Journal of Volcanology and Geothermal Research 324 (2016) 156–168 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores Evidence of recent deep magmatic activity at Cerro Bravo-Cerro Machín volcanic complex, central Colombia. Implications for future volcanic activity at Nevado del Ruiz, Cerro Machín and other volcanoes John Makario Londono Colombia Geological Survey (CGS), Servicio Geológico Colombiano (SGC), Avenida 12 de Octubre 15-47, Manizales, Colombia a r t i c l e i n f o Article history: Received 29 October 2015 Received in revised form 2 June 2016 Accepted 8 June 2016 Available online 10 June 2016 Keywords: Magmatic activity Nevado del Ruiz volcano Machin volcano Magma intrusion Volcanic activity a b s t r a c t In the last nine years (2007–2015), the Cerro Bravo-Cerro Machín volcanic complex (CBCMVC), located in central Colombia, has experienced many changes in volcanic activity. In particular at Nevado del Ruiz volcano (NRV), Cerro Machin volcano (CMV) and Cerro Bravo (CBV) volcano. The recent activity of NRV, as well as increasing seismic activity at other volcanic centers of the CBCMVC, were preceded by notable changes in various geophysical and geochemical parameters, that suggests renewed magmatic activity is occurring at the volcanic complex. The onset of this activity started with seismicity located west of the volcanic complex, followed by seismicity at CBV and CMV. Later in 2010, strong seismicity was observed at NRV, with two small eruptions in 2012. After that, seismicity has been observed intermittently at other volcanic centers such as Santa Isabel, Cerro España, Paramillo de Santa Rosa, Quindío and Tolima volcanoes, which persists until today. Local deformation was observed from 2007 at NRV, followed by possible regional deformation at various volcanic centers between 2011 and 2013. In 2008, an increase in CO2 and Radon in soil was observed at CBV, followed by a change in helium isotopes at CMV between 2009 and 2011. Moreover, SO2 showed an increase from 2010 at NRV, with values remaining high until the present. These observations suggest that renewed magmatic activity is currently occurring at CBCMVC. NRV shows changes in its activity that may be related to this new magmatic activity. NRV is currently exhibiting the most activity of any volcano in the CBCMVC, which may be due to it being the only open volcanic system at this time. This suggests that over the coming years, there is a high probability of new unrest or an increase in volcanic activity of other volcanoes of the CBCMVC. © 2016 Elsevier B.V. All rights reserved. 1. Introduction The study of ascending of magma from depth to the surface is one of the most fascinating and intriguing issues in volcanology. Such magma ascent can be studied from many different perspectives and with different focus. Geology and petrology can help to understand how magma ascended in the past. During an eruptive episode it may be possible to infer parameters for magma ascent if petrological and petrographic data are available (Jaupart, 1998; Uto et al., 2001; Saito et al., 2010; Takeuchi, 2011). Geophysics and geochemistry by means continuous monitoring can help to detect changes in the magmatic system, and possible onset of magma ascent (Bräuer et al., 2005; Chaussard and Amelung, 2012, 2014; Thomas and Neuberg, 2014; Acocella et al., 2015; Kazahaya et al., 2015; Christopher et al., 2015). In this work, geophysical and geochemical data collected by Colombia Geological Survey (CGS) from continuous monitoring of E-mail address: [email protected]. http://dx.doi.org/10.1016/j.jvolgeores.2016.06.003 0377-0273/© 2016 Elsevier B.V. All rights reserved. Cerro Bravo-Cerro Machin Volcanic Complex (CBCMVC) are analyzed to infer magmatic activity. CBCMVC is located in central Colombia (Fig. 1). Some of the most active and dangerous volcanoes in Colombia belong to this complex (Méndez et al., 2002), such as Nevado del Ruiz volcano (NRV), Cerro Bravo volcano (CBV) and Cerro Machín volcano (CMV). The eruptive products of the CBCMVC are andesitic to dacitic with only a smaller proportion of rocks of different composition. During the last 2 Ma, these volcanic complexes have shown a variety of eruptive styles. In the last 0.5 Ma NRV has exhibited vulcanian explosive activity (Thouret et al., 1985; Thouret and Gourgaud, 1990), while CMV has produced six Plinian eruptions over the last 5000 years (Rueda, 2004). The most recent period of eruptive activity started with the 1985 eruption of NRV (VEI = 3). This eruption generated a lahar that killed more than 23,000 people. Since 1985 onward volcanoes such as NRV and CMV have shown multiple signs of unrest. Other volcanoes belonging to the CBCMVC can be considered active volcanoes, for example, Santa Rosa volcano (SRV), Santa Isabel volcano (SIV), Cerro España volcano (CEV), Nevado del Quindío volcano (NQV), and Nevado del Tolima volcano (NTV) (Fig. 1). Although few studies J.M. Londono / Journal of Volcanology and Geothermal Research 324 (2016) 156–168 157 Fig. 1. Location of Cerro Bravo-Cerro Machín Volcanic Complex (CBCMVC) and monitoring networks in operation during the two study periods. 