Lucia Capra

Debris avalanches and debris flows transformed from collapses in the Trans-Mexican Volcanic Belt, Mexico – behavior, and implications for hazard assessment

Volcanoes of the Trans-Mexican Volcanic Belt (TMVB) have yielded numerous sector and flank collapses during Pleistocene and Holocene times. Sector collapses associated with magmatic activity have yielded debris avalanches with generally limited runout extent (e.g. Popocatepetl, Jocotitlan, and Colima volcanoes). In contrast, flank collapses (smaller failures not involving the volcano summit), both associated and unassociated with magmatic activity and correlating with intense hydrothermal alteration in ice-capped volcanoes, commonly have yielded highly mobile cohesive debris flows (e.g. Pico de Orizaba and Nevado de Toluca volcanoes). Collapse orientation in the TMVB is preferentially to the south and northeast, probably reflecting the tectonic regime of active E^W and NNW faults. The differing mobilities of the flows transformed from collapses have important implications for hazard assessment. Both sector and flank collapse can yield highly mobile debris flows, but this transformation is more common in the cases of the smaller failures. High mobility is related to factors such as water content and clay content of the failed material, the paleotopography, and the extent of entrainment of sediment during flow (bulking). The ratio of fall height to runout distance commonly used for hazard zonation of debris avalanches is not valid for debris flows, which are more effectively modeled with the relation inundated area to failure or flow volume coupled with the topography of the inundated area. = 2002 Elsevier Science B.V. All rights reserved.

The cohesive Naranjo debris-flow deposit (10 km3):: A dam breakout flow derived from the Pleistocene debris-avalanche deposit of Nevado de Colima Volcano (México)

Abstract Mass movement processes on volcanic terrains such as landslides and debris avalanches can cause the obstruction of main drainages producing the formation of temporary dams. A good example of this occurred 18.5 ka ago when the eastern flank of the Nevado de Colima Volcano collapsed producing a debris-avalanche deposit that was previously described as one of the largest in the world. The deposit extended from the volcanic summit as far as the Pacific coast, 120 km away. New stratigraphic, sedimentological, and componentry data suggest that the volcanic collapse of Nevado de Colima resulted in a debris avalanche that traveled 20 km southeast to the Naranjo River. There it crashed against a topographic barrier consisting of Cretaceous limestones (Cerro la Carbonera) and the flow direction was diverted to the south down the Naranjo River channel for another 25 km before the avalanche came to a halt. The obstruction of the drainage produced a temporary dam that stored ca. 1 km3 of water and deposited fluvial and slack-water sediments. Some time after the damming, the accumulated water-sediment load was able to overtop the obstructing material and to release a breakout flow with a calculated initial flow discharge of 3.5 million m3/s. The resulting flood (cohesive debris flow) followed the channel of the Naranjo River and, due to the high erodibility of the channel and introduction of substrate material, the debris flow progressively increased its volume up to 10 km3, six times its initial volume. This study highlights the relevance of evaluating the potential remobilization of debris-avalanche deposits to initiate large magnitude cohesive debris flows. Therefore, the hazard and risk analysis of future potential events of this nature must consider the pre-eruption conditions and the topography surrounding a volcano.

The 1997 and 2001 lahars of Popocatépetl volcano (Central Mexico): textural and sedimentological constraints on their origin and hazards

