Large block slide at San Juan Grijalva, Northwest Chiapas, Mexico

Recent Landslides Landslides (2011) 8:109–115 DOI 10.1007/s10346-010-0212-1 Received: 21 May 2009 Accepted: 26 March 2010 Published online: 4 May 2010 © Springer-Verlag 2010

Víctor Hernández-Madrigal I Juan Mora-Chaparro I Víctor Garduño-Monroy

Large block slide at San Juan Grijalva, Northwest Chiapas, Mexico

Abstract On November 4, 2007, a large block slide occurred on the south face of the Cerro La Pera at San Juan Grijalva (SJG), northwest Chiapas, Mexico. The SJG landslide has an area of 1.11 km2 and a volume of 50 Mm3, making it one of the largest landslide of its type in the twentieth century. The landslide created a dam over 80 m high and 1,170 m wide across the Grijalva River, backing up the water and forming a 49 km2 lake. Landslide-generated tsunamis up to 15 m high destroyed the village of SJG, and the newly formed lake flooded 21 villages located upstream. The landslide killed 16 people and caused around 3,600 to be evacuated with incalculable economic losses. It was perhaps the most catastrophic landslide in the history of Mexico. The probable trigger of the landslide was cumulative precipitation of about 67% of the average annual rainfall over the preceding 30 days. The associated potentially causative factors include a M4.5 earthquake that occurred 5 days before the landslide and a water-level drawdown at the Grijalva River generated by the release of water from the Peñitas dam located 14 km downstream. Keywords Large block slide . Water-level drawdown . San Juan Grijalva . Mexico Introduction On November 4, 2007, a large landslide formed in the southern slope of the Cerro La Pera, in the municipality of Ostuacan, Chiapas, Mexico (Fig. 1), burying the village of San Juan Grijalva (SJG). This landslide, a block slide (Cruden and Varnes 1996) killed 16 people and caused the evacuation of nearly 3,600 inhabitants of the 21 communities that were flooded by the reservoir it created, making it perhaps the most catastrophic single landslide event recorded in Mexico. In the present paper, we describe the chronology, effects, mechanisms, and factors considered to be potential triggers of the SJG landslide because of their impact on slope stability. The account is based on field surveys, photointerpretation, review of reports from the Comision Federal de Electricidad (CFE), and interviews with the victims of the landslide. Geology setting The landslide is located within the western sector of the Montañas del oriente physiographic region (Fig. 1), on the face of a monoclinal structure with NW–SE direction and a 10–30o SW dip, in a sedimentary terrain folded by compressive deformations from the Miocene and Pliocene known as Chiapaneco and Cascadiano cycles, respectively (Sánchez-Montes 1969, 1980). Before the slide, the topography of the landslide area was characterized by a 5°–20° slope on the upper hillside, and a 20°–40° slope on the adjacent right bank of the Grijalva River. The

