S33E-07:
Upper Plate Reverse Fault Reactivation and the March-April 2014 Pisagua Earthquake Sequence in Northern Chile
Wednesday, 17 December 2014: 3:10 PM
Gabriel Gonzalez1, Pablo Salazar1, John P Loveless2, Richard W Allmendinger3, Felipe Aron3 and Mahesh Narayan Shrivastava1, (1)Universidad Católica del Norte, National Research Center for Integrated Natural Disaster Management, Antofagasta, Chile, (2)Smith College, Northampton, MA, United States, (3)Cornell University, Ithaca, NY, United States
Abstract:
The March 16–April 3, 2014 Pisagua earthquake sequence ruptured the middle part of the southern Peru-northern Chile seismic gap in the curved segment of the Andean subduction zone. The sequence began on March 16, with an Mw 6.7 earthquake that struck near the central section of the gap. A few hours after this initial event, intense seismic activity began and persisted for two weeks, showing a progressive northward migration towards the position of the main Mw 8.1 shock of this sequence. Two days after the main shock, a second large Mw 7.7 earthquake ruptured at the southern part of the area.. We relocated 336 events of this sequence by using data from a local network (Integrated Plate boundary Observatory Chile, IPOC). From NonLinLoc, HypoDD and waveform analysis, we relocated the Mw 6.7 in the upper plate at 12-14 km depth. Coulomb stress analysis shows that this initial event was able to trigger seismicity on the megathrust leading finally to the Mw 8.1. The initial Mw 6.7 event and some foreshocks have an unusual focal mechanism in comparison to the strike of the subduction interface. The nodal planes are rotated anticlockwise with respect to the slab surface strike. The shortening axes of the upper plate events plunges ~30° towards 220°, highly oblique to P axes of the foreshocks and main shock on the plate interface. On-shore regional structures are dominated by trench orthogonal reverse faults indicating the prevalence of long-term trench parallel shortening in the area. Upper plate micro seismic events, captured before this sequence, have focal mechanisms with E-W striking nodal planes and trench parallel shortening. Our analysis supports the idea of a causative relationship between upper plate long-term structures and the reactivation of the subduction megathrust. The concave seaward geometry of the margin and the trench orthogonal striking reverse faults observed in northern Chile are also common in the Cascadia margin, therefore we propose that both type of margin are able to trigger great subduction earthquakes by reactivation of upper plate structures.