1: Romeral volc (RV), 2:Cerro Bravo volc. (CBV), 3: Nevado del Ruiz volc. (NRV), 4: St. Isabel volc. (SIV), 5:St. Rosa volc. (SRV), 6: Cerro Espana volc. (CEV), 7: Nevado del Quindío volc. (NQV), 8: Nevado del Tolima volc. (NTV), and 9: Cerrro Machín volc. (CMV). have been done on these volcanoes, the Colombia Geological Survey (CGS) has recently initiated research on these volcanoes. Volcano monitoring of active volcanoes in Colombia started formally in 1986. NRV, CMV, and NTV were the first volcanoes to be monitored in Colombia. Since then, baselines of various monitoring parameters have been established by CGS, such as seismicity, geochemistry and deformation. These long-running baselines are key to detecting temporal changes in volcanic activity. With the increasing availability of sophisticated instruments and techniques over the last few decades, it is possible to detect or infer new magma input in volcanic complexes (Lough et al., 2013). Sophisticated monitoring networks have been in place since 2000, and several temporally complete datasets are available for volcanoes in the CBCMVC. Analysis of changes in the baseline of multiple monitoring parameters, such as seismicity, deformation and geochemistry, suggests a possible change in volcanic activity in the CBCMVC. In this work I propose that the geophysical and geochemical data collected over the last 15 years (2000–2015) highlight the onset of renewed magmatic activity at CBCMVC. 2. Sources of information and data Various sources of data were used for the assessment of the ongoing volcanic activity in the CBCMVC: a) Published tiltmeter results from CGS (2007–2015); b) GPS data by Ordoñez et al. (2015); c) InSAR by Lundgren et al. (2015a, 2015b); d) Geochemistry data by Inguaggiato et al. (2014) and CGS (SGC, 2000 to 2015); e) Seismic data from NRV and CMV (SGC, 2014, Londoño and Dionicio, 2011; Londoño and Castaño, 2014); and finally, f) the seismic database of the Colombia Geological Survey (CGS). Some of the SO2 data used in this study were produced from the Giovanni online data system, which is developed and 158 J.M. Londono / Journal of Volcanology and Geothermal Research 324 (2016) 156–168 maintained by NASA (http://giovanni.gsfc.nasa.gov/giovanni/). Seismicity, radon gas, CO2 gas, pH, SO4/Cl and tiltmeter data were reprocessed, analyzed and integrated. Time series before 2000 are not temporally complete for the entire volcanic complex. For NRV and CMV continuous seismic data are available since 1986 and 1989, respectively. For NRV, CMV, NTV, and CBV volcanoes, only sporadic geochemistry and hydrochemistry samples and dry tilt measurements are available. With these time series I observed no important changes before 2000 for almost all volcanoes, except for NRV. Therefore, the studied time period will be focused from 2000 to 2015. Fig. 1 shows the location of CBCMVC, the main volcanoes of the complex, and the networks used for volcano monitoring. The basic configuration of the monitoring networks has not changed significantly between 2000 and 2015. At some seismic stations the sensor has been upgraded from short-period to broad-band instruments, and the number of near-field stations has been increased at NRV and CMV to improve coverage and improve earthquake locations at these volcanoes. To create continuous and internally consistent baselines, the same stations have been used for data analysis and statistics throughout the period 2000 to 2015. Electronic tiltmeters have been used intermittently since 2000, and since 2000 dry tiltmeters, precision leveling vectors and EDM have been used at several volcanoes, however, measurements were not continuous during this time. A complete set of electronic tiltmeter data is available after 2007 for NRV, CMV and NTV volcanoes. GPS instruments have been deployed by CGS since 2010 at CMV, and since 2011 at NRV (SGC, 2014); therefore, GPS data is not complete, however, the trend is the same as that showed by the electronic tiltmeter network (SGC, 2015a, 2015b). Geochemistry data is available since the early 1990s mainly for NRV. Periodic measurements of pH, Cl, SO4 and temperature at hot springs are available for some volcanoes since 2008. Since 2004 until 2015, satellite derived SO2 data has been available. From 2009 to 2015 DOAS instruments were deployed at NRV. Radon gas and CO2 in soil has been available from 2005 to 2009 for NRV, CMV and CBV. At CMV data for CO2 in soil detection for N 400 points around the volcano are available from two field campaigns in 2013 and 2014. Due to the longevity and consistency of these seismic, deformation and geochemical datasets, any temporal or spatial changes in these baselines is likely due to changes in volcanic activity rather than changes due to instrumentation (Fig. 2). Some variables such as temperature Fig. 2. Time evolution of monitoring networks and data from volcanic centers of the CBCMVC. Vertical bars represent number of stations and color represents the volcano. J.M. Londono / Journal of Volcanology and Geothermal Research 324 (2016) 156–168 159 in hot springs, EDM, dry tiltmeters, or precision leveling vectors, were not included in the analysis because either no changes were detected during the study period (i.e. a constant value, or measurements within instrument error), or data were not complete. Fig. 2 shows the timeline of monitoring networks used for analysis. 