Volcanic hazard zonation of the Nevado de Toluca volcano, México

Journal of Volcanology and Geothermal Research 176 (2008) 469–484

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Journal of Volcanology and Geothermal Research j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j v o l g e o r e s

Volcanic hazard zonation of the Nevado de Toluca volcano, México L. Capra a,⁎, G. Norini a, G. Groppelli b, J.L. Macías c, J.L. Arce d a

Centro de Geociencias, Universidad Nacional Autónoma de Mexico, Campus Juriquilla, 76230 Queretaro, Mexico C.N.R. - Istituto per la Dinamica dei Processi Ambientali, Milano, Italy c Instituto de Geofísica, UNAM, CU, Mexico DF, Mexico d Instituto de Geologia, UNAM, CU, Mexico DF, Mexico b

A R T I C L E

I N F O

Article history: Received 18 December 2007 Accepted 19 April 2008 Available online 6 May 2008 Keywords: Nevado de Toluca volcano Trans-Mexican Volcanic Belt volcanic hazard computer simulations

A B S T R A C T The Nevado de Toluca is a quiescent volcano located 20 km southwest of the City of Toluca and 70 km west of Mexico City. It has been quiescent since its last eruptive activity, dated at ∼ 3.3 ka BP. During the Pleistocene and Holocene, it experienced several eruptive phases, including five dome collapses with the emplacement of block-and-ash flows and four Plinian eruptions, including the 10.5 ka BP Plinian eruption that deposited more than 10 cm of sand-sized pumice in the area occupied today by Mexico City. A detailed geological map coupled with computer simulations (FLOW3D, TITAN2D, LAHARZ and HAZMAP softwares) were used to produce the volcanic hazard assessment. Based on the final hazard zonation the northern and eastern sectors of Nevado de Toluca would be affected by a greater number of phenomena in case of reappraisal activity. Block-and-ash flows will affect deep ravines up to a distance of 15 km and associated ash clouds could blanket the Toluca basin, whereas ash falls from Plinian events will have catastrophic effects for populated areas within a radius of 70 km, including the Mexico City Metropolitan area, inhabited by more than 20 million people. Independently of the activity of the volcano, lahars occur every year, affecting small villages settled down flow from main ravines. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The Quaternary Mexican volcanism is concentrated along the TransMexican Volcanic Belt (TMVB) (Fig. 1A), a continental volcanic arc that has been active since 14 Ma (Ferrari et al., 1994). At least 14 active volcanoes are present in Mexico, and most of these are located along the TMVB (Fig. 1A). At present, only Volcán de Colima and Popocatépetl exhibit persistent activity, with small eruptive columns (up to 5–8 km in each case) and short-runout pyroclastic flows that have not affected populated areas (Saucedo et al., 2005; Macías and Siebe, 2005; Macías et al., 2006). The other volcanoes, including Nevado de Toluca (hereafter, NdT), are quiescent and apparently represent no threat to the surrounding populations. However, the sudden reactivation of El Chichón volcano in 1982 after a 550 yr quiescence period killed over 2000 people (Macías et al., 2008), suggesting that long-term dormant volcanoes can become active in a very short time, with catastrophic consequences due to previous dearth of basic studies, hazard maps, emergency information programs, etc. The Popocatépetl and Colima volcano hazard maps were the first maps prepared in Mexico as a consequence of the eruptive crises that occurred in these volcanoes in 1994 and 1991, respectively (Macías et al., 1995; Del Pozzo et al., 1996; Sheridan et al., 2001a; Navarro et al., 2003). More recently, Sheridan et al. (2001b, 2004) produced a hazard map for the Pico de Orizaba volcano in ⁎ Corresponding author. E-mail address: [email protected] (L. Capra). 0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.04.016

which the hazard delineation was based primarily on flow simulations that took into account the more recent eruptive activity. At present, these maps represent a fundamental scientific document that civil defense authorities use in case of future volcanic crises. For the case of Nevado de Toluca, the only available hazard map was reported by Capra et al. (2000, 2004) and Aceves Quesada et al. (2007). These maps were mostly based on the stratigraphic record and morphology of the volcano. The Nevado de Toluca is a quiescent volcano, located 20 km southwest of Toluca, and 70 km west of Mexico City (Fig. 1B). The volcano has been silent since its last eruptive activity, dated at ∼3.3 ka BP (Macías et al.,1997), although minor fumarolic activity was reported during the nineteenth century (Bloomfield and Valastro, 1977). The reactivation of NdT could threaten more than 20 million people, including the Mexico City metropolitan area, that some 10.5 ka years ago was blanketed by more then 10 cm of pumice from the Upper Toluca Pumice eruption, one the most violent Plinian eruption occurred during the Holocene (Fig. 1B) (Cas and Wright, 1988; Arce et al., 2003). The aim of this work is to present a hazard assessment that includes future eruptive scenarios deduced from a detailed geological map (Garcia-Palomo et al., 2002; Bellotti et al., 2004; Norini, 2006) coupled with computer flow simulations for debris avalanches, blockand-ash flows, falls, and lahars. All simulations were carefully tested based on spatial distributions and thickness of past flows, and in some cases the paleotopography was restored in digital elevation models (DEMs) to better reproduce past events. The resulting hazard zonation benefits from accurate calibration and validation of the numerical

