The Recent Seismo-Volcanic Activity at Deception Island Volcano

ARTICLE IN PRESS

Deep-Sea Research II 50 (2003) 1611–1629

The recent seismo-volcanic activity at Deception Islandvolcano

Jesus! M. Iban! ˜ ez*, Javier Almendros, Enrique Carmona, Carmen Mart!ınez-Arevalo,! Miguel Abril Instituto Andaluz de Geof!ısica, Campus de Cartuja s/n, Universidad de Granada, 18071 Granada, Spain

Received14 October 2002; receivedin revisedform 10 December 2002; accepted13 December 2002

Abstract

This paper reviews the recent seismic studies carried out at Deception Island, South Shetland Islands, Antarctica, which was monitoredby the Argentinean andSpanish Antarctic Programs since 1986. Several types of seismic network have been deployed temporarily during each Antarctic summer. These networks have consisted of a variety of instruments, including radio-telemetered stations, autonomous digital seismic stations, broadband seismometers, and seismic arrays. We have identified two main types of seismic signals generated by the volcano, namely pure seismo- volcanic signals, such as volcanic tremor andlong-period(LP) events, andvolcano-tectonic (VT) earthquakes. Their temporal distributions are far from homogeneous. Volcanic tremors and LP events usually occur in seismic swarms lasting from a few hours to some days. The number of LP events in these swarms is highly variable, from a background level of less than 30/day to a peak activity of about 100 events/h. The occurrence of VT earthquakes is even more irregular. Most VT earthquakes at Deception Island have been recorded during two intense seismic crises, in 1992 and 1999, respectively. Some of these VT earthquakes were large enough to be felt by researchers working on the island. Analyses of both types of seismic events have allowed us to derive source locations, establish seismic source models, analyze seismic attenuation, calculate the energy andstress dropof the seismic sources, andrelate the occurrence of seismicity to the volcanic activity. Pure seismo-volcanic signals are modelled as the consequence of hydrothermal interactions between a shallow aquifer and deeper hot materials, resulting in the resonance of fluid-filled fractures. VT earthquakes constitute the brittle response to changes in the distribution of stress in the volcanic edifice. The two VT seismic series are probably related to uplift episodes due to deep injections of magma that did not reach the surface. This evidence, however, indicates the high potential for future volcanic eruptions at Deception Island. r 2003 Elsevier Science Ltd. All rights reserved.

Contents

1. Introduction ...... 1612 2. Seismicity of Deception Island...... 1615

*Corresponding author. Tel.: +34-958-248910; fax: +34-958-160907. E-mail address: [email protected] (J.M. Iban! ˜ ez).

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3. Review of recent seismic studies ...... 1618 3.1. History of seismic monitoring at Deception Island...... 1618 3.2. Seismic antenna surveys ...... 1621 3.3. Seismic attenuation studies ...... 1621 4. The 1999 seismic crisis ...... 1622 5. Source models ...... 1624 5.1. Model for pure volcanic signals ...... 1624 5.2. VT earthquake source model ...... 1625 6. Conclusions ...... 1626 Acknowledgements ...... 1626 References...... 1627