3. Results Fig. 3 shows temporal variation of the geophysical and geochemical parameters analyzed at each volcano, and Table 1 summarizes the main changes over time. In Fig. 3 data have been normalized with respect to the maximum value (taken as unity) for each volcano during the period 2000–2015. This was done to display all the available data in a unique simple plot, because about 40 individual time series were available. First, the yearly average was calculated for each available time series across all stations at each volcano throughout the study period. Then the average was normalized with respect to the maximum value of each dataset and plotted as a color bar. The color represents the volcano. For the number of earthquakes, all small earthquakes with ML b 1.3 were removed from the catalog at all volcanoes. This was done to eliminate the effect of varying sensitivity of the instruments, due to occasional changes from short-period seismic stations to broad-band seismic stations throughout the study period. Fig. 3 and Table 1, show that several volcanic centers exhibited changes after 2007, while the period 2000–2007 did not show any significant changes in those parameters. As mentioned previously, the basic monitoring networks have been well established since 2000 (Fig. 2). While there have been some improvements to monitoring networks, in particular seismic networks, since 2007, these changes alone are not sufficient to explain the changes in observed geophysical and geochemical monitoring data (Fig. 3). Not all volcanoes have the same monitoring networks for each geophysical or geochemical variable, and there may be some bias towards volcanoes with higher activity being more closely monitored and hence having more complete data (Fig. 2). Therefore this study focuses on those volcanoes with the most complete data such as NRV, CMV, NTV and CBV. Based on the results shown in Fig. 3, further analysis of the data was divided into two periods; from 2000 to 2006, and from 2007 to 2015. 3.1. Seismicity Before comparing spatial distribution of seismicity for the two chosen periods, the seismic catalog must be homogeneous. The entire catalog ranged from − 2.5 to 4.7 ML. The completeness magnitude (Mc) trough time was calculated by using the Maximum curvature method, taking a sampling window of 500 data points, with an overlap of 4 (Wiemer and Wyss, 2002). Fig. 4 shows the variation in time of completeness magnitude (Mc). Average Mc for the seismic catalog for all studied period (2000–2015) was − 0.19, including all the available earthquakes. This means that the seismic catalog is complete at very low magnitudes almost all the time. To ensure comparison of a complete catalog for both study periods, and to remove any potential instrument detection effects (e.g. short-period vs. broad-band), mainly the effect of very small earthquakes detected by broad-band stations, only earthquakes with ML N 1.3 were used for seismic analysis, guaranteeing a homogeneous catalog for both study periods. Accordingly, earthquakes smaller than ML1.3 were removed to avoid the over-detection of events at broadband stations as compared to short period seismic stations. From Fig. 4 it is possible to observe that only a small portion of 2001 had a Mc higher than 1.3; the remaining Mc values are lower than 1.3. This means that a low cut off value of ML = 1.3 is reasonable to homogenize and complete the seismic catalog from 2000 to 2015, ranging from 1.3 to 4.7 ML. Smaller values of ML may bias the seismic analysis for some volcanoes where new broad-band stations have been deployed through time. Fig. 3. Time series of geophysical and geochemical parameters for each volcano. Color bars represent volcanoes. Vertical axes are normalized units with respect to the maximum value, except for SO4/Cl and pH. Right vertical axes of SO4/Cl time series corresponds to SRV. The seismic networks are sufficiently similar for both study periods. Although the number of seismic stations increased over time, and some of the sensors were changed from short-period to broad-band during 2007–2015, any additional or upgraded stations were located in the 160 Table 1 Main changes observed for geophysical and geochemical parameters at CBCMVC for the period 2000–2015. NRV: Nevado del Ruiz, CMV: Cerro Machin Volcano, CBV:Cerro Bravo, NQV: Nevado del Quindío volcano, NTV: Nevado del Tolima volcano, SIV: St. Isabel volcano, SRV: St. Rosa volcano, and CEV: Cerro Espana volcano. Year 2000 Main Background activity at changes CBCMVC observed 2001 Background activity at CBCMVC 2003 Background activity at CBCMVC 2004 Background activity at CBCMVC 2005 CMV Small increase in earthquakes at CMV (volcano-tectonic earthquakes) 2006 Background activity at CBCMVC 2013 SIV Large and deep (20 km) deformation source, close to Santa Isabel Volcano (SIV) detected by InSAR analysis (Lundgren et al., 2015a, 2015b). NRV High seimicity and SO2 flux at NRV. December 2013. Earthquake swarm of volcano-tectonic earthquakes, located 5 2014 January–December 2014. Increase in seismic activity at NRV, NTV, NQV, SIV, SRV, CEV. CMV June 2014. Earthquake. Depth = 190 km. ML = 4.4. Located 5 km SE of CMV. km. ML = 4.0, Located 6 km) beneath the active crater. Hydro-thermal activity. No eruption. 2008 CBV April 2008. Increasing CO2 emission (5% vol) and Radon (N1000 pCi/L) in soil at CBV. PSRV September 2008. Earthquake, depth = 133 km. ML = 5.8. Located 15 km W of CBCMVC, close to SRV. CMV November 2008. Earthquake, depth = 4 km. ML = 5.7. Located few km W of CMV. CBV December 2008. Deep LP (DLP) seismicity at CBV. 2009 NRV November 2009. Earthquake swarm of volcano-tectonic earthquakes, located 5 km e W of NRV. Depth = 6-7 km 2010 March 2010. Earthquake. Depth = 120 km. ML = 4.9. Located 15 km W of CBCMVC. CMV Deep (15-20 km) persistent seismicity located 5 km SE of CMV. NRV January–September 2010. Increase in SO2 flux at NRV, reaching high values in October (N5000 Ton/day). NQV November 2010. Increase in rate of shallow seismicity at NQV 2011 SIV Large and deep (20 km) deformation source, close to Santa Isabel Volcano (SIV) detected by InSAR analysis (Lundgren et al., 2015a, 2015b). 