RESEARCH ARTICLE

The November 2002 eruption at Piton de la Fournaisevolcano, La Réunion Island: ground deformation, seismicity,and pit crater collapse

Marc-Antoine Longpré & Thomas Staudacher &

John Stix

Received: 26 October 2004 /Accepted: 5 June 2006 / Published online: 13 September 2006# Springer-Verlag 2006

Abstract An eruption on the eastern flank of Piton de laFournaise volcano started on 16 November, 2002 after10 months of quiescence. After a relatively constant levelof activity during the first 13 days of the eruption, lavadischarge, volcanic tremor and seismicity increased from 29November to 3 December. Lava effusion suddenly ceasedon 3 December while shallow earthquakes beneath theDolomieu summit crater were still recorded at a rate ofabout one per minute. This unusual activity continued andincreased in intensity over the next three weeks, endingwith the formation of a pit crater within Dolomieu. Basedon ground deformation, measured by rapid-static andcontinuous GPS and an extensometer, seismic data, andlava effusion patterns, the eruptive period is divided intofive stages: 1) slow summit inflation and sporadic seismic-ity; 2) rapid summit inflation and a short seismic crisis; 3)rapid flank inflation, onset of summit deflation, sporadicseismicity, accompanied by stable effusion; 4) flankinflation, coupled with summit deflation, intense seismicity,and increased lava effusion; and finally 5) little deflation,

intense shallow seismicity, and the end of lava effusion. Wepropose a model in which the pre-intrusive inflation ofStage 1 in the months preceding the eruption was caused bya magma body located near sea level. The magma reservoirwas the source of an intrusion rising under the summitduring Stage 2. In Stage 3, the magma ponded at a shallowlevel in the edifice while the lateral injection of a radial dikereached the surface on the eastern flank of the basalticvolcano, causing lava effusion. Pressure decrease in themagmatic plumbing system followed, resulting in upwardmigration of a collapse front, forming a subterraneancolumn of debris by faulting and stoping. This causedintense shallow seismicity, increase in discharge of lava andvolcanic tremor at the lateral vent in Stage 4 and, eventuallythe formation of a pit crater in Stage 5.

Keywords Piton de la Fournaise volcano .

Ground deformation . Pit crater . Seismicity . Stoping .

Subterranean cavity . Lateral dike

Introduction

Piton de la Fournaise is a basaltic shield volcano located onLa Réunion Island in the Indian Ocean, about 750 km eastof Madagascar (Figs. 1 and 2). It is the present-dayexpression of the hotspot that produced the Deccan Trapsat ∼65 Ma. It is one of the most active volcanoes on Earth,erupting on average once every year (Bachèlery 1981).Eruptions have occurred more frequently since 1998. Themorphology of this volcano is characterised by at least threelarge collapse structures and their associated landslide scars(Fig. 2). Most of the recent activity is confined within theyoungest of these volcanic depressions, called the EnclosFouqué, which formed less than 5,000 years ago (Bachèlery

Bull Volcanol (2007) 69:511–525DOI 10.1007/s00445-006-0087-0

Editorial responsibility: H Delgado

M.-A. Longpré : J. StixDepartment of Earth and Planetary Sciences, McGill University,3450 University Street,Montréal, Québec H3A 2A7, Canada

T. StaudacherObservatoire Volcanologique du Piton de la Fournaise,Institut de Physique du Globe de Paris,14 RN3, le 27e Km,97418 La Plaine des Cafres, La Réunion, France

M.-A. Longpré (*)Department of Geology, University of Dublin, Trinity College,Dublin 2, Irelande-mail: longprem@tcd.ie

Present address:

1981). At the summit of the main edifice 2631 m above sealevel lie two summit craters, Bory and Dolomieu. Bory hasa diameter of ∼200 m and a maximum depth of 50 m, andhas been relatively inactive during the last century.Dolomieu, on the other hand, has been the location ofmany recent eruptions; it has a diameter of ∼1 km and amaximum depth of 160 m. This summit feature formedprogressively, mainly during the 20th century (Bachèlery1981). In 1977, a large eruption, which occurred outside thenatural barrier of Enclos Fouqué, destroyed houses in thelittle town of Piton Ste-Rose. As a result, a modernobservatory, the Observatoire Volcanologique du Piton dela Fournaise (OVPF), was established in 1979 to monitor thechanging mood of the volcano. It is now equipped with amonitoring network including seismometers, tiltmeters, GPSreceivers, and extensometers (Fig. 2).

Although Piton de la Fournaise has been studied andmonitored extensively in the past 25 years, the source ofmagma, the plumbing system and the eruption mechanismsare still matters of debate (Lénat et al. 1989; Sapin et al.

1996; Nercessian et al. 1996; Aki and Ferrazzini 2000). Inmost eruptions at Piton de la Fournaise there is no evidenceof magma transfer directly from mantle depths, sinceseismicity is absent at these levels (Lénat et al. 1989; Sapinet al. 1996). Thus, magma must be stored in some mannerat shallow levels in the crust. Several workers haveattempted to describe this storage system. (Lénat et al.1989) suggested the presence of a complex of sills anddikes above sea level. Sapin et al. (1996) and Nercessian etal. (1996) investigated the possibility of a small magmachamber just below sea level. Aki and Ferrazzini (2000)proposed a complex model consisting of two different pathsto the eruption sites, a summit path and a rift zone path,with two distinct magma reservoirs. However, all thesestorage systems must be replenished periodically from adeeper source during large eruptions, the most recent ofwhich occurred in 1977 and 1998 (Lénat et al. 1989; Sapinet al. 1996; Staudacher et al. 1998; Aki and Ferrazzini2000).