1. Introduction central basin is markedby a series of seamounts between Deception andBridgeman Islands,many Deception Islandis a volcanic islandlocatedat of which have recently been active (Gracia et al., 62590S and60 410W in the South Shetland 1996; Lawver et al., 1996; Gracia et al., 1997). Islands region (Fig. 1). It constitutes a back-arc However, the crustal thickness of 12–30 km is stratovolcano with a basal diameter of B30 km. consistent with extrusion through riftedcontinen- The volcano rises B1400 m from the seafloor to a tal crust rather than seafloor spreading (Bialas maximum height of 540 m above sea level. The 15- et al., 1990; Gradet al., 1992 ; Barker andAustin, km-diameter island is horseshoe-shaped and dis- 1994; Gradet al., 1997 ; Janik, 1997; Barker and plays a flooded caldera (Port Foster) with dimen- Austin, 1998; Prieto et al., 1998; Barker et al., sions of about 6 10 km andmaximum depthof 2001). Basedon the seismicity andvolcano spatial 190 m. The caldera wall is breached by a 500-m- distribution, Barker andAustin (1998) have wide passage named Neptune’s Bellows. Glaciers suggestedsouth-westwardpropagation of the cover almost half of the island, mainly on Mount BransfieldRift. The NW-SE extension of PondandMount Kirkwoodin the east andsouth, B10 mm/yr has been attributedto rollback of respectively. the subducted slab (Barker andAustin, 1998 ; The geodynamic setting of the Deception Island Dietrich et al., 2001). region is characterizedby interactions among The seismicity in BransfieldStrait has charac- small tectonic units. The Drake microplate (Fig. teristics of rift extension, subduction, and volcan- 1) represents the remnants of a subduction that ism. Virtually all earthquakes in the region are once extended beneath the western margin of the either shallow (o40 km depth) and consistent with Antarctic Peninsula. This subduction process rifting, or deep and consistent with subduction of finishedprogressively from southwest to north- the Drake plate (Pelayo andWiens, 1989 ; Iban! ˜ ez east, but a slow subduction activity at the South et al., 1997). Many of the shallow earthquakes ShetlandTrench, northwest of the South Shetland tendto cluster near volcanoes, indicating a likely Islands, still continues (e.g., Barker, 1982; Pelayo volcanic or volcano-tectonic origin (Pelayo and andWiens, 1989 ; Kim et al., 1995; Lawver et al., Wiens, 1989; Robertson et al., 2001). Seismic 1995; Robertson et al., 2001). activity is specially pronouncedat Deception The BransfieldStrait is a consequence of the Islandvolcano ( Pelayo andWiens, 1989 ; Lee rifting andseparation between the South Shetland et al., 1998; Robertson et al., 2001). microplate andthe Antarctic Peninsula. There are Most rocks on Deception Islandhave been three active extensional basins that show evidence classified relative to the undated caldera-forming of volcanism andnormal faulting. The axis of the eruption (Mart!ı andBaraldo,1990 ). All exposed ARTICLE IN PRESS

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Fig. 1. Location of Deception Islandin the South ShetlandIslandsregion andmap of Deception Islandshowing the main volcanological features. ARTICLE IN PRESS

1614 J.M. Ibań˜ez et al. / Deep-Sea Research II 50 (2003) 1611–1629 rocks have normal magnetic polarity, indicating coastline. In support of this model, Rey et al. that they are younger than 700 ka; the oldest dated (1995) and Mart!ı et al. (1996) note the absence of age of the pre-caldera deposits is 200 ka (Keller circular sections of the postulatedring fault and et al., 1991). The standard sequence has been radial dikes and fractures within the caldera. They developed by Birkemajer (1992), andcorrelated infer that nearly all the post-caldera eruptions with shallow-penetration seismic reflection sec- have been locatedon linear faults, some of which tions in Port Foster by Rey et al. (1997). The pre- lie outside the hypothetical ring fault, while others caldera rocks are exposed on sea cliffs and parts of cut the caldera. One implication of their model is the caldera wall, and are composed of tuffs and that the caldera may be underlain by much less agglomerates with interspersedlava flows and extensive andperhaps discontinuous magma scoria horizons. The post-caldera deposits include reservoirs whose position is controlledby the basaltic lavas andscoria at higher elevations and location of faults. pyroclastic cones aroundPort Foster where the Deception Islandis the most active volcano of magma interactedwith seawater. It is generally the South ShetlandIslandsregion, having erupted accepted that the post-caldera deposits have been at least 6 times since it was first visited160 years controlledby the ring-fracture system, with most ago. All historical eruptions have been relatively eruptions occurring near the margins of the small in volume. They have occurredat locations caldera (Baker et al., 1969; Smellie, 1990). near the coast of the inner bay, all aroundthe The caldera at Deception Island volcano has caldera (Fig. 1). Some of the eruptive episodes been traditionally described as a classic example of have included simultaneous eruptions of chemi- collapse caldera that formed about a ring fracture cally distinct lavas from multiple vents (Newhall following one or more voluminous eruptions of andDzurisin, 1988 ). The first report of an eruption andesitic magma (Baker et al., 1975; Smellie, at Deception Islanddatesback to 1842, when a 1988). However, there are other models of caldera ‘‘wall of fire’’ was observedby whaling ships, formation, for example the incremental growth in relatedto a set of craters that formedbelow Mt. response to a series of moderate-sized eruptions Kirkwoodin the southwest part of the island (e.g., Walker, 1984). On the basis of the structural (Roobol, 1973). From ice records at surrounding data and other lines of evidence, Rey et al. (1995) islands, explosive eruptions are inferred to have and Mart!ı et al. (1996) argue that the Deception occurredin 1912 and1917 ( Orheim, 1972). The ice Island caldera is a tectonic depression that was recordin James Ross Island,200 km from Decep- formedprogressively by passive normal faulting tion (Aristarain andDelmas, 1998 ), suggests that along sets of nearly orthogonal normal faults that an important eruption occurredin the 19th cut across the islandandfollow the regional century. This eruption may have been the largest trends. Structural mapping and shallow-penetra- one on the islandin the last 350 years. Three tion seismic reflection studies within Port Foster eruptions between 1967 and1970 were observed show that the architecture of Deception Islandis directly and are well documented. In December controlledby three major fault systems ( Rey et al., 1967, two vents developed simultaneously at sites 1997). The first one is a NE–SW striking system located B2 km apart. One of them was a that is consistent with the regional extensional submarine eruption that gave birth to a new island regime of BransfieldStrait andcontrolledthe vent in Telefon Bay, while the other occurredinland, alignment during the 1967 and 1970 eruptions. between Telefon Bay andPendulum Cove. A Several faults in this system cut across the caldera. seconderuption occurredin February 1969, when A secondsystem strikes at B120N, defines the fissures openedin the ice on the west-facing slopes caldera wall near Mount Kirkwood in the south, of Mt. Pond, accompanied by pyroclastic emis- andparallels an axis of submarine volcanic cones sions. The last eruption took place in August 1970, in the southern part of Port Foster. A thirdsystem when additional activity along the northern edge strikes at B170N, andincludesthe Macaroni of Telefon Bay formeda chain of new craters and fault that forms an 8 km length of the island’s NE modified the coast line. ARTICLE IN PRESS

