Implications of Geomorphological Research for Recent and Prehistoric Avalanches and Related Hazards at Huascaran, Peru

Nat Hazards (2009) 50:193–209 DOI 10.1007/s11069-008-9330-7 ORIGINAL PAPER Implications of geomorphological research for recent and prehistoric avalanches and related hazards at Huascaran, Peru Jan Klimesˇ Æ Vı´t Vilı´mek Æ Marek Omelka Received: 11 June 2008 / Accepted: 24 November 2008 / Published online: 13 December 2008 Ó Springer Science+Business Media B.V. 2008 Abstract Detailed research of superficial deposits below the northern peak of Huascaran (Cordillera Blanca) provides new information on the limits of a paleo-avalanche originating from this mountain. Geomorphological mapping of the sediments identified glacial deposits, deposits from historical rock-debris avalanches and huge boulders from a paleo- avalanche. Schmidt Hammer rock-hardness tests were used to distinguish between the several generations of rock-debris avalanches, but largely failed to distinguish between the much older moraine and the paleo-avalanche sediments. Thus, only the field geomorphological mapping proved to be reliable for identifying the limits of the paleo-avalanche. The limits identified as granite boulders deposited over volcanic rocks were found to extend 30 m further up the opposite valley slope than previously had been mapped. This larger extent implies a greater volume and/or greater mobility for the prehistoric event. Keywords Slope movements Á Natural hazards Á Schmidt Hammer tests Á Huascaran Á Cordillera Blanca Á Peru 1 Introduction The Cordillera Blanca in Peru is part of the highly tectonically active Andean mountain chain formed by collision of the Cocos, Nazca, Antarctic, and South American lithosphere plates. Intense endogenic processes form suitable conditions for highly active and dangerous geomorphic processes. The Cordillera Blanca forms the NE boundary of the valley J. Klimesˇ (&) Institute of Rock Structures and Mechanics, Academy of Sciences, Prague, Czech Republic e-mail: [email protected] V. Vilı´mek Department of Physical Geography and Geoecology, Faculty of Science, Charles University, Prague, Czech Republic M. Omelka Jaroslav Hajek Center for Theoretical and Applied Statistics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic 123 194 Nat Hazards (2009) 50:193–209 of the Santa River (Callejoń de Huaylas, Fig. 1), which has been affected by many historical natural disasters (Zapata 2002). They vary by type (landslides, rock falls, rock and ice avalanches, debris-flows, outburst floods, and floods), size (Table 1), as well as trig- gering factors. Rock/ice falls and landslides are connected mostly with large magnitude earthquakes (Silgado 1978) mobilizing mass movements on wide areas. Landslides can be also triggered by seasonal rain in the period from November to April with peak Fig. 1 Location map of the study area Table 1 Overview of characteristics of recorded rock-debris avalanches from the north peak of Huascaran, western wall (N/A—not ascertained; reported event from 1917 was not included since no detailed data are available—Zapata 2002) Date Preparatory Volume Speed Run-out Size Total Reference condition/ (9106 m3) (km/h) distance classification area trigger (km) *** (km2) 1962 N/A 13 170 15.5* 8 6* Bolt et al. 1975 1970 earthquake 50 320 16* 8 25.4** Bolt et al. 1975 50–100 280–335 N/A 9 Plafker and Ericksen 1978, Plafker et al. 1971 1987 Precipitations 3.5 N/A 10.5* 7 1.1* Zapata 2002 1989 Precipitations N/A N/A 15 N/A N/A Zapata 2002 * Calculated in GIS ** Calculated in GIS, does not include river channel below the hit point filled by the sediments *** According to Jakob (2005): volume = 106 m3—class 7; volume = 107 m3—class 8; volume = 108 m3—class 9 123 Nat Hazards (2009) 50:193–209 195 precipitation in March (Mark 2002), or with precipitation influenced by the El Ninõ effect (Vilı´mek et al. 2000). Another frequent dangerous combination of processes are glacial lake outbursts producing floods or debris-flows (Ames 1998; Zapata 2002; Vilı´mek et al. 2005). The greatest known historical disaster in the Santa River Valley was the 1970 earthquake (ML 7.7) and subsequent rock-debris avalanches from the north peak of Nevado Huascaran (6,655 m). These events followed a smaller avalanche in 1962. In addition, some authors (Cluff 1971; Plafker and Ericksen 1978) note occurrences of an unknown number of large paleo-avalanches in the same area. They describe event of great magnitude with estimated deposition area of 30 km2, terminal velocity of 140 km/h and deposit thickness on the west bank of the Santa River of about 45 m. The evidence of long-term instability of the northern summit of Huascaran along with an increasing population density in the area raises questions about the frequency and magnitude of potential future events and their risk to the local people. To better understand the paleo-avalanches, the known events were examined, and in situ relative dating Schmidt Hammer (SH) tests, geomorphological mapping, and sedimentological analyses were conducted. The primary aim was to better delineate the paleo-avalanche first outlined in Plafker and Ericksen (1978). The catastrophic slope failures occurring on the northern summit of Huascaran combine several different processes which affect their classification. Some authors classify them as avalanches (Bolt et al. 1975) or debris-flows (Lomnitz 1971), and some others use the term rock slides (Browning 1973). Havenith et al. (2003) used the term ‘‘rock-debris avalanche’’ which we think describes best the nature of the events and therefore adopt in this article. 