VOLCANIC Hazard Zonation of the NTV Capra

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Volcanic hazard zonation of the Nevado de Toluca volcano, Mxico

L. Capra a,, G. Norini a, G. Groppelli b, J.L. Macas c, J.L. Arce d

a Centro de Geociencias, Universidad Nacional Autnoma de Mexico, Campus Juriquilla, 76230 Queretaro, Mexicob C.N.R. - Istituto per la Dinamica dei Processi Ambientali, Milano, Italyc Instituto de Geofsica, UNAM, CU, Mexico DF, Mexicod Instituto de Geologia, UNAM, CU, Mexico DF, Mexico

A B S T R A C TA R T I C L E I N F O

Article history:

Received 18 December 2007Accepted 19 April 2008Available online 6 May 2008

Keywords:

Nevado de Toluca volcanoTrans-Mexican Volcanic Beltvolcanic hazardcomputer simulations

The Nevado de Toluca is a quiescent volcano located 20 km southwest of the City of Toluca and 70 km west ofMexico City. It has been quiescent since its last eruptive activity, dated at 3.3 ka BP. During the Pleistoceneand Holocene, it experienced several eruptive phases, including ve dome collapses with the emplacement ofblock-and-ash ows and four Plinian eruptions, including the 10.5 ka BP Plinian eruption that depositedmore than 10 cm of sand-sized pumice in the area occupied today by Mexico City. A detailed geological mapcoupled with computer simulations (FLOW3D, TITAN2D, LAHARZ and HAZMAP softwares) were used toproduce the volcanic hazard assessment. Based on thenal hazard zonation the northern and eastern sectorsof Nevado de Toluca would be affected by a greater number of phenomena in case of reappraisal activity.Block-and-ash ows will affect deep ravines up to a distance of 15 km and associated ash clouds couldblanket the Toluca basin, whereas ash falls from Plinian events will have catastrophic effects for populatedareas within a radius of 70 km, including the Mexico City Metropolitan area, inhabited by more than20 million people. Independently of the activity of the volcano, lahars occur every year, affecting smallvillages settled down ow from main ravines.

2008 Elsevier B.V. All rights reserved.

1. Introduction

The Quaternary Mexican volcanism is concentrated along the Trans-Mexican Volcanic Belt (TMVB) (Fig.1A), a continental volcanic arc thathas been active since 14 Ma (Ferrari et al., 1994). At least 14 activevolcanoes are present in Mexico, and most of theseare located along theTMVB (Fig. 1A). At present, only Volcn de Colima and Popocatpetlexhibit persistent activity, with small eruptive columns(up to 58kmineach case) and short-runout pyroclastic ows that have not affectedpopulated areas (Saucedo et al., 2005; Macas and Siebe, 2005; Macaset al.,2006). The other volcanoes, including Nevado de Toluca(hereafter,NdT), are quiescent and apparently represent no threat to thesurrounding populations. However, the sudden reactivation of ElChichn volcano in 1982 after a 550 yr quiescence period killed over2000 people (Macas et al., 2008), suggesting that long-term dormantvolcanoes can become active in a very short time, with catastrophicconsequences due to previous dearth of basic studies, hazard maps,emergency information programs, etc. The Popocatpetl and Colimavolcano hazard maps were the rst maps prepared in Mexico as aconsequence of the eruptive crises that occurred in these volcanoes in1994 and 1991, respectively (Macas et al., 1995; Del Pozzo et al., 1996;Sheridan et al., 2001a;Navarro et al., 2003). Morerecently,Sheridan et al.(2001b, 2004) produced a hazard map forthe Pico de Orizaba volcano in

which the hazard delineation was based primarily on ow simulationsthat took into accountthe more recenteruptiveactivity.At present, thesemaps represent a fundamental scientic document that civil defenseauthoritiesuse incase of future volcanic crises.For the case of NevadodeToluca, theonly available hazardmap wasreported by Capraet al. (2000,2004) and Aceves Quesada et al. (2007). These maps were mostly basedon the stratigraphic record and morphology of the volcano.