2012 SIV Large and deep (20 km) deformation source, close to Santa Isabel Volcano (SIV) detected by InSAR analysis (Lundgren et al., 2015a, 2015b). NRV Local deformation at NRV. March April, May–June. Increase in seismic activity and SO2 flux at NRV. Two small phreato-magmatic eruptions. km W of NRV. Depth = 2007 NTV January 2007. Earthquake. depths b10 km W of NTV. CMV April 2007. SO2 flux at CMV = 30 Ton/day. NRV June 2007. SO2 flux at NRV = 370 Ton/day November 2007. About 50ur of deformation at NRV 2015 January–September. 2014. Increase in seismic activity at NRV, NTV, NQV, SIV, SRV, CEV. NRV April 2015. Earthquake swarm of volcano-tectonic earthquakes, located 5 km W of NRV. Depth = NRV Local deformation at NRV. High SO2 flux at NRV Sep. 2014. Earthquake swarm of volcano-tectonic earthquakes, located 5 6-7 km CMV km W of NRV. Depth = Increase in helium 6–7 km isotope at CMV (Inguaggiato et al., 2014) 6–7 km Local deformation at NRV High SO2 flux at NRV. J.M. Londono / Journal of Volcanology and Geothermal Research 324 (2016) 156–168 2002 NRV April–June, September 2002. Increase in rate of volcanic earthquakes (Hybrids) at NRV, located at shallow depths (b2 J.M. Londono / Journal of Volcanology and Geothermal Research 324 (2016) 156–168 161 Fig. 4. Completeness Magnitude (Mc) for the study period. Mc was calculated using Maximum curvature method, using a sampling window of 500 data points with an overlap of 4. Vertical bars represent the standard deviation. near-field (at NRV and CMV) to improve hypocenter precision inside that particular volcanic network, but would have little effect on the seismic hypocenters at other volcanoes and on a more regional scale. I.e. the azimuthal coverage of the entire seismic network at CBCMVC did not change significantly with the introduction of new near-field stations inside a particular volcano's network, such as at NRV and CMV. Furthermore, earthquakes with ML N 1.3 were well detected and well located during the entire 2000–2015 period (the seismic network that was deployed between 2000 and 2006 in the CBCMVC had a total of 20 permanent seismic stations). Robustness of the pre-2007 network was tested by removing the new stations that were installed after 2007 (26 additional stations in 2015), and relocating all the earthquakes from 2000 to 2015. The hypocenters remained in the same place, with little or negligible variation. Therefore, I consider that any changes in observed seismicity are not an effect of changes to the seismic network configuration, but are due changes to volcanic activity at CBCMVC during this time. No earthquakes with ML N 4.5 were recorded between 2000 and 2006 at the CBCMVC (SGC, National Seismic Network reports and database http://seisan.sgc.gov.co/RSNC/index.php/consultas/consulexp). However, several earthquakes with ML N 4 were recorded after 2007 at the CBCMVC. Here I shall describe some of the most relevant earthquakes: On June 2007, an earthquake with ML = 4.0 and depth b 10 km, was located 6 km W of Nevado del Tolima volcano (NTV) (Fig. 1). In November 2008, an earthquake of ML 4.7 (depth = 4 km) occurred at Cerro Machín volcano (south of CBCMVC). In November 2009, December 2013, September 2014, and April 2015, earthquake swarms were located 5 km W of NRV at 6 km depth. This seismogenic location has been reported as a precursor to increasing activity at NRV, and was active before eruptions on November 1985, September 1989, and periods of minor ash emissions (Londoño and Sudo, 2003). Changes in seismicity were observed at Cerro Bravo volcano (CBV) on December 2008, when several deep long period (DLP) earthquakes were registered beneath it. These changes in seismicity across the CBCMVC suggest a change in behavior of seismogenic processes beneath the CBCMVC after 2007. In addition, two earthquakes at possible mantle depths were recorded just beneath the CBCMVC; one occurred on September 2008, with ML 5.7 (depth = 133 km), at about 15 km W of the CBCMVC, close to the Paramillo de Santa Rosa volcano (PSRV); and the other occurred in March 4, 2010, with ML 4.9 (depth = 120 km), located 15 km W of CBCMVC. It is uncertain if these two deep earthquakes affected the volcanic activity at CBCMVC, but it is worth mentioning the presence of such unusual seismicity simultaneously with an increase in crustal seismicity in that region. Due to the uncertainty of its effect on the volcanic activity, these two earthquakes were not included in the analysis and calculations. For seismic data (ML N 1.3) it is interesting to note a migration of earthquake locations from the edges of the CBCMVC to the more central regions. Cerro Bravo volcano (CBV) and Nevado del Tolima volcano (NTV) are both located close to the northern and southern borders, respectively, both had higher seismicity rates between 2000 and 2006, while after 2007 seismicity increased at those volcanoes located in between the CBV and NTV (Figs. 3 and 5). Fig. 5 shows the hypocenter distribution for earthquakes with ML N 1.3 for both study periods, 2000–2006 and 2007–2015. It is possible to observe an important change in the seismicity between the two study periods. For the period 2007–2015 there is increased seismic activity at NRV and CMV, as well as Santa Isabel volcano (SIV), Santa Rosa volcano (SRV), and Cerro