Relationship between tilt changes and effusive-explosive episodes at an andesitic volcano: the eruption at Volcán de Colima, México

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1 Bull Volcanol (2011) 73:91 99 DOI /s RESEARCH ARTICLE Relationship between tilt changes and effusive-explosive episodes at an andesitic volcano: the eruption at Volcán de Colima, México Vyacheslav M. Zobin & Juan José Ramírez & Hydyn Santiago & Eliseo Alatorre & Carlos Navarro Received: 20 November 2009 / Accepted: 31 August 2010 / Published online: 18 September 2010 # Springer-Verlag 2010 Abstract Continuous tilt changes during the effusive-explosive episodes at Volcán de Colima (México) were recorded simultaneously by two tiltmeters installed on opposite sides of the volcano at elevations of 2200 m and 3060 m above sea level. Data indicate that the 2004 lava extrusion was preceded by an inflation accompanied by a deflation. The 2005 explosion sequences were associated with a deflationary inflationary tilt. The period between the 2004 extrusion and the 2005 explosions was characterized by an inflationary tilt during a 3 month period. Two deformation sources were located. The first was situated at a depth between 300 m and 1800 m beneath the crater at the northern flank of the volcano and was responsible for volcano deformation during the preliminary September 2004 stage, the October 2004 extrusion, and the initial stage of the transition period and the March 2005 explosion sequence. The second source was located at a depth between 1800 m and 2800 m beneath the crater at the southern flank of the volcano and was responsible for volcano deformation during the final stage of the transition period and the May June 2005 explosion sequence. Keywords Tilt change. Extrusion. Explosion. Volcán de Colima Editorial responsibility: B. van Wyk de Vries V. M. Zobin (*) : J. J. Ramírez : H. Santiago : E. Alatorre : C. Navarro Observatorio Vulcanológico, Universidad de Colima, Colima, Col 28045, Mexico vzobin@ucol.mx Introduction Tilt monitors are widely used to assess activity at many volcanoes (e.g. Dvorak et al. 1981; Chadwick et al. 1988; Ishihara 1990; Beauducel and Cornet 1999; Bonaccorso et al. 2004; Marchetti et al. 2009; Wadge et al. 2006). Tilt changes generally accompany the major eruptive phases, and continuous tilt monitoring may help identify the processes of magma movement within a volcano edifice. Chadwick et al. (1988) showed that six lava extrusions at Mount St. Helens, occurring during May 1981 to August 1982, were preceded by inflationary tilt changes. The instruments installed within 50 m of the dome ( m from the vent) generally recorded a total of μrad of tilting. Outward tilting began several weeks before each extrusion, accelerating sharply for several days and then changed direction to inward tilting before eruptive activity. Four-day-long inflationary tilt changes were observed before lava extrusion at Pinatubo volcano on 7 June 1991 (Ewert et al. 1996). Genco et al. (2008) noted that during the February March 2007 effusive Stromboli eruption, tiltmeters recorded a small inflation, followed by a gentle ground deflation which lasted for 5 days. Bonaccorso et al. (2008) observed that the inflationary tilt change during the first days of eruption was in good correspondence with the lava effusion rate. Bonaccorso et al. (2008) modeled the deflation, inferring a depressurizing vertically elongated source centered under the volcano edifice at about 2.8 km depth below sea level. Single short inflationary-deflationary tilt changes associated with individual volcanic explosions were recorded

2 92 Bull Volcanol (2011) 73:91 99 at Sakurajima and Suwanoseijima volcanoes in Japan (Ishihara 1990; Iguchi et al. 2008) and Semeru volcano in Indonesia (Nishi et al. 2007; Iguchi et al. 2008). Genco et al. (2008) observed that the major explosion of 15 March 2007 at Stromboli was associated with a large inflation impulse. The source of deformation, as deduced from tilt observations at an andesitic volcano Soufriére Hills, Montserrat, during a dome-growth eruption, was located at a depth of km beneath the crater (Widiwijayanti et al. 2005). Works cited above show that measurements of tilt change is a popular approach to volcano monitoring, but despite the approach s popularity, a continuous tilt record for episodes including (1) lava extrusion, (2) the transition to explosive activity and (3) a long-lived period of explosive activity are not known. As noted 30 years ago by Dvorak et al. (1981), the drift of tilt components and the short base-line of the sensor as well as characteristics of the site installation may not allow dependable determinations of long-term tilt changes of small amplitude. Published data usually describe the tilt change before and