The Nevado de Toluca is a quiescent volcano, located 20 kmsouthwest of Toluca, and 70 km west of Mexico City (Fig. 1B). Thevolcano has been silent since its last eruptive activity, dated at3.3 kaBP (Macaset al.,1997), although minorfumarolic activity wasreportedduring the nineteenth century (Bloomeld and Valastro, 1977). Thereactivation of NdT could threaten more than 20 million people,including the Mexico City metropolitan area, that some 10.5 ka yearsago was blanketed by more then 10 cm of pumice from the UpperToluca Pumice eruption, one the most violent Plinian eruptionoccurred during the Holocene (Fig. 1B) (Cas and Wright, 1988; Arceet al., 2003). Theaim of this work is to present a hazardassessment thatincludes future eruptive scenarios deduced from a detailed geologicalmap (Garcia-Palomo et al., 2002; Bellotti et al., 2004; Norini, 2006)coupled with computerow simulations for debris avalanches, block-and-ash ows, falls, and lahars. All simulations were carefully testedbased on spatial distributions and thickness of past ows, and in somecases the paleotopography was restored in digital elevation models(DEMs) to better reproduce past events. The resulting hazard zonationbenets from accurate calibration and validation of the numerical

Journal of Volcanology and Geothermal Research 176 (2008) 469484

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

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models coupled with high-resolution geological data and reliablecomputer ow simulations. This compiled methodology can betterpredict the extent of products of future volcanic activity in contrast toprevious research, which tended to be based either on numericalmodeling or on geological mapping (Waythomas and Waitt, 1998;Moreno, 2000; Sheridan et al., 2004; Soeld, 2004).

2. The Nevado de Toluca volcano

2.1. Morphostructural features

The Nevado de Toluca is 4680 m high and is characterized by anopen crater,1.5 to 2 km in diameter, EW elongated, which developed

in response to intense tectonic activity (Tenango Fault System, Fig. 2;Garca-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 anksand two main morphological domains can be dened (Norini et al.,2004). The southern ank of Nevado de Toluca volcano has an irregularmorphology, relativelyat anddissected by deeprectilinear valleys withNNWSSE strikes associated to the TaxcoQueretaro Fault System(Garca-Palomo et al., 2000; Bellotti et al., 2006). The altitude of the thisank varies between 3800 and 2500 m a.s.l. and more than 50% of thisarea has slopes greater than 20 (Norini et al., 2004). Ravines are deeplyeroded with depths up to 450 m (Barranca del Muerto,Fig. 2). All thesefeatures led previous authors to consider this area as the remains of an

older volcanic structure called Paleonevado (Cantagrel et al., 1981;Garca-Palomo et al., 2000). The western, northern, and eastern NdTanks, in conjunction with the crater area, depict the second mor-phological domain, and constitute the present active cone of the NdTvolcano. The northern and northwestern sectors of the volcano aregentle,with slopes of 68, mostly formed by pyroclastic fans (Fig. 3B,C)on the Toluca basin. The main drainages are radial with respect to thecone. The eastern sector has a gentler slope but is more dissected, withravines up to 70 m deep (Arroyo Grande ravine, Fig. 2) due to thepresence of the Tenango Fault System (Garca-Palomo et al., 2000), anactive left-lateral transtensive structure (Norini et al., 2006).

The present morphological arrangement of the volcano may have agreat inuence on the hazard assessment, because the distributionand runout distances of volcanic ows will depend primarily on the

ow dynamics and topography.

2.2. Stratigraphic record

The volcano started to grow at 2.6 Ma with andesitic to daciticeffusive activity that ended at 1.1 Ma and led to the formation ofPaleonevado edice (Cantagrel et al., 1981; Garcia-Palomo et al., 2002;Martnez-Serrano et al., 2004). After an intense erosive stage thatoriginated the emplacement of voluminous epiclastic sequences,including two sector collapses (Capra and Macas, 2000), magmaticactivity was renewed 42 ka ago whit the formation of the recentactive cone of the NdT and the emplacement of the Pink Pumice Flow(PPF) deposit (Macas et al., 1997).Fig. 4shows a detailed geologicalmap 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 ve dome collapses dated at 37, 32, 28, 26.5, and13 ka (Macas et al., 1997; Garcia-Palomo et al., 2002), at least threelateral collapses (Macas et al., 1997; Norini, 2006), and four Plinianeruptions at 36 ka (Ochre Pumice) (Garcia-Palomo et al., 2002), 21.7 ka(LowerToluca Pumice, LTP) (Bloomeld 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) (Macas et al., 1997; Arce et al., 2003). Theeruptive sequence is crowned by a phreatomagmatic surge depositdated at 3.3 ka BP (Macas 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 illustrateexpected events in case of renewed volcanic activity. The activity ofthe Paleonevado, characterized by voluminous lava ows, endedapproximately 1.2 Ma ago, a time interval too long to be considered asa possible locus of a new eruption.

