par- 24 janvier 2013 ( dernière mise à jour : 14 mars 2013 )
Ecuador, a territory of ∼600 km × 500 km, is a seismically active country, with destructive earthquakes occurring both along the subduction zone (e.g. Esmeraldas, 1906, Mw 8.8) and in the central part of the territory, in the shallow crust (e.g. Riobamba, 1797, Mw 7.6).
ISTerre is working with the Geophysical Institute in Quito, who is in charge of estimating the seismic hazard over the national territory.
Our work aims at evaluating the probabilistic seismic hazard (PSH) for the country. The estimation of PSH requires models of occurrence of earthquakes in time, space, and magnitude. They rely notably on earthquake catalogs covering the longest time period possible. In Ecuador, seismic history extends over five centuries in the Cordillera, and roughly over one century on the Coastal region.
Since 2007, we have been working on the building of a unified and homogeneous earthquake catalog. After a careful analysis of available international and local catalogs, we proposed a priority scheme for merging the instrumental catalogs. The historical earthquakes have been appended to the instrumental catalog, resulting in a five centuries-long catalog. Next,
homogenization of magnitudes and removal of aftershocks was performed. We find in the Cordillera region within −2.5° and 1° latitude that the average occurrence of an earthquake with Mw ≥ 6.0 is 10 to 20 years.
Over 30 historical events have previously been studied, for which epicentral locations and equivalent moment magnitudes have been determined. Large earthquakes (6 < Mw ≤ 7.6) seem to be related to strike slip faults between the North Andean Block and stable South America to the east, while moderate earthquakes (Mw ≤ 6) seem to be associated with to thrust faults located on the western internal slopes of the Interandean Valley. Below is presented the example of the Ibarra earthquake (1868), equivalent Mw 7.1-7.7, rupture around 70 km long (squares are intensity observations from degree 5 to 9, countours are corresponding to 50, 67 and 95% probability that the epicenter is inside this area).
A seismotectonic model has been established, proposing a delineation for crustal sourc zones, inslab zones, and subduction interface planes. Based on the unified and homogeneized earthquake catalog, on the historical large earthquakes, and on the active tectonics, recurrence curves have been developed for these source zones. This work has led to a first calculation of probabilistic seismic hazard for the country, at different return periods and for different ground motion frequencies (PSHA code used : Openquake). These hazard maps have served in 2011 as a reference for the National Committee in charge of establishing the new seismic building code for Ecuador (MIDUVI-CCQ, 2011).
We are currently updating the seismotectonic model and the recurrence models for earthquakes inside each source zone, and soon this study leading to updated PSHA estimates will be detailed in an article.