The 1998 eruption, which occurred after an unusuallylong repose period of 5.5 years, lasted for 6.5 months andproduced ∼6×107 m3 of basaltic lava. It was preceded inSeptember 1996 by deep earthquakes, located northwest ofthe summit at a depth of 16 km below sea level, and in the35 h prior to the eruption by a swarm of volcano-tectonicearthquakes that migrated from a depth of 5–6 km to sealevel. This activity suggests magma replenishment fromdeep levels (Staudacher et al. 1998; Vlastélic et al. 2005;Battaglia et al. 2005a). The main vent, Piton Kapor, andMaurice et Katia Krafft crater were active on the north flankof Piton de la Fournaise while another vent, Hudson crater,was simultaneously active for about three weeks to thewest–southwest of the central cone (Fig. 2) (Staudacher etal. 1998). Only Piton Kapor remained active for the wholeeruptive period. Bureau et al. (1999) and (Vlastélic et al.2005) argued that Hudson was fed with magma from adeeper source than the other two craters. These character-istics make the 1998 eruption an atypical event. Generally,a typical eruption of Piton de la Fournaise is not precededby sporadic seismicity at depths between sea level and 4 kmbelow sea level, nor is there pre-intrusion inflation in thesummit region during the months prior to the eruption(Lénat et al. 1989; Sigmundsson et al. 1999). Such aneruption is usually signalled by a pre-eruptive seismic crisislasting from several tens of minutes up to about 5–6 h (Akiand Ferrazzini 2000). The events in this type of crisis havelow magnitudes (M<3) and are located under the centralcone at depths near and above sea level (Sapin et al. 1996;Nercessian et al. 1996). Large amounts of summit inflationtypically occur during such a swarm of clustered earth-quakes (Lénat et al. 1989; Sigmundsson et al. 1999;Battaglia and Bachèlery 2003; OVPF unpublished data1999–2002). The onset of volcanic tremor, which is closely

Fig. 1 Tectonic setting of La Réunion Island in the western IndianOcean. The hotspot track is indicated by the thick arrows, and the agesassociated with it are indicated in millions of years. The track was cutin two segments between 35 and 48 Ma by the formation of a mid-oceanic ridge. The Deccan Traps flood basalts are also shown

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related to eruptive activity at Piton de la Fournaise(Battaglia et al. 2005b), signals the beginning of theeruption on the flanks and/or the summit of the volcano,and is frequently accompanied by rapid deflation of thesummit. Several en échelon fissures may open briefly andemit lava, and most of the activity usually shifts to thelowest-altitude vent within a day or so of the eruption’sbeginning (OVPF unpublished data 1999–2002). Theinitially high amplitude of the volcanic tremor decreasesby about an order of magnitude within the first day (OVPFunpublished data 1999–2002). The eruption then progressesthrough a relatively stable phase characterised by occasion-al volcano-tectonic earthquakes, overall constant tremorand lava discharge, and continuous deflation of the summit(OVPF unpublished data 1999–2002). Progressive disap-pearance of the tremor usually indicates the end of theeruption (Battaglia et al. 2005b; OVPF unpublished data1999–2002). Obviously, this description of a “typical”eruption at Piton de la Fournaise is a generalisation, andan individual eruption may have unique attributes, as is thecase for the November 2002 eruption, which was charac-terised by the formation of a pit crater.

Pit craters are common features in volcanic areas. Theyconsist of sub-circular depressions bounded by steep faults(Roche et al. 2001). The emptying of an underlying magma

reservoir is generally considered to be the cause of pit craterformation, so that they may be thought of as small-scaleanalogues of calderas (Roche et al. 2001). Pit craters andcalderas can form as a result of flank eruptions (Filson et al.1973; Hirn et al. 1991; Roche et al. 2000; Geshi et al.2002). Hirn et al. (1991) described the formation of a pitcrater as a result of a flank eruption at Piton de la Fournaisein 1986.

The purpose of the present work is to examine theactivity of Piton de la Fournaise related to the November2002 eruption. Using a collection of datasets, includingground deformation and seismicity, we describe theNovember 2002 eruption and the subsequent formation ofthe pit crater. We then develop a model relating thepressurisation of the shallow plumbing system to theeruptive activity and pit crater formation.

Data collection

A brief description of geophysical data collection andtechniques used in this study is presented below. Appro-priate references for more detailed information on theOVPF monitoring network and geophysical techniques arealso provided.

Fig. 2 Shaded relief map of thePiton de la Fournaise massifshowing three collapse struc-tures (Rivière des Remparts,Plaine des Sables, Enclos Fou-qué), the two summit craters(Dolomieu, Bory), and the ventsfrom the 1998 (triangles) andNovember 2002 (inverted trian-gle) eruptions. The circles rep-resent the main seismometers ofthe OVPF monitoring network.The circle labelled “SFR” alsocorresponds to the location ofthe extensometer and permanentGPS used in this paper (Fig. 3).The inset shows a map of LaRéunion Island. UTM (Gauss–Laborde Réunion) coordinatesare used

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Extensometer network

The fracture network of Piton de la Fournaise is exploitedfor geophysical monitoring. In the last decade, OVPFpersonnel have installed several extensometers that measurethe opening, shearing, and vertical movements of pre-existing fissures in response to volcano deformation.Extensometers have proven to be extremely precise devicesand sensitive to the pressurisation state of the volcanoedifice (Staudacher et al. 2003). One device (SFR exten-someter), in particular, shows variations one order ofmagnitude higher than the other instruments, permittingthe detection of small variations due to inflation or deflationof the edifice during the evolution of eruptions. The SFRextensometer is placed on a large, north-trending, radialfissure to the north of the central cone (Fig. 2), about 50 min length, with a depth exceeding 6 m, and a width of∼60 cm (Bachèlery et al. 2005). The fissure width ismeasured with a commercial instrument (model REPP fromTELEMAC) equipped with a plastic track potentiometerwith a 25 mm maximum range. The device is fixed onstainless steel rods cemented in >1 m thick massive basalticflows. Measurements were performed every minute, col-lected by a DATATAKER processor, and sent to the OVPFvia radio. Data from the SFR extensometer is presented inFig. 3a. Daily variations, on the order of 0.03 mm due totemperature change, can be observed in the inset as a step-like pattern, setting an upper limit for the uncertaintiesinvolved. Larger steps, clearly related to ground deforma-tion, represent permanent variations, and therefore, errorbars were not judged relevant.

Permanent GPS station

Only one permanent GPS station was installed on Piton dela Fournaise volcano in 2002. As with the SFR extensom-eter, the SFR GPS station is located on the north rim ofDolomieu crater, near Soufrière crater (Fig. 2). Position iscalculated relative to station 1B80 on the Enclos Fouquérim and reported to the OVPF every 30 s via radio. Fig. 3bshows mean daily values, which have a precision of ∼1 cm.The theoretical background and uncertainties describedbelow for the use of the differential rapid-static GPS alsoapply for permanent GPS.