Dome collapse events have been characterized by summit domegrowth and destruction, associated or not with an explosivecomponent. The 37- and 28-ka events correspond to major dome-destruction episodes that originated lithic-rich block-and-ash owdeposits that traveled up to 20 km from the source emplacing up to30 m thick deposits (Fig. 6A). Thesedeposits are grouped in the map inone unit because they usually represent vertical sections, beingimpossible to lay out their individual distribution on the geologicalmap. Main outcrops are limited to quarries where material is

extracted for construction (Fig. 3C); it is thus difcult to determine

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: ColimaVolcanic Complex; Pa: Parcutin; NT: Nevado de Toluca; Jo: Jocotitln; Mx: Mexico City; Iz: Iztacchuatl; Po: Popocatpetl; PdO: Pico de Orizaba; CdP: Cofre de Perote. B) Landsatimage (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 Popocatpetl, both active volcanoes.White dotted lines refer to the 10-cm isopach map of three mayor plinian eruptions occurred at the Nevadode Toluca volcano: the Lower Toluca Pumice (LTP, at 21 ka BP), the MiddleToluca 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: TenangoFault; 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.

470 L. Capra et al. / Journal of Volcanology and Geothermal Research 176 (2008) 469484


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the exact distribution of such deposits. Despite this limitation, it isclear that these twomain events were radially distributed as they cropout along all main ravines. Macas et al. (1997) estimated a totalvolume of 3 km3 for each event, which includes several ow unitsthat correspond to a VEI of 4 (Volcanic Explosivity Index,Newhall andSelf, 1982). The deposit associated with the younger 13 ka BP domecollapse crops out mainly on the eastern and northeast sectors. It iseasy to recognizebecause of its brilliant gray color andits stratigraphicposition, between the Lower Toluca Pumice (LTP) and the UpperToluca Pumice (UTP) Plinian fall deposits. It consists of a main-channelfacies up to 10 m thick, with several ow units of clasts-rich block-

and-ash ow deposits, and a lateral facies up to 4 m thick consisting of

a sand-sized unit with small amounts of pumices (Fig. 3C). This owtraveled a maximum distance of 15 km and was dispersed mostly tothe north-eastern sector of the volcano. A volume estimation yielded avalue of 0.11 km3 (D'Antonio et al., 2008), which corresponds to a VEIof 3, being a moderate eruption. All of these pyroclastic ows show asimilar runout that corresponds to an H/L(relation between the dropheight and the maximum runout) of 0.12. The 13-ka event was used tocalibrate computer simulations for pyroclastic ows because of itsstratigraphic control and good exposures. Previous works (Metcalfeet al., 1991; Macas et al., 1997; Newton and Metcalfe, 1999; Caballeroet al., 2000) reported an ash ow deposit on the lacustrinesequence of

the Toluca basin. This deposit probably represents the product of an

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 faulttraces related to the Tenango Fault System (Norini et al., 2006).

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ash cloud associated to the main block-and-ash ow. For such kind of

deposits, anH/Lof 0.1 was estimated.Duringthe growthof the modernedice, theextrusion of the 28-ka

summit dome, produced the collapse of the edice with theemplacement of a debris avalanche deposit followed by a sequenceof block-and-ash ows (Caballero, 2007). The best examples of thiscollapse areoutcrops on the eastern sector of the volcano (Fig. 6C). Theresulting debris avalanche deposit, primarily outcropping on theZaguanravine, hasa volume of 0.35 km3 and an H/L value of 0.14. Otherdebris avalanche deposits wereobserved on the northwest and easternvalleys, with estimated volumes of 0.3 km3 and similar H/Lratios of0.14 (Arroyo Grande and El Nopal deposits,Fig. 4,Norini, 2006).

