INTERNAL TECTONIC STRUCTURE OF THE CENTRAL AMERICA WADATI-BENIOFF ZONE BASED ON ANALYSIS OF AFTERSHOCK SEQUENCES Aleš Špičák, Václav Hanuš, Jiří Vaněk Institute of Geophysics, Academy of Sciences of the Czech Republic, Prague MARGINS, June 18 – 22, 2007, Heredia, Costa Rica

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

OUTLINE 1)region of interest 2)global seismological data – their benefits for understanding structure, recent dynamics and tectonic history of convergent plate margins 3)method of hypocentral data analysis 4)aftershock sequences – selection, data analysis, determination of geometrical parameters and tectonic regime 5)conclusions - tectonic interpretation of aftershock sequence parameters 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

16 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

94° 82°90°85°[W] [N] 94° 82°90°85°[W] [N] Cocos Plate Nicaragua Costa Rica Guatemala El Salvador Honduras Mexico Panama Guatemala Basin smooth segment Fischer Ridge seamount segment Cocos Ridge 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) 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 m b ~ 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 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

ISC hypocentral determinations 1964 – 2005 relocated by Engdahl et al. (1998) m < 4 4 < m < < m < 5 5 < m < 6 m > 6 DATA 1 (EHB data) fault plane solutions of Harvard Centroid Moment Tensor Solutions (1977 – 2005) DATA 2 (HCMTS data) 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

Global seismological data reflect SUBFAC operation (SE Asia – Krakatau vers. Merapi volc.) Krakatau volcano depth [km] SSWNNE [km] depth [km] Merapi volcano SN [km] 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

16 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION 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),

) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

10 aftershock sequences in the Wadati-Benioff zone beneath Central America between : aftershock sequenceno.depthm W of eventsrange [km] 1970 Chiapas Osa Nicaragua Quepos Nicoya El Salvador ) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

transformation of coordinates 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

slab dip = 29° slab strike = 294° 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

slab dip = 41° slab strike = 303° 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

10 aftershock sequences in the Wadati-Benioff zone beneath Central America between : aftershock sequenceno.depthm W of eventsrange [km] 1970 Chiapas Osa Nicaragua Quepos Nicoya El Salvador ) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

position of aftershock sequences along the plate margin (epicentral map) 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

sq2000 (Nicoya earthquake) (a) standard epicentral map step 1: transformation of coordinates (b) projection to the slab surface step 2: approximation of foci by a (fault) plane (d) position of approximating planes (fault planes) in the slab (c) vertical section in the slab 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

sq1983 (Osa earthquake) (a) standard epicentral map step 1: transformation of coordinates (b) projection to the slab surface step 2: approximation of foci by a (fault) plane (d) position of approximating planes (fault planes) in the slab (c) vertical section in the slab 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION

1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) 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. 1) REGION OF INTEREST 2) GLOBAL SEISMOLOGICAL DATA 3) METHOD4) AFTERSHOCK SEQUENCES 5) TECTONIC INTERPRETATION