España volcano (CEV). These last three volcanoes have not had any significant seismicity over the previous few decades. In the earlier study period (2000– 2006), seismicity was focused around NRV, CMV, CBV and NTV, with a few deeper (N10 km) earthquakes located around the volcanic complex (possibly associated with tectonic faulting). To demonstrate more clearly the change in seismicity between these study periods the seismic energy release (E) was calculated for each volcano using the magnitude-energy relation: Log10(E) = 1.5 ML + 4.8. Then, the cumulative value of seismic energy (Ce) with time was obtained as the progressive summation of all the available seismic energy data for each volcano (Ce = ΣE) from 2000 to 2015. Fig. 6 shows the cumulative seismic energy release for earthquakes ML N 1.3 for each volcano for the entire study period, and for all the volcanoes combined for both periods. Fig. 6 demonstrates that the seismic energy released for the period 2007–2015 was two orders of magnitude larger than during the 2000–2006 period. 3.2. Deformation Unfortunately, there is not a complete catalog of electronic tiltmeter data for the period 2000–2006, but the available data from the dry tiltmeter network at NRV, CMV CBV, as well as some short vector leveling profiles (precision leveling) and EDM data, suggest that no important deformation was observed at CBCMVC during this time (SGC, 2007), with values within measurement error. This is in agreement with the low activity of those volcanoes during 2000–2006, except for NRV, for which shallow seismic activity in 2002 and 2003, was associated with the hydrothermal system (SGC, 2003). On the contrary, changes in deformation, by using electronic tiltmeter networks were detected locally for NRV, CMV and NTV 162 J.M. Londono / Journal of Volcanology and Geothermal Research 324 (2016) 156–168 Fig. 5. Hypocenter distribution of crustal earthquakes for the period 2000–2006 (a) and 2007–2015 (b). Red solid line represents the NS profile direction. Filled squares represent seismic stations used for hypocenter location. Green and black lines represent geologic faults. Triangles and numbers represent volcanoes (see Fig. 1 for names). (Ordoñez et al., 2015; SGC, 2012, 2013, 2014), and at regional scales in other volcanoes by using InSAR interferometry (Lundgren et al., 2015a, 2015b) for the period 2007–2015. Fig. 7 shows the time series of electronic tiltmeters at NRV, CMV, and NTV during the 2007–2015 period (SGC, 2015a, 2015b), as well the location of those tiltmeters and a zone of possible regional deformation for the CBCMVC detected by InSAR interferometry by Lundgren et al. (2015a, 2015b). Fig. 7 shows that changes in deformation occurred during the period 2007–2015. A change in local deformation occurred at NRV in 2007 (Ordoñez et al., 2012). Then in 2009 another deformation episode took place (SGC, 2009). This was followed by low energy seismicity located W of NRV. Local and shallow (b2 km depth) deformation (2007–2015) was about 4 times larger at NRV than CMV, CBV and NTV (Fig. 7b), while deeper and wider deformation (2011–2015) was larger and closer at SIV during this time (Lundgren et al., 2015a, 2015b). This pattern suggests that different sources of deformation are acting at CBCMVC at the present. Lundgren et al. (2015a, 2015b) suggested that for the period 2008– 2011 no deformation was detected at CBCMVC by using ALOS PALSAR interferometry, but for the period 2011–2014 by using InSAR interferometry, a change in deformation was detected in a wide zone covering several volcanoes of the CBCMVC, with a center to the NE of SIV and to the SW of NRV, at a depth of about 14 km. It is interesting to note that while local shallow deformation at NRV was detected by an electronic tiltmeter network for the period 2007–2009, ALOS PALSAR did not detect those changes. It is possible that the presence of an ice cap at NRV, and the shallowness of the deformation source (b2 km depth; SGC, 2009), did not enable good results for ALOS PALSAR interferogram (see Fig. S2 of Lundgren et al., 2015a, 2015b). J.M. Londono / Journal of Volcanology and Geothermal Research 324 (2016) 156–168 163 Fig. 6. Cumulative seismic energy release for the earthquakes of Fig. 5 with ML N 1.3 for CBCMVC. a) Cumulative seismic energy release for each volcano. Right vertical axis corresponds to Ruiz (NRV), Machin (CMV), C. Bravo (CBV) and Tolima (NTV) volcanoes. Left vertical axis corresponds to C. Espana (CEV), St. Isabel (SIV), N. Quindio (NQV) and St. Rosa (SRV) volcanoes. b) Cumulative total seismic energy release for the two study periods for all volcanoes. Left Y axis and lower X axis correspond to 2000–2006 period, and right Y axis and upper X axis correspond to 2007–2015 period. Note that cumulative seismic energy release for the period 2007–2015 is almost two orders of magnitude N2000–2006 period. Seismic energy (E) was calculated according Guttenberg and Richter formulation (Log10(E) = 1.5 ML + 4.8). Despite these discrepancies, it is important to note that a wide deep intermittent deformation source is present beneath the CBCMVC, as well as other shallower localized deformation sources. Based on the concurrence of increasing seismicity at most of the same places where those deformation zones are present, such deformation could be attributed to new magmatic activity in this area. The extension of the deep deformation zones surrounding several volcanoes is suggestive of an ascending magma body affecting not only the seismic activity, but also the geochemical behavior of those volcanoes, as it is observed particularly from 2010 to 2015. 