The UTP represents the most violent and voluminous eruption ofNdT, 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 collapsingpumice ows that emplaced on main ravines (Fig. 4 and 7a). TheOmbligoDomewasextrudedat theendof this activity(Arceetal.,2003).Reconstructions of this event givea VEI= 6 (Arce et al., 2003), which maybe comparable to the Plinian phase of the 1991 Pinatubo eruption thathad climatic global effects. In fact, Arce et al. (2003)argued that theYounger Dryas period, the last major glacial event at the limit ofHolocene, 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 Tolucabasin (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

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 thewestern 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 byglacier 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 exposedthe 13 ka BP block-and-ash ow deposit that consists of up to ve different units, and it is crowned by the UTP.



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Fig. 4.Geological map showing the distribution of the younger (b42 ka BP) volcanic sequence of the NdT. The vertical black arrow in the legend indicates that block-and-ash owdeposits are interbeded with Plinian deposits. For the UTP and the MTP deposits, only pumice ow deposits arerepresented. The evident absence of recent volcaniclastic deposits onthe 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 (Modied fromBellotti et al., 2004).

Fig. 5.Diagram showing a simplied 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 fromMacas et al., 1997; Garcia-Palomo et al., 2002).



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known phreatomagmatic deposit was dated at 3.3 ka and has beendescribed 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(Macas et al., 1997) (Fig. 6B). This deposit crops out in only one pointon the western ank of the volcano. This eruption is probablyassociated with a low- to medium-magnitude phreatomagmaticeruption with the formation of a small eruptive column.

Primary lahars have been always associated with mainly eruptiveevents, and originated from the remobilization of falls, pumice ows,block-and-ashows, and debris avalanche deposits in main ravinesand up to a distance of 15 km from the crater (Figs. 4 and 8A).

Finally, during the past century, heavy rains have remobilized largeamounts of unconsolidated material generating secondary lahars. Forexample, in 1952, a rain-triggered ood affected the village of PuebloNuevo,on theeasternsectorof thevolcano (personal communication fromlocal people). In this same drainage, local authorities are now arranging

sand dikes to prevent futureoods (Fig. 8B) that occur every rainy season.

3. Possible hazardous scenarios

Based on the stratigraphic record of the NdTeruptive activity duringthe last 42 ka BP, at least 12 main eruptions have occurred at intervalsvaryingfrom1to8ka(Fig.5). Theanalysis of thecomposite stratigraphicsection of the volcano allows assessment of possible scenarios in thecase of a future eruption of the volcano. Plinian fallouts rest directly ontop of thick paleosols aroundthe volcano, with almost no other previouspyroclastic ow deposits at their base (Fig. 7C). This might indicate thatbefore Plinian eruptions at NdT, the crater was open, without a centraldome inside as attested by the small amounts of accidental clasts in thelower 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 the0.001 km3 Ombligo Dome on its interior that was emplaced just afterthe UTP eruption. Most of the block-and-ash ow deposits recorded inthe stratigraphy of the volcano were probably generated by discrete

collapses of central domes, as those observed in the 28 ka event. A

Fig. 6. Pictures showing someexamplesof pyroclastic deposits at NdT. A) Section at Zacangoquarry, showing the 37 and28 ka BP block-and-ashowdepositscrownedby theLTPandUTP pumice fall deposits. B) 3.3 ka BP pyroclastic ow at Raices quarry. C) El Zaguan debris avalanche deposit.



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similar sequence of events was observed in other volcanoes, where a

growing dome and the associated hydrothermal system weakened themain edice, inducing its collapse(Voightet al., 2002). At NdT, the domeremnants reect a growing path guided by tectonic lineaments,indicating a structural control during the magma ascent, as for thedirection of main collapses, which was guided by geometry andkinematics of tectonic structures (Norini, 2006).

Based on the age of the described eruptions, a regular period ofrecurrence cannot be easily identied, because this depends on thetype of volcanic activity prior to each quiescent period. Generally, itcan be stated that the volcano has remained silent up to 8 ka followedby explosive eruptions. This volcano behavior indicates the need tocarefully evaluate its potential hazards. Several other volcanoesexhibited long quiescent periods before cataclysmic eruptions, suchas 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) thatmight be followed by 1) explosive activity with the formation of asustained column (VEI 46); or 2) opening of new eruptive fractures(associated with active faults) and dome growth with the collapse ofthe old edice, followed by the destruction of the new dome withemplacement of block-and-ash ow deposits. Independently of thepotential eruptive scenario, supercial water will remobilize uncon-solidated volcaniclastic material forming lahars. Because of theabsence of lava ows 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, pyroclasticows (block-and-ashows),

Fig.7. Picturesof mainfall deposits. A) UTPsequenceshowingintercalation of pumice fall andowdeposits. B)Detail ofthe LTP, characterized bya fall basal unit anda sequence offalland surge layers atop. C) Picture displaying fall deposits resting directly on top of thick organic paleosols.