hillside height with respect to the bottom river was about 300 m, and the hillside length was about 1,200 m (Fig. 2). The rock types in the area include alternating conglomerates, sands, silts, clays, and marls of Eocene and Miocene age, which overlie Jurassic and Cretaceous rocks. During the Quaternary, plutonic bodies linked to the Pacific subduction affected the region, starting the volcanic activity that formed the Arco Volcanico Chiapaneco (Mora et al. 2007). More recent volcanic activity formed the Chichonal volcano that erupted in 1982, with significant devastation resulting from its explosive phases and subsequent lahar deposits. The landslide in SJG shows the following sequence of rock types from the landslide toe to the head scarp source area (east; Figs. 2 and 3): (1) conglomerates with a depth of 3 m from the Grijalva River water level, with predominance of well rounded clasts of quartz, limestone, and terrigenous rocks within a wellcemented sandy matrix; (2) alternating coarse sandstone and clay layers of light green or gray color with variable thicknesses of up to 1 m and a 12° dip to the SE direction (Fig. 2). The sandstones contain bedding and microfolds, and the clayey layers represent horizons of weakness on which slope failures can occur. In a lower level of these clayey alternations, the sliding surface of the unstable body can be identified by striations formed during translational displacement; (3) large banks of well-cemented coarse and fine sandstone with syndepositional and postdepositional sedimentary structures containing plant remains and bioturbations [these cemented sandstones form slopes of up to 90o (scarps)]; and (4) lateritic soils up to 2 m in thickness, a product of the weathering of volcanic, and terrigenous rocks and limestones. Ash deposits from the Chichonal are observed topping the sequence. Hydrology The humid tropical climate of the state of Chiapas with yearround abundant rain causes the numerous tributaries that merge to form two of the largest rivers in Mexico: the Usumacinta and Grijalva rivers. The Grijalva River, with a length of 766 km from its source on the western slopes of the Sierra Madre de Chiapas to its mouth in the Gulf of Mexico, is used to generate electricity in four hydroelectric dams (Fig. 1): Angostura (with an effective installed capacity of 900 Mw), Chicoasén (2,400 Mw), Malpaso (1,080 Mw), and Peñitas (420 Mw; CFE 2009). The SJG landslide is located 56 km downstream of the Malpaso dam and 14 km upstream of the Peñitas dam (Fig. 1), in an area where the level of the Grijalva River is influenced by changes in the Peñitas reservoir level. Landslide history The geomorphological map of the landslide study area (Fig. 4) reveals four large and older landslide areas (paleolandslides) near

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Fig. 1 Location of the San Juan Grijalva landslide in the context of the physiographic regions of the state of Chiapas (lower right box). The central image shows the landslide and the flooded zone

the recent SJG landslide: to the SE, a rock flow with an area of 0.87 km2 and an escarpment direction perpendicular to the La Pera monoclinal; to the SW, two rotational landslides, each with an area of 0.13 km2, on to the left bank of the Grijalva River; and to the NW, a probable debris flow 750 m long covering 0.24 km2,

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which has moved perpendicular to the Grijalva River. There are also several smaller (secondary) landslides in the area. The presence of these other, older landslides suggest that the area has a high susceptibility to deep-seated landsliding, which may be linked to changes in groundwater level caused by the Grijalva

Fig. 2 Cross section through the San Juan Grijalva landslide, with NW–SE direction. See Fig. 4 for the location

River and to the steep terrain, geological structural conditions, acting together with the characteristic high precipitation of the region. According to local inhabitants, the Cerro La Pera is known as the “enchanted mountain” because of the frequent underground noises heard there. These noises may have been caused by incipient, precursory ground movements that preceded the SJG landslide. Chronology of events Based on the incipient deformation of the summit of the Cerro La Pera in the slope of the enchanted mountain zone, as well as on the antislope scarps, fractures and small landslides were identified based on detailed analysis of satellite images taken before the landslide event. We consider that a creep instability was possibly caused by a continuous deformation in the clayey alternations of the sandstone layers, which could have been the first event of the translational landslide of SJG. At about 7 P.M. on Sunday, November 4, 2007, rumbling noises coming from the western flank of the SJG village alerted a group of its inhabitants who, assuming their cattle were being stolen, travelled by boat on the Grijalva River to the site where the noises came from. The thundering became louder during the initial part of their journey, which made them return to the village for weapons, lamps, and reinforcements. At 8 P.M., when the reinforced group of men was once more heading toward the source of the rumble, the landslide occurred, instantly ending the lives of these men and other people from the village. Because the landslide occurred at night, the

chronology of events is somewhat uncertain. However, based on subsequent field surveys and aerial photographs taken after the event, it has been established that the landslide possibly started with rotational sliding events at the base of the slope (landslide toe) and the subsequent translational sliding events followed by rock slides and debris flows. Landslide features and size The SJG landslide caused the development of a main scarp at the head and lateral scarps on both flanks of up to 600 m in length and 50 m in height. The maximum width and length of the displaced mass is 1,170 and 1,570 m, respectively. Its total area was of 1.11 km2, of which 0.76 km2 corresponded to the landslide itself and 0.35 km2 corresponded to the dam formed across the Grijalva River (Figs. 2, 4, and 5). Assuming an average depth of 50 m, the volume of the displaced mass is estimated to be about 50 Mm3, comparable in size to the 60 Mm3 of the landslide at Armero, Colombia (Sidle and Ochiai 2006), but greater than the volume of the landslides in Huascarán, Peru, in 1962 (13 Mm3) and 1970 (40 Mm3) and in Saleshan, China (35 Mm3), in 1983 (Sidle and Ochiai 2006; Fig. 6). The SJG landslide may be one of the largest block slides of the twentieth century. Effects of the landslide The emplacement of the toe of the landslide over the bed of the Grijalva River generated at least two waves up to 15 m in height that dragged people and houses from the SJG village and from

