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URI permanente para esta colecciónhttp://10.0.96.45:4000/handle/11056/20585

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Precursory seismic signals before two catastrophic landslides at Irazú Volcano, Costa Rica
(Latin American and Caribbean Seismological Comission, 2024) Chaves Sibaja, Esteban; Pacheco, Javier; Schwartz, Susan; Chavarría, Nathalie; Noah, Finnegan; Higman, Bretwood
Massive rock landslides impose a continuous impact on the socio-economic growth of developing cities, public infrastructure and telecommunications. Every year, millions of dollars are invested in reverting the negative effects associated with their occurrence all over the world. At volcanoes, complex landslide instabilities triggered in some cases by nearby earthquakes, rainfall, local deformation, or a combination of all, may induce changes in the lithostatic pressure of the edifice, suddenly affecting their internal dynamics and increasing the risk of catastrophic eruptions as occurred during the 80’s in Mt. San Helens. Near field geodynamic monitoring, including broadband seismic, GNSS stations and other techniques, is then essential to better characterize ground failure and to improve landslide hazards assessments. The southwest flank of the Irazú volcano in central Costa Rica has been recognized to host massive rock landslides in the past. Just in December 2014 and August 2020, a total combined of 53 million m3 of mass wasting were deposited along the basement of “Río Sucio”, one of the main tributaries of the Sarapiquí river. Using the near field and geodynamic monitoring network that OVSICORI-UNA operates at Irazú and Turrialba volcanos since 2011, we show unique seismological observations that shed light about the rupture initiation and dynamic evolution of the mass movement. Our results demonstrate that during the 2014 and 2020 events, the nucleation phase initiate weeks prior to the catastrophic collapse with the generation of low frequency earthquakes (LFEs), events dominated by velocity weakening frictional properties that likely represent the stick-slip failure of small asperities localized along the basement. Among the observed LFEs, we report the occurrence of 10 repeating earthquake families, each formed by multiple LFEs that rupture approximately the same asperity at different times, generating identical waveforms. Our observations show that as the mass accelerate with time 1) the number of repeating families increases progressively and 2) the inter-event time between LFEs decreases linearly until they merge forming a tremor signal that initiate 30 min prior to the collapse. Near-field seismic data exhibits an exponential increase in tremor amplitude, and thus, seismic moment, that suddenly reduces and become quiescent for 20 seconds before the impulsive mass detachment and failure. We posit that transient embrittlement is the mechanism responsible for such a unique observation. As the slip rate increases, faulting regions with predominantly stable-sliding (aseismic) frictional properties become unstable (seismic), as previously observed in subduction zones and laboratory experiments. As a result, the number of unstable asperities and/or the total effective area of contact between the sliding mass and the basement increases dramatically, modulating tremor amplitude with time. The shear strength from the elastically couple asperities is enough to provide temporal (~20 s) stability to the entire mass, inducing the seismic quiescence. However, the accrued shear stresses imposed by the slow slip episode on the weak asperities overcomes the frictional strength, inducing the catastrophic failure. Our results provide direct evidence of the mechanics that controlled landslide nucleation and resemble those from laboratory experiments and fault zones during tectonic earthquakes.

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Examinando Contribución a Congreso por Autor "Chaves Sibaja, Esteban"