3.3. Geochemistry Changes in gas geochemistry and hydrochemistry were observed on several volcanoes from 2007 onwards as well. CBV showed an increase in CO2 and Radon emissions in soil in March and April 2008 (Figs. 1 and 3). Radon gas in soil showed an increase from 2008 to 2012 for NRV, CMV and CBV as well. A decrease in pH was detected at NRV and NTV between 2009 and 2012 (Figs. 1 and 3)0.1 At NRV some stations (2.5 and 8 km away from the active crater) reached pH values as low as 0.9 in 2010 (SGC, 2011). Moreover, an increase in SO4/Cl ratio was observed at SRV in 2013 and 2015, and CMV in 2013 (Figs. 1 and 3). All pH and SO4/Cl values were measured at hot springs close to the volcanoes (Fig. 1). It is worth of mention that the source of those waters at NRV was previously associated to magmatic gases at depth (Sturchio et al., 1988). From 2000 to 2006, SO2 was detected in very small amounts, at CBCMVC from satellite (NASA GES DISC) and increased after 2012 (Fig. 3). SO2 data available from DOAS instruments also showed temporal changes; In June 2007, a measurement using mobile DOAS instrument showed a value of about 370 Ton/day for NRV. Interestingly small amounts of SO2 flux were detected at CMV in April the same year, by using the same mobile DOAS instrument, with about 30 Ton/ day of SO2. Importantly, this was the first time ever that SO2 was detected at Cerro Machin volcano (SGC, 2007) since gas monitoring sampling began in 1989. During 2008 and 2009 small amounts of SO2 were detected at NRV. Regular measurements were taken from the end of 2009 onwards when DOAS instruments were deployed permanently at NRV. Early in 2010, an increase in SO2 emissions was detected at NRV, eventually reaching values N20.000 Tons/day in 2012. In general, SO2 emissions were highly variable, ranging from few thousand Tons/ day up to several thousands (20,000–30,000 Ton/day), a trend that 164 J.M. Londono / Journal of Volcanology and Geothermal Research 324 (2016) 156–168 Fig. 7. a) Deformation (tilt) observed at NRV (grey), CMV (Red), NTV (green) and CBV (Blue) from electronic tiltmeters from 2007 to 2015. Resultant vectors are shown for each station. b) Accumulated total observed deformation (absolute values of resultant vectors including inflation and deflation) at NRV, CMV NTV and CBV. Left vertical axes corresponds to NRV; right vertical axis corresponds to CMV, NTV and CBV c) Location of tiltmeters (black circles), volcanoes (white triangles) and deformation zone (dashed line) observed at CBCMVC by Lundgren et al. (2015a, 2015b) by using InSAR interferometry. Deformation zone taken from an interferogram courtesy of Sergey Samsonov (see Lundgren et al., 2015a, 2015b for details). CBRAVO tiltmeter is not plotted (Latitude = 5.0229° N; Longitude = 7.5298° W). continues today since 2010. Fig. 8 shows accumulated SO2 flux emissions and seismicity at NRV from 2010 to 2015. On some occasions SO2 release is associated with increasing seismicity, however, often it is not. In addition, a clear change in the magmatic signature in the shallow fluids was observed at CMV during the period 2007–2015. During two sampling campaigns, two measurements of helium isotopes were taken during 2011 and 2013 at CMV. The value of the helium isotope was 4.61 R/Ra, measured in 2011, while the helium isotope reached a value of 5.65 R/Ra in 2013. Simultaneously with the change in helium isotope data, a decreasing of log C/3He ratio from 10.81 to 10.76 was also observed. Such changes in geochemistry suggest magmatic activity at CMV between 2011 and 2013 as was proposed by Inguaggiato et al. (2014, personal communication). Fig. 8. Cumulative SO2 flux (from DOAS instruments) and cumulative number of earthquakes at NRV from 2010 to 2015. VT = volcano tectonic earthquakes. LP = long period earthquakes. J.M. Londono / Journal of Volcanology and Geothermal Research 324 (2016) 156–168 4. Discussion and concluding remarks Geophysical and geochemical changes from 2007 to 2015 at the CBCMVC suggest that possible new magmatic input is currently occurring in that region, and is affecting various volcanic centers. It is interesting to note that these changes are neither simultaneous nor occur at one particular volcano, but rather they occur at different times and places throughout the CBCMVC. Moreover, these changes in geophysical and geochemical parameters do not occur at the same time as changes or improvement in the monitoring networks. Thus, supporting the hypothesis that these changes are associated with volcanic activity rather than an artifact of monitoring networks. The observations presented in this study indicate magmatic signals from deep sources. Geochemical sampling in 2011 at CMV (Inguaggiato et al., 2014); the magmatic origin of SO2 flux at NRV since 2012; a deep source of deformation in between several volcanoes of CBCMVC from 2011 to 2013 (Lundgren et al., 2015a, 2015b) and an increase in the seismicity in almost all volcanoes of the volcanic complex not associated with configuration or improvement of seismic networks, among other changes all suggest that new magma is ascending beneath the CBCMVC. The most affected center until now is NRV. One of the possible reasons for why NRV is currently the most active volcano of the CBCMVC can be because NRV is the only open system within the complex. Geomorphology, recent activity, and monitoring data suggest