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The chemical differences observed between the the local seismicity has been classifiedin two main lavas suggest that the eruptions were fedby small groups: volcano-tectonic (VT) earthquakes and isolatedmagma reservoirs ( Roobol, 1979). Like- long-period(LP) seismicity (which includesLP wise, the synchronicity of some historical erup- events andvolcanic tremor). A thirdgroup of tions andthe fact that they occurredthroughout seismo-volcanic events are namedhybrids, since the caldera are most easily explained if the they share characteristics from both of the main volcanic activity resultedfrom small magma groups. bodies that rose up from a large magma chamber VT earthquakes are local earthquakes generated extended across the whole caldera. Leaks of silicic within the volcanic edifice. At Deception Island, magma along ring fractures have preceded some for example, they usually display S2P times large caldera-forming eruptions (e.g., Bacon, smaller than 3–4 s. VT earthquakes are dominated 1985), andone couldspeculate that the recent by a double-couple mechanism, and often char- activity at Deception Islandmay be a long-term acterizedby impulsive P-wave onsets andclear S precursor of a larger eruption. The most accepted arrivals. The spectral content is broad, with interpretation, however, assumes that Deception significant energy up to 30 Hz. In Fig. 2 we plot Islandis presently in an early stage of the infilling three examples of different types of tectonic of the caldera by small-volume eruptions (Roobol, earthquakes recorded at Deception Island. Note 1982). the clear differences in the temporal and spectral Evidences for present-day volcanic activity at signatures. The hypocenters of VT earthquakes are Deception Island include fumaroles and hydro- not homogeneously distributed at Deception thermal activity (Fig. 1), resurgence of the floor of Island. For example, Vila et al. (1992, 1995) Port Foster, andseismicity. Fumaroles andhot provided a map of the seismicity for the 1986– springs with temperatures up to 110C encircle 1991 periodshowing epicenters locatedall over the Port Foster, while hydrothermal vents have been island; Iban! ˜ ez et al. (2000) locatedearthquakes reportedoutsidethe island( Smellie, 1990). The recorded during the period 1994–1998 along a analysis of repeatedbathymetric surveys of Port NE–SW fracture system near the Spanish base; Foster obtainedbetween 1949 and1993 allowed andfinally Iban! ˜ ez et al. (2003) locatedthe Cooper et al. (1998) to identify three regions that epicenters of VT earthquakes belonging to the are shallowing at rates of up to 0.5 m/yr. High 1999 series, which were confinedmostly to Port rates of shallowing in near-shore regions to the Foster. VT earthquakes at Deception Islandare north of Fumarole Bay andin Telefon Bay could usually shallow, low-magnitude events. However, be entirely due to sedimentation, but a region of some earthquakes with magnitude greater than 3.5 rapidshallowing in NE Port Foster is attributedto have been recorded, and there is evidence of the volcanic resurgence. Uplift has continuedmore occurrence of larger earthquakes near Deception recently in this area, with 5 m measuredbetween Island, with magnitudes greater than 5.0 (Pelayo 1990 and1999. This region lies within an EW- andWiens, 1989 ). The source of the VT earth- trending belt of earthquake epicenters across Port quakes is associatedto the brittle failure of parts of Foster (Vila et al., 1992; Iban! ˜ ez et al., 2003), the volcanic edifice in response to changes in the displays high seismic attenuation in the shallow- distribution of local or regional stresses. Stresses most crust (Vila et al., 1995), andis site of may change for example due to interactions of magnetic andgravity anomalies ( Ortiz et al., water with hot materials (Vila et al., 1995; Correig 1992). et al., 1997) or to the effects of shallow magma injections (Ortiz et al., 1997; Iban! ˜ ez et al., 2003). LP seismicity is characterizedby a quasi- 2. Seismicity of Deception Island monochromatic spectral content. The most frequent type of LP seismicity at Deception Island Basedon the experience from the successive field are the LP events, which constitute signals with a surveys carriedout at Deception Islandvolcano, spindle-shaped envelope and durations smaller ARTICLE IN PRESS