Abstract Popocatepetl volcano is the most active volcano in central Mexico, and represents a high risk for more than 40 million people, including Mexico City. In 1994, volcanic activity at Popocatepetl renewed with the formation of ash-rich plumes up to 7 km high. In April 1996, lava emissions filled the crater and were accompanied by a series of explosions that produced eruptive columns up to 8 km high. Associated with explosive events in 1997 and 2001, two major lahars events occurred, leaving debris flow deposits along Huiloac Gorge for as far as 15 km, to the town of Santiago Xalitzinta. The 1997 debris flow deposit originated after a prolonged emission of ash which caused glacier melt and a rapid release of water (1×107 m3). The amount of melting water was sufficient to gradually erode the river bed causing a flood that gradually transformed from a debris flow to a hyperconcentrated flow. In contrast, the 2001 debris flow that originated from a post-depositional remobilization of a pumice flow deposit, did not experience any flow transformation and carried 25% water at maximum. The different behavior of these two lahars has important hazard implications. Both lahars reached Xalitzintla town, but at that point, the 1997 lahar had already transformed into a sediment-loaded stream flow. The 2001 lahar, by contrast, maintained the characteristics of a debris flow, being more competent, and with greater destructive power. What happened with these lahars demonstrates how important it is to take into consideration secondary volcanic phenomena. Even though they were not large flows, they were capable of threatening populated areas, even during periods of volcanic quiescence or reduced magmatic activity.

Holocene plinian eruption of La Virgen volcano, Baja California, Mexico

A plinian eruption occurred approximately 6500 yr ago at La Virgen Volcano, the youngest volcano of the Tres Virgenes Volcanic Complex (TVVC), located in Baja California, Mexico. Deposits of the eruption suggest a sequence of events that started with the opening of the volcanic conduit, and development of a plinian eruption column up to 18 km in height. This eruption column produced a fallout deposit with a dispersal axis toward the southwest, an areal extent of about 500 km 2 , and a minimum volume of 1.14 km 3 . Vulcanian activity (hydromagmatic) followed the plinian phase, producing pyroclastic surge and fallout deposits. The eruptive activity ceased after a basaltic-andesite lava flow was emplaced closing the eruptive activity. Petrological and geochemical evidence indicates that the eruption was triggered by magma mixing processes. Our studies confirm that La Virgen is a dormant volcano with the potential for future violent eruptions. The present study provides important information for the construction of a volcanic hazards map. Significant hazards are presented to the population living within a distance of 30 km from the volcano, together with the interstate road connecting the entire peninsula of Baja California, which runs at a distance of only 3 km from the volcano.

The 26 May 1982 breakout flows derived from failure of a volcanic dam at El Chichón, Chiapas, Mexico

The eruptions of El Chichon between 28 March and 4 April 1982 produced a variety of pyroclastic deposits. The climactic phase, on 3 April at 07:35 (4 April at 01:35 GMT), destroyed the central andesitic dome and fed pyroclastic surges and flows that dammed nearby drainages, including the Magdalena River. By late April, a lake had formed, 4 km long and 300–400 m wide, containing a volume of 26 × 10 6 m 3 of hot water. At 01:30 on 26 May, the pyroclastic dam was breached and surges of sediment and hot water soon inundated the town of Ostuacan, 10 km downstream. This hot flood was finally contained at Penitas Hydroelectric Dam, 35 km downstream, where one fatality occurred and three workers were badly scalded. Stratigraphic and sedimentologic evidence indicates that the rapidly draining lake initially discharged two debris flows, followed by five smaller debris flows and water surges. The main debris flows became diluted with distance, and by the time they reached Ostuacan, they merged into a single hyperconcentrated flow with a sediment concentration of ∼30 vol%. Deposits from this hyperconcentrated flow were emplaced for 15 km, as far as the confluence with another river, the Mas-Pac, below which the flow was diluted to sediment-laden streamflow. The minimum volume of the breakout-flow deposits is estimated at 17 × 10 6 m 3 . From high-water marks, flow profiles, and simulations utilizing the DAMBRK code from the National Weather Service, we calculated a maximum peak discharge of 11,000 m 3 /s at the breach; this maximum peak discharge occurred 1 h after initial breaching. The calculations indicated that ∼2 h were required to drain the lake.