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(D'Antonio et al., 2008). The TITAN2D simulations were performed toobtain theow thickness and distribution based on a mass volume of0.11 km3 and an estimated basal friction angle between 8 and 10(Table 2). These values were determined essentially based on the massrunoutof past ows (H/L) and compared withvaluesusedfor the sametype ofows in previous works to obtain an estimation of the basalfriction angle (Rupp et al., 2006). The internal friction angle was xedat30, normalvalue for granularmaterial, even if is not too relevant forsmall granularow (Hutter et al.,1995).Fig. 9A shows the distributionof simulated ows where their depth is also indicated. These resultsare similar to the thickness and runout of the observed deposits(Fig. 4). The largest runout ows are observed in shallow ravines withgentle topographic gradients, which these ows can easily inundate(e.g. Cienega ravine, Fig. 9). The ow trajectories are different withrespect to past events, because of the actual conguration of the

crater. For instance, the Zacango ravine, one of the most studied placesbecause of its widely exposed pyroclastic sequence, probably will benot greatly affected by block-and-ash ows in the future. In fact, adacitic dome is acting as a topographic barrier forcing any gravita-tionalows far away from this ravine (white star inFig. 4).

As described above, lacustrine sediments on the Toluca basin areinterbedded with ash layers, probably emplaced by the ash cloudsaccompanying the main pyroclastic ows. As previously reported(Malin and Sheridan,1982; Saucedo et al., 2005), the energy cone canbe used to estimate the possible extension of this heavily diluteddensity current. In this case, we used an H/Lratio of 0.1 (Fig. 9B).

4.1.2. Debris avalanchesTo simulate this type of ow, we calibrated the simulation

parameters for TITAN2D considering the El Zagun and Arroyo Grandedebris avalanche deposits as prototypes (Fig. 4). We choose the ArroyoGrande deposit to reconstruct the paleotopography and perform simu-lations to exactly reproduce its extent (Fig.10). In fact, debris avalanchedeposits are capable to completely change the topography, making itimpossible to simulate past ows over present DEMs. The paleotopo-graphy of the Arroyo Grande gully was restored by eliminating thethickness of the Arroyo Grandedeposit andof the following overlappingvolcanic succession, as measured in several stratigraphic sections, afterwhich simulations with the FLOW3D and TITAN2D routines wereperformed. Thebest-t H/L was0.14, from which a basal friction angle of8was extrapolated (Table 2). For the FLOW3D routine, viscous andturbulent coefcient wereset equal to0 (Table2). Bothsimulations gavesimilar results, with ows moving about 9 km from the source, and

stayedconnedinto the valley where thicknesses of 100 m are observed

on the proximal area. The concordance between these two simulationssuggests that for such low-volume (0.3 km3) granular ows, thetraveled distance does not depend strictly on the mass volume, aspreviously generalized by other authors for large rock-falls (Dade andHuppert, 1998). This point will be analyzed further in the next section,because it has important hazard assessment implications. TITAN2D wasalso used to simulate debris avalanche ows on other ravines, whereknown deposits crop out (Fig. 10C, D), giving similar results between

simulated outputs and observed deposits.

4.2. Lahars: LAHARZ

Heavy rains are the most common triggering mechanism of lahars,although other water sources are possible, such as the rupture of craterlakes (Manvilleet al.,1999), temporaryvolcanic dams(Macaset al., 2004;Capra, 2007), and glacial water outburst (Gudmundsson et al., 1998).Depending on the sediment concentration, debris ows can form andgradually transform to hyperconcentratedows (Scott, 1988). LAHARZ isa semi-empirical model, designed as a rapid, objective, and reproducibleautomatedmethod for mappingareas of potentiallahar inundation basedon the ow volume (Schilling, 1998; Iverson et al., 1998). It has alreadybeen used by several authors to produce hazard maps (Sheridan et al.,2001b; Samaniego et al., 2004; Hubbard et al., 2006).