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Fig. 1. A) Map of the Trans-Mexican Volcanic Belt (TMVB) showing the location of the Nevado de Toluca and other active volcanoes. Abbreviations are: Ce: Ceboruco; CVC: Colima Volcanic Complex; Pa: Parícutin; NT: Nevado de Toluca; Jo: Jocotitlán; Mx: Mexico City; Iz: Iztaccíhuatl; Po: Popocatépetl; PdO: Pico de Orizaba; CdP: Cofre de Perote. B) Landsat image (RGB combination) of the central sector of the TMVB. Mexico City is located 70 km northwest of Nevado de Toluca and 40 km northeast of Popocatépetl, both active volcanoes. White dotted lines refer to the 10-cm isopach map of three mayor plinian eruptions occurred at the Nevado de Toluca volcano: the Lower Toluca Pumice (LTP, at 21 ka BP), the Middle Toluca Pumice (MTP, at 12.5 ka B.P.) and the Upper Toluca Pumice (UTP, at 10.5 ka BP) that covered the area occupied today by Mexico City. Abbreviations are: TF: Tenango Fault; TLF: Tenango Lava Flow; CVF: Chichinautzin Volcanic Field; CR: Las Cruces Range. Black dotted line refers to the main Tenango fault intersecting Nevado de Toluca Volcano.

models coupled with high-resolution geological data and reliable computer flow simulations. This compiled methodology can better predict the extent of products of future volcanic activity in contrast to previous research, which tended to be based either on numerical modeling or on geological mapping (Waythomas and Waitt, 1998; Moreno, 2000; Sheridan et al., 2004; Sofield, 2004). 2. The Nevado de Toluca volcano 2.1. Morphostructural features The Nevado de Toluca is 4680 m high and is characterized by an open crater, 1.5 to 2 km in diameter, E–W elongated, which developed in response to intense tectonic activity (Tenango Fault System, Fig. 2; García-Palomo et al., 2000, Norini et al., 2004; Bellotti et al., 2006; Norini et al., 2006). The Ombligo dome, residing in the crater's interior, separates two lakes (Moon Lake and Sun Lake, Fig. 3A). The volcano shows striking morphological differences on its flanks and two main morphological domains can be defined (Norini et al., 2004). The southern flank of Nevado de Toluca volcano has an irregular morphology, relatively flat and dissected by deep rectilinear valleys with NNW–SSE strikes associated to the Taxco–Queretaro Fault System (García-Palomo et al., 2000; Bellotti et al., 2006). The altitude of the this flank varies between 3800 and 2500 m a.s.l. and more than 50% of this area has slopes greater than 20° (Norini et al., 2004). Ravines are deeply eroded with depths up to 450 m (Barranca del Muerto, Fig. 2). All these features led previous authors to consider this area as the remains of an older volcanic structure called Paleonevado (Cantagrel et al., 1981; García-Palomo et al., 2000). The western, northern, and eastern NdT flanks, in conjunction with the crater area, depict the second morphological domain, and constitute the present active cone of the NdT volcano. The northern and northwestern sectors of the volcano are gentle, with slopes of 6–8°, mostly formed by pyroclastic fans (Fig. 3B,C) on the Toluca basin. The main drainages are radial with respect to the cone. The eastern sector has a gentler slope but is more dissected, with ravines up to 70 m deep (Arroyo Grande ravine, Fig. 2) due to the presence of the Tenango Fault System (García-Palomo et al., 2000), an active left-lateral transtensive structure (Norini et al., 2006). The present morphological arrangement of the volcano may have a great influence on the hazard assessment, because the distribution and runout distances of volcanic flows will depend primarily on the flow dynamics and topography.

2.2. Stratigraphic record The volcano started to grow at ∼2.6 Ma with andesitic to dacitic effusive activity that ended at 1.1 Ma and led to the formation of Paleonevado edifice (Cantagrel et al., 1981; Garcia-Palomo et al., 2002; Martínez-Serrano et al., 2004). After an intense erosive stage that originated the emplacement of voluminous epiclastic sequences, including two sector collapses (Capra and Macías, 2000), magmatic activity was renewed ∼42 ka ago whit the formation of the recent active cone of the NdT and the emplacement of the Pink Pumice Flow (PPF) deposit (Macías et al., 1997). Fig. 4 shows a detailed geological map compiled from previous works (Garcia-Palomo et al., 2002; Bellotti et al., 2004; Norini, 2006), representing the last 50 ka of volcanic activity that was characterized by different eruptive phases (Fig. 5), including five dome collapses dated at 37, 32, 28, 26.5, and 13 ka (Macías et al., 1997; Garcia-Palomo et al., 2002), at least three lateral collapses (Macías et al., 1997; Norini, 2006), and four Plinian eruptions at 36 ka (Ochre Pumice) (Garcia-Palomo et al., 2002), 21.7 ka (Lower Toluca Pumice, LTP) (Bloomfield et al., 1977; Capra et al., 2006), 12.1 ka (Middle Toluca Pumice, MTP) (Arce et al., 2005), and 10.5 ka (Upper Toluca Pumice, UTP) (Macías et al., 1997; Arce et al., 2003). The eruptive sequence is crowned by a phreatomagmatic surge deposit dated at ∼3.3 ka BP (Macías et al., 1997). 2.3. Origin, distribution and magnitude of major volcanic events We next describe in detail past eruptions and associated deposits related to the activity of the active cone of NdT (b50 ka), to illustrate expected events in case of renewed volcanic activity. The activity of the Paleonevado, characterized by voluminous lava flows, ended approximately 1.2 Ma ago, a time interval too long to be considered as a possible locus of a new eruption. Dome collapse events have been characterized by summit dome growth and destruction, associated or not with an explosive component. The 37- and 28-ka events correspond to major domedestruction episodes that originated lithic-rich block-and-ash flow deposits that traveled up to 20 km from the source emplacing up to 30 m thick deposits (Fig. 6A). These deposits are grouped in the map in one unit because they usually represent vertical sections, being impossible to lay out their individual distribution on the geological map. Main outcrops are limited to quarries where material is extracted for construction (Fig. 3C); it is thus difficult to determine

