Geofísica Internacional ISSN: 0016-7169 [email protected] Universidad Nacional Autónoma de México México Frischbutter, A. Structure of the Managua graben, Nicaragua, from remote sensing images Geofísica Internacional, vol. 41, núm. 2, april-june, 2002, pp. 87-102 Universidad Nacional Autónoma de México Distrito Federal, México Available in: http://www.redalyc.org/articulo.oa?id=56841202 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative Geofísica Internacional (2002), Vol. 41, Num. 2, pp. 87-102 Structure of the Managua graben, Nicaragua, from remote sensing images A. Frischbutter GeoForschungsZentrum Potsdam, Potsdam, Germany Received: July 10, 2001; accepted: March 15, 2002 RESUMEN Como resultado de varias interpretaciones de datos de sensores remotos combinados con datos sismológicos del terremoto de 1972, se llega a la conclusión que la zona de fallas del lago de Managua es una estructura compuesta de importancia regional con dirección norte-sur. El borde este de la zona de fallas del lago de Managua está formado por estructuras de menor orden en que la importancia de movimiento lateral se disminuye hacia el este favoreciendo estructuras de fallamiento normal. Las fallas N-S se presentan en su mayoría como fallas oblicuas con componentes dextrales de movimiento lateral. En combinación con fallas marginales en dirección NW de la depresión de Managua, resulta un régimen de transpresión de la zona del lago de Managua. Movimientos sinistrales en estructuras con rumbos de NNE a NE fueron reproducidos claramente en datos sismológicos del terremoto de 1972 (extensión en substructuras: por ejemplo el graben de Managua, el graben del Aeropuerto). Planos con rumbo N, NW y NE se consideran activados recientemente dentro de la región de interés. Del punto de vista de la amenaza sísmica hay que tomar en cuenta las relaciones cinemáticas. Por su régimen extensional los planos con rumbo NE tienen importancia especial. En los planos bajo compresión en dirección NW son possibles mayores concentraciones de estrés. Se postula que en el evento de 1972 se activó solamente la parte oeste del graben de Managua. Como consecuencia de esta reactivación - además de otras deducciones - aumentó posiblemente la concentración del estrés a lo largo de la parte este del graben. PALABRAS CLAVE: Percepción remota, neotectónica, sismología, graben de Managua, Nicaragua. ABSTRACT From remote sensing interpretations, combined with seismological data of the 1972 earthquake, the Managua Fault Zone is seen as a N-S - trending, composite structure of regional importance. The eastern margin of the Managua Fault Zone is formed by lower-order, N-S - trending graben structures such as the Managua graben or the Airport graben, which are bordered by oblique faults. The intensity of lateral movements on these N-S trending faults seems to decrease to the east as normal faulting increases. Lateral components on N-S trending faults are mainly dextral in recent times. In combination with the NW - trending marginal faults of the Nicaragua depression this represents a transpressional regime for the Managua Fault Zone. Components of left lateral movements on NNE- to NE - trending structures are clearly indicated from seismological data for the 1972 Managua earthquake (extension within substructures like the Managua graben, Airport graben). N-, NW- and NE - trending planes should be regarded as recently active in the region. These kinematic relations must be taken into account for seismic hazard assessment: NE - trending planes are emphasized because of their extensional regime. It is postulated that during the 1972 event only the western part of the Managua graben was activated. Higher recent stress concentration along the eastern margin of the graben may be a consequence, among others. KEY WORDS: Remote sensing, neotectonic, seismology, Managua graben, Nicaragua. INTRODUCTION sults until 1982, recording more than 15 000 local and re- gional earthquakes. It collapsed in the following years, due Nicaragua, situated in Central America between the to economical, political and military crises that struck the Pacific ocean and the Caribbean sea, became a region of in- country. Similar problems finally inhibited the continuation tense geoscientific research after the devastating earthquake of geoscientific data collection, the full use of the new data (M=6.2) in December 1972 that destroyed Managua, the capi- base and of new emergent techniques for the evaluation of tal of the country. In the years after the earthquake several the seismic risk of Managua and all of Nicaragua. seismological, geological and tectonic projects have investi- gated the