2 Methodology Numerous (120) in situ Schmidt Hammer (SH) tests were performed and statistically analyzed to reveal relative ages of the different rock-debris avalanche deposits. To validate and interpret the results, several other field and laboratory methods were applied. These included field geomorphologic mapping; satellite-imaging and air-photo interpretation; and rock and sediment sampling and analyses. Thus, much information about the landform types and their activity over the last 40 years were collected to form good base for better understanding the future landscape development. 2.1 Schmidt Hammer tests Probably the most frequent use of the Schmidt Hammer in geomorphological research is in relative dating of rocky landforms (Evans et al. 1999; Mentlı´k 2005; Shakesby et al. 2006; Engel and Traczyk 2008). The obtained rebound-values (R) can be interpreted in terms of mechanical strength, which for a given rock type decreases with duration of weathering, so that lower R-values correspond to more weathered surfaces (Mentlı´k 2005). We have used an N-type Schmidt Hammer for relative dating of mixed moraine and gravitational deposits originating through glacial processes and rock-debris avalanches. The SH-test followed the methodological procedure of Goudie (2006) and Mentlı´k (2005). We divided the measured boulders into three groups based on the type and chronology of the depositional processes identified through field investigations, geomorphologic mapping, and satellite-image interpretation. The first group of measured boulders consisted of moraine material containing boulders with the longest inferred weathering 123 196 Nat Hazards (2009) 50:193–209 Fig. 2 Top of the lateral moraine ridge below the north peak of Huascaran with example of boulders tested by the Schmidt Hammer history (Fig. 2); the second group was formed by boulders most likely deposited in the paleo-avalanche. The last group contained boulders transported during the 1962 and 1970 rock-debris avalanches. For each measured boulder, we acquired its size, volume, location, and basic petrologic description. 2.2 Statistical analysis of SH-test results We used a linear mixed model to estimate the mean and standard deviation of R-value measurements of boulders in the genetically defined groups. This model enabled us to account for varying numbers of measurements on different boulders and to test for possible differences in means and standard deviations across the groups. We tried several methods of classification analyses to see whether we could discrimi- nate among the genetically defined groups of boulders based on the means and standard deviations of R-values, and boulder volumes. Balancing the simplicity of the classification rules and the overall performance, we chose simple classification trees which used only mean R-values. In the classification analyses, we established a rule based on acquired data which could be applied to assign new measured boulders into the different genetic groups. This rule was tested only on the same data as were used for rule calculation, which may bias the overall classification results toward more optimistic ones. Similarly, we performed an automatic cluster analysis to see whether the unsupervised learning process (with no prior knowledge of classes) formed classes similar to the original classes, or at least whether the results of the classification analyses could be reproduced. From the various methods of cluster analysis, we decided to choose the simple methods of K-means with the means of R-values as the only variable. All the statistical analyses were performed using the R computation environment (http:// www.r-project.org). 123 Nat Hazards (2009) 50:193–209 197 2.3 Geomorphological mapping and grain-size analyses Landform mapping and description were undertaken during field work as well as through interpretation of 2006 and 1984 SPOT satellite images and 1970 infra-red aerial photo- graphs (scale 1:42,000). These investigations focused on identifying and describing the main landforms and their contained materials to explain the origins of the boulders tested by the SH device. Recent glaciations and ice-scoured bedrock were delineated from the satellite images where these areas appear as white and light grey polygons, whereas the adjacent moraines have darker tones of grey and brown and usually form clearly visible ridges. Possible confusion among these forms may be due to the shadows below sub-vertical slopes and continuous transitions among the color tones.

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