In order to simulatethe most probable scenario,the minimum andmaximum ow volumes used were 500,000 m3 and 5,000,000 m3

respectively (Fig. 11). The minimum value corresponds to the 1952Pueblo Nuevo lahar deposit, while the maximum value representsolder lahar deposits exposed on the northern ank of the volcano(Fig. 4) and associated with the 37 ka, 28 ka, and 13 ka dome collapseevents. The LAHARZ routine simulates the distal lahar inundationzones (LIZ, Fig. 11), therefore, the red area on the volcano edicecorresponds to anH/Lvalue of 0.17 (LPHZ,Fig. 11), which limits theproximal area where erosive processes take place and where laharsstart to bulk and transform from uviatile or hyperconcentratedows to debris ows. The main parameter that determines the limitof this proximal area is the break in slope that separates the maincone from the volcanic apron where deposition takes place. Based on

the nal map, lahars will primarily affect the eastern and northernsector of the volcano, including several villages. On the southernanks, ravines are so deep that laharscannot inundatevast areas, butcontrarily 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 atmo-sphere in two dimensions from discrete point source (Macedonioet al.,2005, http://datasim.ov.ingv.it/). The program determines theparticle settling velocity and based on eruption parameters (massdischarge, column height, wind prole) and on physical conditions(atmospheric diffusion coefcient and column shape) the mass

distribution is obtained. Specically, for the present case, we basedthe simulation on the PC2 layer of the UTP eruption (VEI 6, Arceet al., 2003), using two different wind proles that dominate thearea, one eastward (during spring and summer), and one westward

Table 1

Eruptive parameters of major eruptions during late PleistoceneHolocene

Type Age Runout(km)

H/L 10-cmisopacharea(km2)

Columnheight(km)

Volume(km3)

VEI Ref

Name

Dome collapse 37 ka yrs 17 0.12 n.a. n.a. 3 4 1Dome collapse 28 ka yrs 17 0.12 n.a. n.a. 3 4 1Flank collapse 28 ka yrs 9 0.14 n.a. n.a. 0.35 3 2

El ZagunPlinian eruption 21.7 ka yrs n.a. n.a. 300 24 0.8 4 3

LTP

Plinian eruption 12.5 ka yrs n.a. n.a. 40 21 1.8 4 4MPF

Dome collapse 13 ka yrs 15 0.12 n.a. n.a. 0.13 3 1, 5El Refugio

Plinian eruption 10.5 ka yrs n.a. n.a. 2000 42 8 6 6UTP

Abbreviations are: LTP: Lower Toluca Pumice; MPF: Middle Toluca Pumice; UTP: UpperToluca Pumice; n.a.; not applicable; VEI: Volcanic Explosivity Index. References are; 1,Macas 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).

Table 2

Parameters used for ow simulations with FLOW3D, TITAN2D and LAHARZ routines

Debris avalanches Pyroclastic ow Lahars

H/L(a0) 0.14 0.120.1 (ash could)

n.a.

b 8 1012 n.a.v0(m/s) 0 15 n.a.Volume (m3) 3.5105 1.1105 51055106

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

http://datasim.ov.ingv.it/http://datasim.ov.ingv.it/


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Fig.10.Image showing computer simulations for debris avalanches. Simulation of the Arroyo Grande debris avalanche with (A) FLOW3D and (B) TITAN2D model. Both simulationswere 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).

Fig. 9. Pyroclasticow simulations. A) TITAN2D simulation for pyroclasticowsshowingow thicknesson mainravines.The parameters werecalibrated with the 13ka BP El Refugiodeposit. 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 foranH/Lvalue of 0.1, the area represents the possible extension ofne ash related to pyroclastic ow.



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(during fall and winter). The meteorological data were obtainedfrom the Air Resource Laboratory web page(http://www.arl.noaa.gov/ready/amet.html). The particle settling velocity was calculatedbased on the granulometric distribution of F2 pyroclastic owemplaced just after the PC2 layer (Arce et al., 2003), which couldgive a good approximation of the column grain-size characteristics(Pfeiffer et al., 2005).Table 3 reports values used during simulation.

Fig.12A shows theisopachmap reproducing the PC2layer. Black dotsindicate thickness measured in theeld (Arce et al., 2003), indicating agood approximation of the simulated fallout. For distal areas it is muchmore difcult to compare real and simulated data because of erosion ofthe nest 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, winddirection that occasionallycharacterizes thearea,even if none of thefourplinian eruptions so far described have this dispersal axis.

