Gravity survey in the IPOC-Array, Iquique, Northern Chile (IPOC-Grav)
DFG-Proposal: GO 380/29-1
Proponents: H.-J. Götze und M. Sobiesiak
Processing: H.-J. Götze, A. Boockmeyer, and the CAU Gravity Group
MIGRA-2011 Field Crew:
Anke Boockmeyer, Hans-Jürgen Götze, Oscar Lücke and Rezene Mahatsente
Motivation
Over the last years specific correlations between residual gravity anomalies and seismological data gained insight into the behaviour of dynamic processes at
convergent plate margins (e.g. Oncken et al., 2006; Alasonati Tašárová, 2007; Tassara, 2010). It was shown that the combination/assimilation of spatially distributed gravity anomalies with areas of seismological moment release bears information on material and structural behaviour of the causing seismological structures. They can affect also the propagation of earthquake rupture zones which is strongly related to the flexural rigidity of the lithosphere. If it is possible to tie these structures with geological and tectonical observations we can hope to make conclusions toward a temporal consistency of these structures, even over several seismic cycles. The detection of such structures and the explanation of their physical relevance for the rupture process provide additional information to investigate the seismic risk - particularly in locally limited areas.
Over the last years several authors described spatial relationships between trench-parallel gravity anomalies (TPGA) and the distribution of maximum co-seismic displacement and/or the maximal reduction of seismic moment (asperities) at the earthquake rupture zone (Wells et al., 2003, Song and Simons, 2003, Fuller et al., 2006, Raeesi and Atakan, 2009). Here, mostly Bouguer-anomalies were analyzed, which image basin structures in the forearc area. Furthermore, they correlate with domains of maximum co-seismic moments. Wells et al. (2003) assume, that these basin structures are proxies for long-term reduction of the seismic moment. Song und Simons (2003) correlate positive trench-parallel topographic anomalies with distributions of maximum co-seismic displacements. These positive correlations are typical for many subduction zones around the Pacific. However, such areas own certain geometrical properties of the plate boundary where the building of sedimentary basins at deep sea terraces is possible. New results show that density anomalies in the upper plate can control the seismological behaviour in subduction zones (e.g. Tassara, 2010).
Abb: New gravity stations in the area of the IPOC network. New gravity stations were measured mainly north and east of Iquique and in the area of Pozo Almonte.
Field campaign
For integrated modelling and the identification and first-order asperities and density heterogeneities at the plate interface we observed approx. 500 new gravity measurements in Northern Chile. These field measurements were conducted in the time between January 17 and February 20, 2011 in the operating area of the "Integrated Plate Boundary Observatory Chile" - IPOC/Iquique-Network *). In the recent campaign gravity observations were performed along all accessible roads and tracks embedding the seismological IPOC stations into the gravity net. For the first time we used our Burris microGal gravity meter (B42) in the field in order to obtain more precise gravity stations in the central part of the area of interest. The other part of the measurements were carried out with a LaCoste & Romberg gravity meter, type G. The new stations were integrated in SFB-267 and the Chilean gravity network (e.g. Götze and Schmidt, 2006). Next steps are a post processing of new data and the calculations of Bouguer-, Free air and isostatic residual fields/anomalies and their interpretation in a constrained 3-dimensional density model after eliminating the extreme strong regional trend in the Bouguer gravity field. Constraints for modelling purposes stem from the formerly results of the "Special research area, Deformation processes in the Andes, SFB 267" and the recent data acquisition of the IPOC-group and their partners. Correlations of domains with high positive seismic b-values and maximum co-seismic moment with positive isostatic residual anomalies yield information on the potential identification of asperities at the subduction interface in the gravity anomalies. Corresponding correlations in the area of the Chilean port Antofagasta south of the recent IPOC- area were obtained by Sobiesiak et al. (2007); their results should be verified in the Iquique segment further to the north.
The field campaign was supported by our colleagues at the Arturo Prat University, Iquique, Department of Physics. We are indebted to Prof. Manuel Olcay, our local partner, of the recent campaign for his help. This project is part of the IPOC activities and is financed by the Deutsche Forschungsgemeinschaft, Bonn
*) http://www.gfz-potsdam.de/portal/gfz/Struktur/Departments/Department+3/sec31/Projekte/Field+studies/IPOC;jsessionid=9EE8F275DDB7D8EBF006D624A324AC65
References:
Alasonati Tašárová Z, 2007. Towards understanding the lithospheric structure of the southern Chilean subduction zone (36̊S-42̊S) and its role in the gravity field. Geophys. J. Int,. 170 (3), pp.995–1014, doi:10.1111/j.1365-246X.2007.03466.x.
Fuller, C.W., S.D. Willett, M.T. Brandon, (2006), Formation of forearc basins and their influence on subduction zone earthquakes, Geology, 34, 2, 65-68, doi:10.1130/G21828.l.
Oncken O., G. Chong, G. Franz, P. Giese, H.-J. Götze, V. Ramos, M. Strecker, P. Wigger, 2006. The Andes – Active Subduction Orogeny. Frontiers in Earth Science Series, Vol 1. Springer-Verlag, Berlin Heidelberg New York, 567pp
Raeesi, M., and Atakan, K. 2009. On the deformation cycle of a strongly coupled plate interface: the triple earthquakes of March 13, 1963, Nov.15,
2006 and Jan.13, 2007 along the Kurile subduction zone. Journal of Geophysical Research, 114, doi:10.1029/2008JB006184.
Schmidt, S. and H.-J. Götze, 2006. Bouguer and isostatic maps of the Central Andes. In Oncken, G. Chong, G. Franz, P. Giese, H.-J. Götze, V. Ramos, M. Strecker & P. Wigger (eds), Frontiers in Earth Sciences, 1, Springer Verlag, p. 559 - 565.
Sobiesiak, M. M., U. Meyer, S. Schmidt, H.-J. Götze, C. Krawczyk, 2007. Asperity generating upper crustal sources revealed by b-value and isostatic residual anomaly grids in the area of Antofagasta. - Journal of Geophysical Research, Vol. 112, B12308, doi: 10.1029/2006JB004796.
Song, T. A. and M. Simons 2003. Large Trench-Parallel Gravity Variations Predict Seismogenic Behavior in Subduction Zones: Science, 301, 630-633.
Tassara, A., (2010), Control of forearc density structure on megathrust shear strength along the Chilean subduction zone, Tectonophysics, doi:10.1016/j.tecto.2010.06.004.
Wells, R.E., R. Blakely, Y. Sugiyama and D. Scholl, P.A. Dinterman, 2003, Basin-centered asperities in great subduction zone earthquakes: A link between slip, subsidence, and subduction erosion?, J. Geophys. Res., 108, B10, 2507, doi:10.1029/2002JB002072.