Mass movements in tropical volcanic terrains: the case of Teziutlan (Mexico)

Abstract During the last decade, soil degradation coupled with global climate changes has increased hydrogeological hazards in Mexico. In tropical volcanic terrains, alteration processes have enhanced the formation of clay minerals that promote water retention and result in soil/rock weakness. Intense seasonal rainfall can trigger the liquefaction and remobilization of these low-resistance terrains. During the first week of October 1999, heavy rains affected eastern Mexico, including Puebla State. As a consequence, approximately 3000 mass movements, consisting of rock and soil slides and slips, debris flows and avalanches were generated in this area. In the town of Teziutlan (Puebla), which is located on volcanic deposits, a single mass-movement event caused approximately 150 deaths. In the present work we identified two types of mass movements in the Teziutlan area—Type 1: superficial erosion of an unwelded ignimbritic sequence forming small detrital fans, and Type 2: thin soil slide/debris flow from the remobilization of a volcanic sequence composed of clay-rich paleosols interbedded with ashfall horizons. The clay-rich volcanic paleosols favored the formation of perched water tables on a hydraulic aquiclude. Positive pore-water pressures triggered the failure. Based on these results, the principal human settlement in the Teziutlan area may be threatened by future debris flows, which could cause serious harm to the dense population and severe damage to its infrastructure. It is necessary to prevent future deaths and damage by installation of mitigative measures based on detailed studies. Without any further study, it will not be possible to prevent and mitigate a natural disaster with the same magnitude as the 1999 catastrophic hydrogeological phenomena.

Textural features as indicators of debris avalanche transport and emplacement, Taranaki volcano

The Pungarehu debris avalanche deposit was emplaced by the largest known collapse of the proto–Taranaki volcano, ca. 25,000 calibrated (cal.) years ago. This debris avalanche deposit displays a highly contrasting sedimentary character between its proximal and distal reaches. Examination of the deposit granulometry, sedimentary structures, and microscopic particle attributes provides new insights into debris avalanche transport and internal evolution processes. Initial collapse of the proto–Taranaki volcano during this event occurred near the Last Glacial Maximum, with snow and ice cover and substantial groundwater present. The collapsing, sliding large blocks of edifice material, “megaclasts,” were highly fractured by the landslide generation and the depressurization event, forming pervasive jigsaw textures. As the megaclasts moved, shear was focused in softer domains between the hardest, lava-dominated lithologies. These crush and shear zones developed a complex pattern of relative motion between horizontal and vertical parts of the landslide, rather than a simple basal shear zone that supported an upper pluglike mass. The sheared zones, concentrated in soft, pyroclastic lithologies, were areas of intense synflow fragmentation, producing a proto-interclast matrix between large blocks of coherent (albeit jigsaw fractured) lavas. Down flow, the interclast matrix component increased to become pervasive by ~23–25 km from the source, enveloping and preserving large megaclasts out to at least 30 km. The most distal exposures, limited by coastal erosion to ~25–27 km, show that the matrix was not completely water saturated, with only superficial penetration of the sand-dominated material into the margins of fractured lava domains, which still contained central void space. Evidence of multiple generations of particle fracturing is seen under scanning electron microscopy of sand-grade clasts, with initial decompression fractures crosscut by later cracks, pits, and scratches produced by collisional and frictional processes during transport. The findings from this study help to explain the formation of the highly irregular topography of debris avalanche deposits, with chaotically distributed (and probably temporary) zones of shear developing where softer lithologies occur in a collapsing mass, thus leading to differential velocity profiles of portions of the flowing mass in vertical and horizontal planes.

Long-range hazard assessment of volcanic ash dispersal for a Plinian eruptive scenario at Popocatépetl volcano (Mexico): implications for civil aviation safety