that other volcanoes of the CBCMVC remain closed systems. It is possible that in the near future, some of those volcanoes become open systems, and that the magmatic activity occurring deeper within the volcanic complex will manifest at the surface as eruptions, ash or gas exhalations. For the purposes of this study an open volcanic system is defined as a volcano with permanent or semi-permanent open conduits; with fumarolic or gas activity releasing into the atmosphere; and with eruptions that vary on a time-scale of days to decades. A closed volcanic system is a volcano with sealed conduits; with very little emission of gas into the atmosphere; and with no eruptions over a long time-scale (centuries or more). Other authors have different definitions for an open volcanic system and this remains an open question, however a central theme to all definitions of open-vent volcanism is that of an established conduit that facilitates eruption, and can respond quickly to changes in the magmatic or hydrothermal system (Jaupart, 1998; 165 Sparks, 2003; Morales et al., 2010; Chaussard et al., 2013; Rodgers et al., 2015). As mentioned previously, CBV registered a sequence of DLP seismicity on December 2008. N35 DLP earthquakes were recorded on 30th December (Fig. 9). DLP events were characterized by low frequencies, ranging from 1.8 to 2.8 Hz. This seismicity was preceded by an increase in CO2 and radon gas in soil emission in April 2008, at several stations close to CBV. DLP seismicity has been associated with deep magma movement (White, 1996; Chouet and Matoza, 2013; Power et al., 2012). Fig. 9 shows temporal change in radon and CO2 in soil and location of some of those DLP events. The CO2, probably of deep origin, may be related to the degassing of a deep magma body (30 km depth, Londoño and Dionicio, 2011; Londoño, 2015), and the DLP events may be the result of such magma or gas movement at depth. It is interesting to note that an increase in radon emission (and CO2) was also detected at CMV from March to May in the same year 2008 (Fig. 3). However, it is uncertain if this increase in radon at CMV is related to the further increase in radon and CO2 at CBV, or with the subsequent occurrence of DLP seismicity. Two CO2 field campaigns in 2013 and 2014 at CMV using a flow meter with a cumulative camera system and a LICOR sensor (SGC, 2013, 2014), showed high values of CO2 of up to 700,000 ppm at some places where vegetation and birds were killed. This high release of CO2 at CMV is believed to come from a deep magmatic source located at about 15 km depth based on CO2 isotopes (δ13C of CO2) study (Inguaggiato et al., 2014). This also suggests that deeper magmatic sources are acting at CBCMVC. Werner et al. (2012) suggested that CO2 and SO2 flux several months prior to the 2009 eruption of Redoubt volcano were associated with magma degassing; although the depth at which that degassing occurred was not well established, the presence of DLP earthquakes (25–38 km depth; Power et al., 2012) and deep deformation sources let them to conclude that the origin of such gases were from a deep magma source. It is possible that a similar situation can be occurring at CBCMVC, but not as accelerated in time as at Redoubt volcano. On the other hand, NRV showed an important increase in seismicity in 2010 that persists to time of writing (May 2016), with N 250,000 volcanic earthquakes recorded between 2010 and 2016. Concurrently, seismic activity was extended to other volcanoes of the complex (Figs. 3, 5 and 6). One possibility of this phenomenon is the transfer of Fig. 9. a) Temporal changes in CO2 and Radon gas in soil at Cerro Bravo volcano (CBV) at two stations (CBRA-1 dashed lines, CBRA-2 solid lines) associated with DLP seismicity. b) Location of stations and some DLP seismicity occurred on 30th Dec. 2008. Rhomboids represent radon and CO2 soil stations (both located at same place). Filled squares represent seismic stations. Black triangle represents volcano. Filled circles represent hypocenters of DLPs. 166 J.M. Londono / Journal of Volcanology and Geothermal Research 324 (2016) 156–168 Fig. 10. Spatial and temporal variation of seismicity at NRV. a) January 2010–July 2012. b) August 2012–Dec. 2015. Plotted earthquakes with ML N 1.0. stress from NRV to the active faults surrounding the volcano (Castaño et al., 2011); but volcanoes such as CMV and CBV are a long way from NRV, and showed increasing seismicity in 2008 and 2009, before the reactivation of NRV in 2010. This supports the idea that wider and deeper magmatic activity was occurring at CBCMVC from 2007 onwards, and not localized only at NRV or close to it. As was pointed out by Lundgren et al. (2015a, 2015b), it is possible that other volcano sources can be responsible for the wide deformation observed in the region close to NRV, for the period 2011–2013 (see below). On the other hand, it is interesting to note that the relationship between seismicity and SO2 flux shows a similar trend from 2010 to the beginning of July 2012 for NRV. After that date, the SO2 flux stabilized but the number of volcano tectonic earthquakes (VT) and long period earthquakes (LP) continued to change, (Fig. 8). This change is possibly associated with a change in volcanic activity. From that date (July 2012) onwards, VT earthquakes were located far from the active crater and increased in number, while the number of LP earthquakes occurring at the active crater decreased. The observed changes in seismicity are possibly