Seismic network

A total of 19 seismometers are installed permanently onPiton de la Fournaise, transmitting data to the observatoryby radio. Three sites, located in the vicinity of the summit,allow detection of the numerous, low magnitude, oftenshallow earthquakes. Because these stations are close to oneanother (<1 km), constraints on focal depth given by the

differential time between near and distant stations islacking. Only one summit station is equipped with athree-component seismometer, allowing a direct real-timedetermination of the distance to the hypocentre using theinterval time between P and S wave arrivals (Nercessianet al. 1996). Hence, for seismic crises characterised by alarge number of low magnitude and superficial earthquakes,as is the case for the November and December 2002seismicity, an approximation of the hypocentre location isobtained. The daily number of earthquakes is presented inFig. 3c as a log plot, due to the large number ofM ≤1 events in the third week of December 2002.Individual events were often superimposed, due to the highrate of activity, making precise arrival picking and exactlocalisation of the sources difficult. Our interpretations ofthe seismicity are thus based on key hypocentre locationscalculated manually and relative change in seismic patternsseen in automated locations, which should be regarded assemi-quantitative (Fig. 4). For a more complete descriptionof the OVPF seismic network, the reader is referred to Akiand Ferrazzini (2000), Nercessian et al. (1996) andBattaglia et al. (2005a).

Rapid-static GPS network

Differential rapid-static GPS measurements are part of theOVPF monitoring program since 1998. Campaigns arecarried out regularly, with increasing frequency duringvolcanic crises. The network consists of about 80 stationsdistributed around the summit and the flanks of thevolcano, each equipped with a stainless steel pin cementedinto massive lava flows.

The technique requires the simultaneous use of at leasttwo receivers (Ashtech Zxtreme). The base receiver was leftat a fixed position (1B80) outside the Enclos Fouquéthroughout the measurement sessions and was set tocontinuously determine its position. The roving receiverwas taken into the field and fixed to the individualbenchmarks for occupation times from 5 to 7 min with afrequency of 1 measurement per second. Generally, threedays are required to complete the survey of all benchmarksby foot, depending on weather conditions and volcanichazards. After the January 2002 eruption which lasted for11 days, we carried out GPS surveys in March 2002,covering the benchmarks around the summit region. Thesame measurements were repeated in September andOctober 2002 in response to the onset of deformationrecorded by continuous tiltmeters and extensometers sincethe beginning of August. Partial surveys, which werelimited due to hazards, were carried out on 25, 28, and 29November for benchmarks located on the summit craterrims and on the southern and western flanks of the centralcone. Complementary survey stations on the eastern,

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western and northern flanks and northern rim of Dolomieuwere visited on 6, 9, and 11 December.

Data collected in the campaigns were processed by theWINPRISME program of ASHTECH, using executionmode “forward and backward”, processing mode “survey”,and processing parameter “static”. The average position ateach station was calculated and the value from the previoussurvey subtracted to obtain a displacement vector. Fig. 5shows the displacement vectors between the campaigns of

a) March and October, b) October and November, and c)October and December 2002.

Under optimal conditions, differential rapid-static GPShas a precision of about 1 cm in the horizontal plane (Mosset al. 1999). In the field, the accuracy can degrade toapproximately 1.5 cm due to set-up errors and troposphericnoise. The errors associated with positioning in the verticalplane are about double those measured horizontally, andtherefore, vertical ground deformation measurements will

Fig. 3 Ground deformation andseismic data collected from 29July to 31 December, 2002. Thefive stages described in the textare illustrated with acorresponding vertical shadedband, showing the beginning ofStage 1 on 4 August and the endof Stage 5 on 23 December.Note the narrow white band,indicating the short duration ofStage 2. a Continuous grounddeformation measured by theSFR extensometer located onthe northern rim of Dolomieu(Fig. 2). Opening of the fissure(mm) is plotted as a function oftime (days). A detailed plot ofStage 5 is shown in the inset,where pit crater formation ismarked by small but sharp de-flation and the step-like patternis due to daily variations relatedto temperature change. b Con-tinuous ground deformationmeasured by the SFR permanentGPS station located on thenorthern rim of Dolomieu. X(east) and Y (north) displace-ments are shown as a function oftime. c Semi-log plot showingthe daily number of earthquakes,reaching a peak of >5,700events on 22 December. A barindicates the presence of volca-nic tremor during Stage 3 andStage 4

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not be discussed in this paper. From one campaign to thenext, the reproducibility of horizontal GPS measurementsin places where no significant deformation has occurredindicates that the difference in position between twocampaigns also has a high level of precision on the orderof 1 to 2 cm. Therefore, the use of error ellipses was not

judged to be necessary. We assume that similar atmosphericconditions prevail along the paths linking one satellite tothe two receivers. For this to be true, the maximum distancebetween the two receptors is limited to 10 km. A completedescription concerning the use and application of GPS isgiven by Hofmann-Wellenhof et al. (1997).

Fig. 4 Semi-quantitative earth-quake hypocentre locations. Da-ta were calculated automaticallyin near real time. OVPF dataindicate that no seismic activitywas recorded below sea level(Valérie Ferrazzini, pers. comm.2003). Most notably, there is aclear shallowing of the seismic-ity occurring from Stage 4 toStage 5. Representative dateswere selected for each stage: a20 September b 15 November c21 November d 29 November e20 December, 2002

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Fig. 5 Horizontal ground deformation measured by rapid-static GPSat Piton de la Fournaise in 2002. Strong asymmetry of the volcanogives rise to preferential bulging on the eastern flank. Only a fewbenchmarks are located on the southeastern flank of the volcano dueto its inaccessibility. a Between the surveys of March and earlyOctober 2002, the vectors indicate slow summit inflation of thesummit region occurring during Stage 1. The source appears to becentred under Dolomieu. b Vectors obtained from surveys of Octoberand 25, 28, and 29 November show the combined effects of a

deflating source under Dolomieu and an inflating source located onthe eastern flank. This is especially apparent for survey stations TD30and T562. The location of Piton Guanyin is also shown for reference.c On the western and northern flanks of the central cone, the vectors,obtained from surveys of October and 6, 9, and 11 December, indicatethe combined effects of a deflating source located under Dolomieucrater and an inflating source on the eastern flank. However, vectorson the eastern flank are dominated by inflation from the source (thelateral dike) located farther south on the eastern flank

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Observations

In this section, we describe the volcanic unrest in thesecond half of 2002, eventually leading up to the November2002 eruption. Five distinct stages of activity can bedefined based on changes in the patterns of grounddeformation, seismicity, and lava discharge.