Fig. 11. Volcanic hazardzonation forlahars at NdTvolcano. The LPHZ corresponds tothe LaharProximal HazardZone, where laharsmight form byerodingmaterialon themaincone;LIZ corresponds with the lahar inundation zone based on ow volumes as indicated in the legend.

Table 3

Main parameters used for fall simulations with HAZMAP routine

Number of settling velocity particles classes 17Total mass emitted from point source (kg) 2E12

Number of vertical steps 100Height of the column (m) 40000Diffusion coefcient (m2/s) 8000Suzuki coefcient (Aand ) 2.5 1

http://www.arl.noaa.gov/ready/amet.htmlhttp://www.arl.noaa.gov/ready/amet.htmlhttp://www.arl.noaa.gov/ready/amet.htmlhttp://www.arl.noaa.gov/ready/amet.htmlhttp://www.arl.noaa.gov/ready/amet.html


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5. Discussion

5.1. Hazard zonation

The nal hazard zonation of NdT volcano was compiled based onthe simulated scenarios previously described. A main problem for the

zonation is how to dene the different hazard levels. One potentialapproach is to identify the more hazardous zones based on thenumber of events that might affect it. In this sense, the main conewould fall within the highest hazard zone, and distal ravines would beexposed to the lowest hazard, since only lahars can inundate it. Butevidently, 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 proles, A) towards NE and B) towards SW(forexplanation, see the text). Black dotted circles in gure a indicate fall deposit thickness measured for the PC2 layer of the UTP eruption (from Arce et al., 2003). Isopachvalues are

in meter .C) Landsat image showing the distribution of the simulated 10 cm isopach from a VEI 6 eruption.



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represent the most probable hazard in the area. Based on theseconsiderations, two different zonations are proposed.

The rst refers to the actual quiescent state of the volcano andconsiders only possible erosive processes that generate lahars. Thisscenario corresponds with the lahar hazard delineation previouslydescribed (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 1952event. Towns such as San Juan Tilapaand Tenango might

also be affected by this hazard, because they are located on the distalfans of these main ravines susceptible to oods during heavy rains. Inthe proximal area, erosive processes could cause signicant damage,including landslides of the walls of main ravines, where several smallvillages are settled. In fact, especially on the southern sector of thevolcano, planar areas are often occupied by greenhouses for cultiva-tion of plants, and many people settled their houses on the steepanks of ravines. At least 50,000 people could be affected by this typeof phenomenon.

Thesecond hazardzonation (Fig.13)depictsthescenariosincaseofafuture magmatic activity. The map delineates different zones based onthe types of events affecting certain areas. By comparing the FLOW3Dand TITAN2D simulations for debris avalanche, an energy line with H/Lof 0.14 was traced to include all possible trajectories and maximumrunout. But considering that past sector collapses have occurred to thewestern, northwestern and easternanks ofNdT(Figs. 4 and 10),as thevolcano responses to the geometry and kinematics of the activebasement faults (Norini et al., 2006), the most feasible sectors forcollapse directions are the east-northwest (red dashed line,Fig. 13).

Even if block-and-ash ows could only affect ravines up to a 15 kmfrom the summit, the associated ash-cloud fallouts (H/L=0.1) couldthreaten areas up to a distance of 30 km, including the towns ofTenango, Metepec, Ixtapan de la Sal and Toluca, where approximately500,000 people live (demographic data from INEGI, http://www.inegi.gob.mx/inegi/default.aspx).

Plinian activity represents the most catastrophic scenario, becauseover a 1 m thick layer of pumice could blanket an area within a 30 kmradius from the vent that would damage the City of Toluca, capital of theState of Mexico. Butin addition to local damage, such aneruptive scenario

might completely paralyze the central part of the country affecting morethan 30 million of inhabitants, including Mexico City. A large number of

fatalities could occur at several rural villages over a radius of more than70km due tothe depositionofN10cmofash(Fig.12), which will resultinthe collapse of roofs, and health problems. Main airports such as thoselocated in Mexico City and Toluca will stop operations, as in 1997 when asmall ash plume from Popocatpetl volcano caused the Mexico Cityairport to shut down for some hours, causing signicant economic loses.Finally, ne ashes will be dispersed on the atmosphere provoking,darkness and temporally climate alteration such as during the 1982

Chichn eruptionthatcaused thetemperature of theglobe todecreaseca.0.5 (Angell and Korshover, 1983).Dome and ank collapses should be more predictable because the

volcano currently lacks a summit dome. In fact, in order to experiencethis type of activity, a new dome must grow inside or outside thecrater. In contrast, the stratigraphic record (Figs. 5 and 7C) indicatesthat Plinian eruptions have taken place with an open conduit similarto the present morphology of the volcano that will be probablypreceded by phreatic and phreatomagmatic eruptions.