Fig. 3 Rock types in the San Juan Grijalva landslide. a Exhumed conglomerates. b Regolith (clay) layers. c Coarse and fine sandstones. See Fig. 5 for location

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Fig. 4 Geomorphological map of the study area

other settlements near the river’s margin (Figs. 2 and 3). In addition, the 1,170 m long and 80 m high landslide dam across the Grijalva River flooded a 49 km2 area upstream (Fig. 1), with flooding rates of between 0.109 and 0.53 m/day over 59 days between November 4, 2007, and February 2, 2008. During that period, the CFE excavated a channel to reconnect the river, thus

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avoiding an even larger catastrophe from a potential dam-break flood resulting from overtopping and failure of the landslide dam. A possible dam-break flood was assessed as representing a serious danger to the hydroelectric dam of Peñitas and to other villages located downstream. The landslide and landslide-generated waves caused 16 deaths, while the flooding affected 21 settlements (Fig. 1)

Fig. 5 Perspective of the SJG landslide and dam formed over the Grijalva River. The lower right bow shows the damages produced by the landslide tsunamis in the SJG village

and caused the resettlement of nearly 3,606 inhabitants. Although the flooding did not cause deaths, the resulting economic losses were incalculable.

sands from the bottom of the river (Fig. 3a), that is, the rotational movement that dragged and raised material from the base of the slope and the bottom of the river, exposing it on the surface through the tilting of the sliding body (Figs. 2 and 5).

Landslide mechanism Rotational movement The instability that developed at the base of the slope was possibly initiated by inferred large changes of the pore-water pressure due to the rising and lowering of water level in the upper dam reservoir at the base of the slope and heavy rainfall. The rotational movement of the toe of the landslide is inferred by the exhumation of well-cemented conglomerates and

nt Va jo

300 250

0

Landslide name

Fig. 6 Volumes of catastrophic landslides

ia

rij G

ci b Ar an

Fo r

k

Ar

m

sh an

Sa le

ua sc ar

50

an

100

Ar ar as H ua sc ar

an

150

al va

er o R ev en ta

do r

200

H

Volume m3 (x 1 000 000)

350

Translational movement of large slide blocks The translational movement was identified by the translational sliding, in a SW direction and with counterclockwise horizontal rotation, of three large bodies or blocks of sandstones in alternation with clays, well-cemented coarse sandstones, and lateritic soils (Figs. 2, 4, and 5). The translational movement originated with the loss of confinement at the base of the slope that occurred in the preceding (initial) rotational movement at the base of the slope. The consequent increase of the shear stress suddenly accelerated the plastic deformation in the clayey layers, thus defining the weakness plane over which the landslide developed. The thrust exerted by these blocks on the rotational deposits at the base of the slope intensified the exhumation of the conglomerates that were raised to heights of 80 m above the water level of the Grijalva River. Secondary landslides In the back and the flanks of the large moving blocks, ample deposits were formed of up to 10 m in diameter of sandstone blocks, debris, sands, and clays that were produced by detachments and secondary landslides. Some of these deposits were carried downslope on the right

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450 400

2000

300 250

1500 200 1000

150

Accumulate ranfall (mm)

2500 350 Daily rainfall (mm)

Fig. 7 Precipitation recorded in the SJG zone (meteorological station of Ocotepec; CONAGUA 2007). Bars represent the daily rainfall; and the continuous line, the accumulated rainfall

100 500 50 0

0 Oct-01

Oct-08

flank by complex debris slide, and flows that covered part of the rotational landslides occurred during the first stage (Fig. 5). Classification The SJG landslide can be classified as a large block slide. According to Ibsen and Brunsden (1996), this class of complex gravitational movement is characterized by the involvement of minor rotational landslides in the foot and head and, occasionally, by mud and debris flows in the flanks of the main translational landslide, features that were observed in the SJG landslide.

