Sofía Rebolledo

New Findings on the 1958 Las Melosas Earthquake Sequence, Central Chile: Implications for Seismic Hazard Related to Shallow Crustal Earthquakes in Subduction Zones

On the 4th of September 1958, a sequence of 3 earthquakes of magnitude 6.7–6.9 struck the Andean Main Cordillera at the latitude of Santiago, Central Chile. The quakes were preceded by a magnitude 6.0 foreshock one week earlier. This seismic sequence provided the only documented effects of strong shaking related to shallow earthquakes in a subduction-zone environment in which seismicity is dominated by interplate and intermediate-depth intraplate earthquakes. The 1958 earthquake sequence is reviewed as part of a project of seismic hazard assessment of the densely populated region of Santiago. We reinterpret historical documents and carried out field observations to obtain new intensity estimates, and we estimate ranges of peak acceleration values based on geotechnical back-analyses of earthquake-induced landslides. Estimated peak intensities of 9 and peak accelerations close to 1 g illustrate the significant seismic hazard in areas around active faults in the region and the need to adapt the building codes to these rare but potentially highly destructive types of earthquakes.

Evolución geológico-geomorfológica cuaternaria del tramo superior del valle del río Laja

La historia geologica del Cuaternario en el tramo superior del valle del rio Laja corresponde a una compleja interrelacion entre volcanismo, procesos de remocion en masa y sedimentarios aluvio-fluviales. El valle fue modelado inicialmente por la accion glaciaria del Pleistoceno inferior en rocas de las formaciones terciarias Cura-Mallin y Trapa-Trapa, ademas de rocas plutonicas, sobre las cuales se disponen en discordancia cuatro grandes unidades cuaternarias; el cono poligenico de Quilleco, una secuencia de rocas volcanicas pleistocenas, el volcan Antuco y el deposito de avalancha volcanica de Antuco. Los depositos del cono poligenico de Quilleco representan a facies mixtas volcano-sedimentarias intermedias y distales de los estratovolcanes que originaron las secuencia de rocas volcanicas pleistocenas con las cuales engranan lateralmente. El volcan Antuco corresponde a un estratovolcan mixto y compuesto, de composicion basaltica y andesitico-basaltica, cuya actividad se inicio ca. 130.000 a A.P. La primera etapa de su desarrollo (Antuco 1) culmino 9.700p 600 a A.P. con el colapso gravitacional lateral del edificio, que origino la gran avalancha volcanica de Antuco, cuyos materiales represaron el desague natural del lago del Laja y de sus quebradas afluentes. El colapso gravitacional fue el resultado de una actividad eruptiva freatomagmatica de tipo Bandai-San la que produjo, casi simultaneamente, flujos piroclasticos turbulentos, de tipo de oleadas de base humeda, compuestos de cenizas basalticas negras cuyos depositos primarios se han denominado Arenas Negras de Trupan-Laja. El volcan actual (Antuco 2) incluye un cono principal de lavas y escorias y la emision de, al menos, tres flujos piroclasticos importantes de poco espesor, localmente separados por depositos de corrientes de barro y coluvios. Posteriormente, debido a la ruptura del represamiento del lago del Laja, las cenizas negras fueron removidas hasta la Depresion Central, donde formaron un gran abanico aluvial de aproximadamente 50 x 60 km2.

Catastrophic, rainfall-induced debris flows in Andean villages of Tarapacá, Atacama Desert, northern Chile

In March 2012, during the rainy season in the Altiplano plateau, a >100-year return period rainfall event affected the deeply incised valleys of the Precordillera of the Tarapacá Region, northern Chile. This extreme event in a very arid region triggered a number of debris and mud flows that caused severe damage and destruction in several small villages along the Camiña and Tarapacá valleys. The highly vulnerable location of the villages on top of alluvial fans due to socioeconomic and cultural reasons is a key factor to explain the level of destruction in most villages. In this paper, this unusual, remarkable landslide event is described, and the hazard faced by these settlements for future rainfall episodes and possible mitigation measures are discussed.

Geohazard Studies for Urban Planning in the Santiago Metropolitan Region, Chile: Some Lessons for Future Interactions Between Engineering Geoscientists and Urban Planners in Developing Countries

A series of novel seismic and landslide hazard studies funded by the Chilean Housing and City Planning Ministry were carried out in the Santiago Metropolitan Region in the aftermath of the 2010 Chile earthquake by university geoscientists and urban planning consultants. The studies aimed to suggest modifications of urban planning regulating plans. Interaction of geoscientists with urban planners, architects and other professionals proved to be challenging in order to get successful results. Issues such as work scale, terminology, lack of experience from policy makers in this kind of studies and shortcomings of the territorial planning instruments in enforcing restrictions in risk areas were identified. In this paper we summarize the conducted geohazard studies and discuss the main experiences arising from interaction between geoscientists and urban planners.