5.2. Computer simulation

An important result of this work resulted from the comparisonbetween the FLOW3D and TITAN2D routines for conned debrisavalanches. This analysis showed that the trajectory and runout of thishazard is very similar (Fig. 10A,B) with both routines, even if FLOW3Ddoes not take into account the mass volume. This is an importantresult because it supports the idea that volume does not likely affectthe travel distance of low-volume, conned, granular ows. ByplottingH/Lvalues vs. volume of volcanic debris avalanches (Fig. 14,data taken from Hayashi and Self, 1992), it is clear that a directrelationship between volume and runout can be observed only forvoluminous debris avalanches. In contrast, low-volume debrisavalanches are scattered in the plot. This contrasting behavior couldindicate that the mobility of small masses is probablymore dependenton other factors, such as the material type, degree of fracturing,presence of pyroclastic material, or clay fraction in the pre-failuremass (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 smallvolcanic debris avalanches less than 1 km3 in volume, a method that

Fig. 14.Diagram showing the relation between theH/Land volume for volcanic debris avalanches (data fromHayashi and Self, 1992).

http://www.inegi.gob.mx/inegi/default.aspxhttp://www.inegi.gob.mx/inegi/default.aspxhttp://www.inegi.gob.mx/inegi/default.aspxhttp://www.inegi.gob.mx/inegi/default.aspx


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can be easily applied without the time-consuming calibration andvalidation of numerical models.

6. Conclusions

The Nevado de Toluca hazard assessment benets from bothgeological and computational analyses. The availability of an excellentstratigraphic record allows evaluation of the expected scenario and

better constraint of the input parameters for numerical modeling ofhazardous volcanic processes. Computer simulations always give anoutput, but these do not necessarily correspond to a good scenario ofthe volcano eruptive behavior. For the case of the Nevado de Tolucahazard zonation, the integration between geological data andcomputer routines provides validation and calibration for numericalmodels. This is the basic condition for reliability and good condenceof the hazard assessment.

Currently, NdT is in a quiescent state without any evidence ofrenewed activity; however, NdT maybecome active in the future, withseismic and volcanic events that can have catastrophic results for thesurrounding communities. Based on the stratigraphic record, thevolcano remained quiescent for periods up to 8000 years, being nowimpossible to predict when the next eruption will be. The populationis now completely vulnerable to such events, because the volcano isnot monitored and no specic civil protection plans are known to bereliable and ready for use. In this sense, the hazard zonationspresented here provides the rst and most basic document for 1)land use planning; 2) civil protection defense plans preparation; and3) volcanic crisis management.

The actual density of the seismic network in the area is too low toensure detection of the rst seismic signals associated with renewal ofvolcanicactivity, and no others typesof monitoring (as interferometryfordeformation of theedice, or gravimetry, etc.) are conductedon a regularbasis. In the case of renewed volcanic activity, there is the possibility thatthreatened populationswill benot alerted intime and will not receive themost useful information. Thus, we encourage better surveillance of NdTand other dormant volcanoes in the TMVB. Most of these volcanoesshould be better studied to understandtheir potential hazards,which can

be assessed in deep only after detailed geological work.

Acknowledgements

This work is dedicated to Armando Garcia-Palomo, our best friendthat largely contributedto the knowledgeof Nevado de Toluca volcano.The work was supported by CONACYT grant (projects 37889 and46340) to L. Capra. The Ministry of Foreign Affairs of Italy and SRE ofMexico provided travel assistance to Gianluca Groppelli and GianlucaNorini. 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 theeldwork. Special thanks to Mike Sheridan and Marcus Bursik whokindly provided TITAN2D and FLOW3D softwares, and Antonio Costa

for assistance during HAZMAP simulation. Marina Manea and EmilioNava also helped during data processing. Revisions by Alicia Felpeto,

Joan Marti and an anonymous reviewer substantially improved themanuscript.

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