during the lava extrusion process or during single explosion events. No description of the tilt change during a long-lived period of explosive activity had been published. In this paper, we report and analyze the tilt changes during three episodes of the eruptive activity of Volcán de Colima, México: the September October 2004 andesitic blocky-lava extrusion with a new dome grown in the crater, the November 2004 February 2005 effusive-to-explosion transition period and the March June 2005 explosive sequence that destroyed this new dome. The activity of Volcán de Colima during The andesitic Volcán de Colima is one of the most active volcanoes in Mexico. It is located in the western part of the Mexican Volcanic Belt, and together with the Pleistocene volcano Nevado de Colima, forms the Colima Volcanic Complex (Fig. 1). Volcán de Colima has a wide spectrum of eruption styles, including small phreatic explosions, major blocky-lavas, and large explosive events. Its eruptions mainly occur from the central crater but flank eruptions with the formation of lava domes (Fig. 1) have occurred on the north-eastern flank (Vulcancito dome, formed during 1889) and on the southern flank (Los Hijos domes, unknown age) (Breton González et al. 2002). The unrest began on 28 November 1997 and developed into three stages of effusive-explosive activity. Each of these stages continued for 2 3 years and involved the extrusion of andesite to form blocky-lava flows and a summit lava dome, followed by the Fig. 1 Monitoring system for the Colima Volcanic Complex. The labelled contour lines at 2000, 3000, and 3500 m show the general relief destruction of the lava dome by explosions (Zobin et al. 2002; 2006a; 2006b; 2008). The most recent stage of eruptive activity at Volcán de Colima began on 30 September 2004 (Zobin et al. 2006b; 2008). The extrusion of andesitic lava during September October 2004 formed a summit lava dome. A series of large explosions (with energy J) began on 10 March and concluded on 7 June The repetitive construction of new small-sized domes was observed during the April to July 2005 explosive activity; each was destroyed by the following explosion. The March September 2005 explosive sequence removed the 2004 lava dome and a sequence of small April July 2005 lava domes, and left a crater 260 m wide and 30 m deep. The ejected material had a volume of about m 3. The simultaneous and continuous records were collected by two tiltmeters installed on the slopes of the volcano during the effusive-explosive episodes. We study the relationship between the tilt changes and the volcanic effusive explosive episodes during the eruption and propose a conceptual model to explain this relationship. System of monitoring at Volcán de Colima used in this study Applied Geomechanics and Sobetra AGI701-2 biaxial tiltmeters were deployed in 1999 in 1-m-deep boreholes at monitoring sites COIA and EHJ1 at a distance of 1.6 km (elevation 3060 m) and 4 km (elevation 2200 m) from the central crater, respectively (Fig. 1). These resistive bubbletype tiltmeters have a resolution of 0.1 μrad, a rate of 30 samples/hour, and included a temperature sensor sealed with

3 Bull Volcanol (2011) 73: the sensor body. The radial components of both tiltmeters were oriented towards the crater; so positive signals indicated inflation whereas negative signals indicated deflation. The tilt stations are very sensitive to external influences; the abrupt drift of tilt components may be related to intense rainfall or local earthquakes. During the period of observations, from September 2004 until July 2005 (Fig. 2), no significant (M 5) local earthquakes were observed. The intense rainy season began in mid-june in Therefore, no abrupt drift of tilt components related to external influence is expected to have affected the September July results. All significant variations in tilt change during the period under study can be attributed to the deformation of the volcanic edifice. Seismic station EZ5 was situated at a distance of 4 km from the crater on the southern flank of the volcano (Fig. 1), and was equipped with a broadband threecomponent GURALP CMG-40TD sensor with a corner frequency of 30 s and a digitizer DM24 with a sampling rate of 100 samples/s. Seismic station EZV4 was situated at a distance of 1.6 km from the crater on the north-eastern flank of the volcano and was equipped with a short-period (natural period of 1 s) vertical seismometer. A Sony model CCD-TRV118 video-camera was installed at an elevation of 4000 m (Fig. 1) andwassituatedata distance of 5.5 kmn from the crater (NEV). Tilt changes during the effusive-explosive episodes Figure 2b and c shows the daily averages of tilt (radial component) recorded by two biaxial tilt stations situated at different elevations on the volcanic