Stage 1: slow summit inflation and sporadic seismicity

Following the January 2002 eruption, seismicity beneathPiton de la Fournaise was extremely low, with less than 3events per day, and continuous ground deformationmeasurements from the SFR extensometer and GPS stationindicated a stable situation. In early August, however, slowsummit inflation started to be recorded, contemporaneouswith seismicity beneath Dolomieu crater (Fig. 3a, c).Hence, we make the beginning of August as the onset ofStage 1. The seismicity gradually increased from earlyAugust to mid-September 2002. Most of October wasseismically quiet, with activity resuming suddenly on 5November (Fig. 3c). Although no precise hypocentrallocations were computed, the OVPF partial data andprevious observations of similar seismicity at this volcanoclearly indicate that these volcano-tectonic earthquakeswere located between sea level and the summit region.Displacement vectors shown in Fig. 5a are also consistentwith inflation of the summit region, with vectors of higheramplitude generally located on eastern flank. The largestvector (3F33, 8.6 cm), however, is situated on the southernslopes of the central cone.

Stage 2: rapid summit inflation and seismic crisis

At 1937 UTC on 15 November, an intense seismic crisisbegan, accompanied by strong inflation of the summit(Fig. 3). We define Stage 2 as that part of the crisis whereground deformation recorded by the SFR permanent GPSshows a sharp northwest movement (Fig. 3b). Notably,many vectors of Fig. 5b also indicate a northwestdisplacement on the northern and eastern rim of Dolomieucrater. In Fig. 3a, Stage 2 can be seen as the short, nearlyvertical segment, indicating sudden inflation at the SFRextensometer site. A total of 1,000 volcano-tectonic earth-quakes of low magnitude (<3) occurred on 15 November(Fig. 3c), again mostly above sea level centred under thesummit (Staudacher 2002, OVPF unpublished annualreport).

Stage 3: flank inflation and stable lava effusion

The sharp closure of the fissure recorded by the extensom-eter (Fig. 3a) marks the beginning of Stage 3, which lasted

from 16 to 29 November. The change from an abruptnorthwest to a moderate southwest movement recorded bythe permanent GPS also is attributed to the start of Stage 3(Fig. 3b). Shortly after this, at 0033 UTC, lava effusioncommenced, as attested by the appearance of the volcanictremor, progressively replacing the 5-hour-long seismiccrisis (Staudacher and the OVPF Team 2002). Four fissuresopened on the eastern flank of the massif at elevationsbetween 1,850 and 1,540 m. The eruptive activity wasrapidly focused at a main vent at 1,540 m altitude, buildinga scoria cone that was named Piton Guanyin (Fig. 2). Aphase of stable activity with nearly constant volcanictremor, sporadic summit seismicity, and regular lavadischarge lasted for 13 days. The SFR extensometerrecorded the closure of the fissure, producing a logarith-mic-shaped curve (Fig. 3a), while the permanent GPS dataindicate a continuous southwest displacement (Fig. 3b)throughout the duration of Stage 3. Gaps in the permanentGPS data are due to power supply problems at the station.No equivalent of the southwest movement can be seen inFig. 5b, but vectors at TM61, TM63, TM64, TM65, andTM51 in Fig. 5c show a similar trend. Both Fig. 5b and cshow large displacements with a major north component onthe eastern rim of Dolomieu (e.g. TD30, 17.6 cm, Fig. 5b)and the eastern flank of the central cone (e.g. 3E40 and

Fig. 6 Model for the November 2002 eruption at Piton de laFournaise volcano, showing schematic east–west cross-sectionsthrough the Enclos Fouqué for the different stages of activity. Arrowsand stars (*) show expected deformation at the surface and expectedlocations of earthquakes, respectively. The insets show a detailedevolution of the situation in the shallow subsurface beneath Dolomieu,leading to the formation of the pit crater. a Stage 1 (4 August–15November): Increasing pressure in the magma chamber near sea level(Nercessian et al. 1996) causes pre-intrusion inflation in the summitregion. Better constraints are necessary to determine the depth andsize of possible deeper magma reservoirs; thus, the second magmachamber sketched in this figure is hypothetical. Inset: A pre-existingweak zone was left behind at shallow levels (∼500 m) by previouseruptions. b Stage 2 (15–16 November): A vertical intrusion risesunder Dolomieu, creating rapid summit inflation. Inset: A pre-existingweak zone may provide space for magma ponding. c Stage 3 (16–29November): Towards the end of the pre-eruptive crisis, the radial dikepropagates laterally, allowing magma to reach the surface on theeastern flank. d Stage 4 (29 November–3 December): Due to magmawithdrawal, magma pressure falls in the main reservoir and thevertical conduit, causing the roof above the vertical intrusion underDolomieu to begin to collapse. Falling blocks contribute to drainagetowards the radial dike, resulting in increased tremor and lavadischarge at the vent. e Stage 5 (3–23 December): On 3 December,lava discharge ceases. The vertical conduit may have drainedcompletely. Subterranean collapse at shallow levels continues andcauses intense shallow seismicity and the development of a column ofdebris, reaching the surface within Dolomieu on 23 December to formthe pit crater initially characterised by overhanging walls. Gravita-tional collapse causes the enlargement of the pit crater, whilecompaction decreases void space in the porous column of debris.Steam emissions appear in the pit crater in April 2003 and intensifyuntil the end of May 2003. This may be due to the heating ofgroundwater by renewed intrusion into the open vertical conduit

�

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2E41, 36.1 cm, Fig. 5c). With the exception of 16November, seismic activity during Stage 3 was fairly low,typically with fewer than 10 events per day (Fig. 3c).

Stage 4: sudden deflation, increase in lava dischargeand summit seismicity

On 29 November, sudden deflation (Fig. 3a) and a renewalin the seismic and eruptive activity occurred (Staudacherand the OVPF Team 2002), marking the beginning of Stage4. Both the volcanic tremor and lava discharge increasedsignificantly, with the main lava flow crossing the nationalroute (RN2 on the east coast) on 30 November, andreaching the Indian Ocean on 1 December. The summitseismicity jumped from fewer than 10 earthquakes per daybefore 29 November to over 1,500 on 1 December(Fig. 3c).