Oct-15

Oct-22

Oct-29

Nov-05

Water-level drawdown of the Grijalva River Five significant water-level drawdowns of the Grijalva River were observed based on the records at the Peñitas dam, each one with a duration of 3 days during October and November of 2007 (Fig. 8 and Table 1). The first four drawdowns had rates from 0.35 to 0.9 m/day (Table 1), while the last, which occurred from November 2 to 5, had a drawdown rate of 1.47 m/day, 4.2 times greater than the average drawdown rate of 0.35 m/day reported by Massarsch et al. (1987) when four landslides occurred in the Fort Henry and Ardclooney dams in Ireland. The SJG landslide took place on November 4, 2007, almost at the end of this last water-level drawdown of the Grijalva River.

Factors Rainfall A rain gauge 35 km SE of the landslide (meteorological station of Ocotepec) measured a cumulative rainfall of 2,638 mm in October (Fig. 7), which is 4.18 times higher than the average monthly rainfall for October (631 mm). The cumulative rainfall of 2,765 mm from October 1 to November 4 was equivalent to 67.12% of the average annual rainfall (4,119.2 mm). Maximum daily rainfall values of 403.4, 308.9, and 250.5 mm were generated by the cold front number 5, from October 28 to 30, at least 4 days before the landslide happened (CONAGUA 2007).

Fig. 8 Water level at the Peñitas dam before and after the landslide. The boxes show water-level drawdown, and labels indicate the change in water level (Δh) and water-level drawdown rate (V) of each drawdown (modified from CONAGUA 2008)

Elevation (msnm)

Seismicity The Servicio Sismológico Nacional (SSN 2007) reported an earthquake of M4.5 with a focal depth of 65 km and a distance of 25 km from the landslide that occurred on October 30, 2007, 5 days before the event.

Conclusions The SJG landslide of November 4, 2007, is considered to be the most catastrophic in recent Mexican history and, perhaps, one of the largest landslide of its type (block slide) worldwide in the twentieth century. In the surroundings of the present landslide are observed older landslides of the rotational type and debris slides and flows that reveal a potentially unstable terrain. A landslide inventory map of this region might have predicted the SJG landslide and lives might have been saved. The complex SJG landslide is classified as a large block slide but also includes a combination of rotational slides and debris flows developed at the base of the slope and along the flanks of the translational slide blocks, respectively. The most important geological preconditioning factor for the landslide was the presence of clayey layers

95 94 93 92 91 90 89 88 87 86 85 84 83 82

Δ h4= 4.54m Δ h1= 2.7m

V3= 0.35m/day

Δ h5= 4.42m

V1= 0.9m/day

V5= 1.47m/day

Δ h2= 1.22m V2= 0.4m/day

11/14/2007

11/10/2007

11/6/2007

11/2/2007

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10/29/2007

10/25/2007

10/21/2007

10/17/2007

10/13/2007

10/9/2007

10/5/2007

10/1/2007

Date

114

V4= 0.91m/day Landslide

Δ h3= 1.06m

Table 1 Identification number (ID), start and end dates, duration (days), change in water level (Δh), and water-level drawdown rate (V) of water-level drawdown events in the Grijalva River between October 13 and November 5, 2007

ID

Start date

End date

1 2 3 4 5

October 13 October 17 October 25 October 30 November 2

October 16 October 20 October 28 November 2 November 5

Days

Δh (m)

V (m/day)