edifice during September 2004 to June We divided the periods of significant volcanic activity into three stages. Stage I shows the tilt changes during the andesitic blocky-lava extrusion and stage III shows the period of large Vulcanian explosions. The time interval corresponding to the transition from effusive to explosive activity is shown as stage II. Tilt changes before the lava extrusion A symmetrical inflationary deflationary impulse preceding the 30 September lava extrusion was recorded at COIA station (Fig. 3). It began on 6 September, reached its maximum (6 μrad) on 10 September and was terminated Fig. 2 a Daily number of degassing events. b and c The daily averages of tilt data (radial components) recorded by two biaxial tilt stations COIA and EHJ1 situated at different elevations on the volcanic edifice during September 2004 June 2005 and corrected for temperature effects. Stage I locates the tilt changes during the andesitic blocky-lava extrusion; stage II shows the period of transition from effusive to explosive stages, and stage III shows the period of large Vulcanian explosions. The number of degassing events was calculated from the seismic records at short-period station EZV4 situated at 1.6 km from the crater. Numbers with arrows show the selected crucial dates for deformation calculations (See Table 2) Fig. 3 a The variations in the lava emission rate. b The variations in explosive activity. c Daily averages of tilt changes (radial components) recorded by tilt station COIA. d Daily averages of tilt changes (radial components) recorded by tilt station EHJ1. The rectangle shows the period of tilt change during the September November 2004 andesitic blocky-lava extrusion

4 94 Bull Volcanol (2011) 73: days before the lava began to flow from the crater. Station EHJ1 did not record any anomaly. Tilt changes during the lava extrusion Figure 3 illustrates the tilt changes during the lava extrusion together with the variations in lava emission rate and explosive activity. The tilt record at station COIA (Fig. 3c) shows a well-expressed negative impulse, which began on 5 October, 6 days after the beginning of the extrusion, indicating deflation of the upper part of volcanic edifice at an elevation of about 3000 m. This symmetrical deflationary inflationary impulse had a duration of about 170 h and reached an amplitude of 8 μradon9october.thepeakin the tilt deflationary impulse occurred just after the lava effusion rate reached its maximum (Fig. 3a). It occurred against a background of a flat record that began simultaneously with lava extrusion from the crater on 30 September. The flat record re- appeared after the deflationary impulse and then continued until 23 October, marking the end of lava effusion. No significant tilt change at station EHJ1, situated at an elevation of 2200 m on the southern side of the volcano, was recorded before or during the lava extrusion (Fig. 3d). Tilt changes during the transition period Immediately after termination of lava discharge, the tilt changes recorded at upper station COIA began to show an intense inflation, which continued for 3 months (Fig. 2; stage II). Tilt amplitude reached about 50 μrad. The record of inflation at lower station EHJ1 began about 50 days later and was characterized by an amplitude of about 20 μrad only. The initial stage of inflation was accompanied by numerous small degassing events (Fig. 2a). Their number increased simultaneously with the amplitude of the inflation anomaly at station COIA (elevation 3060 m) but then gradually decreased when the inflation at station EHJ1 (elevation 2200 m) began. Tilt changes during the stage of large Vulcanian explosions The stage, during which there were large explosions with energy J, was divided into two substages: the large explosions that occurred in March, 2005 (Fig. 4, stage IIIA) and during May June, 2005 (Fig. 4, stage IIIB). A list of explosions, with the values of energy estimated from the broadband seismic records in (Zobin et al. 2006b; 2007), is presented in Table 1. We do not consider here the tilt change related to any individual explosions. It was shown (Zobin et al. 2007) that the individual explosions of this sequence were associated Fig. 4 Daily averages of tilt changes (radial components) recorded by two tilt stations COIA and EHJ1 during the March June, 2005 explosion sequence. The rectangles show two periods of large Vulcanian explosions (IIA, March; IIB, May June). Diamonds indicate the position of large explosions (Energy>10 12 J) with rather large (from 1 to μrad, depending on the energy of explosions) inflationary deflationary tilt changes over the period of a few minutes at station COIA. It is impossible to see these short impulses on the daily averaged curves. Thus, we discuss only the daily averaged tilt change during the series of explosions. Table 1 The 2005 large explosions of Volcán de Colima, México Date,yyyy_mmddhhmm Energy (Joules) 2005_ E _ E _ E _ E _ E _ E _ E _ E+12 The method of calculation explosion energy is described in Zobin et al. (2006a, b) The energy estimations were published in (Zobin et al. 2007). The errors in the energy estimations are equal to ±0.5 log. unit