Stage 5: end of lava effusion, gradual increase in summitseismicity, and formation of a pit crater

The effusion of lava from Piton Guanyin, together with therapid deformation in the summit region, stopped on 3December, representing the transition between Stage 4 and5. By contrast, earthquakes under the central cone stilloccurred at a high rate of about one per minute (Staudacherand the OVPF Team 2002). According to semi-quantitativeautomated hypocentral locations, seismicity also shifted toshallower levels (∼500 m beneath Dolomieu crater floor) atthis time (Fig. 4, Staudacher 2002, OVPF unpublishedannual report). This intense seismicity occurred at anincreasing rate for three weeks, reaching a peak of about5,700 earthquakes (4 per minute) on 22 December (Fig. 3c).The seismicity ended with the formation of a pit crater of∼100 m diameter and ∼10 m depth within Dolomieu crater on23 December accompanied by a M3 event, the strongestearthquake of the crisis. Afterwards, the activity rapidlydeclined. In the following months, the size of the pit craterincreased by gravitational collapse of the walls and compac-tion of the floor, and steam emissions (50–60°C) began totake place within it in April 2003. By the end of May 2003,the diameter, depth, and volume of the pit crater had reached∼200 m, ∼25 m, and ∼8×105 m3, respectively. Only slightdeformation occurred after 3 December. Most notably, theextensometer indicated a small but sharp closure of thefissure while the permanent GPS recorded a clear southeastmovement on 23 December (Fig. 3a, b).

Interpretations and model

Based on the data presented above, we propose a model forthe November 2002 eruptive episode at Piton de la

Fournaise volcano. The model integrates ground deforma-tion, seismic, and eruptive patterns observed during thevolcanic unrest from August to December of 2002, as wellas previous work on the eruptive mechanisms and theinternal structure of Piton de la Fournaise volcano. The fivestages are used to illustrate the model (Fig. 6).

Stage 1

Horizontal ground deformation data presented in Fig. 5a isconsistent with inflation of the summit region during Stage1 due to a source located beneath Dolomieu crater.Deformation is larger on the eastern flank, probably dueto the pronounced east–west asymmetry of the volcano,which causes the seaward side to bulge preferentially(Battaglia and Bachèlery 2003). The nature of the sourceof inflation may be a shallow crustal magma reservoir.Seismic wave propagation and earthquake distributionstudies by Nercessian et al. (1996) revealed the existenceof a high-velocity plug, ∼1.5 km in diameter, centred underDolomieu crater above sea level and surrounded by alower-velocity ring. The pre-eruptive seismicity is mostlyrestricted to this high-velocity zone (Nercessian et al.1996). In addition, the presence of a deeper ∼1 km-sizelow-velocity body just below sea level, above which thepre-eruptive swarms are clustered, suggests that this regionis generally aseismic yet surrounded by a few hypocentresabove a depth of about 1.5 km below sea level which aretemporally related to eruptive activity (Nercessian et al.1996). Nercessian et al. (1996) and Sapin et al. (1996)interpreted this low-velocity aseismic volume as a likelycandidate for a shallow reservoir supplying magma to mosteruptions. Data from melt inclusions trapped in the olivinesof the November 2002 lava support this interpretation(Vigouroux et al. in prep.). Nercessian et al. (1996) alsosuggested that the high-velocity plug corresponds to asolidified intrusive complex which is sufficiently strong andcohesive for seismic failure to occur in response to upwardpropagation of magma. The lower-velocity ring wasattributed to an accumulation of lava flows building thetopographic relief (Nercessian et al. 1996). Earlier work by(Lénat et al. 1989) described the shallow magma storagesystem as a complex of sills and dikes having an upperlimit around 1–2 km below the summit. This geometryexplained the different roots to the 9 June and 14 June 1985intrusions, as well as small chemical differences among thelavas of the different 1985–1987 eruptions. The depthestimate by Lénat et al. (1989) for the upper limit of thismagma reservoir is not consistent with that of Nercessian etal. (1996). It was determined by the wavelength of the pre-eruptive inflation, which is now thought to be caused bymagma intrusion (Aki and Ferrazzini 2000; Battaglia andBachèlery 2003), rather than by inflation of the magma

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storage region itself. Moreover, the presence of the high-velocity plug may argue against a complex of sill-like anddike-like magma pockets at depths of ∼1 km below thesummit, which would likely have a low-velocity characterif not fully solidified. Pre-eruptive swarms usually startingaround 500–800 m above sea level suggest that the upperlimit of the reservoir is located above sea level, but at adepth of ∼2 km below Dolomieu.

Figure 6a is a schematic east–west cross-section throughthe Enclos Fouqué collapse structure showing the principalcharacteristics of Stage 1. The shallow magma chamber isconsistent with the data of Nercessian et al. (1996). Forsimplicity, the reservoir is illustrated as an ellipsoidal body.However, it also could be regarded as a plexus of sills anddikes similar to that proposed by Lénat et al. (1989), butlocated at slightly greater depth. Increasing magma pressuremay be responsible for the inflation of this reservoir,resulting in the slow inflation of the volcano summit.The stars (�) represent the sporadic earthquakes occurringthrough Stage 1, which may be related to magma pressureand/or readjustment of the edifice structure (Nercessianet al. 1996). The presence of a pre-existing cavity, or weakzone of collapse debris, located at shallow levels ∼500 munderneath Dolomieu, is proposed based on the seismicpattern of eruptions occurring between 1998 and November2002. For several recent eruptions, summit seismicityoccurred at a late stage of the eruptions and was usuallyaccompanied by increasing tremor and lava discharge(OVPF unpublished data 1999–2002). These events resem-ble those of 1986 prior to the formation of a pit crater,suggesting that subterranean collapse occurred periodicallyforming a cavity (OVPF unpublished data 1999–2002).