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Fig. 2. Shaded digital elevation model of the Nevado de Toluca Volcano (50 m in pixel resolution) showing main morphological and structural features. Black lines indicate the fault traces related to the Tenango Fault System (Norini et al., 2006).

the exact distribution of such deposits. Despite this limitation, it is clear that these two main events were radially distributed as they crop out along all main ravines. Macías et al. (1997) estimated a total volume of ∼3 km3 for each event, which includes several flow units that correspond to a VEI of 4 (Volcanic Explosivity Index, Newhall and Self, 1982). The deposit associated with the younger 13 ka BP dome collapse crops out mainly on the eastern and northeast sectors. It is easy to recognize because of its brilliant gray color and its stratigraphic position, between the Lower Toluca Pumice (LTP) and the Upper Toluca Pumice (UTP) Plinian fall deposits. It consists of a main-channel facies up to 10 m thick, with several flow units of clasts-rich blockand-ash flow deposits, and a lateral facies up to 4 m thick consisting of

a sand-sized unit with small amounts of pumices (Fig. 3C). This flow traveled a maximum distance of 15 km and was dispersed mostly to the north-eastern sector of the volcano. A volume estimation yielded a value of 0.11 km3 (D'Antonio et al., 2008), which corresponds to a VEI of 3, being a moderate eruption. All of these pyroclastic flows show a similar runout that corresponds to an H/L (relation between the drop height and the maximum runout) of 0.12. The 13-ka event was used to calibrate computer simulations for pyroclastic flows because of its stratigraphic control and good exposures. Previous works (Metcalfe et al., 1991; Macías et al., 1997; Newton and Metcalfe, 1999; Caballero et al., 2000) reported an ash flow deposit on the lacustrine sequence of the Toluca basin. This deposit probably represents the product of an

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Fig. 3. A) Picture showing the crater of the volcano, with the Ombligo Dome, emplaced after the UTP plinian eruption, separating the Sun and the Moon Lakes. Picture taken from the western rim of the crater; B) Panoramic view of the NdT from the east. Notice the large amphitheater formed after the last sector failure and dome collapses, subsequently shaped by glacier activity. C) Panoramic view of the El Refugio quarry on the northern sector of the volcano, one of the best exposures of the eruptive sequence of the last 40 ka. Here is exposed the 13 ka BP block-and-ash flow deposit that consists of up to five different units, and it is crowned by the UTP.

ash cloud associated to the main block-and-ash flow. For such kind of deposits, an H/L of 0.1 was estimated. During the growth of the modern edifice, the extrusion of the 28-ka summit dome, produced the collapse of the edifice with the emplacement of a debris avalanche deposit followed by a sequence of block-and-ash flows (Caballero, 2007). The best examples of this collapse are outcrops on the eastern sector of the volcano (Fig. 6C). The resulting debris avalanche deposit, primarily outcropping on the Zaguan ravine, has a volume of 0.35 km3 and an H/L value of 0.14. Other debris avalanche deposits were observed on the northwest and eastern valleys, with estimated volumes of 0.3 km3 and similar H/L ratios of 0.14 (Arroyo Grande and El Nopal deposits, Fig. 4, Norini, 2006). The UTP represents the most violent and voluminous eruption of NdT, with the emission of 8 km3 of magma (Dense Rock Equivalent) and the formation of a 42-km-high Plinian column that dispersed material toward the E–NE, blanketing the area occupied today by

Mexico City with a layer of pumice more than 10 cm thick (Arce et al., 2003) (Fig. 1B). Associated with this eruptive column were collapsing pumice flows that emplaced on main ravines (Fig. 4 and 7a). The Ombligo Dome was extruded at the end of this activity (Arce et al., 2003). Reconstructions of this event give a VEI = 6 (Arce et al., 2003), which may be comparable to the Plinian phase of the 1991 Pinatubo eruption that had climatic global effects. In fact, Arce et al. (2003) argued that the Younger Dryas period, the last major glacial event at the limit of Holocene, in Mexico occurred exactly after the UTP eruption. The LTP (Fig. 8b) and the MTP represent minor Plinian eruptions, with volumes (DRE) of 0.8 km3 and 1.8 km3 and column heights of 24 km and 21 km, respectively, which dispersed material toward the east, over the Toluca basin (Fig. 1B). These values give a VEI of 4 for such eruptions (Table 1). Few data are available for phreatomagmatic events of NdT, although this type of eruptive activity may occur in the future, because two permanent lakes are situated in the crater area. The only

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Fig. 4. Geological map showing the distribution of the younger (b 42 ka BP) volcanic sequence of the NdT. The vertical black arrow in the legend indicates that block-and-ash flow deposits are interbeded with Plinian deposits. For the UTP and the MTP deposits, only pumice flow deposits are represented. The evident absence of recent volcaniclastic deposits on the southern sector of the volcano could be due to the high topographic gradient that is favoring erosion. Abbreviations on the map refer to debris avalanche deposits (DAD): AG: Arroyo Grande; EZ: El Zaguan; EN: El Nopal (Modified from Bellotti et al., 2004).

