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MARGINS, June 18 – 22, 2007, Heredia, Costa Rica

INTERNAL TECTONIC STRUCTURE OF THE CENTRAL AMERICA WADATI -B ENIOFF ZONE BASED ON ANALYSIS OF AFTERSHOCK S EQUENCES Ale š Špičák, V áclav Hanuš, Jiří Vaněk Institute of Geophysics , Academy of Sciences of the Czech Republic , Prague. MARGINS, June 18 – 22, 2007, Heredia, Costa Rica. MOTIVATION

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MARGINS, June 18 – 22, 2007, Heredia, Costa Rica

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  1. INTERNAL TECTONIC STRUCTUREOF THE CENTRAL AMERICA WADATI-BENIOFF ZONEBASED ON ANALYSIS OF AFTERSHOCK SEQUENCESAleš Špičák, Václav Hanuš, Jiří VaněkInstitute of Geophysics,Academy of Sciences of the Czech Republic,Prague MARGINS, June 18 – 22, 2007, Heredia, Costa Rica

  2. MOTIVATION - to contribute to the understanding of the internal tectonic structure of subducting slab of the Cocos plate by defining fault zones that rupture during strong earthquakes

  3. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION • OUTLINE • region of interest • global seismological data – their benefits for understanding structure, recent dynamics and tectonic history of convergent plate margins • method of hypocentral data analysis • aftershock sequences – selection, data analysis, determination of geometrical parameters and tectonic regime • conclusions-tectonic interpretation of aftershock sequence parameters

  4. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION 16

  5. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION 94° 90° 85° [W] 82° Mexico Guatemala [N] [N] Honduras El Salvador Nicaragua Guatemala Basin Costa Rica smooth segment Panama Fischer Ridge Cocos Plate seamount segment Cocos Ridge 94° 90° 85° [W] 82°

  6. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION • GLOBAL SEISMOLOGICAL DATA • 44 years ( since 1964) of continuous observation and standard data processing (since 1976 accompanied by focal mechanism determination of stronger events) • homogeneous coverage of the whole Earth from mb~ 4.0 • precision of hypocentral determination is sufficient to contribute to understanding the structure and dynamics of convergent plate margins • enable comparative studies among individual convergent margins • with global seismological data, both SUBFAC and SEIZE problems • (and some others) can be addressed

  7. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION m < 4 4 < m < 4.5 4.5 < m < 5 5 < m < 6 m > 6 DATA 1 (EHB data) ISC hypocentral determinations 1964 – 2005 relocated by Engdahl et al. (1998) DATA 2 (HCMTS data) fault plane solutions of Harvard Centroid Moment Tensor Solutions(1977 – 2005)

  8. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION [km] Merapi volcano [km] Krakatau volcano depth [km] S N depth [km] SSW NNE Global seismological data reflect SUBFAC operation (SE Asia– Krakatau vers. Merapi volc.)

  9. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION 16 Global seismological data reflect SUBFAC operation (Costa Rica – volcán Irazú) ŠPIČÁK, A., et al. (2005), Source region of volcanism and seismicity pattern beneath Central American volcanoes. N. Jahrbuch fuer Geol. Palaeont, 236 (1/2), 149-172.

  10. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION 1 5 10 15 20 24

  11. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION • 10 aftershock sequences in the Wadati-Benioff zone • beneath Central America between 1964 - 2005: • aftershock sequence no. depth mW • of events range [km] • 1970 Chiapas 58 6-87 7.4 • 1983 Osa 27 6-96 7.4 • 1985 10 25-67 5.6 • 1992 Nicaragua 136 4-76 6.2 • 1993 48 16-73 7.2 • 1996 18 24-89 6.5 • 1998 34 4-96 6.5 • 1999 Quepos 10 9-46 6.9 • 2000 Nicoya 18 5-49 6.4 • 2001 El Salvador 61 37-88 7.7

  12. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION transformation of coordinates

  13. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION slab dip = 29° slab strike = 294°

  14. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION slabdip = 41° slabstrike = 303°

  15. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION • 10 aftershock sequences in the Wadati-Benioff zone • beneath Central America between 1964 - 2005: • aftershock sequence no. depth mW • of events range [km] • 1970 Chiapas 58 6-87 7.4 • 1983 Osa 27 6-96 7.4 • 1985 10 25-67 5.6 • 1992 Nicaragua 136 4-76 6.2 • 1993 48 16-73 7.2 • 1996 18 24-89 6.5 • 1998 34 4-96 6.5 • 1999 Quepos 10 9-46 6.9 • 2000 Nicoya 18 5-49 6.4 • 2001 El Salvador 61 37-88 7.7

  16. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION position of aftershock sequences along the plate margin (epicentral map)

  17. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION step 1: transformation of coordinates step 2: approximation of foci by a (fault) plane sq2000 (Nicoya earthquake) (b) projection to the slab surface (a) standard epicentral map (d) position of approximating planes (fault planes) in the slab (c) vertical section in the slab

  18. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION step 1: transformation of coordinates step 2: approximation of foci by a (fault) plane sq1983 (Osa earthquake) (b) projection to the slab surface (a) standard epicentral map (d) position of approximating planes (fault planes) in the slab (c) vertical section in the slab

  19. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

  20. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD 4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION • CONCLUSIONS – tectonic interpretation • -distribution of foci in aftershock sequences is quasiplanar; the geometry of the respective planes – fracture zones (FZ) - can be determined ! • -all mainshocks and almost all events with available CMT (95 %) are of normal character in the coordinate system anchored to the subducting slab • -a majority of fault planes offer nodal planes parallel to the trench • occurrence of events off the main hypocentral clusters („outliers“) point to seismogenic rupturing along two or more faults in fracture zones • geometrical parameters of fracture zones probably reflect the interaction of the subducting slab with the continental wedge of the Caribbean Plate in the hanging wall: • - areas with smooth sea floor morphology – slab-parallel FZ • - seamounts and ridges on the sea floor – steeply inclined FZ oriented quasiperpendicular to the trench • Špičák, A., Hanuš, V., and Vaněk J. (2007), J. Geophys. Res., in press.

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