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Si us plau utilitzeu sempre aquest identificador per citar o enllaçar aquest document: https://hdl.handle.net/2445/204560
Volcanic risk assessment at Irazú volcano, Costa Rica
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[eng] Irazu is the highest volcano in Costa Rica (3427 m.a.s.l.); it constitutes an andesitic shield, with two main craters at the top and several pyroclastic cones cuspidal, and adventitial on the South Flank. The first historical eruption was registered in 1723, and other eruptive episodes took place in 1917-1921, 1924, 1928-1930, 1933, 1939- 1940 and 1963-1965. These eruptive events were characterized by phreatomagmatic, strombolian and phreatic phases that generated pyroclastic fall deposits, pyroclastic density current (PDC) deposits and lahars. The 1963-1965 eruptions had considerable effects on the population, crops, and pastures, as well as on civil works (collapse of roofs and bridges). The most considerable economic losses occurred in the Greater Metropolitan Area (GAM), mainly due to ash and lahars.
We have reconstructed the tephro-chronostratigraphy of the last 2.6 ka of the Irazu volcano, these data allowed us to estimate that this volcano has had an eruptive frequency that ranges between 23 and 100 years, with a major event every 85 years (VEI between 1 and 3). Furthermore, this eruptive reconstruction was the basis for carrying out a long-term hazard assessment, which has been conducted by us in two steps a) a spatial analysis (susceptibility) and b) a temporal analysis.
Regarding susceptibility, we determined that the highest values correspond to the Main Crater area, from where all historical eruptions have been vented. On the southern flank we identified an area with medium susceptibility values due to the presence of fissures and scoria cones that were active during the Holocene, therefore, we cannot rule out a possible future eruptive foci on this flank.
In the case of the temporal analysis, we could expect for a five-year window a magmatic unrest that results in a magmatic eruption in the Main Crater with VEI ≥1 (that is, VEI between 1 and 3), with pyroclastic falls, short PDCs, ballistics and lahars (mainly on the south and southwest flanks), similar to the eruptions of 1723-1724, 1939-1940, or 1963-1965. A geothermal unrest (the least likely) culminating in a phreatic explosion is also conceivable. In the case of a three-years forecasting window, we obtained a significant probability of an eruption of VEI=3 with ashfall,
lahars, PDCd and ballistics, all of an extension similar to the eruptive episode of
1963- 1965.
The other probable scenarios (VEI between 1 and 3) could range between ashfall and lahars of medium to large extent, in lower ballistic grades and low magnitude earthquakes (between 3.1-5.0 Mw). It is important to highlight that, for all possible scenarios, the ashfall would mainly affect the southwest flank, while in the case of lahars it would affect the south and southwest flank, and for the PDCd and ballistics, they would be restricted to the Main crater area.
The inputs generated with the spatial and temporal analysis were the basis for determining that the 1963-1965 eruptive scenario (VEI=3) is the one that represents the greatest hazard, so we reconstructed and simulated all the hazards that were registered in that episode, which allowed us to define three hazard areas: high, medium, and low. This information was the basis for carrying out an analysis of population, land use and critical infrastructure exposure.
Among the most relevant results we have that currently more than 2.5 million people live in the GAM, mainly at urban areas in the cities of San Jose and Alajuela, where the development of the industrial sector is concentrated. On the other hand, in the surroundings of Irazu , rural communities base their economy on agricultural activities. Likewise, there are more than 150 informal settlements exposed to the lahar hazard. Regarding educational and health centers, the majority are located in sectors of the GAM exposed mainly to the impact of ashfall.
Finally, we provide an essential contribution to territorial planning, which must consider hazards of volcanic origin within the planning processes for updating and improving emergency plans to face future volcanic crises.
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CAMPOS DURÁN, Daniela. Volcanic risk assessment at Irazú volcano, Costa Rica. [consulta: 3 de desembre de 2025]. [Disponible a: https://hdl.handle.net/2445/204560]