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Fig. 2. Example of vertical-component velocity seismograms andarray-averagedspectrograms for (a) a VT earthquake; (b) a regional earthquake; and (c) an intermediate-deep earthquake. All these signals have been recorded at Deception Island volcano. Spectrograms are obtained by dividing the seismograms into overlapping 2.56-s windows and averaging over the whole seismic array.

than 60 s. In some cases, a high-frequency phase (2000) have identiﬁed two different types of precedes these events. In Fig. 3 we show examples spectral behavior in the low-frequency bandof of three types of LP events with different spectral these signals. The most common corresponds to a content andduration. The location of the source series of non-regularly spacedpeaks, while the of these pure volcanic signals has been derived secondshows regularly spacedharmonics of a from seismic antenna analyses (Almendros et al., fundamental peak. In the ﬁrst group two subsets 1997, 1999). Alguacil et al. (1999) and Iban! ˜ ez et al. are differentiated by the frequency of the main ARTICLE IN PRESS

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Fig. 3. Example of vertical-component velocity seismograms andarray-averagedspectrograms for LP events recordedat Deception Island, with frequency contents centered at (a) 5 Hz; (b) 2.5 Hz; and (c) 1.5 Hz.

peak. The source processes that generate the LP resonance of open conduits have been complicated seismicity involve a volumetric component due to by introducing different geometries of the reso- the resonance of fluid-filled cavities (Chouet, nance system. Although they are enough to 1996). explain some tremor signals, in other cases the Volcanic tremor is a monochromatic signal with geometrical considerations need to be completed duration longer than that observed for LP events. by including in the tremor models data about the Episodes of tremor that last from minutes to reology of the fluids and their dynamics (Chouet, several hours anddayshave been observedat 1996). Some results, which integrate observations Deception Islandvolcano. In Fig. 4 we show a 3-h of tremor andLP, show evidencesthat both type sample of volcanic tremor. Models of tremor of events share similar source regions andpro- generation are basedon degassing, fluctuations cesses (Almendros et al., 1997, 2001). Tremor and of the gas, resonance of conduits, etc (e.g. Chouet, LP events are supposedto be differentmanifesta- 1992; Julian, 1994). Those models considering the tions of the same process. An LP event is the ARTICLE IN PRESS

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Fig. 4. Example of vertical-component velocity seismogram corresponding to continuous recording of 3 h of volcanic tremor at Deception Islandvolcano. response to a sudden pressure transient within a generation, which constitutes our only clue as to fluid-filled crack, while tremor is the response to the hybridcharacter of the signals. In reality, only continuous fluctuations of pressure. a detailed analysis of the source mechanisms using Hybridevents are signals that contain both a network with appropriate coverage couldsettle double-couple and volumetric components. They the question of whether these signals are LP events are characterizedby an initial high-frequency or hybrids. phase similar to a VT earthquake, followedby a monochromatic signal similar to those shown by the LP events (Fig. 5). In some cases, LP events 3. Review of recent seismic studies with an energetic initial high-frequency onset couldbe confusedwith hybrids. While the low- 3.1. History of seismic monitoring at Deception frequency signature is originatedin both cases by Island the resonance of a fluid-filled cavity, in the case of hybridevents the initial pressure step that triggers The seismic monitoring of Deception Island the resonance is causedby a brittle failure process. volcano began in the 1950s when a seismometer Recently, Iban! ˜ ez et al. (2003) have distinguished in was deployed at the Argentinean base. Data from Deception Islandsignals that might be interpreted this station were recorded continuously on smoked as hybrids. They have found a few VT earthquakes paper drum until the end of the 1960s when the that are closely followedby LP events. Both parts series of eruptions that took place in the island of the signals share a similar source region. This forcedthe evacuation of the bases. Seismic records region coincides with an area of VT earthquake obtainedduringthis periodshow the frequent ARTICLE IN PRESS

