Physics
Scientific paper
Nov 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007e%26psl.263..235n&link_type=abstract
Earth and Planetary Science Letters, Volume 263, Issue 3-4, p. 235-245.
Physics
5
Scientific paper
Precise locations of aftershocks following a large subduction earthquake offers insights into the nature of the seismogenic zone interface and stress redistribution within the subducting plate and its overburden. In 1995 a large magnitude Mw = 8.0 earthquake in Antofagasta, northern Chile marked the beginning of an aftershock sequence recorded for 2 months on a dense temporary network surrounding the rupture region. In this study we use cross-correlation and a double-difference relocation technique to obtain high-precision locations for this aftershock dataset. Relocation of the aftershock sequence reveals topography of the subducting Nazca plate and indicates that the slab is deeper to the south. The majority of the seismicity is located in a thin (˜ 1 1.5 km) region on the seismogenic interface. We also find seismicity located just above the seismogenic interface in the overlying continental crust. Particularly a streak of seismicity that extends ˜ 8 km vertically from the seismogenic interface into the overlying continental crust and is in close proximity to a region of large VP/VS ratio interpreted as the migration of trapped subduction fluid. Analysis of the seismicity streak yields a strong correlation between time and focal depth for events within this streak and indicates a fluid flow of ˜ 105 m/day giving a continental crust permeability of 10- 14 10- 15 m2. Focal mechanism data shows predominantly dip slip motion for these events, in contrast to thrust motion along the seismogenic interface, accommodating stress redistribution in the overriding plate. One could interpret this streak as the migration of fluid along a previous weak fault zone. Alternatively, the seismicity streak is located close to the bottom of the 1995 rupture and therefore one would expect large stress unloading in this area. The normal faulting in the overlying continental plate could be secondary faulting an effect of stress reorientation after the main thrust event. This relaxation of stress could have re-activated previous weak fault zones not only in the overlying continental crust but also in the subducting slab.
Nippress Stuart E. J.
Rietbrock Andreas
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