causes of the event and the implications for the In the nineties the seismic network was reestablished seismic risk of Managua. In 1975 a seismological observa- on a higher technical level and geoscientific research began tion network was installed, which worked with excellent re- to emerge mainly under the auspices of the Instituto 87 A. Frischbutter Nicaraguense de Estudios Territoriales (INETER), the lead- posed, starting from Jurassic times (but at least since Creta- ing geophysical institution of the country. Besides trying to ceous): rifting in connection with the separation of Laurasia improve its own research capabilities, the institute is co-op- and Gondwana, the recent convergence in NW- to WNW - erating with many geoscientific researchers from various direction between the South- and North-American plates as countries, especially in the field of seismology, tectonics and well as subduction of the Pacific- under the American plates. seismic hazard assessment. The triangular Cocos plate is bordered in the west by the East Pacific rise against the Pacific plate, towards the south This paper presents initial results of a project carried against the Nazca plate by the Galapagos rift and in the North- out in the context of German activities related to the Interna- east against the Caribbean- and North-American plates by tional Decade of Natural Disaster Reduction (IDNDR). In- the Central American subduction zone along the Middle terpretations of remote sensing images are made, especially America trench. The territory of Nicaragua represents the with respect to features of the structural pattern, to obtain a southern part of the Chortis block, which is bordered to the part of the basic information required for seismic hazard as- NW by the sinistral Motagua-Polochic-Faults (including the sessments for the territory of Nicaragua. Cayman trough) and in the Southeast by the dextral Hess- Escarpment (Figure 1). REGIONAL GEOLOGICAL SUMMARY The crustal structure of Nicaragua is the result of an interaction of several plates, starting with the breakup of The interaction of the Cocos-, Nazca- and Caribbean Pangea at 250 Ma (Burke et al., 1984 ), followed by the col- plates between the North- and South-American plates pro- lision of the Farallon plate with Protocaribbean crust (primi- duces a complicate structural pattern of the Earth’s crust in tive island arc tholeiites) in Albian to Santonian times Central America. Several geotectonic events are superim- (Seyfried et al., 1991). The crust beneath the Caribbean sea Fig. 1. Sketch of the regional geological setting of Nicaragua. The chain of active volcanoes (dotted line) as well as average drift velocities for different positions are shown. 88 Structure of the Managua graben was considered as a remnant of an oceanic flood basalt pla- Caribbean plate. Volcanic activity in Central America dates teau, wedged between the North- and South American plates back to Quaternary times. The magmatic arc consists mainly (Sinton et al., 1998). The Farallon plate was broken into the of basaltic-andesitic volcanic rocks (predominantly Cocos- and the Nazca-plates in Lower Miocene times. An pyroclastics with subordinated solid lava sheets). Moreo- old medium- to low grade, partly highly deformed ver, the belt follows the axis of the Nicaragua depression. metamorphicum of probably Paleozoic age, including gra- The Nicaragua depression is a morphologically well ex- nitic to intermediate intrusive rocks (Bermúdez et al., 1992) pressed NW-SE - trending lowland between El Salvador is involved in the magmatic arc evolution of the crust along and Costa Rica (up to 50 km wide), which was interpreted the border with Honduras (Figure 2). The Atlantic coastal as a tectonic half-graben by McBirney & Williams (1965) plain is formed mainly by Miocene to Quaternary up to or a full graben structure due to coast-parallel strike slip Holocene deposits, overlying a Tertiary, sedimentary-mag- faulting (Cruden, 1989). Van Wyk de Vries (1993) explained matic succession, which continues towards the east as the the depression by isostatic readjustment related to the mass Nicaragua ridge under the Caribbean sea. Within the Inner of Tertiary volcanics deposited in the Interior Highlands. Highlands of Nicaragua are widespread volcanic rocks of The depression is filled with Miocene to Quaternary, pre- Tertiary age, which generally become younger from east to dominantly volcano-clastic deposits. The marginal struc- west and parallel to the Middle America trench. tures of the depression are covered mostly by the youngest deposits. The thickness of Holocene deposits is in the or- A belt of volcanoes