Stage 2

The pre-eruptive seismic crisis of 15–16 November wasaccompanied by strong inflation of the summit for a shorttime span of less than 5 h (Fig. 3). The rapid transition frompre-eruptive inflation to deflation of the summit area, bestshown by the north component of Fig. 3b, emphasises thevery short duration of Stage 2. Pre-eruptive seismic swarmsand deformation of the summit area have been known tooccur simultaneously several tens of minutes to ∼5–6 hbefore eruptions at Piton de la Fournaise and have beeninterpreted to reflect intrusion of magma (Aki and Ferrazzini2000; Battaglia and Bachèlery 2003). Figure 6b illustratesthe intrusion causing the inflation and seismicity duringStage 2 of the November 2002 eruption. The northwestground displacement shown in Fig. 3b suggests that thecentre of inflation (the intrusion) was located to the south-east of the SFR permanent GPS station. The intrusion mayhave been driven by overpressure in the magma reservoirnear sea level, while the earthquakes were probably caused

by magma pressure, flow to the surface, readjustment of theedifice structure, and hydraulic fracturing (Nercessian et al.1996). Ponding of magma at shallow levels may haveoccurred under Dolomieu crater. In our model, the pre-existing cavity created by previous eruptions providesspace for magma accumulation. Alternatively, magma mayhave reached high levels in the edifice without accumula-tion. Shallow earthquakes, which may have been due tomagma movement, suggest that the intrusion reachedhigher levels in the edifice than at the eruption’s mainvent, Piton Guanyin (OVPF unpublished data 2002).

Stage 3

Magma reached the surface on the eastern flank of Piton dela Fournaise at a remote site ∼1 km below and ∼4 km to theeast of the summit region. We propose that a lateral dikewas injected from the vertical intrusion (Stage 2) towardsthe eastern flank, and that the sharp shift from a rapidnorthwest to a moderate southwest movement (Fig. 3b) atthe permanent GPS station represents the onset of lavaeffusion at Piton Guanyin.

When compared to data collected in October, differentialrapid-static GPS measurements in the campaigns of 25, 28,and 29 November and 6, 9, and 11 December indicateisotropic deformation of the summit, supporting themigration of a radial dike to the eastern flank. Firstly, thedeformation shown in Fig. 5b cannot be explained simplyby a single deflating source beneath the summit crater. Thevectors on the northern and eastern rim of Dolomieu have anon-negligible north component, suggesting an inflatingsource to the south to southeast. Furthermore, the bench-marks on the southern rim and flank also appear to havebeen pushed to the west by an inflating source to the east.

In addition, we suggest that the deformation shown inFig. 5c took place mostly in Stage 3, even though some ofit must have occurred in Stage 4 as attested by continuousground deformation measurements (Fig. 3a, b). For in-stance, vectors on the northern flank of the central coneshow a southwest displacement, consistent with the signalfrom the permanent GPS (Fig. 3b). We also propose that thesource of the strong deformation on the eastern flank ismost likely the lateral dike, propagating in the shallowsubsurface at the beginning of Stage 3.

Lateral injections of magma at Piton de la Fournaise arecommon occurrences, frequently resulting in flank eruptionsgenerally within a 1–3 km radius of the central cone(Nercessian et al. 1996; Peltier et al. 2005). However, thefinal radial magma flow to the vents usually occurs aseismi-cally due to the low strength of surface material, built up byaccumulations of lava flows and pyroclastics, and theabsence of water, preventing hydraulic fracturing (Nercessianet al. 1996; Aki and Ferrazzini 2000).

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Indeed, no clear seismic signal allowed tracing thetrajectory of the lateral propagation of magma during theNovember 2002 episode. The most plausible explanationis that magma travelled sub-horizontally from the verticalintrusion towards the eastern flank at elevations between1,850 m and 1,540 m where the fissures opened (Fig. 6c).Other possibilities include magma migrating obliquelyfrom the reservoir near sea level to the eastern flank orreaching higher levels in the edifice before following thetopographic gradient downslope towards the site of PitonGuanyin. After the intense pre-eruptive seismicity, only afew volcano-tectonic earthquakes were recorded each daybetween 16 and 29 November, indicating that steady stateconditions were present in the edifice after the onset oflava effusion.

Stage 4

The complexity of Stage 4, with the heightened activitydescribed in the Observations section above, is interpretedto be the result of a change in the shallow plumbingsystem, with the magma pressure falling below lithostatic.This pressure disequilibrium in the chamber near sea leveland in the vertical intrusion may have caused the roofabove the conduit to collapse. Hirn et al. (1991) attributedthe progressive stoping at the top of a subterranean cavityto a pressure decrease in a magma reservoir following aflank eruption at Piton de la Fournaise in 1986. The rapidincrease in summit seismicity may have been due to theonset of the collapse and to the instability in the plumbingsystem, as suggested for Piton de la Fournaise in 1986 andfor Fernandina caldera, Galapagos Islands, in 1968 (Hirnet al. 1991). Falling pieces of the roof in the verticalconduit may have enhanced the drainage of magma intothe radial dike, leading to an increase in lava discharge andvolcanic tremor. According to the caldera collapse modelof Druitt and Sparks (1984), roof collapse may result inincreased magma withdrawal. This situation may havecontinued until the conduit was drained to a level belowthe junction with the lateral dike or, alternatively, until thedebris blocked the entrance to the lateral dike, bringinglava effusion to an end. As roof blocks collapsed, magmaalso may have been forced upward in the debris-filledconduit, although magma did not reach the surface inDolomieu. Abrupt deflation of the summit (Fig. 3a),consistent with the increased magma withdrawal charac-terising Stage 4, nearly ceased with the end of lavaeffusion on 3 December, suggesting that magma drainagealso stopped. The strong inflation on the eastern flank mayhave been augmented during forceful magma injectiontowards Piton Guanyin (Fig. 5c). Figure 6d summarisesthe processes inferred to have occurred between 29November and 3 December.

Stage 5

Typically, the end of lava effusion at Piton de la Fournaiseis accompanied by a few volcano-tectonic earthquakes andthe disappearance of the volcanic tremor (Hirn et al. 1991;Battaglia et al. 2005b). For this eruption, however, thesummit seismicity persisted and increased in intensity up to23 December (Fig. 3c). Seismicity shifted to higher levelsin the edifice from Stage 4 to Stage 5, with the earthquakesmainly very shallow at a depth of ∼500 m underneathDolomieu (Fig. 4, Staudacher 2002, OVPF unpublishedannual report). This progression suggests acceleratingupward stoping and upward migration of the shallowcollapse front at the top of the subterranean column ofdebris. The acceleration of the stoping process mayindicate that the material in the last few tens of metresof the subsurface was weaker, perhaps already faultedand fractured, making the stoping easier. This is inagreement with the presence of the incoherent lava pilefrom recent eruptions within Dolomieu crater. This phasespanned slightly more than three weeks and ended whenthe collapse front reached the surface and formed the pitcrater on 23 December, accompanied by an earthquake ofmagnitude 3. Ground deformation during Stage 5 wasminor but continuous, consistent with progressive col-lapse, and characterised by sudden closure of the fissuremonitored by the SFR extensometer and a sharpsoutheastward movement at the permanent GPS locationassociated with the pit crater formation (Fig. 3a inset, b).Comparison of data from October and December indicatesthat the inflation of the eastern flank due to the lateral dikewas permanent or at least mostly not recoverable (Fig. 5c),suggesting that the radial dike became frozen. Figure 6eschematically represents the main features characterisingStage 5.

