Intermediate deep earthquakes 5.8 Mw 6.4 from flat to normal sections in Chile-Argentina Nazca plate

SPAGNOTTO, S. L.; TRIEP, E.G.; NACIF, S. V.; FURLANI, R.

Foz do Iguassu, Brazil

Congreso; Meeting of the Americas; 2010

American Geophysical Union (AGU)

We report teleseismic waveform modeling focal mechanisms and slip distributions for seven earthquakes deeper than 100km between latitudes 31°--32°S and between 33°-34°S. We relate locations, nodal planes and type of mechanisms to previously obtained and improved plate morphology between latitudes 31°-34°S.  The northern flat slab section resume the normal dip subduction to the east at about ~67.5°W, to the southeast between ~32°-33.5°S and ~67.5°-70°W, and to the east-southeast south of ~33.5°S. North of ~31°S the slab increase its dip to the north, but more gently. All mechanisms are normal, four are pure dip-slip, one with important strike-slip component, and two of them almost pure strike-slip. Generally, for each earthquake at least the strike of one nodal plane closely correlates with the corresponding constant depth contours lines of the slab. Two pure normal-dip earthquakes are not too far of the flat slab western end and have very similar mechanisms: one nodal plane dipping 35° southeast and the other 56° northeast. It is difficult to choose one of them as the fault plane, because the combination of the slab negative buoyancy, the deformation suffered nearby where the slab had been not horizontal and the possible inheritance of faults from the outer ridge. The earthquake in the normal section of the slab near ~34°S, just below active volcanoes, has almost pure dip-slip mechanism with a plane in the down-dip direction. The three earthquakes with strike slip component are associated to strong contortions of the plate: one near the eastern tip of the flat slab, one down-dip to the east of this one, and one in the transition zone of the slab dip change to normal. Two additional earthquakes mechanisms, Md ~4.5 and ~3.9, obtained by local P-wave first motion at depths of ~217 km and ~199km are located within the strongest  slab bend at the center of the transition zone. They have strike-slip mechanisms with thrust component, the P axis are consistent to the bending. The normal dip mechanism with a little strike-slip at ~33.6°S, ~68.9°W, and 160km depth, near the boundary between the transition and normal zones, behaves differently to all the others. The strikes of the nodal planes are near perpendicular to the contour lines. If we choose the nodal plane dipping to the southeast this could be evidence of some breaking between the two zones. There is a similar CMT mechanism of another earthquake up-dip towards northeast which could reinforce that interpretation. Also this could agree with the proposed geological processes that suggest slab steepening south of ~33°-33.5°S. We conclude that slab pull and/or slab lateral distortion determine the occurrence of the analyzed earthquakes except probably those in the western end of the flat slab. We pick up the depth of the earthquakes from the corresponding minimum rms value of the inversion, tried different rupture velocities, Vr, and initial source time functions. Vr varies from 1.9 to 3.6 km/sec. We constrained the slip distribution by the focal mechanism and rupture velocity obtained in the inversion, and then choosing one of the nodal planes. The recovered source time function shows time duration ~13sec for all the events considered as only one shock. Some of these functions show one foreshock and/or one aftershock. When the function has two picks we tried again the inversion with two shocks. In these cases the time duration is ~20-24sec. We present three of these double events. We looked for the ruptured area length along strike, L, and along the down-dip, W. Four earthquakes have L>W, two L~W, and one L<W. In the flat slab the two western earthquakes have W~18km and the one to the east of it W~10km. So, if the thickened oceanic crust could reach these values then the rupture should be inside of it. The earthquake in the normal section has W~16km. Here the crust probably has ~6-7km. The other four earthquakes, three of them in the down-slop, have W from 25km to 29km. Thus, for these earthquakes the rupture goes well into the mantle of the subducting lithosphere.