Fig. 5. Diagram showing a simplified stratigraphic column of the b 42 ka BP eruptive sequence and their C14 ages (with relative error) highlighting volcanic quiescent intervals of 8000 yrs (i.e. between the LTP and the 13 ka BP dome collapse) (data compiled from Macías et al., 1997; Garcia-Palomo et al., 2002).

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Fig. 6. Pictures showing some examples of pyroclastic deposits at NdT. A) Section at Zacango quarry, showing the 37 and 28 ka BP block-and-ash flow deposits crowned by the LTP and UTP pumice fall deposits. B) 3.3 ka BP pyroclastic flow at Raices quarry. C) El Zaguan debris avalanche deposit.

known phreatomagmatic deposit was dated at 3.3 ka and has been described as a surge deposit, dark gray in color, up to 1 m thick, lithic rich, with a laminated basal layer and a more dilute upper horizon (Macías et al., 1997) (Fig. 6B). This deposit crops out in only one point on the western flank of the volcano. This eruption is probably associated with a low- to medium-magnitude phreatomagmatic eruption with the formation of a small eruptive column. Primary lahars have been always associated with mainly eruptive events, and originated from the remobilization of falls, pumice flows, block-and-ash flows, and debris avalanche deposits in main ravines and up to a distance of 15 km from the crater (Figs. 4 and 8A). Finally, during the past century, heavy rains have remobilized large amounts of unconsolidated material generating secondary lahars. For example, in 1952, a rain-triggered flood affected the village of Pueblo Nuevo, on the eastern sector of the volcano (personal communication from local people). In this same drainage, local authorities are now arranging sand dikes to prevent future floods (Fig. 8B) that occur every rainy season.

3. Possible hazardous scenarios Based on the stratigraphic record of the NdT eruptive activity during the last 42 ka BP, at least 12 main eruptions have occurred at intervals varying from 1 to 8 ka (Fig. 5). The analysis of the composite stratigraphic section of the volcano allows assessment of possible scenarios in the case of a future eruption of the volcano. Plinian fallouts rest directly on top of thick paleosols around the volcano, with almost no other previous pyroclastic flow deposits at their base (Fig. 7C). This might indicate that before Plinian eruptions at NdT, the crater was open, without a central dome inside as attested by the small amounts of accidental clasts in the lower parts of the Plinian fallouts (Arce et al., 2003; Arce et al., 2005; Capra et al., 2006). At present, the volcano has an open crater with the ∼0.001 km3 Ombligo Dome on its interior that was emplaced just after the UTP eruption. Most of the block-and-ash flow deposits recorded in the stratigraphy of the volcano were probably generated by discrete collapses of central domes, as those observed in the 28 ka event. A

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Fig. 7. Pictures of main fall deposits. A) UTP sequence showing intercalation of pumice fall and flow deposits. B) Detail of the LTP, characterized by a fall basal unit and a sequence of fall and surge layers atop. C) Picture displaying fall deposits resting directly on top of thick organic paleosols.

similar sequence of events was observed in other volcanoes, where a growing dome and the associated hydrothermal system weakened the main edifice, inducing its collapse (Voight et al., 2002). At NdT, the dome remnants reflect a growing path guided by tectonic lineaments, indicating a structural control during the magma ascent, as for the direction of main collapses, which was guided by geometry and kinematics of tectonic structures (Norini, 2006). Based on the age of the described eruptions, a regular period of recurrence cannot be easily identified, because this depends on the type of volcanic activity prior to each quiescent period. Generally, it can be stated that the volcano has remained silent up to 8 ka followed by explosive eruptions. This volcano behavior indicates the need to carefully evaluate its potential hazards. Several other volcanoes exhibited long quiescent periods before cataclysmic eruptions, such as Tambora volcano (Stothers, 1984; Sigurdsson and Carey, 1989). Based on these observations, and according to the present morphol-

ogy of the volcano, the most probable scenario may be a small pheatomagmatic explosion as the 3.3 ka BP event (vent opening) that might be followed by 1) explosive activity with the formation of a sustained column (VEI 4–6); or 2) opening of new eruptive fractures (associated with active faults) and dome growth with the collapse of the old edifice, followed by the destruction of the new dome with emplacement of block-and-ash flow deposits. Independently of the potential eruptive scenario, superficial water will remobilize unconsolidated volcaniclastic material forming lahars. Because of the absence of lava flows in the stratigraphic record of the recent cone, they were not included in the hazard evaluation. 4. Computer simulations The main hazardous events of NdT, in accordance with its eruptive history are debris avalanches, pyroclastic flows (block-and-ash flows),

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Fig. 8. A) Textural features of debris flow deposits at the Ojo de Agua laharic sequence. B) Sand dikes to reduce flood effects close to Pueblo Nuevo village (black arrow).