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Fig. 5. Example of vertical-component velocity seismogram andarray-averagedspectrogram for a signal interpretedas a hybridevent (Iban! ˜ ez et al., 2003). occurrence of local seismicity, including VT earth- with deep hot materials. In average, the energy of quakes, LP events, andepisodesof volcanic the tectonic andvolcanic quakes recordedbetween tremor. The rate of occurrence of seismic events 1986 and1991 was quite low. increasedjust before each of the 1967–1970 This situation changedin January 1992, eruptions, which emphasizes the usefulness of when an important increase in the number and seismic monitoring as a forecasting tool at Decep- magnitude of the seismic events was detected tion Island. Regional and teleseismic earthquakes (Ortiz et al., 1997). More than 750 VT earthquakes also were detected, but due to the lack of adequate were recorded in less than 2 months. Some of coverage only earthquakes with magnitude 5 or these earthquakes andeven a few episodesof above couldbe located. volcanic tremor were felt. During this survey From 1970 to 1986 there are no seismic records the only available seismic instrument was a three- from Deception Island. In 1986 the monitoring of component short-periodseismometer deployed the seismic activity was re-establishedthrough near the Argentinean base. Although the epicen- summer fieldsurveys carriedout by Argentinean tral area of the recorded activity could not andSpanish researchers, with the result of a be determined accurately, indirect evidence temporary control of the seismic activity of the shows that the source area was probably located volcano (Fig. 6). Several types of seismic deploy- under Port Foster, just 2–3 km from the shoreline. ments have been used( Fig. 3). Between 1986 and Observations of small gravity irregularities 1988 a vertical short-periodseismic station record- andmagnetic anomalies correlatedwith the ing on a thermal paper drum was used, at a successive seismic swarms suggest that a magmatic location near the Argentinean base. Between 1989 injection took place during this increase of and 1991, seismic data were recorded by a network seismic activity (Ortiz et al., 1992, 1997; Garc!ıa composedof five short-periodseismic stations with et al., 1997). However, it hadnot enough energy to radio telemetry. In this period, several VT earth- reach the surface. Changes in fumarolic emissions quakes anda few volcanic tremor episodeswere anda possible deformation process near the recorded. Vila et al. (1992, 1995) and Correig et al. Argentinean base support the hypothesis of mag- (1997) studied this seismic activity and provided matic injection. The seismic andgravimetric the first epicentral map of the VT activity of evidences of this activity started to decline in Deception Islandvolcano. These authors also February 1992. proposeda modelof the occurrence of the activity During the 1992–1993 and1993–1994 field basedon the degasification of an aquifer in contact surveys new seismic stations were deployed near ARTICLE IN PRESS

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Fig. 6. Histogram of the daily number of seismo-volcanic earthquakes recorded at Deception Island during the summer ﬁeld surveys in the period1989–1999, with the exception of the 1997–1998 survey. In the 1998–1999 survey we show separately the VT earthquakes (dark gray) and LP events (light gray). the Argentinean andSpanish bases to studythe pre-1992 level (Fig. 6). However, a change in the continuity of the seismic swarm of January characteristics of the seismic activity was observed. 1992, although the activity haddecreasedto the Starting on December 1992, the number of VT ARTICLE IN PRESS

J.M. Iba´n˜ez et al. / Deep-Sea Research II 50 (2003) 1611–1629 1621 earthquakes clearly decreased, while the number of seismic antennas has allowedfor the analysis of LP events andtremor increased( Felpeto et al., the LP seismicity generatedin differentregions 1994). Since the seismic stations deployed on the of the island. Moreover, the source locations of islandwere powerless to locate or analyze the LP several hundreds of VT earthquakes recorded seismicity, the objectives of the seismic experi- in January andFebruary 1999 were estimated ments were modiﬁed and the use of seismic using an inverse ray-tracing procedure (Iban! ˜ ez antennas was introduced. et al., 2003).

