The the risks and the areas subject to disaster

The Nevado del Ruiz, also known as La Mesa de Herveo, or Kumanday in the language of the local pre-Columbian indigenous people, is a volcano located on the border of the departments of Caldas and Tolima. 

Last eruption: 2016

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Elevation: 17,425? ft

First ascent: 1936

Location: Caldas & Tolima, Colombia

Volcanic arc/belt: Northern Volcanic Zone; Andean Volcanic Belt

Did you know: Nevado del Ruiz has the fourth-deadliest volcanic eruption in history by death toll (23,000).

A relatively small eruption of Nevado del Ruiz volcano, located in the Central Cordillera of Colombia, South America, took place on November 13, 1985. The materials ejected during the eruption melted part of the glacial ice cap at the summit of the volcano, releasing a series of lahars (volcanic mudflows and debris flows). The lahars descended through steep, well-defined, and relatively narrow river canyons, reaching speeds of up to 45 km/hr or 28mph.

Major flows descended the eastern side of the volcano through the valleys of the Azufrado, Lagunillas, and Gualí rivers. The flow through two of these channels, the Azufrado and Lagunillas rivers, merged approximately halfway down the mountainside to form a large flow that continued along the Lagunillas River valley. This flow disgorged through a narrow canyon onto a gently sloping alluvial fan and adjacent floodplain that extend along the eastern front of the mountains.

The lahar flow devastated the city of Armero, built on the alluvial fan approximately 2.0-2.5 km downstream from the mouth of the Lagunillas River canyon. Estimates varied and claimed that between 20,000 and 24,000 people perished at Armero, most of them crushed or buried in their homes. Another lahar flow descended the western slope of the volcano through the narrow canyon of the Chinchina River, destroying 400 houses and causing as many as 3,000 fatalities near the town of Chinchina. This event is thought to be the deadliest recorded lahar in human history. Nevado del Ruiz has been active through the Quarternary4, with at least three VEI-4 eruptions in the past 2,000 years, in 1350 AD, 200 BC and 850 BC5. An eruption of similar magnitude would cause the melting of the massive ice sheet that caps the volcano and would generate larger lahars than that of 1985 with catastrophic consequences for the area.

The risk of an eruption like that which occurred on November 13, 1985 was well recognized prior to the event. Extensive effort went into defining the risks and the areas subject to disaster and to promoting emergency preparedness for evacuation and disaster relief. Although many did much to prepare, it seems clear that the risk under which Armero lived its last days was never recognized by local city officials. Additionally, the officialBottom of Form warning system in place the night of the disaster failed to get word to the citizens of Armero, most of whom were in bed for the evening after ashfall and other volcanic events of the day were thought to have ceased. An estimated 5,000 people did survive and were displaced by the disaster.

The continuation of sporadic volcanic activity recorded since the November 13, 1985 eruption, combined with the extensive ice cap still remaining around the summit, could result in future lahars with equally devastating consequences for the large population still living in proximity to the volcano.

The study team characterized the magnitude and extent of the flows, catalogued much of the damage, and analyzed the nature of the geologic processes that led to both the initiation of the lahars and the extensive damage resulting from them. In addition, the team interviewed many survivors and participants in the emergency preparedness and recovery efforts. The lessons learned can be summarized as follows:

The Nevado del Ruiz volcano continues to present a threat to the region from the formation of additional and potentially larger lahars to other volcanic-related phenomena such as ash fallout. Longer-term research to continuously assess the Ruiz hazard is needed.

Special care must be given to properly educate as well as warn those members of the community who will be required to take action during an emergency situation to effectively communicate the potential risk.

Risk communication and warnings are taken more seriously when accompanied by materials that provide adequate information, specifically about both the nature of the hazard and the steps that must be taken to mitigate the effect of the hazard. These factors are particularly important for hazards that have not manifested themselves in the lifetimes of the people who face them.

The recovery efforts in many ways had characteristics similar to other disaster recovery efforts. The study and analysis of these efforts would have been enhanced had collection of socioeconomic data been undertaken prior to the event.Nevado del Ruiz volcano (NRV), Columbia, is one of the most dangerous volcanoes in the world and caused the death of 25,000 people in 1985. Using a new algorithm for repeated tomography, we have found a prominent seismic anomaly with high values of the Vp/Vs ratio at depths of 2–5 km below the surface, which is associated with a shallow magma reservoir. The amplitude and shape of this anomaly changed during the current phase of unrest which began in 2010. We interpret these changes as due to the ascent of gas bubbles through magma and to degassing of the reservoir. In 2011–2014, most of this gas escaped through permeable roof rocks, feeding surface fumarole activity and leading to a gradual decrease of the Vp/Vs ratio in the reservoir. This trend was reversed in 2015–2016 due to replenishment of the reservoir by a new batch of volatile-rich magma likely to sustain further volcanic activity. It is argued that the recurring “breathing” of the shallow reservoir is the main cause of current eruptions at NRV.

Beginning in 2010, NRV has been in a phase of volcanic unrest with intense seismic activity, surface deformation and gas venting. The SO2 flux reached extreme values of 30 ktons per day at times and the cumulative output of SO2 from 2012 to 2015 is estimated to be 7 10 tons. It’s estimated that the flux of H2O and CO2 exceeds this value by a factor of at least 10.

Following the catastrophic 1985 event, Colombian authorities have made important efforts to improve the monitoring of volcanic activity at NRV through the deployment of a large number of instruments. Inclinometers have revealed large amounts of surface deformation close to the main Arenas crater between May 2012 and the present6. Deformation climaxed with a lava dome eruption in the eastern sector of the crater from September to November 2015. Beginning in 2015, drumbeat seismicity, a long series of events repeating themselves at regular time intervals with identical waveforms, has been recorded in the crater area. Ground inflation over a broad area has been occurring at a rate of ~4 cm per year with a deformation center that lies ~10 km to the south of the NRV edifice at a depth of ~14 km.

Since the 1980s, NRV has been monitored by a permanent seismic network that has been expanded gradually. Data from this network have been used to determine the locations of seismic events, their spectral characteristics, time changes of seismic attenuation beneath the volcano as well as a three-dimensional seismic crustal model. Compared to this model, which was derived from body-wave data recorded before 2002, the present work yields important new information. The extensive data set that has been acquired since 2002 allows large improvements in the resolution of tomographic inversions. Furthermore, the seismic velocity structure of NRV has changed considerably as a result of volcanic and magmatic activity, especially during the latest phase of unrest that began in 2011. Using large amounts of data over a long time interval and a new algorithm for repeated tomography, we have derived a new detailed 3-D seismic model and have determined changes of seismic velocity beneath the volcano.


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