lahars and fallouts (Table 1). Several computational flow simulations are discussed in the literature (Valentine and Wohletz, 1989; Dobran et al., 1994; Itoh et al., 2000), but the programs used here for volcanic flows are those that are particularly suitable to flow over real topography and that give the best reproducibility. Several works have already showed the importance of the DEM resolution on computational routines (Stevens et al., 2002; Hubbard et al., 2006; Davila et al., 2007). For the present work, computational routines were performed on a DEM with a resolution of 50-m pixel obtained from vectorial data at 1:50,000 scale (INEGI) (Fig. 2). 4.1. Dry granular flow: FLOW3D and TITAN2D Block-and-ash flows are gravity-driven, partially fluidized, and composed of a basal dry-grain flow layer that follows topographic features, and an upper, more dilute, turbulent layer that can reach greater distances than the underlying flow. Debris avalanche flows are dry granular flows originated by the collapse of a volcanic edifice. Different models have been proposed to explain the mobility of such flows and how the energy generated dissipates during flowage (i.e. Hayashi and Self, 1992). The relationship between the drop height and the maximum runout (H/L), also known as the apparent coefficient of

friction (Hsu, 1975), better describes their mobility, which generally increases as the mass increases (Dade and Huppert, 1998). Interstitial fluid is generally absent or present at b10% of the total mass, so it does not play a dominant role during transport. Two routines are used to simulate debris avalanches and the basal grain flow of pyroclastic density currents: FLOW3D (Kover, 1995) and TITAN2D (Patra et al., 2005). The main difference between these two routines is that FLOW3D calculates the shear resistance based on basal friction (a0), viscosity (a1) and turbulence (a2) to reproduce sliding block trajectory (velocity and maximum runout), but obviating the collapsing mass. In contrast, TITAN2D simulates the change in flow thickness because it considers the initial volume of the collapsing mass driven downslope by gravity where the resistance forces are given by basal and internal friction angles of the collapsing material. Comparisons between these two routines are provided for some examples. Table 2 describes the parameters used during simulations. 4.1.1. Block-and-ash flows Computer flow simulations were calibrated based on the distribution of the 13 ka block-and-ash deposit, since this represents the youngest and best-exposed event. Although it does not represent the most catastrophic event, it may represent the most realistic scenario

L. Capra et al. / Journal of Volcanology and Geothermal Research 176 (2008) 469–484 Table 1 Eruptive parameters of major eruptions during late Pleistocene–Holocene Type

Age Name

Dome collapse Dome collapse Flank collapse Plinian eruption Plinian eruption Dome collapse Plinian eruption

37 ka yrs 28 ka yrs 28 ka yrs El Zaguán 21.7 ka yrs LTP 12.5 ka yrs MPF 13 ka yrs El Refugio 10.5 ka yrs UTP

Runout (km)

H/L

10-cm isopach area (km2)

Column height (km)

Volume (km3)

VEI

Ref

17 17 9

0.12 0.12 0.14

n.a. n.a. n.a.

n.a. n.a. n.a.

3 3 0.35

4 4 3

1 1 2

n.a.

n.a.

300

24

0.8

4

3

n.a.

n.a.

40

21

1.8

4

4

15

0.12

n.a.

n.a.

0.13

3

1, 5

n.a.

n.a.

2000

42

8

6

6

477

on the proximal area. The concordance between these two simulations suggests that for such low-volume (∼0.3 km3) granular flows, the traveled distance does not depend strictly on the mass volume, as previously generalized by other authors for large rock-falls (Dade and Huppert, 1998). This point will be analyzed further in the next section, because it has important hazard assessment implications. TITAN2D was also used to simulate debris avalanche flows on other ravines, where known deposits crop out (Fig. 10C, D), giving similar results between simulated outputs and observed deposits. 4.2. Lahars: LAHARZ

Abbreviations are: LTP: Lower Toluca Pumice; MPF: Middle Toluca Pumice; UTP: Upper Toluca Pumice; n.a.; not applicable; VEI: Volcanic Explosivity Index. References are; 1, Macías et al. (1997); 2, Caballero (2007); 3, Capra et al. (2006); 4, Arce et al. (2005); 5: D'Antonio et al. (2008); 6: Arce et al. (2003).