3.2. Seismic antenna surveys 3.3. Seismic attenuation studies The increasing contribution of LP events and tremor to the overall seismicity of Deception Mart!ınez-Arevalo! et al. (2003) studied the Islandvolcano motivatedthe introduction of seismic attenuation in the short-distance and seismic antennas. Seismic antennas allow complete high-frequency range using P; S; andcodawaves. analyses of the seismic wavefieldandare able to They applied different techniques, including both provide useful information about the nature of LP frequency-dependent and frequency-independent events andvolcanic tremors. In 1994, a small- methods such as broadening of the pulse for direct aperture seismic antenna was set up near the P-andS-waves, coda normalization for S-waves, Spanish Base ‘‘Gabriel de Castilla’’ to track the single back-scattering for coda waves, and the seismo-volcanic sources. This antenna was de- spectral method. The results show that Q values ployedagain for the summer fieldsurveys between are significantly smaller, for the entire frequency 1995 and1997. Preliminary reports obtainedfrom range analyzed(6–30 Hz), than those foundin analyses of the array data (Alguacil et al., 1999) other volcanic andtectonic areas. The attenuation confirmedthe existence of VT earthquakes, LP for P-waves is greater than for S-waves in the events, andsustainedtremors near the array site, frequency-independent methods, with a Qb=QP in an area south of Cerro Caliente (see Fig. 1). ratio that ranges between 1.9 and3.2. Q factors Iban! ˜ ez et al. (1997) reportedthe presence of obtainedfor S-waves show clear differences intermediate depth (30–120 km) and shallow depending on the method used. The coda normal- earthquakes (0–30 km) in the area. Almendros ization methodhas suppliedsignificantly higher Q et al. (1997) showedthat some volcanic tremor values (Qd) than the other two methods (Qb). This episodes were generated by the multiple occur- discrepancy is due to the fact that coda normal- rence of low-energy LP events. Almendros ization andsingle back-scattering methodselim- et al. (1999) introduced a modification of inate the contribution of the near-surface the zero-lag cross-correlation technique taking attenuation. Comparing Qb and Qd; the near- into account the circular geometry of the incoming surface attenuation under the recording site, Qk; wave fronts. Using this technique, the epicentral was estimated. Qd displays an anomalous fre- locations of a subset of LP events were quency dependence, with a minimum value at estimatedto be close to the antenna, at distances 21 Hz. This pattern was interpretedas the effect of of a few hundred meters southwest of the array strong scattering of the seismic waves in the source site. area. Qc values clearly depend upon frequency and In 1998, two semi-circular seismic antennas were lapse time. The lapse time dependence could be deployed along the inner coast of the western side interpreted as a depth dependence of the seismic of Deception Island( Fig. 7), at a distance of about attenuation in Deception Islandvolcano. Separat- 2 km. The objective of the two-antenna deploy- ing the contribution of intrinsic andscattering ment was the investigation of the shallow velocity attenuation, Mart!ınez-Arevalo! et al. (2003) found structures under the arrays and the use of that the scattering attenuation is predominant over a joint location methodto improve the location the intrinsic effects. All these observations suggest capabilities of the antennas. The presence of the a highly heterogeneous structure for Deception ARTICLE IN PRESS

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Fig. 7. Map of Deception Island volcano showing the position of the seismic stations and antennas deployed during the different seismic experiments carriedout since 1994.

Islandvolcano. Similar observations were doneby reachedits maximum dailynumber of events on Havskov et al. (2003) using different techniques. January 20, with the occurrence of 80 VT earthquakes. When the fieldsurvey finished,the seismic activity was still high, more than 50 earthquakes/ 4. The 1999 seismic crisis day. At the bottom of Fig. 8 we show a histogram of the local earthquakes recorded by a seismic The seismic crisis that occurredin January– station deployed at the Spanish Base, which February 1999 represents a goodexample of the continuedits operations for up to 2 months after intense seismic swarms that occasionally shake the main field survey ended. We can infer from this Deception Islandvolcano. During the month of figure that, although in March the activity was December 1998 only a few events were recorded (8 decreasing, at the end of this month there was a VT earthquakes), a rate similar to that reportedby new increase. During the summer periodmore Iban! ˜ ez et al. (2000) for previous years. However, than 3000 earthquakes were detected and 863 have this trend was suddenly modified on January 1, been analyzed. Several earthquakes had magni- 1999, when the number of local earthquakes tude-momentum greater than 2.5. At least two of suddenly increased (Fig. 8). The seismic swarm them were felt by the staff working on the island, ARTICLE IN PRESS