5 Bull Volcanol (2011) 73: During the March 2005 explosion sequence, upper station COIA recorded a deflationary inflationary anomaly which began on 9 February and continued up to 7 April with the maximum apparent deflation amplitude of about 25 μrad recorded on 18 March. Station EHJ1 recorded a smalldeflationary tendency reaching on 18 March about 4 μrad only. The large explosions occurred on 10 and 13 March during the maximum development of a deflationary anomaly. During the May June 2005 explosion sequence, upper station COIA recorded rather low-amplitude variations in tilt while lower station EHJ1 recorded a well- expressed deflationary anomaly from 15 April to 3 July with the apparent deflation amplitude of about 76 μrad on 5 June. The anomaly began about 30 days before the first large explosion of this sequence. All May June large explosions occurred during the development of deflationary anomalies. The maximum deflation was correlated with the largest explosion of this sequence which occurred on 5 June, with an energy of about J. Location of the deformation sources We have the observations of only two tiltmeters; therefore, we cannot model the surface deformation. However, an approximate estimation of the position of a deformation source as a simple spherical model may be made. We use the tilt vectors and tilt amplitudes calculated for six crucial dates in the eruption development during September 2004 to March These selected dates were the following (Table 2, Fig. 2): 10 September 2004, when the preliminary anomaly, associated with the magma intrusion, reached its maximum; 15 October 2004, when the anomaly observed during the lava extrusion reached its minimum; 15 November 2004, when the significant positive tilt was observed at station COIA; 31 December 2004, when the positive tilt began to be recorded also at station EHJ1; 18 March 2005, when the negative tilt recorded at COIA station during the March explosive sequence reached its minimum; and 15 May 2005, during the development of significant negative tilt at EHJ1 station at the initial period of the May June explosive sequence. For calculations, the mean values of tilt within three-day windows were taken, either side of the selected day. The position of deformation sources from tilt vectors The tilt vectors were calculated using the mean values of radial and tangential components of tilt recorded at the same station that were transformed from angular into linear values using the equation: D R;T ¼ L Sinq R;T ð1þ where D is the length of displacement component (radial R or tangential T) at tilt station, L is the distance between tilt station and volcanic crater and θ is the tilt in degrees (Manual 2001). The length of vector V was calculated as: V ¼ R 2 þ T 2 0:5 ð2þ Its azimuth Az was calculated as (Montes de Oca 1994): Az ¼ tan 1 ðrt = Þ ð3þ and then corrected for the real position of tilt components according the volcano crater: for station COIA Az corr ¼ Az þ 300 for station EHJ1 Az corr ¼ Az þ 123 ð4þ ð5þ Table 2 Deformation characteristics at different stages of the Colima eruption No Date, yyyy_mm_dd θ 1 /θ 2 Station Vector, m Azimuth, deg _09_ COIA EHJ _10_ COIA EHJ _11_15 33 COIA EHJ _12_ COIA EHJ _03_ COIA EHJ _05_ COIA EHJ

6 96 Bull Volcanol (2011) 73:91 99 Figure 5 shows the deformation vectors obtained for two tilt stations at the different stages of the eruption. During the intrusion extrusion episodes, the vectors at station COIA were larger in length than the same at EHJ1 station (Fig. 5a). The stable position of the vectors at COIA station directed to (or from) the same source indicates the common source for these eruption episodes. The transition stage from effusive to explosive activity is characterized generally by vectors directed away from the central part of volcanic edifice (Fig. 5b); this could be a result of a new body of magma intrusion. During the November 2004 activity, more intense deformation was observed near the crater at station COIA, but during the December 2004 activity, the deformation at station EHJ1 prevailed. During May 2005, the vector length at EHJ1 station became significantly larger than the COIA vector indicating the deeper source of deformation comparing with the March episode. The EHJ1 vector again was not directed at the crater but toward the central part of the volcanic edifice, which may indicate the presence of a deeper source. The