(D'Antonio et al., 2008). The TITAN2D simulations were performed to obtain the flow thickness and distribution based on a mass volume of 0.11 km3 and an estimated basal friction angle between 8˚ and 10˚ (Table 2). These values were determined essentially based on the mass runout of past flows (H/L) and compared with values used for the same type of flows in previous works to obtain an estimation of the basal friction angle (Rupp et al., 2006). The internal friction angle was fixed at 30˚, normal value for granular material, even if is not too relevant for small granular flow (Hutter et al., 1995). Fig. 9A shows the distribution of simulated flows where their depth is also indicated. These results are similar to the thickness and runout of the observed deposits (Fig. 4). The largest runout flows are observed in shallow ravines with gentle topographic gradients, which these flows can easily inundate (e.g. Cienega ravine, Fig. 9). The flow trajectories are different with respect to past events, because of the actual configuration of the crater. For instance, the Zacango ravine, one of the most studied places because of its widely exposed pyroclastic sequence, probably will be not greatly affected by block-and-ash flows in the future. In fact, a dacitic dome is acting as a topographic barrier forcing any gravitational flows far away from this ravine (white star in Fig. 4). As described above, lacustrine sediments on the Toluca basin are interbedded with ash layers, probably emplaced by the ash clouds accompanying the main pyroclastic flows. As previously reported (Malin and Sheridan, 1982; Saucedo et al., 2005), the energy cone can be used to estimate the possible extension of this heavily diluted density current. In this case, we used an H/L ratio of 0.1 (Fig. 9B). 4.1.2. Debris avalanches To simulate this type of flow, we calibrated the simulation parameters for TITAN2D considering the El Zaguán and Arroyo Grande debris avalanche deposits as prototypes (Fig. 4). We choose the Arroyo Grande deposit to reconstruct the paleotopography and perform simulations to exactly reproduce its extent (Fig. 10). In fact, debris avalanche deposits are capable to completely change the topography, making it impossible to simulate past flows over present DEMs. The paleotopography of the Arroyo Grande gully was restored by eliminating the thickness of the Arroyo Grande deposit and of the following overlapping volcanic succession, as measured in several stratigraphic sections, after which simulations with the FLOW3D and TITAN2D routines were performed. The best-fit H/L was 0.14, from which a basal friction angle of ∼8˚ was extrapolated (Table 2). For the FLOW3D routine, viscous and turbulent coefficient were set equal to 0 (Table 2). Both simulations gave similar results, with flows moving about 9 km from the source, and stayed confined into the valley where thicknesses of 100 m are observed

Heavy rains are the most common triggering mechanism of lahars, although other water sources are possible, such as the rupture of crater lakes (Manville et al.,1999), temporary volcanic dams (Macías et al., 2004; Capra, 2007), and glacial water outburst (Gudmundsson et al., 1998). Depending on the sediment concentration, debris flows can form and gradually transform to hyperconcentrated flows (Scott, 1988). LAHARZ is a semi-empirical model, designed as a rapid, objective, and reproducible automated method for mapping areas of potential lahar inundation based on the flow volume (Schilling, 1998; Iverson et al., 1998). It has already been used by several authors to produce hazard maps (Sheridan et al., 2001b; Samaniego et al., 2004; Hubbard et al., 2006). In order to simulate the most probable scenario, the minimum and maximum flow volumes used were 500,000 m3 and 5,000,000 m3 respectively (Fig. 11). The minimum value corresponds to the 1952 Pueblo Nuevo lahar deposit, while the maximum value represents older lahar deposits exposed on the northern flank of the volcano (Fig. 4) and associated with the 37 ka, 28 ka, and 13 ka dome collapse events. The LAHARZ routine simulates the distal lahar inundation zones (LIZ, Fig. 11), therefore, the red area on the volcano edifice corresponds to an H/L value of 0.17 (LPHZ, Fig. 11), which limits the proximal area where erosive processes take place and where lahars start to bulk and transform from fluviatile or hyperconcentrated flows to debris flows. The main parameter that determines the limit of this proximal area is the break in slope that separates the main cone from the volcanic apron where deposition takes place. Based on the final map, lahars will primarily affect the eastern and northern sector of the volcano, including several villages. On the southern flanks, ravines are so deep that lahars cannot inundate vast areas, but contrarily they do have larger runouts. 4.3. Fallout: HAZMAP HAZMAP is a computer program for simulating the diffusion, transport and sedimentation of volcanic particles in the atmosphere in two dimensions from discrete point source (Macedonio et al., 2005, http://datasim.ov.ingv.it/). The program determines the particle settling velocity and based on eruption parameters (mass discharge, column height, wind profile) and on physical conditions (atmospheric diffusion coefficient and column shape) the mass distribution is obtained. Specifically, for the present case, we based the simulation on the PC2 layer of the UTP eruption (VEI 6, Arce et al., 2003), using two different wind profiles that dominate the area, one eastward (during spring and summer), and one westward

Table 2 Parameters used for flow simulations with FLOW3D, TITAN2D and LAHARZ routines Debris avalanches

Pyroclastic flow

Lahars

H/L (a0)

0.14

n.a.

ϕb v0 (m/s) Volume (m3)

8 0 3.5 ⁎ 105

0.12 0.1 (ash could) 10–12 15 1.1 ⁎ 105

n.a. n.a. 5 ⁎ 105–5 ⁎ 106

Abbreviations are: ϕb: basal friction angle; v0: initial velocity; n.a. not applicable.

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Fig. 9. Pyroclastic flow simulations. A) TITAN2D simulation for pyroclastic flows showing flow thickness on main ravines. The parameters were calibrated with the 13 ka BP El Refugio deposit. Higher thicknesses are exposed on the proximal areas on the cone slopes. CR: Cienega Ravine; ZR: Zaguan Ravine. B) Energy cone obtained with the FLOW3D application for an H/L value of 0.1, the area represents the possible extension of fine ash related to pyroclastic flow.

Fig. 10. Image showing computer simulations for debris avalanches. Simulation of the Arroyo Grande debris avalanche with (A) FLOW3D and (B) TITAN2D model. Both simulations were performed over a reconstructed paleotopography (see text for details). Note that distribution and runout are practically the same in both simulations. (C) Distribution of debris avalanche deposits at NdT that is comparable with the TITAN2D simulations (D).