Discussion

Magma dynamics

Our model describes an inflating source (Fig. 6a), creatingpre-intrusive inflation of the summit area. In the absence ofevidence for magma transfer from mantle depths since1998, a mechanism other than influx of magma from depthis required to produce the inflation of the source and tosupply magma to the surface. Tait et al. (1989) argued thatinflation in the eruptive cycle of Kilauea volcano may bethe result of vesiculation associated with crystallisation in achamber, in the absence of any new magma inflow. Whenoverpressure is great enough to fracture the chamber walls,magma intrudes to shallower levels. A small amount ofcrystallisation is sufficient to cause an eruption, and an

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isolated chamber can produce up to several tens oferuptions, each expelling a small fraction of the totalvolume (Tait et al. 1989). This is an attractive mechanismfor Piton de la Fournaise, since most eruptions lack seismicevidence for magma transfer from depth.

Overpressure in the magma reservoir near sea level maylead to an intrusion underneath Dolomieu which isassociated with seismic swarms preceding eruptions atPiton de la Fournaise. In the case of the November 2002eruption, a pre-existing shallow cavity at ∼500–700 mbeneath Dolomieu may have facilitated the ponding ofmagma during the intrusion (Fig. 6b). Moreover, thepresence of the high-velocity plug, which could representa dense intrusive complex, may have impeded the upwardmigration of magma underneath Dolomieu, thereby con-tributing to magma accumulation in the cavity above thevertical conduit. In such a configuration, the system maynot be able to accommodate the magma supply rate in thevertical intrusion, and the magma may find a zone ofweakness to migrate elsewhere, i.e., to the flanks. A similarcase was recently invoked for Stromboli volcano to explainthe transition between the explosive and effusive activity of2003 (Ripepe et al. 2005). The low-velocity ring on theperiphery of Piton de la Fournaise’s central cone, which isthought to represent a highly fractured zone built byeruptive material, may be an ideal medium for lateralmagma propagation.

The intense renewal of activity characterising Stage 4 iscomplex and unusual for Piton de la Fournaise. We suggestthat the simultaneous increasing lava discharge, volcanictremor, and summit seismicity are related and were causedby the onset of roof collapse above a vertical magmaconduit in which pressure had dropped below lithostaticlevels, producing forceful drainage of remnant magma toPiton Guanyin. Alternatively, the increasing lava dischargeand volcanic tremor could have been produced by a newinflux of magma from the chamber near sea level.However, such an explanation is not consistent with theintense seismicity and the deflation of the summit area from29 November to 3 December (Fig. 3). The very shallowearthquakes instead suggest subterranean collapse due to apressure decrease in the plumbing system. Such a pressuredecrease is inconsistent with a new batch of magma comingfrom the magma reservoir.

Pit crater formation

Although hundreds of calderas and summit pit craters havebeen identified in volcanic regions, very few caldera or pitcrater formation events have been observed directly(Kennedy and Stix 2003; Hirn et al. 1991). Therefore, thestudy of such a collapse event is important to ourunderstanding of pit craters and caldera-forming eruptions.

Among the few subsidence events that have occurred inhistoric times, geophysical data were collected at Katmaicaldera, Alaska (1912), Fernandina caldera, GalapagosIslands (1968), Mt. Pinatubo, Philippines (1991), andMiyakejima caldera, Japan (2000) (Simkin and Howard1970; Mori et al. 1996; Hildreth and Fierstein 2000; Geshiet al. 2002). The 1986 event at Piton de la Fournaise,although of much smaller magnitude, also was monitoredand attributed to the progressive stoping at the top of a cavitycaused by a pressure disequilibrium (lithostatic>magma) in amagma reservoir following a flank eruption (Hirn et al.1991). Hirn et al. (1991) interpreted the intense shallowseismicity of the 10 days preceding the 1986 collapse eventto represent tapping of magma deeper in the edifice. Onlythe final 36-hour paroxysmal crisis, accompanied bysummit deflation, was attributed to the collapse process.This interpretation contrasts with the December 2002collapse phase, which we infer to have occurred over thecourse of three weeks. Similarities in the seismic patterns ofthe Fernandina collapse (Simkin and Howard 1970; Filson etal. 1973) with that of Piton de la Fournaise in 1986 led Hirnet al. (1991) to suggest a comparable collapse mechanismfor the two events, but at different spatial and temporalscales. The process of caldera formation described by Geshiet al. (2002) for Miyakejima volcano shares many featureswith the model suggested for Piton de la Fournaise andFernandina. A flank intrusion and small submarine eruptioncaused the withdrawal of magma from the reservoir beneaththe summit of Miyakejima, carrying it to the northwest ofthe volcano. A summit caldera began to grow during thisintrusion, continuing to enlarge for over a month andleading to a depression of 1.6 km in diameter and 450 m indepth (Geshi et al. 2002). Geshi et al. (2002) proposed thatunderground stoping migrated upward from the roof of themagma reservoir to ∼3 km below the summit before surfacesubsidence started to occur.

Recent experimental analogue models also provideinsight. Roche et al. (2001) examined sub-surface structureof pit craters and their collapse mechanism, using a sand–plaster mixture over a cylinder of silicone, as analogue rockand reservoir in pressure disequilibrium (“lithostatic”>sili-cone), respectively. Depending upon the roof aspect ratio(roof thickness/roof width), their experiments revealed twobasic collapse mechanisms. If the aspect ratio is low(≤1), outward dipping reverse faults propagate upwardthrough the entire roof thickness and cause the subsidenceof a single piston. On the other hand, at high aspect ratios(>1.2), collapse propagates upward by stoping, formingboth reverse faults and tension fractures. In this case, theinitial underground collapse is restricted to a zone above thesilicone cylinder, forming a subterranean cavity under astable roof. An intermediate mechanism was obtained inexperiments with aspect ratios between 1.1 and 1.2. In this

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case, stoping leads to the formation of a cavity with a thin,unstable roof which suddenly collapses. The depressionformed is characterised by overhanging walls. In all cases,the diameter of the collapse area was approximately thesame as that of the silicone reservoir.

