Predicción In Silico de actividad biológica

Recent geophysical studies have imaged two potential magma chambers, or magma storage regions, beneath Volcán de Colima (Gardine, 2010; López-Loera, 2012). Aeromagnetic data collected during three campaigns in 1963, 1983 and 1999 covering an area of 11,568 km3 over the CVC, reveals anomalies associated with Volcán de Colima and Nevado de Colima (López- Loera, 2012).

López-Loera (2012) modelled a 2¾ dimension section trending north-northeast – south- southwest along the magnetic anomalies (Figure 5.15). The 2¾ dimension model adds constraints on the shape of the modelled body, perpendicular to the 2D section. By using the magnetisation and magnetic susceptibility of local rocks (debris-avalanche deposits, andesitic lava flows, basement limestones and marine sediments), López-Loera (2012) interpreted one of the anomalies to represent a magma chamber 5.5 km thick, at a depth of 4.8 km beneath Volcán de Colima (Figure 5.15).

Figure 5.15 2¾ dimension magnetic model along a section through the north-south trending anomalies beneath Volcán de Colima and Nevado de Colima modified from López-Loera (2012).

The anomaly is defined by the contrast in magnetic susceptibility between the rock types (López-Loera, 2012). The body is elongated in a north-south direction with a length of over 6.8 km, extending 5.6 km to the south of the CVC with an average thickness of 0.54 km (López- Loera, 2012). In order to better constrain their model, the authors used a previous viscoelastic model by Cabrera-Gutiérrez and Espinolda (2010) who proposed the presence of a magma chamber of ~1.96 km in radius at ~5.6 km below the crater. This shallow magma chamber is

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consistent with seismicity, which reveals a quiet zone at 3 to 7 km depth below the crater of Volcán de Colima (Nuñez-Cornú et al., 1994; Zobin et al., 2002; López-Loera, 2012).

Beneath Nevado de Colima, another anomaly exists which has been interpreted as representing a collapsed magma chamber (López-Loera, 2012). The anomaly extends >5.6 km in a north- south orientation, with a maximum thickness of 1.2 km, at a depth of 5.2 km below the remnant crater of Nevado de Colima (Figure 5.15).

Gardine (2010) carried out a P-wave tomographic study on the CVC using regional and local earthquakes ranging in magnitude from 1.5 to 5.1. Inversions were carried out on 8660 arrivals from 198 regional earthquakes associated with the subducting Rivera plate, and 101 shallow crustal earthquakes (Gardine, 2010). Two low-velocity zones were imaged, one at 4 to 10 km depth and a second at 15 to 30 km depth (Figure 5.16). Low-velocity zones have been imaged at other volcanoes (e.g. Husen et al., 2004; Waite and Moran, 2009), and are commonly interpreted as reflecting the presence of a magma chamber, as increased temperatures and partial melt slow down seismic waves (Husen et al., 2004; Waite and Moran, 2009; Gardine, 2010). The shallow low-velocity zone imaged beneath Volcán de Colima covers an area of ~ 30×30 km, with a depth extending from just below sea-level (~4 km below the crater) to 10 km, giving a volume of ~9000 km3. The total estimated volume of erupted magmas from the CVC stratovolcanoes (Volcán Cántaro, Nevado, Paleofuego and Volcán de Colima) is ~490 km3, which is ~5% of the volume of the low-velocity zone. The magma chamber volume estimate at Soufriere Hills volcano, Montserrat, based on seismic tomography and thermal modelling is 13 km3 (Paulatto et al., 2012). Tomography studies at Mt St Helens volcano yielded magma chamber volume estimates of ~100 km3 (Lees, 1992; Waite and Moran, 2009). Larger magma chamber volumes of ~1200 km3 have been estimated for Teide volcano (De Barros et al., 2012) and 1000 to 1400 km3 for Los Humeros Caldera, Mexico (Verma and Gomez-Arias, 2013). In comparison to these magma chamber volume estimates at other volcanoes, and given the total estimated volume of erupted products, it seems unlikely that this anomaly represents a shallow- level magma chamber. Based on the large volume of this anomaly, Gardine (2010) concluded that it more likely reflects the regional geology, as the CVC sits in the Colima Rift, which is infilled by sediments which would have lower velocities than the basement rocks exposed in the graben walls (Gardine, 2010).

The second low-velocity zone is situated ~20 km southeast of Volcán de Colima (Gardine, 2010). Based on a lower-bound estimate of P-wave velocity changes in mantle olivine of 0.5 m/s/˚C at temperatures below 700˚C (Isaak, 1992), Gardine (2010) attributed the deeper low- velocity zone, with P-wave velocities 0.2 km/s slower than background, to a storage region with temperatures ~400˚C above the background. Gardine (2010) used the Mexican subduction zone

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thermal models of Currie (2002) to estimate a background temperature of ~500˚C at ~25 km depth below the CVC. This is consistent with the new thermal models of Ferrari et al. (2012) which estimate the temperature at ~25km depth to be between 500 and 600˚C. The lower- velocity zone, therefore, has a temperature of ~900˚C.

Figure 5.16 The low-velocity isosurface plot from the tomography results from Gardine (2010).

Volcán de Colima is indicated by the blue triangle at (0,0). The shallow low-velocity zone is under Volcán de Colima from 0 to 10 km depth. The deeper low-velocity zone is at 15-30 km depth, and is located ~20 km southeast of Volcán de Colima.

Partial melt also reduces the P-wave velocity; however, the velocity is reduced by 3.6% per 1% of partial melt (Hammond and Humphreys, 2000; Gardine, 2010). It is therefore unlikely that partial melt plays a strong role, given the small velocity anomaly displayed by the lower- velocity zone, although it may play a part. Gardine (2010) calculated an error of ±200˚C on the temperature to account for up to 50% partial melt contributing to the reduced velocities.

This value of 900 ± 200˚C is consistent with the estimated eruption temperatures for the Group I and Group II CVC magmas, which yielded 910 to 1050˚C (see Sections 2.3 and 3.3); and with estimates from previous petrological studies (Savov et al., 2008; Luhr et al., 2010), and experimental petrology (Moore and Carmichael, 1998) which estimated eruption temperatures of ~950˚C.

The presence of shallow storage regions at ~ 5 km depth and another at ~15 to 30 km depth are consistent with the interpretation of the petrological data which reveal multiple magma storage regions at the CVC.

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