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Fig. 8. Histograms of the daily number of LP events (top) and VT earthquakes (middle) detected during the 1999 crisis by the Fumarole seismic antenna, andVT earthquakes recordedbythe continuous station at the Spanish Base between December 1998 and April 1999 (bottom). one of them at the beginning of the series, on epicenter; andthe apparent slowness providesthe January 11, andthe secondin January 20, with incidence angle. The hypocentral distribution (Fig. magnitudes of 2.9 and 3.4, respectively. Both 9) indicates that the seismicity is clustered at a events represent a qualitative change in the seismic focal depth of B2 km near the Fumarole array pattern, because since the 1991–1992 crisis no andmostly towardthe northeast. More than 90% earthquakes of this magnitude had been reported of the events are clusteredin a volume of around (Ortiz et al., 1997). LP seismicity appearedin 10 km3. However, deeper and more distant events swarms that lasteda few days andalternatedwith also were found, with focal depths reaching 10 km rest periods of several days. A total of 1800 LP andepicentral distances up to 18 km. The main events were recorded, with peak activity reaching cluster has two denser regions (see the dashed lines about 150 events/day. Using a lower limit of 30 in Fig. 9). The first one extends about 3 km in a events/day to define an LP swarm (Iban! ˜ ez et al., direction of about 45N, with focal depths 2000), there were nine swarms distributed ran- between 1 and4 km. The secondis longer but domly throughout the recording period (Fig. 8). more diffuse, and extends toward 80N. Most part This pattern coincides with that previously ob- of the seismicity is locatedwithin the inner bay of servedfor the LP seismicity (see Fig. 6), and Deception Islandandapparently dipsat an angle therefore the LP activity in 1998–1999 was of about 45. Hybridevent locations coincidewith considered normal for the area. the cloudof VT earthquakes. Most of them are Hypocenters of the VT earthquakes andhybrids containedin the branch of seismicity extending were determined using the apparent slowness, toward80 N. back-azimuth, and S2P time to perform an The VT earthquakes analyzedspan a magnitude inverse ray-tracing source location (Iban! ˜ ez et al., range from 0.8 to 3.4. These magnitudes allow us 2003). The S2P time fixes the distance to the to track the energetic evolution of the seismic source; the back-azimuth is the direction to the series, as well as estimate the cumulative seismic ARTICLE IN PRESS

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Fig. 9. Hypocentral locations of the VT earthquakes (dots) and hybrid events (squares) recorded by the Fumarole Bay seismic antenna during the 1999 crisis (from Iban! ˜ ez et al., 2003). The dashed lines represent the alignment of epicenters along preferred directions. moment. Except for the two larger earthquakes involvedin the generation of earthquakes during mentionedabove, the energy release is more or less the seismic series. These tiny fractures are primar- steady in time. The distribution of magnitudes is ily concentratedin a small focal volume, insideof nearly the same throughout the series. The stress which we have detected hundreds of events. drop of the VT earthquakes, derived from the magnitude analysis, is very low. More than 95% of the stress drop values are concentrated between 0.1 5. Source models and4 bar. Corner frequencies range from 40 Hz for the smallest events to 6 Hz for the largest ones In this section, we will describe the models (these values do not include those events that generally assumedto explain the origin of the LP saturated the records). Using Brune’s model seismicity andthe generation of VT earthquakes at (Brune, 1970) we foundthat the source dimensions Deception Island. We also will explain the reason range from 10 m up to 200 m, with an average of for their occurrence in swarm-type sequences. 50 m. Using Madariaga’s model (Madariaga, 1977) the dimensions obtained are smaller, and 5.1. Model for pure volcanic signals range from 5 to 120 m, with an average of 30 m. In any case, the estimates of the source dimensions The conceptual model proposed by Iban! ˜ ez et al. indicate clearly the small size of the fractures (2000) to explain the origin of the pure volcanic ARTICLE IN PRESS

