HR: 1340h
AN: S53A-0180    [Abstracts]
TI: Investigating active faulting in the south-central Chilean forearc by local seismicity and moment tensor inversions
AU: * Rietbrock, A
EM: A.Rietbrock@liverpool.ac.uk
AF: Universtiy of Liverpool, Department of Earth and Ocean Sciences, 4 Brownlow Street, Liverpool, L69 3GP United Kingdom
AU: Bohm, M
EM: bohm@gfz-potsdam.de
AF: GFZ-Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Echtler, H
EM: helle@gfz-potsdam.de
AF: GFZ-Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Melnick, D
EM: melnick@gfz-potsdam.de
AF: GFZ-Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Bruhn, C
EM: bruhn@geo.uni-potsdam.de
AF: University of Potsdam, Institute of Geosciences, Postfach 60 15 53, Potsdam, 14415 Germany
AU: Bataille, K
EM: bataille@udec.cl
AF: Universidad de Concepcion, Dept. Ciencias de la Tierra, Casilla 160-C, Concepcion, CL Chile
AB: The seismological ISSA experiment is giving a detailed insight into the seismicity distribution of southern Chile, where major earthquakes (M>8) have repeatedly ruptured the surface, involving vertical offsets of several meters. During a nearly 5-month observation period in 1999 and 2000 a temporary seismic network recorded approximately 350 local earthquakes. Two localized areas, North and South of the Arauco peninsula, showed a very high seismic activity in and above the interplate seismic zone of the Nazca-South America convergent margin. We used a double-difference relocation technique to obtain detailed images of the seismicity distribution in these areas. We also determined fault plane solutions to interpret the observed alignment of earthquakes hypocenters. Due to the low signal to noise ratio reliable first motion reading were difficult to achieve, which only very few clear readings. To overcome this problem we used moment tensor inversions to estimate reliable source mechanisms. However, for small magnitude earthquakes ($<$5) the biggest obstacle is the alignment of synthetic and observed waveforms. Inverting only for the amplitude spectrum, and therefore dropping the information in the phase spectrum can mostly circumvent the alignment problem. The two clusters investigated show high waveform correlation coefficients for most of the earthquakes indicating that possibly changes in fluid pressure can be responsible for triggering the events. After relocation most of the hypocenters in each of the two clusters align on a eastward dipping fault. Source mechanisms obtained indicate thrust faulting, where one of the possible fault planes aligns with the steep eastward dipping fault based on the seismicity distribution. These faults are reaching down to the top of the seismogenic zone and may serve as pathways for ascending fluids released in the subduction process. Active crustal-scale faulting below and active uplift of the coast account for active tectonic segmentation and are discussed in relation with active basal accretion and active shortening in the South-Central Chilean forearc.
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
DE: 8164 Stresses--crust and lithosphere
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting