, spectra of noise prior to and after the occurrence of the tremor were also averaged and subtracted from the averaged spectrum of the tremor. Thus, periodically, samples of noise had to be taken in order to apply this method successfully. Figure 9 shows two examples of the spectral amplitude of a tremor, the spectral amplitude of the preceding noise and the corresponding residual for a tremor of high signal/noise ratio (A) and low signal/noise ratio (B). Other types of tremors recorded at Deception Island, not yet analyzed, are of shorter duration than those shown in Figures 7 and 8. Figure 10A shows a tremor quite similar to a surface wave train (LP-event). This type of event was recorded very often at the island and no correlation with other seismic events was observed. Figure 10B shows the sudden start of a long-duration tremor with a short-duration signal of high-frequency contents. This feature was often observed. Figure 11 shows the FFT spectral analysis of the events of Figure 10. In both cases the spectra show a well-defined peak that allows the distinction of shortduration tremors from small earthquakes. Figure 12 shows a similar situation as in Figure 10B. Origin o f tremors at D e c e p t i o n Island

corded. Observing all these factors, a presumable but not deterministic relation between seismicity and tremors was found, a tremor very often being preceded or followed by an increase in the local seismic activity, and in some cases by seismic swarms. In addition, events of different characteristics can appear superimposed on the tremor (Vila, 1992). As this type of tremor is of long duration, a complete record of all of them could not be obtained with the digital recorders available at Deception Island. The method employed for their analysis consisted in obtaining several records at different intervals of time during the tremor activity. The records were then processed indepen-

The origin of the tremors is not yet well understood and the models proposed for their explanation, which include oscillations of magmatic chambers, resonance in the emission ducts or geothermal noise produced in the uppermost ducts by gas phases, or magma intrusions, are based upon the comparison of the characteristics of the tremor with physical phenomena caused by similar sources to those existing in volcanic areas. Bearing in mind the spectral characteristics of the tremors analyzed and their duration, and according to the geological characteristics of Deception Island, the existence of shallow

98

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Fig. 10. Example of volcanic tremors. (A) Volcanic tremor, similar to a surface wave trend (LP event), with a span of less than one minute, recorded at station FUM on February 18, 1990. (B) Sudden start of a long-duration tremor recorded at station BAS on January 26, 1990. The tremor began with a short high-frequency content (arrow). Instrumental gain is 66 dB, and the Y-axis units are of 14-bit D/A converter (213 D/A units correspond to 10-3 cm/s). (B)

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Fig. 11. Spectral analysis. (A) is the FFT spectrum of the event shown in Fig. 10A, and (B) is the FFT spectrum of the event shown in Fig. 10B. Both spectra have been computed using a 4 s Hanning window, centred at 7 and 10 s, respectively: they show a very well-defined peak. The existence of this peak permits the distinction of short-duration tremor from small seismic events. aquifers ( M a r t i n i a n d G i a n n i n i , 1988 ) a n d the lack o f e v i d e n c e o f m a g m a m o v e m e n t s , we p r o p o s e t h a t the origin o f the l o n g - d u r a t i o n t r e m o r s m a y be a s s o c i a t e d with the geotherm a l noise o r i g i n a t e d in the u p p e r m o s t d u c t s o f the f u m a r o l i a n system, associating t h e surfi-

cial d u c t s to h a r m o n i c oscillators which, e i t h e r c o m b i n e d in a s u p e r p o s i t i o n o r i n d i v i d u a l l y , m a y a c c o u n t for the spectra o f the seismic signals as well as for t h e i r t e m p o r a l stability. A q u a n t i t a t i v e e x p l a n a t i o n for this t y p e o f volcanic t r e m o r s is given b y Seidl et al. ( 1981 ),

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and further generalized by Schick et al. (1982a), in terms of a radiation formula which describes the observed spectra in terms of a superposition of n sources in the form: n

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-Mi/J2)exp (-Dill)

1

where A is the amplitude of the ground velocity, Aoi is the source strength of i-th source, v is the frequency, Ni is the _IV,-_~order of i-th source, equal to 1 for a monopole source, M, is the space-time coherence factor of i-th source and Dg is the attenuation factor along the wave path from i-th source to the recording station. In order to fit this model to the observed data (the ground velocity amplitude), a non-linear iterative process (Bevington, 1969) was applied. The iterative process was stopped when the variation of the Z 2 was less than 1% of the previous X2.Z 2 represents the precision of the fit and is defined as:

where Yi are the observations, y (x,.) the predictions of the model and a,- are the fluctuations ofyi about the expected values < y i > .