3 3 3 3 3

2.7 1.22 1.06 1.7 4.42

0.9 0.4 0.35 0.57 1.47

The landslide started on November 4 during the fifth water-level drawdown, with an average rate of 1.47 m/day

intercalated with sandstones, dipping at 12° from the Cerro La Pera ridge toward the valley bottom. However, the exceptional accumulated rainfall during the cold fronts 2, 4, and 5 and the water level rise and fall of the Grijalva River were the most likely triggers of the SJG landslide. The low level of shaking associated with the 65 km deep M4.5 earthquake 5 days before the landslide suggests that it was not a significant causative factor in the initiation of the rapid slope failure on 4 November. It will be necessary to make a rigorous slope-stability analysis of the failure mechanism of the landslide in order to quantitatively determine the influence of the various factors that contributed to the failure. Acknowledgments We thank the Comision Federal de Electricidad for providing favorable conditions during fieldwork and Luis Manuel Garcia Moreno, Subsecretario de Protección Civil of the state of Chiapas, for the logistic support and materials given to us. Thanks to Programa de Mejoramiento del Profesorado (PROMEP), project UMSNH-PTC-212, and projects CONACYT 48506-F and PAPIT IN 103909-3. Thanks to Mr. Hancox and Dr. Havenith (reviewers), and Mauri McSaveney (editor) for the helpful comments and suggestions.

References Comisión Federal de Electricidad (CFE) (2009) Hidroeléctricas. http://www.cfe.gob.mx/ es/LaEmpresa/queescfe/Listadodecentralesgeneradoras/. Accessed 20 April 2009 Comisión Nacional del Agua (CONAGUA) (2007) Bitácoras Mensuales de Fenómenos meteorológicos No. 010 y 011. Subdirección General Técnica. SMN, Subgerencia de Pronóstico Meteorológico. http://smn.cna.gob.mx/boletin/bitacora/bitacora-sep07. html Comisión Nacional del Agua (CONAGUA) (2008) Reporte de niveles San Juan Grijalva. http://www.cna.gob.mx/Deault.aspx. Accessed 27 January 2008 Cruden DM, Varnes DJ (1996) Landslides types and processes. In: Turner AK, Schuster RL (eds) Landslides: investigation and mitigation. Special Report 247, Transportation research board. US National research council, Washington, DC, pp 36–75 Ibsen ML, Brunsden D (1996) Block slide. In: Dikau R, Brunsden D, Schrott L, Ibsen ML (eds) Landslide recognition, identification, movement and causes. Wiley, pp 64–77 Massarsch KR, Broms BB, Fives MO, Nilsson A (1987) The Shannon embankments— effects of sudden drawdown and seepage on the stability. In: Hanrahan ET, Orr TLL, Widdis TF (eds) Groundwater effects in geotechnical engineering. Proceedings of the 9th European Conference on Soil Mechanics and Foundation Engineering, pp 465–468 Mora JC, Jaimes-Viera MC, Garduño-Monroy VH, Layer P, Pompa V, Godinez ML (2007) Geology and geochemistry characteristics of the Chiapanecan Volcanic Arc (Central Area), Chiapas Mexico. J Volcanol Geotherm Res 162:43–72 Sánchez-Montes de Oca R (1969) Estratigrafía y paleogeografía del Mesozoico de Chiapas: Seminario sobre exploración petrolera, v. 3. Instituto Mexicano del Petróleo, México, p 31 Sánchez-Montes de Oca R (1980) Geología petrolera de la Sierra de Chiapas. Bol Asoc Mex Geol Petrol 31(1–2):67–77 Sidle RC, Ochiai H (eds) (2006) Landslides, processes, prediction, and land use. Water Resource Monograph 18. American Geophysical Union Servicio Sismológico Nacional (SSN) (2007) Catálogo de sismos. http://www.ssn.unam. mx/website/jsp/catalogo.jsp. Accessed 1 December 2007 V. M. Hernández-Madrigal ()) : V. H. Garduño-Monroy Departamento de Geología y Mineralogía, Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo, Planta Alta, Edificio “U2”, C. U., Morelia, Michoacán, Mexico e-mail: [email protected] J. C. Mora-Chaparro Instituto de Geofísica, Universidad Nacional Autónoma de México, Mexico City, Mexico

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