TRIGGERED SEISMICITY IN THE ANDEAN ARC REGION VIA STATIC STRESS VARIATION BY THE MW=8.8, FEBRUARY 27, 2010, MAULE EARTHQUAKE

SPAGNOTTO, SILVANA; TRIEP, ENRIQUE; GIAMBIAGI, LAURA; LUPARI, MARIANELA

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2015 vol. 63 p. 36 - 47

0895-9811

In a narrow longitudinal Andean arc area, between 35°S and 36°S, crustal seismicity increased by more than nine times in the three-month period after the great Mw 8.8 Maule earthquake when compared to a similar period before the event. Here we explore how changes in Coulomb conditions associated with the mega-thrust earthquake triggered subsequent upper-plate events in the region.The localized seismicity defines an area of ~80 km long and ~18 km wide and NNW to NNE trend. Md magnitudes varied from 0.7 to 3.1 except for two earthquakes with Mw of 3.9 and 4.5, localized in the northern end of the area. We calculated the focal mechanisms for these two events as normal/strike-slip and strike-slip respectively.During 2011, a 13 temporary station network was installed in the trasarc region in Malargüe, Argentina. 60 earthquakes were localized in the study region during an 8 month period. Static stress change calculations were performed using Coulomb criterion, assuming major previously proposed structures as receiver faults. We used previously published earthquake source parameters and slip distribution for the Maule quake. The largest contribution to total static stress, up to 5 bars, comes from the normal component, suggesting that the unclamping was the main process responsible for seismicity activation. Our results are totally consistent with co-seismic dilatational deformation inferred from GPS observations in the region and subsidence in nearby volcanoes caused by magma migration.Three different Quaternary tectonic settings ?extensional, strike-slip and compressional- have been proposed for the arc region at these latitudes. We found that the strike-slip regime is the most favorable, and only due to the unclamping produced by the Maule quake could the regime have temporarily changed to a normal/strike-slip regime. The activation of Quaternary NNE to N-trending dextral strike-slip faults with dextral transtensional movement is inferred to be caused by the unclamping phenomenon.