In all cases, the best fit of this model to observations is achieved by considering a single term ( i = 1 ). Figure 13 displays the model that best fits the residual tremors presented in Figure 9, and Table 2 gives the numerical values of the parameters. The fact that only one term is needed in fitting observations indicates that the motion is basically unidirectional (Schick et al., 1982a, b), as is to be expected in the degasification of an aquifer. As shown in Figure 13, it is a common feature that, despite the original differences in amplitude of the analyzed tremors, the residuals have similar characteristics, displaying predominant frequencies whose values show

TABLE 2 Numerical values of the best geothermal noise model fit (Seidl et al., 1981 )

A N M D Z2

S/N ratio ~ 1 (January 16, 1988)

S/N ratio > 2 (January 29, 1988)

70.9 1.39 0.022 0.167 410

154.4 0.80 0.01 0.217 1329

1 0 0

J.

E E

Signal/Noise ratio ) 2 •

200-

Station: BAS

150. E3 ~1000<~

50-

,

,

2

,

i

4

,

i

6

,

i

8

,

,

10

,

i

12

,

FREQUENCY (Hz)

i

14

,

r

16

,

r

18

(B)

E

250-

Signal/Noise

200 -

Station: BAS Date: February 16, 1988

ratio

~ I

~ 150~100-

50-

0

,

,

2

,

,

4

,

,

6

,

,

8

,

~

10

,

J

12

,

,

14

,

,

16

,

,

18

FREQUENCY (Hz) Fig. 13. Acoustic model. (A) The acoustic emission model (line) for the residual tremor shown in Figure 9A. (B) The acoustic emission model for the residual tremor shown in Figure 9B. Asterisks are experimental data and line the predictions from the deduced model. Although (A) corresponds to a signal/noise ratio > 2 and (B) corresponds to a signal/noise ratio~ 1, both fits are quite similar (see Table 2 ).

very small variations• Taking this observation into account, one may apply the classic expressions for the frequency of emission of an organ-pipe resonance from an open-ended conduit: /)_

ET

AL.

value into account, length L of the pipe ranges between 100 and 200 m, values compatible with those observed in fields of fumaroles (Martini and Giannini, 1988). On the other hand, because the temperature of the aquifer is known, by applying the Clausius-Clapeyron equation:

(A) 250-

V I L A

C

4L

where v is the frequency of the emission, c is the velocity of sound in the medium and L is the length of the pipe. Assuming that the medium where the tremor originates is a mixture of water and steam at 200°C (Martini and Giannini, 1988 ), the velocity of the sound in the mixture will be slower than the velocity of the sound in water (v ~ 1500 m / s ) . Taking this

dp L12 d T - T ( V 2 - V1) and assuming that the variation of pressure is due to a hydraulic column that maintains the water in a liquid state at 200°C, a column length of the order of 200 m is obtained. The values obtained with both expressions are in good agreement. Small differences in the location of maxima of spectra, such as those shown in Figure 13, may be attributed to the different site location of the events• Bearing in mind the fact that the resolution of the spectrum is of 0.5 Hz, a difference such as the one observed means a variation of approximately 20 m in the value of L, which is included in the experimental error• The secondary peaks may be interpreted as disturbances in the emission ducts which cause the generation of non-fundamental frequencies (see e.g. Schick et at., 1982a, b). Conclusions There is significant seismic activity at Deception Island, with numerous events located on the island, with magnitudes ranging from 0.1 to 2.2 and distributed along the main fractures of the island, in good agreement with the regional structural features of the Bransfield Strait back-arc. The seismic events appear in clusters, in space as well as in time. Volcanic tremors recorded at Deception Island are well characterized by the stability of their high spectral contents, with few and welldefined spectral peaks. Even though the origin of the tremors is still not well understood, if we consider the location of the stations that usually recorded tremors, as well as the correla-