deformation vector distributions during different stages of the eruption may, therefore, indicate the presence of at least two sources of deformation situated at different depths. The position of deformation sources from the tilt amplitude ratio Widiwijayanti et al. (2005) proposed a method for locating a shallow magma body, based on Mogi s (1958) model, using the simultaneous tilt records at two stations. According to Widiwijayanti et al. (2005), for a spherical nucleus of deformation, buried at depth z below the surface of a halfspace, the following equation may be used: q 1. q 2 ¼ r 1 ðr 2 þ zþ 2:5 = r 2 ðr 1 þ zþ 2:5 ð6þ Fig. 5 Deformation vectors at tilt stations COIA and EHJ1 calculated for three stages of the eruption at Volcán de Colima. a Lava extrusion stage 1; b Effusive to explosive transition stage 2, and c Explosive stage 3. The contour lines at 2000, 3000, and 3500 m show the generalized relief of Volcán de Colima. The gray vectors point to the deflationary direction. The scale of vector length is shown separately for each stage where θ 1 and r 1,andθ 2 and r 2, are the radial surface tilts and the surface traces of the vectors joining the nucleus of strain, situated within the volcanic edifice, with the recording locations COIA and EHJ1, respectively (Fig. 6). The tilt amplitude ratios were rather large during the September 2004 intrusion and the October 2004 extrusion, and during the November 2004 transition period and the March 2005 explosions (6.7, 6.7, 33 and 5.8, respectively). They were low during the December 2004 February 2005 transition period and the May June 2005 explosions (0.5 and 0.1, respectively). The Eq. 6 allows an approximation of the position of the nucleus of strain by selecting the best solution to fit the tilt amplitude ratios θ 1 / θ 2. Based on the minimum of the misfit function, the best models for two deformation sources were selected. The position of the upper source, constrained by tilt amplitude ratios between 5.8 and 33, is estimated to lie between the depths of 300 m to 1800 m beneath the crater. The deep source, constrained by a tilt amplitude ratio of 0.1 and 0.5, is located between the depths of 1800 m to 2800 m beneath the southern flank of the volcano. Figure 7 shows the temporal variations of tilt amplitude ratios indicating the participation of these two sources in eruptive process.

7 Bull Volcanol (2011) 73: Fig. 6 Approximate positions of the strain sources during the eruption at Volcán de Colima shown on a north south profile of Volcán de Colima and model of the volcanic system. The source areas correspond to the position of the sources that were constrained by the minimum of the misfit function of the tilt amplitude ratios. The schematic view of the volcanic system consisting of the dyke conduit and lava dome is shown also Discussion The tilt change observations during the effusive explosive episodes allow us to formulate the following results: 1. The eruption episodes were associated with the deformation produced by two sources situated (1) at a depth between 300 m and 1800 m beneath the crater at the northern flank of the volcano and (2) between 1800 m and 2800 m beneath the southern flank of the volcano. 2. The upper source is inferred to be responsible for volcano deformation during the preliminary September 2004 stage, the October 2004 extrusion, the initial stage of the transition period, and the March 2005 explosion sequence. The deep source is interpreted as being responsible for volcano deformation during the final December 2004 February 2005 stage of the transition period and the May June 2005 explosions. Based on these results, we propose the following conceptual model for the volcanic system acting during the eruption (Fig. 6). Schematically, it consists of three elements: a N-S trending dyke, filled with magma, which connects at the depth of about 2,000 m with the cylindrical volcanic conduit, and a lava dome at the upper part of the conduit. This model follows in its structural elements the model proposed by Costa et al. (2007a, b) for the volcanic system feeding lava dome eruptions. Costa and colleagues model supposes that the extrusions are mainly fed by dykes at depth, with cylindrical geometries developing only at shallow levels. Our proposed sources of deformation, which may be associated with the magma storage areas, are situated (1) within the volcanic conduit and the lava dome base and (2) at the southern part of the dyke, respectively. We suggest the following scenario for the effusive explosive activity: the September 2004 intrusion of magma from the dyke to the conduit caused inflation near COIA station; in October 2004 extrusion of magma from the crater was accompanied by ascent of magma from the magmatic storage area along the conduit, with a