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479

Fig. 11. Volcanic hazard zonation for lahars at NdT volcano. The LPHZ corresponds to the Lahar Proximal Hazard Zone, where lahars might form by eroding material on the main cone; LIZ corresponds with the lahar inundation zone based on flow volumes as indicated in the legend.

(during fall and winter). The meteorological data were obtained from the Air Resource Laboratory web page (http://www.arl.noaa. gov/ready/amet.html). The particle settling velocity was calculated based on the granulometric distribution of F2 pyroclastic flow emplaced just after the PC2 layer (Arce et al., 2003), which could give a good approximation of the column grain-size characteristics (Pfeiffer et al., 2005). Table 3 reports values used during simulation. Fig. 12A shows the isopach map reproducing the PC2 layer. Black dots indicate thickness measured in the field (Arce et al., 2003), indicating a good approximation of the simulated fallout. For distal areas it is much more difficult to compare real and simulated data because of erosion of the finest material, but previous authors reported about 10 cm of fall deposits in Mexico City, as can be extrapolated from the simulation

(Fig. 12C). Fig. 12B shows the result of the simulation towards SW, wind direction that occasionally characterizes the area, even if none of the four plinian eruptions so far described have this dispersal axis.

Table 3 Main parameters used for fall simulations with HAZMAP routine Number of settling velocity particles classes Total mass emitted from point source (kg) Number of vertical steps Height of the column (m) Diffusion coefficient (m2/s) Suzuki coefficient (A and λ)

17 2E12 100 40000 8000 2.5 1

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5. Discussion 5.1. Hazard zonation The final hazard zonation of NdT volcano was compiled based on the simulated scenarios previously described. A main problem for the

zonation is how to define the different hazard levels. One potential approach is to identify the more hazardous zones based on the number of events that might affect it. In this sense, the main cone would fall within the highest hazard zone, and distal ravines would be exposed to the lowest hazard, since only lahars can inundate it. But evidently, if no magmatic activity occurs at the volcano, lahars will

Fig. 12. Fallout simulation obtained with HAZMAP computer routine with same input parameters (Table 3), but with two different wind profiles, A) towards NE and B) towards SW (for explanation, see the text). Black dotted circles in figure a indicate fall deposit thickness measured for the PC2 layer of the UTP eruption (from Arce et al., 2003). Isopach values are in meter .C) Landsat image showing the distribution of the simulated 10 cm isopach from a VEI 6 eruption.

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481

Fig. 13. Volcanic hazard map of NdT volcano displaying the areas of pyroclastic flows, fall, debris avalanches and lahars. The dimension of the gray circles is proportional to the number of inhabitants, as indicated in the scale. Abbreviations are: LPHZ, lahar proximal hazard zone; LIZ, lahar inundation zone.

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represent the most probable hazard in the area. Based on these considerations, two different zonations are proposed. The first refers to the actual quiescent state of the volcano and considers only possible erosive processes that generate lahars. This scenario corresponds with the lahar hazard delineation previously described (Fig. 11). Towns such as San Juan de las Huertas, Zacango, and Villa Guerrero will be affected by lahars produced by heavy rains, as in the 1952 event. Towns such as San Juan Tilapa and Tenango might also be affected by this hazard, because they are located on the distal fans of these main ravines susceptible to floods during heavy rains. In the proximal area, erosive processes could cause significant damage, including landslides of the walls of main ravines, where several small villages are settled. In fact, especially on the southern sector of the volcano, planar areas are often occupied by greenhouses for cultivation of plants, and many people settled their houses on the steep flanks of ravines. At least 50,000 people could be affected by this type of phenomenon. The second hazard zonation (Fig.13) depicts the scenarios in case of a future magmatic activity. The map delineates different zones based on the types of events affecting certain areas. By comparing the FLOW3D and TITAN2D simulations for debris avalanche, an energy line with H/L of 0.14 was traced to include all possible trajectories and maximum runout. But considering that past sector collapses have occurred to the western, northwestern and eastern flanks of NdT (Figs. 4 and 10), as the volcano responses to the geometry and kinematics of the active basement faults (Norini et al., 2006), the most feasible sectors for collapse directions are the east-northwest (red dashed line, Fig. 13). Even if block-and-ash flows could only affect ravines up to a 15 km from the summit, the associated ash-cloud fallouts (H/L = 0.1) could threaten areas up to a distance of 30 km, including the towns of Tenango, Metepec, Ixtapan de la Sal and Toluca, where approximately 500,000 people live (demographic data from INEGI, http://www.inegi. gob.mx/inegi/default.aspx). Plinian activity represents the most catastrophic scenario, because over a 1 m thick layer of pumice could blanket an area within a 30 km radius from the vent that would damage the City of Toluca, capital of the State of Mexico. But in addition to local damage, such an eruptive scenario might completely paralyze the central part of the country affecting more than 30 million of inhabitants, including Mexico City. A large number of

