2016 IASPEI-LACSC Abstract

Zahradník, J., J. Janský, H. Tavera, L. Barros, E. Sokos, and C. Bina, Seismic source model of a deep Mw 7.6 earthquake, Nov. 24, 2015, Brazil-Peru border, Abstracts of the 2016 Meeting of the Latin American and Caribbean Seismological Commission, International Association of Seismology and Physics of the Earth's Interior, Costa Rica, 95, O137, 2016.


Two Mw 7.6 earthquakes occurred within five minutes on November 24, 2015 at Brazil-Peru border; we studied the first. Using pP-P arrival times, hypocenter depth was first constrained to 600+/-10 km. Then, using P waves from 18 stations at distances ∼340-1560 km, epicenter was located at 10.59°S, 70.98°W. Same stations were used for full-waveform inversion. Centroid position was found at 10.65°S, 70.95°W, 600+/-5 km, shifted ∼20 km from GCMT solution. Strike, dip, and rake angles 2°/40°/-64° (Plane 1), 150°/55°/-110° (Plane 2), Mw 7.6, Mo=2.8x1020 Nm, and DC>95%, were similar to those of GCMT. Using iterative deconvolution at 0.02-0.10 Hz, the source appeared to be composed of three subevents, tightly clustered to each other (<15 km), with total duration of 15-20 s, featuring almost identical focal mechanisms and high DC%. When grid-searching in Planes 1 and 2 for two major subevents and their moment-rate time functions simultaneously (non-negative least-squares), the source appeared less compact, indicating rupture propagation from hypocenter at azimuth of ∼140°, either horizontally (in Plane 2), or slightly downward, with plunge -30° (in Plane 1). This direction is nearly perpendicular to slip vector. The same was confirmed by directivity effects derived from angular variation of P-wavegroup duration at 8-16 Hz, and by kinematic location of the stopping point of rupture. The latter resolved the rupture length (40+/-10 km), and provided rupture speed of 1.5-2.5 km/s, as low as 40% of shear-wave speed. The event is consistent with regional stress model calculated from 16 GCMT focal mechanisms of nearby deep events at Lat 5°S to 14°S: Sigma 1 and 3 axes with (azimuth, plunge) = (61°, 77°) and (256°, 13°), respectively, and low friction (0.1-0.2). These results may elucidate processes involving buoyancy forces and metastable mineral phases, upon further analysis with an appropriate thermo-mechanical model.

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