VOLCANICTREMORSATDECEPTIONISLAND

tion of the seismic noise with the tremors and the geological characteristics of Deception Island, we can suggest that the tremors are generally associated with the geothermal noise originated in the uppermost ducts of the fumarolian system. By associating surficial ducts to harmonic oscillators which account for the spectrum of the seismic signal as well as for its temporal evolution, it can be seen that the motion is basically unidirectional, as is to be expected in the degasification of an aquifer. Taking into account the observed main frequencies, and by applying the classical mechanical expressions of organ pipes, the resulting pipe length agrees with the value obtained by using the ClausiusClapeyron equation to determine the length of the water column needed to maintain the equilibrium. Other types of tremors recorded at Deception Island are of shorter duration and very often are triggered by seismic events. They also show a very large stability in their frequency contents, allowing for the distinction between short-duration tremors and small earthquakes. Acknowledgements We would like to thank C.A. Rinaldi and the "Instituto Ant~rtico Argentino", for helping us in the logistics and subsistence arrangements for the field seasons. R. Scandone is thanked for the helpful review of the manuscript. This paper was supported by the Spanish Antarctic Program under Grant ANT88-0303-C and ANT89-0828-E, and by the DGICYT under Grant PB90-0599-C03-03. References Aki, K., Fehler, M. and Das, S., 1977. Source mechanism of volcanic tremors, fluid driven crack models and their application to the 1963 Kilauea eruption. J. Volcanol. Geotherm. Res., 2: 259-287. Ashcroft, W.A., 1972. Crustal structure of the South Shetland Islands and Bransfield Strait. Br. Antarct. Surv. Sci. Rep., 66, 43 pp.

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Parra, J.C., Gonz~ilez-Femin, O. and Bannister, J., 1984. Aeromagnetic survey over the South Shetland Islands, Bransfield Strait and part of the Antarctic Peninsula. Rev. Geol. Chile, 23: 3-20. Pelayo, A. and Wiens, D., 1989. Seismotectonics and relative plate motions in the Scotia Sea region. J. Geophys. Res., 94: 7293-7320. Richter, C., 1958. Elementary Seismology. Freeman, San Francisco, Calif., 758 pp. Schick, R. and Mugiono, R. (Editors), 1991. Volcanic Tremor and Magma Flow. Forschungszentrum Jiilich GmbH, 200 pp. Schick, R., Cosentino, M., Lombardo, G. and Patane, G., 1982a. Volcanic tremor at Mount Etna--a brief description. Mere. Soc. Geol. Ital., 23:191-196. Schick, R., Lombardo, G. and Patane, G., 1982b. Volcanic eruptions and shocks associated with eruptions at Etna (Sicily), September 1980. J. Volcanol. Geotherm. Res., 14: 261-279. Seidl, D., Schick, R. and Riuscetti, M., 1981. Volcanic tremors at Etna, a model for hydraulic origin. Bull. Volcanol., 44: 43-46. Seidl, D., Kirbani, S.B. and Brtistle, W., 1990. Maximum entropy spectral analysis of volcanic tremor using data from Etna (Sicily) and Merapi (Central Java). Bull. Volcanol., 52: 460-474. Shaw, H.R., 1980. The fracture mechanisms of magma

J. VILA ET AL.

transport from the mantle to the surface. In: R. Hargraves (Editor), Physics of Magmatic Processes. Princeton University Press, Princeton, N.J., pp. 201-264. Smellie, J.L., 1988. Recent observations on the volcanic history of Deception Island, South Shetland Islands. Br. Antarct. Surv. Bull., 81: 83-85. Smellie, J.L., 1989. Deception Island. In: I.W.D. Dalziel (Editor), Tectonics of the Scotia Arc, Antarctica. Field Trip Guidebook T 180, 28th Int. Geol. Congr. American Geophysical Union, Washington, D.C., pp. 146t52. Tarney, S.D., Saunders, A.D., Pankhurst, R.J. and Barker, P.F., 1982. Volcanic evolution of the northern Antarctic Peninsula and Scotia arc. In" R.S. Thorpe (Editor), Andesites. John Wiley, Norwich, pp. 371400. Vila, J., 1992. Estudis Geoflsics a l'Illa Decepci6n, Shetland del Sud, Ant/trtida. Ph.D. Thesis, University of Barcelona, 187 pp. Vila, J., Ortiz, R., Correig, A.M. and Garcia, A., 1992. Seismic activity at Deception Island. Proc. 6th Int. Symp. Antarctic Earth Sciences, NIPR, Japan, in press. Weaver, S.D., Saunders, A.D., Pankhurst, R.J. and Tarney, J., 1979. A geochemical study of magmatism associated with the initial stages of back-arc spreading. The Quaternary volcanics of Bransfield, from South Shetland Islands. Contrib. Mineral. Petrol., 68:151169.