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differences in the source time function of the pressure pulse that triggers the oscillation of the fluid-filled crack. When the rise time is sharp, we observe the highest energy of the high-frequency waves. A smooth source time function will reduce the high-frequency contribution to the spectrum. When the pressure step appears isolated, the duration of the resulting signal is short and produces an LP event. In the case of continuous or multiple activation of the source, the resulting signal may last for minutes or hours, producing volcanic tremor. Note that some of the seismo- volcanic events recorded at Deception Island during the 1994–1997 period were classified as hybrids (Iban! ˜ ez et al., 2000) basedon their spectral properties; however, they may not represent true Fig. 10. Sketch of the seismic source model proposed by Iban! ˜ ez hybrids as described for example by Lahr et al. et al. (2000) to explain the origin of the LP seismicity at (1994) for Redoubt volcano. They can be ex- Deception Islandvolcano (see text for explanations). plainedby the same source mechanism described above, andtherefore may be better consideredas LP events with an energetic high-frequency onset. signals (Fig. 10) can be summarizedas follows: a Finally, we must point out that the conceptual sudden pressure step occurring at depth in a fluid- model of single crack in Fig. 10 is an over- filledcrack puts the crack itself in auto-oscillation. simplification. In reality, the source may be This pressure step may be causedfor example by a composedof a complex crack system with sudden phase change of the hydrothermal fluids. A different sizes and orientations. two- or three-dimensional source model is neces- Due to its characteristic spectral properties, the sary to explain the spectral characteristics of the quasi-monochromatic tremor may be explainedby acoustic resonance. The high-frequency waves are a different mechanism, consisting of the resonance produced by the phase change occurring at some of a pipe-like conduit (Chouet, 1985). The signals position along the lower edge of the crack. The show spectral peaks that are overtones of a low-frequency waves are generatedby the reso- fundamental frequency. This spectral pattern nance of the whole crack andtherefore propagate reveals that one dimension of the source is with similar back-azimuths, depending on the dominant over the other two, and therefore an spatial orientation of the crack. The complex organ pipe source model would be adequate to low-frequency wave pattern is due to the mixed explain the spectral features of the tremor. contribution of P, SV, andSH waves radiatedby the source (according to the model developed by 5.2. VT earthquake source model Chouet, 1986, 1988, 1992) andto the generation of surface waves in the multiple-layeredstructure of To understand a fracture process involving low the island, composed of frozen soil and different stress drop, small fault dimensions, and occurrence volcanic layers. This model could provide a unique of multiplets, as observedin Deception Island, source mechanism for LP events andvolcanic Iban! ˜ ez et al. (2003) invokedthe role of the fluids. tremors. The different spectra can be associated In this framework the release of seismic energy is with different crack dimensions and properties. associatedwith lubrication of a pre-existing zone Assuming the model developed by Chouet (1992), of weakness. Lubrication reduces the friction the different energy of the initial high-frequency coefficient anddecreases the effective normal stress wave trains of the signals is explainedby over the fault surface. This can in turn trigger the ARTICLE IN PRESS

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probably not magmatic, but water embedded in the geological structure. Martini andGiannini (1988) reportedthe existence of a shallow aquifer near the epicentral area, anda deeperaquifer may be present as well (Martini andCasselli, pers. comm.).

6. Conclusions

The data obtained since 1986 during several Fig. 11. Sketch of the deep magma intrusion model proposed by Iban! ˜ ez et al. (2003) for the origin of the 1999 VT series at seismic fieldsurveys at Deception Islanddemon- Deception Islandvolcano (see text for explanations). strate that this is a seismically active volcano. Both LP seismicity andVT earthquakes have been recorded and analyzed, leading to knowledge of different aspects of the volcano. The present paper earthquake. Iban! ˜ ez et al. (2003) proposeda describes this seismicity and proposes models that possible scenario that explainedthe origin of the may realistically explain the release of seismic seismic crisis basedon the volcanic nature of energy within the volcano. Deception Island. The seismic series was caused by The most intense seismic activity at Deception stress generatedby the uplift of the source area in Islandoccurredduring the seismic crises of 1992 response to the injection of magma at depth and1999. This means that in about 15 years of (Fig. 11). The presence of a magma reservoir in seismic monitoring, two intense volcanic crises the northeast area of Port Foster has been have been observed. If we take into account that suggestedby several authors ( Blanco, 1997; there are no permanent stations on the island, and Garc!ıa et al., 1997). An increase of pressure within seismic measurements can be performedonly the magma reservoir, or the upwarddisplacement during 3 months/yr (from December to February), of magma, disturbs the stress field in the region then similar periods of intense seismic activity over the reservoir. When the stress reaches a might be occurring more often than what has been thresholdvalue, fracture processes begin, follow- detected so far. Deception Island, therefore, seems ing pre-existing zones of weakness. The absence of to be a very active volcano, at least from a visible evidence of a subsequent eruption and the seismological point of view, although no eruption lower level of earthquake activity observedsince has been reportedsince 1970. Given the frequent 1999 imply that the injection of magma stopped presence of scientific teams, andalso thousandsof before reaching of the surface. The uplift hypoth- tourists (especially during the summer season), esis is compatible with the majority of the greater efforts shouldbe put into seismic and geophysical observations. The small size of the geophysical monitoring of Deception Islandin fractures involvedis explainedby lubrication of order to reduce the risk associated with volcanic the microfaults due to pressurized fluids. The hazards. increase of pore pressure in the very shallow crust overlying the magmatic system couldhave induced the repeatedactivation of the fracture network. The presence of fluids in the source area not only Acknowledgements expedites the fracture processes, but also explains the origin of the hybrids. Their low-frequency This work was partially supportedby projects segments are produced by resonances of fluid-filled ANT98-1111, REN-2000-2897, EU project e- cracks. The fluidinvolvedin the lubrication of the Ruption EVR1-2001-00024, andREN-2001-3833. microfaults andthe generation of hybridevents is We thank five anonymous reviewers andthe ARTICLE IN PRESS

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