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Institute of Geophysics AS CR, v. v. i.
Deep structure, seismotectonics and tectonic evolution at convergent plate margins
Staff involved in this research: Marie Běhounková, Václav Hanuš, Aleš Špičák, Jiří Vaněk
Subduction zones at convergent plate margins generate the world’s largest and most destructive earthquakes, most of the world’s tsunamis, and most of the world’s explosive volcanoes. They are also the sites where much of the world’s population is concentrated and, over geologic time, where the most of the earth’s continental crust and mineral resources have been generated. Despite the societal and economic importance of convergent margins, many aspects of earthquake generation process at convergent plate margins remain poorly understood. Progress is hindered by the sheer scope of the problems, by the space and time scales and by complexities of the process. At present, many national and international programs and projects are devoted to the investigation of geodynamics of convergent plate margins, e.g. the German SFB 574 project, U.S. NSF-MARGINS program, NOAA projects, IODP projects, projects of Japanese geoscientists at IFREE, JAMSTEC.
The plate tectonics group of the Institute of Geophysics, Prague has devoted many years of research namely to the analysis of uneven distribution of earthquake foci at convergent plate margins to contribute to the overall knowledge of the causes of some of the above-mentioned phenomena – disastrous earthquakes (e.g. Vaněk et al., 2000; Špičák et al., 2007b), seismo-volcanic interactions (e.g. Špičák et al., 2005a, b, 2008), accumulation of metals (e.g. Hanuš et al., 2000). We consider the global seismological data as an optimum material for our work; they are freely accessible, they cover all convergent plate margins with comparable homogeneity, and they cover time interval of more than 40 years, revealing thus the decisive trends in seismotectonics of individual convergent plate margins. We confront/relate the existing seismological evidence with available knowledge of particular volcanic activity and regional tectonics and geodynamics. Our work benefits from a data set of precise teleseismic earthquake hypocentre determinations denoted as EHB (after the relocation procedure by Engdahl, van der Hilst and Buland, BSSA, 1998 – available on request to the first author at engdahl[at]colorado.edu), comprising relocated ISC (International Seismological Centre) data filtered of weak and mislocated events, covering the period 1964-2005. The EHB data set has not yet been systematically applied for the interpretation of hypocentral clusters of deep earthquakes, although it supports almost all recent tomographic studies in respective regions. It is convenient to supplement the set of EHB hypocentral determinations with focal mechanism determinations by Harvard Seismological Center (HCMTS), covering the period 1976-recent. Data on volcanoes are taken from the Catalogue of the Active Volcanoes of the World (Neumann van Padang, 1951), Volcanoes of the World (Simkin et al., 1981), and Volcanic Activity Reports (Venzke et al., 2002).
Our analysis of spatial distribution of earthquake foci is usually performed by suitably oriented, relatively dense system of swaths of about 50 km width. Hypocentres of events, the epicentres of which belong to a respective swath, are depicted in a vertical section. A set of parallel neighbouring vertical sections gives us a suitable 3-D view on earthquake distribution with much higher resolution than what can be achieved by computer 3D graphics at computer screen.
We focus our attention namely on four subduction zones – SE Asia, Andean South America, Central America and Izu-Bonin-Mariana arc. The choice of the investigated areas and topics reflects recent interests of international geoscience community expressed in running research projects and held workshops.
Our experience in interpretation of global seismological data can be documented by our recent achievements. We revealed the existence of an intermediate-depth aseismic gap in the Wadati-Benioff zone in the subducting slab and seismically active columns under some volcanoes, explained the interrelation of these two phenomena and related them to the process of generation of the primary magma of calc-alkaline volcanoes (Hanuš et al., 1996, Špičák et al., 2002, 2004, 2005 a, b). We revealed the deepest ever recorded earthquake swarm beneath a chain of seamounts in the Izu-Bonin island arc, related its origin with transport of magma to the sea floor and we pointed to potential environmental risk of poorly monitored submarine volcanoes and seamounts (Špičák et al., 2008). We denoted the seismically active fracture zones in the continental wedge above the subducting slab as responsible for large accumulations of metals in hypogene ore deposits of Peru, Chile and Argentina (Hanuš et al., 2000). We attributed the disastrous 1999 Puebla, Mexico earthquake to a fracture zone in the continental wedge above the subducting slab and pointed to potential danger of similar seismically active structures above subducting slabs all over the world (Vaněk et al., 2000). We noticed anomalous seismic activity ocean-wards off the Wadati-Benioff zone in the Sunda arc, its specific interrelation to the Wadati-Benioff zone, and we attributed this observation to the onset of a new subduction cycle (Špičák et al., 2007a). We also contributed to the knowledge of the internal tectonic structure of the shallow part of the subducting slab of the Cocos plate beneath Middle America by analysis of aftershock sequences of several strong earthquakes (Špičák et al., 2007b).
All the above referenced results of our group were achieved by the analysis of global seismological data and thanks to their irreplaceable features – namely the length of the time interval covered by the data (see below for details) and homogeneous coverage of all convergent margins for magnitude > 4 events.
Hanuš, V., Vaněk, J. and Špičák, A. (1996): Sumatran segment of the Indonesian subduction zone: morphology of the Wadati-Benioff zone and seismotectonic pattern of the continental wedge. J. Southeast Asian Earth Sci., 13, 39-60.
Hanuš, V., Slancová, A., Špičák, A. and Vaněk, J. (1999): Discontinuous nature of the lower part of the South American Wadati-Benioff zone in the Arica Elbow region. Studia Geoph. Geod., 43, 163-184.
Hanuš, V., Vaněk, J. and Špičák, A. (2000): Seismically active fracture zones and distribution of large accumulations of metals in the central part of the Andean South America. Mineralium Deposita, 35, 2-20.
Špičák, A., Hanuš, V. and Vaněk, J. (2002): Seismic activity around and under Krakatau volcano, Sunda Arc: Constraints to the source region of island arc volcanics. Studia Geoph. Geod., 46, 545-565.
Špičák, A., Hanuš, V. and Vaněk, J. (2004): Seismicity pattern: an indicator of source region of volcanism at convergent plate margins. Phys. Earth Planet. Int., 141, 303-326.
Špičák, A., Hanuš, V. and Vaněk, J. (2005a): Source region of volcanism and seismicity pattern below Central American volcanoes. N. Jb. Geol. Paläont. Abh., 236 (1/2), 149-172.
Špičák, A., Hanuš, V. and Vaněk, J. (2005b): Seismotectonic pattern and the source region of volcanism in the central part of Sunda Arc. J. Asian Earth Sci., 25, 583-600.
Špičák, A., Hanuš, V. and Vaněk, J. (2007a): Earthquake occurrence along the Java trench in front of the onset of the Wadati-Benioff zone: beginning of a new subduction cycle? Tectonics, 26, TC1005, doi: 10.1029/2005TC001867.
Špičák, A., Hanuš, V., Vaněk, J. and Běhounková, M. (2007b): Internal tectonic structure of the Central American Wadati-Benioff zone based on analysis of aftershock sequences. J. Geophys. Res., 112, B09304, doi: 10.1029/2006JB004318.
Špičák, A., Vaněk, J. and Hanuš, V. (2008): Volcanic plumbing system and seismically active column in the volcanic arc of the Izu-Bonin subduction zone. Submitted to Geochem. Geoph. Geosyst.
Vaněk, J., Špičák, A. and Hanuš, V. (2000): Position of the disastrous 1999 Puebla earthquake in the seismotectonic pattern of Mexico. Bull. Seismol. Soc. Amer., 90, 786-789.