corresponding deflation at station COIA; in November 2004, the second magma intrusion from the northern part of the feeding dyke began to inflate the magma storage area. This process caused a re-distribution of magma and the intrusion of a new portion of magma within the second source at the southern part of the dyke that produced the inflationary deformation near both stations EHJ1 and COIA. The subsequent fragmentation of a new magma batch generated the March 2005 explosion sequence. This explosion sequence removed a great amount of magmatic material from the first magma storage area, causing deflationary deformation. The May June 2005 explosions are inferred to have been generated later from the second deep magma storage area that was again filled. The eruption of magmatic material during this explosion sequence thus produced the deflationary anomaly at station EHJ1. Fig. 7 Variations in the tilt amplitude ratios during the Colima eruption. The date of the tilt amplitude ratios measuring are indicated. Stages I, II, III of the eruption are the same as in Fig. 2

8 98 Bull Volcanol (2011) 73:91 99 The presence of two magma storage areas during the eruption at Volcán de Colima may be partially supported by the studies of earlier activity of the volcano. The old lava domes on the volcanic flanks (Vulcancito and Los Hijos) are situated just above the position of the supposed magma storages (Fig. 6). More than one possible source of deformation was also found by the horizontal GPS and vertical levelling work of Murray and Wooler (2002). They suggested their data could be modeled from vertical-motion data as a deflating source but that the horizontal GPS data suggested either an inflation or the interplay with other sources of deformation, such as compaction, lateral spreading, and down-slope creep. Note that the measurements made by Murray and Wooler (2002) are isolated measurements over a long time period, whereas ours are continuous and thus will detect much shorter timescale events, such as rapid magma intrusion, or withdrawal. Instrumental drift, however, prevents tilt meters from detecting long-term gravitational effects. The proposed N-S trend of the feeding dyke coincides with the sub-meridional direction of the main tectonic structure of the region, the Colima Rift zone (Garduño- Monroy et al. 1998) as well as with the position of the old flank lava domes (Fig. 1). The deflationary inflationary periods during the Colima eruption are similar to those observed at the Soufrière Hills Volcano from InSAR and GPS measurements (Wadge et al. 2006) showing deflation during the December 1999 December 2000 lava effusion and inflation in times of quiescence (the April 1998 November 1999 period of no lava effusion). In our case, we have the periods of extrusion, a transition period that may be considered as quiescence, and additionally, a period of large explosions. So we add to the two stages of deformation during lava dome eruptions, noted for Soufrière Hills Volcano, the period of deflation corresponding to the stage of large explosions. Conclusions The study of continuous tilt changes at two stations, installed on the opposite sides of Volcán de Colima, during the effusive explosive episodes allows us to show a difference in the tilt changes during the effusive and explosive eruption stages as well as during the transition period from one stage to another. Two possible deformation sources were located at a depth between 300 m and 1800 m beneath the crater at the northern flank of the volcano and at a depth between 1800 m and 2800 m beneath the crater at the southern flank of the volcano. A proposed conceptual model of the eruption describes the interaction during the effusive explosive episodes between two magma storage areas corresponding to these deformation sources and situated within the N-S trending dyke. Acknowledgments The comments by JDL White, B van Wyk de Vries, J Gottsmann and two anonymous reviewers helped to improve the manuscript. One of the anonymous reviewers improved our English grammar. This study was partially supported by the European Commission, 6th Framework Project VOLUME', Contract No References Beauducel F, Cornet FH (1999) Collection and three-dimentional modeling of GPS and tilt data at Merapi volcano, Java. J Geophys Res 104: Bonaccorso A, Campisi O, Falzone G, Gambino S (2004) Continuous tilt monitoring: Lesson learned from 20 years experience at Mt. Etna. In: Bonaccorso A, Calvary S, Coltelli M, Del Negro C and Falsaperla S (eds), Etna: Volcano Laboratory. AGU Monograph 143, Washington, pp Bonaccorso A, Gambino S, Guglielmino F, Mattia M, Puglisi G, Boschi E (2008) Stromboli 2007 eruption: Deflation modeling to infer shallow-intermediate plumbing system. 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