associated with magma intrusion pulses, located N 7 km depth and to the NW of NRV. Fig. 10 shows this change in seismicity at NRV. This could mean that other sources of magma are responsible for the SO2 flux rather than the shallow magma system at NRV (Stix et al., 2003). Moreover, relatively high SO2 flux rates released by NRV continuously since 2004 (Fig. 3) with no associated large eruption (passive degassing), are suggestive of SO2 influx from a deep magma chamber, possibly basaltic, as being responsible for the excess sulfur, as was pointed out by Shinohara (2008). Additionally, strong evidence of magma ascent at CBCMVC is the recent observation of a small lava dome that has extruded at the surface of NRV since September 2015, and continues to grow until time of writing (March 2016). This new dome was detected by the Italian Space Agency (ASI) COSMO-SkyMed radar slant-range, with an estimated pseudovolume of 12,000 m3 (Lundgren et al., 2015b; SGC, 2015b). This dome seems to be the surface manifestation of the change in seismicity observed since 2010 at NRV. Similarly, at CMV spatial distribution of seismicity also changed. From 2000 to 2008, seismicity was concentrated primarily at the main dome inside the crater, but from 2009 to 2015, a shift in the seismicity Fig. 11. Spatial and temporal variation of seismicity at CMV. a) January 2000–Dec. 2008. b) January 2009–Dec. 2015. Plotted earthquakes with ML N 1.0. J.M. Londono / Journal of Volcanology and Geothermal Research 324 (2016) 156–168 167 depth seismicity (15–30 km depth) throughout the volcanic complex except for in the CMV area (Figs. 5b, 11). Our current study of temporal variations of seismic velocities from seismic 3D tomography of P and S velocity for CBCMVC, aims to detect possible temporal changes related to the suggested new magmatic activity. In the last two centuries, no large eruption (N5 VEI) has taken place at CBCMVC (Méndez, 2003), but the potential is high, based on the geological record of several volcanoes of that complex (Méndez et al., 2002; Regnier, 2014). NRV and CMV are two dangerous active volcanoes. The last important eruption of NRV (VEI = 3) was on November 1985. Although it was small, N 23,000 people were killed by a lahar. The last eruption of CMV was about 900 years ago; geological records show that there is a significant eruption at this volcano approximately every 900 years (VEI N 5; Rueda, 2004). Given this background, it is crucial that any hint of increasing activity at the CBCMVC is considered carefully. According to Phillipson et al. (2013), any change of any geophysical or geochemical variable above a predetermined background level, should be considered as unrest in a volcano. Based on this, I suggest that CBCMVC is in a state of unrest, with several volcanic centers showing signs of activity that must be considered in the hazard and risk assessment of this region of Colombia. A rough estimation indicates that N 3 million people (DANE, 2012) live close to those volcanoes and could potentially be affected by their awakening. Acknowledgements Fig. 12. P-wave velocity profile (a) and cross section of spatial mapping of b-value (b) of Central Colombia (Lat = 4.7° N). Configuration of tectonic plates and possible location of deep magmatic chambers of CBCMVC. a) Colors represent velocity (km/s) and contours represent velocity perturbation (%). b) Colors represent b-value and contours represent standard error of b-value. Fig. 12a modified from Londoño and Dionicio (2011). Fig. 12b modified from Londoño (2015). Numbers represent volcano locations: 1:C. Machín, 2:N. Quindío, 3:St. Rosa, 4:St. Isabel, 5:N. Ruiz, and 6:C. Bravo. Dashed circles represent the possible deep magmatic chambers of CBCMVC. was detected towards the SE and events became deeper, changing from 3 to 5 km depth at the crater to 15–20 km depth to the SE. Fig. 11 shows the change in spatial distribution of seismicity at CMV. This change in spatial distribution in seismicity supports the idea that new magmatic activity is occurring beneath CBCMVC, and not only at NRV. Other possible evidence of magmatic activity at CBCMVC is the presence of a deep magmatic chamber beneath the complex. Based on a study of regional 3D tomography of seismic velocity, Londoño and Dionicio (2011), suggested that deep magmatic bodies are emplaced at CBCMVC at about 20–40 km depth. Moreover, Londoño (2015), observed a zone with high b-values almost at the same location (beneath CBCMVC) of the low-Vp values of Londoño and Dionicio (2011). Fig. 12 shows a profile located at latitude 4.7° N, showing the results of seismic 3D tomography of P-wave velocity with a resolution of 30 × 30 × 10 km and a cross section in the same direction with the results of spatial mapping of b-values with a resolution of 5 × 5 × 5 km. This figure shows a zone with low-velocity for P-wave and high b-value, just beneath the CBCMVC. This zone is associated with the deep main source of the magma supply for the active volcanoes at CBCMVC. It is known that deep magma can ascend, and that some portions of such ascent is aseismic (Chouet and Matoza, 2013; Kumagai et al., 2003). This suggests that the seismicity observed until now at CBCMVC, which is continuous, and at high levels from time to time, is indicative that magma is moving upward, and probably at different depth levels, from deep (20–40 km) to relatively shallow depths (2–3 km), with some aseismic gaps at some depths. 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