Popocatépetl is one of Mexico’s most active volcanoes threatening a densely populated area that includes Mexico City with more than 20 million inhabitants. The destructive potential of this volcano is demonstrated by its Late Pleistocene–Holocene eruptive activity, which has been characterized by recurrent Plinian eruptions of large magnitude, the last two of which destroyed human settlements in pre-Hispanic times. Popocatépetl’s reawakening in 1994 produced a crisis that culminated with the evacuation of two villages on the northeastern flank of the volcano. Shortly after, a monitoring system and a civil protection contingency plan based on a hazard zone map were implemented. The current volcanic hazards map considers the potential occurrence of different volcanic phenomena, including pyroclastic density currents and lahars. However, no quantitative assessment of the tephra hazard, especially related to atmospheric dispersal, has been performed. The presence of airborne volcanic ash at low and jet-cruise atmospheric levels compromises the safety of aircraft operations and forces re-routing of aircraft to prevent encounters with volcanic ash clouds. Given the high number of important airports in the surroundings of Popocatépetl volcano and considering the potential threat posed to civil aviation in Mexico and adjacent regions in case of a Plinian eruption, a hazard assessment for tephra dispersal is required. In this work, we present the first probabilistic tephra dispersal hazard assessment for Popocatépetl volcano. We compute probabilistic hazard maps for critical thresholds of airborne ash concentrations at different flight levels, corresponding to the situation defined in Europe during 2010, and still under discussion. Tephra dispersal mode is performed using the FALL3D numerical model. Probabilistic hazard maps are built for a Plinian eruptive scenario defined on the basis of geological field data for the “Ochre Pumice” Plinian eruption (4965 14C yr BP). FALL3D model input eruptive parameters are constrained through an inversion method carried out with the semi-analytical HAZMAP model and are varied by sampling them using probability density functions. We analyze the influence of seasonal variations on ash dispersal and estimate the average persistence of critical ash concentrations at relevant locations and airports. This study assesses the impact that a Plinian eruption similar to the Ochre Pumice eruption would have on the main airports of Mexico and adjacent areas. The hazard maps presented here can support long-term planning that would help minimize the impacts of such an eruption on civil aviation.

Hazard assessment at San Martín volcano based on geological record, numerical modeling, and spatial analysis

The San Martín shield volcano, located in the Los Tuxtlas Volcanic Field, has experienced effusive shield-building activity, as well as explosive eruptions, as evidenced by direct observations during the last eruption in 1793. The threat to the surrounding villages consists principally of lahars, especially because of the tropical climate in the region. Ash fallout and lava flows represent additional hazards. In addition, the surrounding Quaternary monogenetic field includes more than 300 scoria cones and about 40 explosion craters (mainly maars) that also represent a hazard source. In the present study we constructed hazard maps using field data, orthophotos, spatial analysis, and specialized software (LAHARZ and HAZMAP) to deliminate lahar inundation zones, areas that could potentially be affected by ash fallout (including the evaluation of houses prone to roof collapse due to ash load), and the most susceptible areas for hosting future monogenetic vent formation.

Seismic characterization of hyperconcentrated flows in a volcanic environment: Seismic characterization of hyperconcentrated flows

We present direct observations and monitoring data of a hyperconcentrated flow that occurred along La Lumbre ravine, one of the most active channels of Volcán de Colima in Mexico. Flow properties were inferred from video images and seismic data recorded by a geophone installed outside the channel. We collected flow samples 400m upstream from the monitoring station and analyzed the variation of sediment concentration and grain-size distribution over time. A joint analysis of hydrological (i.e. flow velocity, wetted perimeter) and rheological (i.e. yield stress τy and dynamic viscosity μm) parameters was performed to characterize the flow. Different flow regimes and sediment transport processes were identified and analyzed in comparison with both the amplitude and spectral features of the seismic signal. We observed differing sediment concentrations at the same discharge, suggesting a decoupling between sediment transport processes and discharge for low-magnitude flows. A straightforward correlation was found between the amplitude of the seismic signal and the sediment concentration, and a value of 1.8 × 10 3 mm/s was identified that can be used as a threshold to recognize the hyperconcentrated phase of the flow. This information was tested on the complete seismic dataset gathered at La Lumbre ravine during the 2015 rainy season. We identified the transition from streamflow to hyperconcentrated flow (and/or vice versa) in 16 low-magnitude events and we validated this result using the video recordings. The correlation between seismic amplitude and sediment concentration is valid at La Lumbre ravine but would need to be tested in other locations for the development of automatic flow classification methods. This work contributes to standardized seismic methods for characterizing flow processes in volcanic environments, also for the development of lahar early warning systems. Copyright