Roche et al. (2001) compared their experimental resultsto the morphology of natural pit craters. The observedsimilarities provide a means to roughly estimate the depthand the diameter of the reservoir responsible for thecollapses. For Piton de la Fournaise, Stage 5 ended withthe formation of a pit crater inside Dolomieu summit crater.Initially, the collapse area had a diameter of ∼100 m and adepth of ∼10 m, and thus a volume of approximately8×104 m3. Further gravitational collapse occurred in themonths afterwards, increasing the size of the collapse area.By 30 May, 2003, the diameter of the crater was ∼200 mand its depth ∼25 m, with a volume of ∼8×105 m3. Theseismicity of Stage 4 and particularly that of Stage 5, aswell as the absence of surface subsidence until 23December, suggest the upward migration of a cavity, orweak zone, rather than the progressive collapse of acoherent block. Therefore, the collapse mechanism resem-bles that described experimentally for an intermediate casewith aspect ratios between 1.1 and 1.2 (Roche et al. 2001).Moreover, the largest earthquake (M=3) of the entire periodof activity occurred on 23 December, which could accountfor the sudden collapse of the thin and unstable roof abovethe shallow cavity. This interpretation suggests that a smallpocket of magma ∼100–200 m in diameter initially intrudedto a depth of 100–300 m below the surface. This is a roughand minimum estimate, and the upper end of the verticalconduit may have been somewhat deeper, as earthquakesassociated with the collapse process occurred at a depth of∼500 m. Then, as magma drained to Piton Guanyin, thepressure dropped in the shallow plumbing system, and thestoping process began. The cavity started to migrateupward to eventually form a pit crater.

The experiments of Roche et al. (2001) also may providean explanation for subterranean collapse without pit craterformation for eruptions from September 1999 to January2002. The experiments with high aspect ratios reproducedan underground cavity with a stable roof. This may indicatethat the upper end of the vertical conduit creating the cavityin September 1999 was either deeper or smaller or both,compared to the situation in November 2002. In this model,the cavity was initially deep, and the critical aspect ratio forsurface collapse was only reached in November 2002 afterearlier eruptive episodes caused the cavity to migrate tohigher levels.

Notably, the volume of the newly formed pit crater atPiton de la Fournaise (∼8×105 m3) is one order ofmagnitude smaller than that of the lava flows (∼5–8×106 m3) of November 2002 (Staudacher 2002, OVPF

unpublished annual report). Observations of subterraneancollapses in mines indicate that surface subsidence iscommonly less than the amount of underground subsi-dence, i.e., 90%, 50%, and 20% for roof aspect ratios of 1,2, and 5 respectively, due to swelling of collapse debris(Whittaker and Reddish 1989; Wright 1973). Therefore,although large voids in the porous debris-filled column mayhave contributed to a smaller volume of the pit crater (25 msurface subsidence <∼30 m underground subsidence frommining estimates), this cannot account for the large volumedisparity. Instead, it suggests that the two volumes are notdirectly related. The vertical conduit, which is inferred to beresponsible for the surface collapse, is not the source ofmagma but rather an intermediate magma storage feature.Therefore, the volume difference can be explained by influxof magma from the deeper chamber near sea level flowingthrough the radial dike towards Piton Guanyin, in amountslargely exceeding the volume of accumulated magma at thetop of the vertical conduit. Other calderas and pit cratersalso have different volumes compared to material erupted inthe corresponding eruption. The 1986 pit crater at Piton dela Fournaise had an approximate volume of 2.5×106 m3

while the eruptions of March 1986 produced ∼15×106 m3

of basaltic lava (Hirn et al. 1991). In contrast, the caldera atMiyakejima volcano had a volume of 6×108 m3 while thetotal volume of eruptive materials was only ∼6.8×106 m3.This difference clearly showed that the subsidence wascaused by intrusion of material into the radial dike (Geshi etal. 2002).

Roche et al. (2001) showed that the diameter of thecollapse is roughly equal to that of the reservoir. Thus, amagma chamber similar to that described by Nercessian etal. (1996) would cause a Dolomieu-size subsidence. It istherefore logical that the formation of Dolomieu summitcrater may be linked to the magma reservoir near sealevel. The chamber roof would then have an aspect ratioof ∼2.5, so Dolomieu also may have formed byprogressive upward stoping and migration of a largeunderground cavity.

Conclusions

We described the volcanic activity of Piton de la Fournaise,one of the Earth’s most frequently active volcanoes, leadingup to a flank eruption and the formation of a pit crater in thesummit region at the end of the year 2002. Geophysicalmonitoring before, during, and after the eruptive episodeallowed the development of a model, dividing the activityinto five distinct stages and presenting new insights formagma transport dynamics within the basaltic shield of LaRéunion Island. This event is also important for under-standing collapse processes taking place at this and other

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volcanoes. Direct studies of subsidence processes are rarebut provide important information for hazard assessmentsuch as the mechanisms, timeframes, and the destructivepotential of collapse events. These phenomena, such as theformation of calderas and pit craters and the occurrence ofgiant landslides, are a fundamental part of the evolution ofisland volcanoes such as Piton de la Fournaise.

Acknowledgements We would particularly like to thank PierreBriole of the Institut de Physique du Globe de Paris for GPS dataprocessing, and Peggy Hellweg and an anonymous referee for theirthorough reviews. We also acknowledge Valérie Ferrazzini of theOVPF, Ben Kennedy and Nathalie Vigouroux of McGill University,and Valentin Troll of Trinity College Dublin for helpful comments.We are indebted to all members of the OVPF, in particular PhilippeKowalski and Philippe Catherine, for their hard work in providinghigh quality data during the stay of M-AL at the OVPF. Financialsupport was provided by GEOTOP-UQÀM-McGill and the OfficeFranco-Québécois de la Jeunesse to M-AL, and grants from theNatural Sciences and Engineering Research Council of Canada to JSand M-AL.

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