fatalities could occur at several rural villages over a radius of more than 70 km due to the deposition of N10 cm of ash (Fig. 12), which will result in the collapse of roofs, and health problems. Main airports such as those located in Mexico City and Toluca will stop operations, as in 1997 when a small ash plume from Popocatépetl volcano caused the Mexico City airport to shut down for some hours, causing significant economic loses. Finally, fine ashes will be dispersed on the atmosphere provoking, darkness and temporally climate alteration such as during the 1982 Chichón eruption that caused the temperature of the globe to decrease ca. 0.5° (Angell and Korshover, 1983). Dome and flank collapses should be more predictable because the volcano currently lacks a summit dome. In fact, in order to experience this type of activity, a new dome must grow inside or outside the crater. In contrast, the stratigraphic record (Figs. 5 and 7C) indicates that Plinian eruptions have taken place with an open conduit similar to the present morphology of the volcano that will be probably preceded by phreatic and phreatomagmatic eruptions. 5.2. Computer simulation An important result of this work resulted from the comparison between the FLOW3D and TITAN2D routines for confined debris avalanches. This analysis showed that the trajectory and runout of this hazard is very similar (Fig. 10A,B) with both routines, even if FLOW3D does not take into account the mass volume. This is an important result because it supports the idea that volume does not likely affect the travel distance of low-volume, confined, granular flows. By plotting H/L values vs. volume of volcanic debris avalanches (Fig. 14, data taken from Hayashi and Self, 1992), it is clear that a direct relationship between volume and runout can be observed only for voluminous debris avalanches. In contrast, low-volume debris avalanches are scattered in the plot. This contrasting behavior could indicate that the mobility of small masses is probably more dependent on other factors, such as the material type, degree of fracturing, presence of pyroclastic material, or clay fraction in the pre-failure mass (Siebert et al., 1987; Ui, 1983) with respect to the mass volume (Dade and Huppert, 1998). Based on this result, we concluded that the energy line concept is still useful for hazard assessment of small volcanic debris avalanches less than 1 km3 in volume, a method that

Fig. 14. Diagram showing the relation between the H/L and volume for volcanic debris avalanches (data from Hayashi and Self, 1992).

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can be easily applied without the time-consuming calibration and validation of numerical models. 6. Conclusions The Nevado de Toluca hazard assessment benefits from both geological and computational analyses. The availability of an excellent stratigraphic record allows evaluation of the expected scenario and better constraint of the input parameters for numerical modeling of hazardous volcanic processes. Computer simulations always give an output, but these do not necessarily correspond to a good scenario of the volcano eruptive behavior. For the case of the Nevado de Toluca hazard zonation, the integration between geological data and computer routines provides validation and calibration for numerical models. This is the basic condition for reliability and good confidence of the hazard assessment. Currently, NdT is in a quiescent state without any evidence of renewed activity; however, NdT may become active in the future, with seismic and volcanic events that can have catastrophic results for the surrounding communities. Based on the stratigraphic record, the volcano remained quiescent for periods up to 8000 years, being now impossible to predict when the next eruption will be. The population is now completely vulnerable to such events, because the volcano is not monitored and no specific civil protection plans are known to be reliable and ready for use. In this sense, the hazard zonations presented here provides the first and most basic document for 1) land use planning; 2) civil protection defense plans preparation; and 3) volcanic crisis management. The actual density of the seismic network in the area is too low to ensure detection of the first seismic signals associated with renewal of volcanic activity, and no others types of monitoring (as interferometry for deformation of the edifice, or gravimetry, etc.) are conducted on a regular basis. In the case of renewed volcanic activity, there is the possibility that threatened populations will be not alerted in time and will not receive the most useful information. Thus, we encourage better surveillance of NdT and other dormant volcanoes in the TMVB. Most of these volcanoes should be better studied to understand their potential hazards, which can be assessed in deep only after detailed geological work. Acknowledgements This work is dedicated to Armando Garcia-Palomo, our best friend that largely contributed to the knowledge of Nevado de Toluca volcano. The work was supported by CONACYT grant (projects 37889 and 46340) to L. Capra. The Ministry of Foreign Affairs of Italy and SRE of Mexico provided travel assistance to Gianluca Groppelli and Gianluca Norini. We greatly acknowledge Fernando Bellotti, Lizeth Caballero, Micaela Casartelli, Marco D'Antonio, Andrea Gigliuto, Riccardo Lunghi, Anna Merlini, and Damiano Sarocchi for their help during the fieldwork. Special thanks to Mike Sheridan and Marcus Bursik who kindly provided TITAN2D and FLOW3D softwares, and Antonio Costa for assistance during HAZMAP simulation. Marina Manea and Emilio Nava also helped during data processing. Revisions by Alicia Felpeto, Joan Marti and an anonymous reviewer substantially improved the manuscript. References Aceves Quesada, F., Martin del Pozzo, A.L., Lopez Blanco, J., 2007. Volcanic hazards zonation of the Nevado de Toluca Volcano, Central Mexico. Natural Hazards 41, 159–180. Angell, J.K., Korshover, J., 1983. Global temperature variations in the troposphere and stratosphere, 1958–82. Monthly Weather Review 111, 901–921. Arce, J.L., Macías, J.L., Vázquez-Selem, L., 2003. The 10.5 ka Plinian eruption of Nevado de Toluca volcano, Mexico: stratigraphy and hazard implications. Geological Society of America Bulletin 115 (2), 230–248. Arce, J.L., Cervantes, K.E., Macías, J.L., Mora, J.C., 2005. The 12.1 ka Middle Toluca Pumice: a dacitic Plinian–subplinian eruption of Nevado de Toluca in Central Mexico. Journal of Volcanology and Geothermal Research 147 (1–2), 125–143.

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