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

Abstract

On the 4 th 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 We reinterpret historical documents and carried out field observations to obtain new intensity estimates, and we estimate ranges of peak acceleration values based on geotechnical back-analyses of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.

Large rockslides in the Southern Central Andes of Chile (32–34.5°S): Tectonic control and significance for Quaternary landscape evolution

“…c) Detail of the south-eastern area. LM: Las Melosas, site of 1958 M 6.9 shallow earthquake (e.g., Sepúlveda et al, 2008), PR: Parraguirre River headwaters, site of the 1987 rockfall and subsequent debris flow through Colorado and Maipo River valleys (e.g., Hauser, 2002). In b) and c), rockslides are marked by squares.…”

“…We recognize two caveats in our interpretation: 1) The present-day distribution of seismicity is taken as a proxy for Quaternary seismicity because paleoseismic analyses are not available, and 2) The paucity of rockslides during historical strong shallow earthquakes in the area (e.g. 1958 M = 6.9 Las Melosas event, Sepúlveda et al, 2008) is contrary to empirical model predictions of landslides (e.g. Keefer, 1994) for earthquakes of similar magnitude.…”

A Neogene giant landslide in Tarapacá, northern Chile: A signal of instability of the westernmost Altiplano and palaeoseismicity effects

“…Although high earthquake intensities (up to around IX) and accelerations close to 1.0 g may be reached in epicentral areas with the different earthquake types that occur in the region Astroza et al, 2005;Sepúlveda et al, 2008), the presence of a flexure immediately next to the landslide suggests that this fault is the most likely seismic source. The area has records of recent shallow earthquakes, such as the 2001 Aroma-Chusmiza earthquake, triggered within the Aroma Flexure about 50 km south of the study area (Farías et al, 2005;Legrand et al, 2007).…”

Hazard investigation of the Portillo Rock Avalanche site, central Andes, Chile, using an integrated field mapping and numerical modelling approach

“…Beneath central Chile and western Argentina (28°S-33°S), where the study area is located, the subducted plate is sub-horizontal, with a dip angle of less than 10°that extends east for hundreds of kilometres at a depth of approximately 100 km before resuming its downwards trend (Cahill and Isacks, 1992). Along this segment, most of the earthquakes are compressional with moderate magnitude and depth focus of ∼ 80 km Barrientos et al, 2004), although very shallow (0-20 km) damaging earthquakes (5.9 ≤ M ≤ 6.9) have also been recorded in the area (Lomnitz, 1961;Barrientos et al, 2004;Sepulveda et al, 2008). It is believed that these shallow events are produced by the deformation of the overriding continental plate in response to the differential coupling that exists in the region due to along-strike variations in the dip angle of the Nazca Plate.…”